|1.0.0 Introduction to the Process||9.0.0 Welding Procedure Variables|
The gas tungsten arc welding (GTAW) process, also known as tungsten inert gas (TIG) welding, uses a non-consumable tungsten electrode to produce the weld. A shielding gas (usually an inert gas such as argon), protects the weld area from atmospheric contamination, and the process normally uses a filler metal, though some welds, known as autogenous (aw-TOJ-uh-nuhs) welds, do not require a filler metal.
A constant-current welding power supply produces energy that is conducted across the arc through a column of highly ionized gas and metal vapors known as plasma. Welders most commonly use TIG to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys.
TIG provides the welder with greater control over the weld than competing procedures such as shielded metal arc welding (SMAW) and gas metal arc welding (GMAW), thus allowing for stronger, higher quality welds. However, GTAW/TIG is comparatively more complex and difficult to master (closer tolerance requirements and filler metal usually added by other hand), and is significantly slower than most other welding techniques as well.
This course will present a basic understanding of the GTAW/TIG process and equipment, along with the key variables that affect the quality of welds. It will also cover core competencies such as setting up equipment, preparing materials, fitting up, starting an arc, welding pipes and plates, and repairing welds. Lastly, you will get an understanding of the safety precautions for GTAW/TIG and an awareness of the importance of safety in welding.
Although this course is very comprehensive, always refer to the manufacturer’s manuals for specific operating and maintenance instructions.
Gas tungsten arc welding (GTAW) is an arc welding process that produces coalescence of metals by heating them with an arc between a tungsten (non-consumable) electrode and the work. Shielding comes from a gas or gas mixture (Figure 1). Both pressure and filler metal may or may not be used. This process is also known as TIG welding, which stands for tungsten inert gas welding, unless you are working in Europe, where you may hear it called WIG welding, using Wolfgram, the German word for tungsten. Throughout this course, the process will be referred to as TIG.
Figure 1 — Gas tungsten arc welding.
The gas tungsten arc welding process is very versatile. This process may be used to weld and a wide variety of metals. It is an all-position welding process. Welding in other than flat positions depends on the base metal, the welding current, and the skill of the welder. The process was developed for the "hard-to-weld" metals and can be used to weld more different kinds of metals than any other arc welding process.
Gas tungsten arc welding has an arc and a weld pool clearly visible to the welder. It produces no slag for entrapment in the weld, and no filler metal carries across the arc, so there is little or no spatter. Because the electrode is non-consumable, you can make a TIG weld by fusing the base metal without a filler wire.
The TIG welding process was invented by Russell Meredith of Northrop Aircraft’s welding group in 1941. Mr. Jack Northrop's dream was to build a magnesium airframe for lighter, faster warplanes. This new process was called "Heliarc," as it used an electric arc to melt the base material and helium (He) to shield the molten puddle. The Linde Division of Union Carbide bought the patents, developed a number of torches for different applications, and sold them under the brand name Heliarc. Linde also developed procedures for using argon (Ar) gas, a more readily available and less expensive gas than helium.
At first, only direct current with a positive electrode was used. However, the electrode tended to overheat and deposit particles of the tungsten electrode in the weld. This ferrous non-ferrous problem was overcome by making the electrode negative, which then also made it satisfactory for welding stainless steel.
During World War II, welding machines producing alternating current and high frequency stabilization were developed. Alternating current with a superimposed high frequency, high voltage current over the basic welding current achieved good quality welding of aluminum and magnesium. With helium largely replaced by argon due to its greater availability, the gas tungsten arc welding process became more widely accepted by the early 1950s, and today is classified by the American Welding Society by that term.
Welders can apply the gas tungsten arc welding process by the manual, semiautomatic, machine, or automatic methods, although the manual method produces the greatest majority of work; the torch is operated by hand, and filler metal, if used, is added with the other hand. A foot pedal is an additional refinement that controls the amount of welding current and switches the current on and off. TIG allows the welder extreme control for precision work by very closely controlling the heat and accurately directing the arc.
Operators can also use TIG semi automatically, that is by operating the torch by hand with a wire feeder adding the filler metal automatically. Semiautomatic gas tungsten arc welding is rarely used; however, machine and automatic methods are becoming increasingly popular for many applications. TIG machine welding occurs when equipment performs the welding only under the control and observation of the welding operator.
Automatic welding occurs when the equipment performs the welding without adjustment or control by a welding operator. The amount of automation or mechanization applied to the process depends on the accessibility of the joint, quality control requirements, number of identical welds to be made, and the availability of capital.
TIG welding generally produces welds far superior to those produced by metallic arc welding electrodes. Especially useful for welding aluminum, it is quite useful for welding many other types of metals as well. The TIG process is most effective for joining metals up to 1/8 inch thick, although you can use it to weld thicker material with appropriate preheating.
Gas tungsten arc welding has many advantages over most other types of welding processes. The outstanding features are the following:
- It makes high quality welds in almost all metals and
- There is no slag, so very little, if any, postweld cleaning is required.
- There is no filler metal carried across the arc, so there is little or no spatter.
- Welding can be performed in all positions.
- Filler metal is not always required.
- Pulsing may be used to reduce the heat input.
- The arc and weld pool are clearly visible to the welder.
- Because the filler metal does not cross the arc, the amount added is not dependent on the weld current level. alloys.
The limitations of the gas tungsten arc welding process include the following:
- The welding speed is relatively slow.
- The electrode is easily contaminated.
- It is not very efficient for welding thick sections because deposition rates are low.
- The arc requires protection from wind drafts that can blow the stream of shielding gas away from the arc.
- To Table of Contents -
TIG uses the heat produced by the arc between the non-consumable tungsten electrode and the base metal. An inert shielding gas supplied through the torch shields the molten weld metal, heated weld zone, and non-consumable electrode from the atmosphere. The gas protects the electrode and molten material from oxidation, and provides a conducting path for the arc current.
An electric current passing through an ionized gas produces an electric arc. In this process, the inert gas atoms are ionized by losing electrons and leaving a positive charge. Then the positive gas ions flow to the negative pole and the negative electrons flow to the positive pole of the arc. The intense heat developed by the arc melts the base metal and filler metal (if used) to make the weld. As the molten metal cools, coalescence occurs and the parts join.
There is little or no spatter or smoke. The resulting weld is smooth and uniform, and requires minimum finishing (Figure 2).
Figure 2 — TIG process.
You do not need to add filler metal when welding thinner materials, edge joints, or flange joints. This is known as autogenous welding. For thicker materials, an externally fed or "cold" filler rod is generally used. The filler metal in gas tungsten arc welding does not transfer across the arc, but is melted by it.
You strike the arc in one of three ways:
- By briefly touching the electrode to the work and quickly withdrawing it a short distance.
- By using an apparatus that will cause the arc to jump from the electrode to the work.
- By using an apparatus that starts and maintains a small pilot arc. This pilot arc provides an ionized path from the main arc.
The torch then progresses along the weld joint manually or mechanically after remaining in one place until a weld puddle forms. Once the welder obtains adequate fusion, the torch moves along the joint so the adjacent edges join and the weld metal solidifies along the joint behind the arc, thus completing the welding process.
The TIG process uses a constant current power source, either direct or alternating current. A constant current welding machine provides nearly constant current during welding, so both stick (SMAW) and TIG (GTAW) can operate from the same power supply. The exception is that you do not need a high frequency attachment, often added for gas tungsten arc welding, to scratch start the arc.
The constant current output is obtained with a drooping volt-ampere characteristic, which means that the voltage is reduced as the current increases. The changing arc length causes the arc voltage to increase or decrease slightly, which in turn changes the welding current. Within the welding range, the steeper the slope of the volt-ampere curve, the smaller the current change for a given change in the arc voltage. Figure 3 shows volt-ampere curves for different welding machine performance characteristics. This shows several slopes, all of which can provide the same normal voltage and current.
Figure 3 — Volt-ampere curves.
Differences in the basic power source design cause the variations in power sources. A machine with a higher short circuit current will give more positive starting. A steep volt-ampere characteristic is generally the most desirable when the welder wants to achieve maximum welding speeds on some welding jobs. The steeper slope gives less current variation with changing arc length, and gives a softer arc.
The types of machines that have this kind of curve are especially useful on sheet metal. These types of machines are also typically used for welding at high current levels. On some applications, such as all-position pipe welding, a welder may want a less steep volt-ampere characteristic for better arc control with high penetration capability. Machines with a less steep volt-ampere curve are also easier to use for depositing the root passes on joints that have varying fitup. This power source characteristic allows the welder to control the welding current in a specific range by changing the arc length. This type of machine also produces a more driving arc.
|Test Your Knowledge
1. The predominant shielding gas used for TIG is _____.
2. How is the arc struck using the manual TIG process?
- To Table of Contents -
A typical TIG welding system usually consists of the following elements:
- Welding power supply
- Welding torch
- Tungsten electrode
- Welding cables
- Gas shielding system
Since welders can apply TIG by various methods with a wide variety of equipment configurations, often they will include several available items of optional equipment such as water circulators, foot rheostats, programmers, motion devices, oscillators, automatic voltage controls (AVC), and wire feeders. Figure 4 shows a diagram of the equipment used for a manual welding setup.
Figure 4 — Equipment for gas tungsten arc welding.
The purpose of the power source or welding machine is to provide the electric power of the proper current and voltage to maintain a welding arc. Manufacturers offer several various sizes and types of power sources for gas tungsten arc welding. Most of these power sources operate on 230 or 460 volt input electric power. Power sources that operate on 200 or 575 volt input power are available as options.
The duty cycle of a power source is defined as the ratio of arc time to total time. For rating a welding machine, a ten minute time period is used. Thus, for a machine rated at a 60% duty cycle, the rated welding current load could be safely applied continuously for six minutes and be off for four minutes. Most power sources used for gas tungsten arc welding have a 60% duty cycle. For the machine and automatic methods, a welding machine with 100% duty cycle rating would be best, but these are not normally available.
The formula for determining the duty cycle of a welding machine for a given current load is:
|% Duty Cycle =||(Rated Current)2||x Rated Duty Cycle|
For example, if a welding machine is rated at a 60% duty cycle at 300 amperes, the duty cycle of the machine when operated at 250 amperes would be.:
|% Duty Cycle =||(300)2||x 60 = 86%|
Figure 5 represents the ratio of the square of the rated current to the square of the load current, multiplied by the rated duty cycle. This chart can be used instead of working out the formula. A line is drawn parallel to the sloping lines through the intersection of the subject machine’s rated current output and rated duty cycle. For example, a question might arise whether a 300 amp 60% duty cycle machine could be used for a fully automatic requirement of 225 amps for a 10-minute welding job. The chart shows that the machine can be safely used at slightly over 230 amperes at a 100% duty cycle. Conversely, there may be a need to draw more than the rated current from a welding machine, but for a shorter period. This graph can be used to compare various machines. All machines should be rated to the same duty cycle for comparison.
Figure 5 — Duty cycle vs. current load.
The type of power source determines the type of current available. The most important factor in selecting the type of current is the type of metal to be welded. The thickness of the metal can also have an influence. You can use either alternating or direct current for both gas tungsten arc welding and high frequency arc ignition, and you may pulse the welding current.
You can connect direct current in one of two ways: electrode negative (straight polarity) DCEN or electrode positive (reverse polarity) DCEP. The electrically charged particles flow between the tip of the electrode and the work (Figure 6). You can use electrode negative for welding all metals.
Note: In AC Arc, 50% of heat in Electrode and 50% in Base Metal
Figure 6 — Negative and positive polarity.
Follow special procedures to weld alloys of magnesium and aluminum, which have a refractory surface oxide that hinders their fusion. You can make welds on aluminum and magnesium with a short arc length using electrode negative and a helium-bearing shielding gas, but you can weld these metals more easily by using electrode positive because this connection breaks down the oxide layers on the surfaces. The main problem with using electrode positive is that the current carrying capacity of the electrode is extremely low. In fact, the electrode will begin to melt if the currents are too high. For this reason, you should rarely use electrode positive except for welding thin sheet metal.
The pulsed current method of TIG employs two levels of welding current instead of a steady current. The welding current switches periodically between the high and low levels to produce a pulsating current or arc. See Figure 7 for a diagram of pulsed direct current. This pulsed current produces a continuously welded seam consisting of overlapping arc spot welds. Figure 8 shows a cross-sectional view of the pulsed current weld bead. Each of the spots is produced by the high level welding current after which the current is switched to the lower level. This lower level allows the weld to solidify partially between spots and maintains the arc to avoid re-ignition problems. Pulsed current may be used with direct or alternating current, but it is most commonly used with direct current.
Figure 7 — Pulsed current terminology.
Figure 8 — Weld produced by pulsed current.
The pulsed direct current method of gas tungsten arc welding has several advantages over steady direct current for welding thin materials. The pulsed method is more tolerant of edge misalignment, normal fixturing can be used with thinner materials, and it gives better distortion control and root penetration. For open root welding, the high pulse provides high current for complete penetration, but the low pulse cools the puddle down to prevent burning through at the root of the joint. Pulsing reduces the heat input to the base metal. This is particularly good for welding thin stainless steel sheet metal, which distorts very easily without pulsed current. Another advantage of pulsed current is that it is very good for welding in the vertical and overhead positions because good penetration is obtainable with less heat input. Pulsing keeps the weld puddle from getting too large to control because of the partial solidification that occurs during the low current.
The number of pulses used can vary from about ten per second down to about one or one-half per second. The length of time the high current is on and the length of time the low current is on are variable, as well as the percentage of low current with respect to the high current.
Alternating current is a combination of both polarities that alternate in regular cycles. In each cycle, the current starts at zero, builds up to a maximum value in one direction, decays back to zero, builds up to a maximum value in the other direction, and decays back to zero. The arc goes out during the zero portion of the cycle, so a high frequency current in the welding circuit reignites the arc.
Using alternating current provides the advantages of both direct current electrode positive (reverse polarity) without the current limitations, and direct current electrode negative without the oxide cleaning problems. For this reason, welders generally use alternating current for manual welding aluminum and magnesium.
However, in the alternating current circuit, there is a tendency for the current to become unbalanced. The arc current flows more easily in one direction because it takes greater energy to obtain electrons from the base metal than from the tungsten electrode. The tungsten electrode emits electrons more easily because it becomes much hotter during welding than the base metal does. The amplitude of the current in the cycle, when the electrode is negative, is normally higher than it is during the cycle when the electrode is positive. This tends to produce an unbalanced current. Operators can use either series connected capacitors or insert a direct current voltage in the welding circuit to balance the current. Balanced current is desirable for some applications like high-speed mechanized welding, but it is not necessary for most manual welding applications. Balanced current flow has three main advantages:
Better oxide-cleaning action
Better and smoother welding action
No reduction in the output rating of a conventional welding transformer
Disadvantages of a balanced current flow are the following:
The high-frequency current is a separate, superimposed current used to maintain a pilot arc and help start the arc. The pilot arc does not do any welding, but it is needed to start the welding arc without touching the electrode to the work when using either direct or alternating current.
When using alternating current, the high frequency current keeps the arc from going out when the alternating current changes cycles, from positive to negative or negative to positive.
When using direct current, the high frequency only helps to start the arc and may be turned off after establishing the arc. Using a high frequency current is the best starting method because touching the tip of the electrode to the work or starting on a piece of carbon can contaminate the tungsten electrode.
When using this superimposed high frequency current with AC TIG, you need to take certain precautions because the high frequency spark gap oscillators in the power sources radiate power at frequencies that can interfere with commercial, police, and aviation radio broadcasts. It can also interfere with television transmissions. Because of this, the operation of high frequency for AC is subject to control by the Federal Communication Commission in the United States, and most other countries have similar regulations.
When installing a welding machine that uses high frequency stabilizers, you must pay special attention to provide earth grounding and special shielding. Manufacturers provide special installation instructions that also require all metal conductors in the area of the machine to be earth grounded. These requirements help limit high frequency radiation. If you follow these instructions carefully, you can post a certificate stating that you reasonably expect the high frequency stabilizer to meet FCC regulations.
Constant current (cc) machines can produce AC or DC welding power; they can be rotating (generators), static (transformer/rectifier), or three phase inverter machines.
For shop use, an electric motor can power a generator welding machine, or an internal combustion engine (gasoline or diesel) can do it for field use. You can adjust generator welding machines intended for shielded metal arc welding to function for gas tungsten arc welding if you add an inert gas and a high frequency attachment.
You can adapt engine-driven, either water- or air-cooled welding machines as well, many of which also provide auxiliary power for emergency lighting, power tools, etc. Generator welding machines can provide DC power, and in some cases both AC and DC power to the arc, depending on the machine design.
You can also adapt alternator welding machines (also called rotating or revolving field machines) for gas tungsten arc welding. These machines consist of an electric generator made to produce AC power.
Transformer-rectifier welding machines are used much more widely for gas tungsten arc welding than motor-generator welding machines. Transformer-rectifier machines provide both AC and DC welding current to the arc. A single phase transformer producing alternating current is connected to the rectifier, which then produces DC current for the arc. The rectifier is an electrical device which changes alternating current into direct current.
Transformer-rectifier welding machines operate on single phase input power (Figure 9), and because of this, an unbalance may be created in the power supply lines, which is objectionable to most power companies.
Figure 9 — Welding machine.
However, this type of welding machine is the most versatile for TIG because you can use it for welding a variety of base metals. A programmable type of transformer-rectifier power source is often used for TIG welding; the welder can select either AC or DC current for the application by simple means of a switch which can change the output terminals to the transformer or to the rectifier.
The transformer-rectifier welding machines are available in different sizes and have several advantages over rotating power sources:
- Lower operating costs
- Lower maintenance costs
- Quiet operation
- Lower power consumption while idling
- No rotating parts
A recently developed machine uses the inverter and different levels of programming. These machines operate on three-phase input power. The three-phase input helps overcome the line unbalance that occurs with the single-phase transformer-rectifier machines. Inverters provide power down to .5 ampere with a very fast response time of one millisecond and less than 1 % ripple. Different programming is available, depending on the complexity of the job. The high frequency inverters are very quiet and provide outstanding arc stability.
Transformer welding machines are not used often for gas tungsten arc welding except at home shops or small job shops where gas tungsten arc welding is used only occasionally. Transformer welding machines produce AC power only and operate on single-phase input power. Like generator welding machines intended for SMAW, you can also adapt transformer welding machines for TIG by adding an inert gas and a high frequency attachment.
The transformer welding machine takes power directly from the line, transforms it to the power required for welding, and by means of various magnetic circuits, inductors, etc., provides the volt-ampere characteristics proper for welding. The main advantage of the transformer is that it has the lowest initial investment cost and uses electric power efficiently. However, movable parts tend to vibrate, wear, and become loose, which creates undesirable noise.
To overcome the arc extinguishing-restriking problem, a square wave AC output power source was developed. Either the conventional constant current type or the constant voltage type of power source can use the square wave output form. In either case, the time for switching from positive to negative or negative to positive current pulse is approximately 50 to 150 microseconds; thus the arc is difficult to restart and is unstable.
Power electronics can be used to vary the positive and negative output of the machine. The area above the zero point on the curve (the direct current positive area) and the area below the curve (the negative area) can be equalized or balanced.
A power source developed specifically for gas tungsten arc and plasma arc welding provides a square-wave output form but also allows a balance or imbalance between the straight polarity and reverse polarity half-cycles of each cycle.
In welding aluminum, the electrode negative (straight polarity half-cycle) gives maximum penetration, whereas the electrode positive (reverse polarity half-cycle) provides for the cleaning action. It is advantageous to provide the most straight polarity half-cycle, and this is possible, as shown in Figure 10. This machine also has programming ability and encloses a high-frequency oscillator plus gas and water valves.
Figure 10 — Square wave output: balanced and unbalanced.
TIG welding machines have some or all of the following controls to operate the welding:
- On-off power switch.
- Polarity selection switch — for machines that produce DC power.
- Welding current control — a knob or tap switch on the front of the welding machine that controls the amount of welding current delivered to the arc.
- Foot pedal — an optional piece of equipment for manual welding. It starts the current flow, varies the current during welding, and reduces the current at the end of the weld. This control also starts the high frequency current when high frequency current is used.
- High frequency control — turns the high frequency current on and off, and selects the type of high frequency current used. Continuous high frequency current is used for AC welding where high frequency current is needed only for arc starting with DC welding current. Also included is a knob to control the amount of high frequency current.
- Hot start — a knob on some welding machines. When in use, this control causes the machine to furnish momentarily a surge of current substantially above the welding current to get the arc initiated. The knob can also set the amount of “hot start” current required.
- Pulsation controls. When pulsed current is desired, several controls are usually needed.
- Up-slope and down-slope controls — optional controls that are timers. The upslope control allows the welding current to build up gradually at a set rate at the beginning of the welding. The down-slope control allows the welding current to decay gradually at a set rate at the end of the welding to prevent crater cracking.
- Shielding gas controls — timers that can be set to start the flow of shielding gas before the welding current starts and to maintain gas shielding after the welding arc has been broken. Both of these controls are used to prevent oxidation of the tungsten electrode and contamination of the weld puddle when hot.
Several or all of these controls are used with a programmable panel (Figure 11) and are available in wide variety depending on the programmer used.
Figure 11 — Programmer.
Torches for TIG welding, designed and used only for this process, are available in a variety of types and sizes. The torch conducts the welding current to the arc and the shielding gas to the arc area. It usually includes various cables, hoses, and adaptors for connecting the torch to the power, gas, and cooling supplies. Manual torches should also have a handle so the welder can manipulate the arc. Figure 12 shows a manual gas tungsten arc welding torch. Manual torches can weigh from as little as three ounces (85 grams) to about sixteen ounces (450 grams), and are rated according to their maximum usable welding current. These torches can utilize various types and sizes of electrodes and nozzles while the angle of the electrode to the handle (the head angle) may vary from torch to torch. The most common head angle is 120 degrees, but some torches use 90- degree head angles and others have adjustable heads.
Figure 12 — Manual TIG torch.
There are two major types of welding torches used for TIG: air-cooled and water-cooled. The air-cooled torches are cooled by the flow of the shielding gas (which means that they really are gas-cooled). The only air cooling occurs from the heat radiating into the atmosphere.
Water-cooled torches have water circulating through the torch, which accounts for most of the cooling (Figure 13); the shielding gas does the rest. Air-cooled torches are usually small, lightweight, and less expensive than water-cooled torches, and with a maximum welding current of 200 amperes, they are used normally for welding thin metal. These torches are more versatile than water-cooled torches because no water is needed, but they are for low duty cycle welding because the tungsten electrode in an air-cooled torch becomes hotter than in a water-cooled torch, which can transfer tungsten to the weld, thus causing inclusions. Water-cooled torches can operate continuously up to about 200 amperes, with some especially designed for welding currents up to 500 amperes. These torches are usually heavier (water hose and connectors usually come with the torch) and more expensive than the air-cooled types.
Figure 13 — Cross-section view of water-cooled torch.
There are four types of nozzles or gas cups used for gas tungsten arc welding: ceramic, metal, fused-quartz, and dual-shield nozzles. They provide shielding gas to the welding electrode and metal. As a general rule the inside diameter of the gas nozzle should be three times larger than the electrodes diameter. Ceramic nozzles are the cheapest and most popular type, but they are brittle.
Ceramic nozzles are the best kind to use with high frequency current to prevent cross-firing to the nozzle.
Metal nozzles can be either the slip-on type or the water-cooled type. The slip-on type is limited to low current welding, whereas the water-cooled nozzles are usable with high welding current.
Fused-quartz nozzles are transparent and some welders prefer them for increased visibility, but the inside of the nozzle can be dulled by vapors when the electrode is contaminated, which impairs the vision.
Dual-shield nozzles allow a small amount of helium or argon around the electrode to shield the immediate weld puddle. Around the central part of the nozzle, an annular grooved section sends an atmosphere of carbon dioxide or nitrogen to keep air from contact with the central inert-gas shield. The industry rarely uses the dual-shield nozzle.
Inside the nozzle is the gas orifice. The gas orifice is a series of holes in the end of the collet body around the electrode that supplies the shielding gas into the nozzle. This gives a more even flow of shielding gas around the electrode (Figure 14).
Figure 14 — Parts of a manual torch.
Orbital welding heads are designed specifically to produce high quality welds in critical welding applications (Figure 15). Because companies related to the aircraft, pharmaceutical, semiconductor, food processing, and related industries require superior weld quality in terms of bead shape, integrity, and cleanliness, these advanced systems incorporate computer technology to control the variables in a weld.
Figure 15 — Tube-to-tube welding heads.
Torch oscillation speed and width are independently adjustable and automatically synchronized to allow precise positioning of filler wire entry into the weld puddle, and compact wire feeders are controlled electronically for accuracy and repeatability.
Single cylinders, portable or stationary manifold systems, or pipes connected to bulk storage torches may supply the shielding gas. The most widely used form of gas flow control is the combination regulator and flowmeter (Figure 16). Flowmeters must be appropriate for the various shielding gases because they must be calibrated for a specific gas. Use only the regulators and flowmeters designed for a specific gas. There is a fundamental difference between the regulators used for oxy-fuel welding and those used for TIG/MIG welding. While both have a gauge that provides a tank/cylinder pressure and a second gauge, with oxy-fuel welding, the second gauge displays pressure as the working unit, and with TIG/MIG, the second gauge displays flow and the working unit. The working pressure on the oxy-fuel regulator is in pounds per square inch (psi), while the regulator for TIG/MIG is in cubic feet per hour (cfh) or liters per minute (lpm). See Figure 16.
Figure 16 — Regulator and flowmeter.
The flowmeter consists of a plastic or glass tube that contains a loosely fitting ball. As the gas flows up the tube, it passes around the ball and lifts it up: the more gas that moves up the tube, the higher the ball lifts.
The shielding gas regulator has a constant outlet pressure to the flowmeter of about 50 psig. This is important because the flowmeter scales are accurate only if the gas entering them is at that approximate pressure. If you use higher inlet pressures, the gas flow rate will be higher than the actual reading. The reverse is true if the inlet pressure is lower than 50 psig; therefore, it is important to use accurately adjusted regulators. With an accurate flowmeter, these regulators can deliver inert gas flows up to 60 cfh; read the scale by aligning the top of the ball with the cfh increment lines.
To obtain an accurate reading, you must mount the meter in a vertical position. Any slant will create an off-center gas flow and result in an inaccurate reading. As already mentioned, you need to use different flowmeters for different gases.
The flow of gas necessary for good TIG welding depends primarily on the thickness of the material, but there are other factors as well, including welding current, size of nozzle, joint design, speed of welding, and a draft-free area in the location of the welding. This last factor can affect gas coverage and usage considerably
Plastic hoses bring the shielding gas to the welding torch because helium will diffuse through the walls of rubber or rubber-fabric hoses. To standardize the hose system, these same plastic hoses are used for argon also. They may connect straight to the torch, or go through the power source or the inert gas attachment to the torch.
The welding cables and connectors connect the power source to the torch and to the work, essentially the same as those used for SMAW. The cables are normally made of copper or aluminum and consist of hundreds of fine wires enclosed in an insulated casing of natural or synthetic rubber. The cable connecting the work to the power source is the work lead, which typically connects to the work by pincher, clamps, bolt, or special connection. The cable connecting the torch to the power source is the electrode lead, and it is part of the torch assembly.
The size of the welding cable used depends on the output capacity of the welding machine and the distance between the welding machine and the work. Cable sizes range from the smallest at NO. 8 to AWG No. 4/0 with amperage ratings of 75 amperes and upward. Table 9-1 shows recommended cable sizes for use with different welding currents and cable lengths.
Table 9-1 ─ Suggested copper welding cable sized for
gas tungsten arc welding.
TIG is a very versatile process, and because of its versatility, there is a need for multiple types of torches, wire feeders, water circulators, and motion devices. The following presents some of the most common devices.
When you use semiautomatic, machine, and automatic welding, and a filler metal is necessary, you need a filler wire feeder. For manual welding, you feed the filler metal by hand. You can feed filler metal into the pool either preheated (hot) or at room temperature (cold).
A cold wire feeding system consists of a wire drive mechanism, a speed control, and a wire guide attachment that directs the wire into the molten weld pool. The wire drive consists of a motor and gear train, which power a set of drive rolls to push the filler wire. A constant speed governor, either electronic or mechanical, functions as the wire feed speed control, and a flexible conduit connected to the drive mechanism usually guides the filler wire to the weld puddle. Often, the wire guide attaches to the torch, and it maintains the angle of approach to the weld puddle. For heavy duty applications, the wire guide is water-cooled.
Filler wires used for this application range from 1/32 inch (0.8 mm) to 3/32 inch (2.4 mm) in diameter. Generally, cold wire feeds into the leading edge of the weld puddle.
The equipment for a hot wire system is similar to that for cold wire, except it electrically preheats the wire with an alternating current from a constant voltage to the desired temperature before it reaches the weld pool. In many cases, a shielding gas protects the filler wire from oxidation.
The TIG hot wire method will give a high deposition rate comparable to using MIG. Sometimes this method is used to weld carbon and low alloy steels, stainless steels, copper alloys, and nickel alloys. Feed hot wire into the trailing edge of the weld puddle, but do not use hot wire for aluminum, aluminum alloys, and copper; they require very high heating currents which cause uneven melting and arc blow.
When you use a water-cooled torch, you must have a continuous water supply via a water circulator or directly from a hose connection to a water tap. Hoses, which may or may not go through a valve in the welding machine, carry the water to the welding torch. Figure 17 shows a water circulator.
Figure 17 — Water circulator.
Machine welding and automatic welding use motion devices to move the welding head, workpiece, or torch depending on the type and size of the work and the preference of the user.
Often, motor-driven carriages run on tracks or directly on the workpiece. Carriages are useful for straight line contour, vertical, or horizontal welding. Side beam carriages are supported on the vertical face of a flat track, and they can be used for straight line welding.
You can use welding head manipulators for longitudinal welds and, in conjunction with a rotary weld positioner, for circumferential welds. These welding head manipulators come in many boom sizes and can be used also for semiautomatic welding with mounted welding heads.
Oscillators are optional equipment used to oscillate the torch for surfacing, vertical-up welding, and other welding operations that require a wide bead. Oscillators can be either mechanical or electromagnetic devices.
Orbital heads are compact, rugged, and clamp on a pipe or tube (Figure 18). To weld the smallest to the largest tubes, you will need a family of heads. These heads will rotate the torch around the pipe, continuously carrying the tungsten electrode. Multiple adjustments and computer control allow for precise positioning
Figure 18 — Orbital welding head designed for low clearances.
|Test Your Knowledge
3. What is the most important factor in selecting power supply?
4. Welding cables are most commonly made of which material?
- To Table of Contents -
A basic knowledge of equipment setup, adjustment, and shutdown is necessary to make effective and efficient welds. This section will give you the basics of setup and electrode preparation. Always refer to the manufacturer’s safety precautions and proper tip preparation. Also, always wear your safety glasses when you are in the welding area.
Attach the remote control to the remote control outlet on the power source.
Check torch cables and connect them to the power source.
Select the appropriate electrode for the job.
Ceriated tungstens (orange band) and lanthanated tungstens (black band) are the recommended alternatives to thoriated tungstens for DCEN applications if they are available. Use pure or zirconiated tungstens for AC welding with conventional sine wave or conventional square wave power sources.
Taper the electrodes for DCEN welding to direct and control the arc. The taper angle of the electrode is the included angle. For most applications, a 30° taper about 2 1/2 to 3 electrode diameters long works well (Figure 19).
Figure 19 — Preparing the tip.
Round off pure and zirconiated electrodes for AC welding with conventional square wave power sources to withstand the heat generate during the electrode positive portion of the AC cycle. The rounded tip should not exceed the diameter of the electrode. Otherwise, the arc may wander around the surface, making it hard to control (Figure 20).
Figure 20 — Rounded tip for conventional square wave.
You can use any of the alloyed tungstens for AC welding with inverters because you can adjust the positive portion of the AC cycle to provide just enough amperage for cleaning without overheating the tip of the electrode. Match the collet and collet body to the electrode diameter. Check the nozzle to make sure it is the proper size and is in good condition. The nozzle should be a minimum of 3 times the diameter of the electrode.
Replace nozzles that are chipped, cracked, or badly worn. Damaged or dirty nozzles can alter the gas flow pattern and cause defects or discontinuities in the weld.
- Thread the collet body into the torch head.
- Insert the collet into the collet body.
- Install the nozzle.
- Insert the electrode so the tip extends about 1/2 inch beyond the nozzle.
- Screw the cap into the back of the torch head and tighten it lightly so the electrode will move with finger pressure.
- Adjust the electrode stickout and tighten the cap to secure the electrode in place.
- Place the torch on its hanger so it will not arc when you turn the power switch on. Do not lay it across the welding table.
Refer to Figure 21 for assembly.
Figure 21 — Torch assembly.
Chain the cylinder in place and remove the cap. Stand away from the valve port. Open and close the valve quickly to blow out any dirt before attaching the regulator. Install the regulator and flow meter assembly. Attach the gas hose to the flow meter. Attach the other end of the hose to the connection on the power source. Open the cylinder valve slowly until pressure registers on the regulator; then open the valve all the way. Turn the power source on and tap the foot pedal to start the flow of gas. Adjust the flow meter to approximately 15 to 20 cubic feet per hour (cfh) for argon. Set the post flow time on the power source (1 second for every 10 amps). Test for leaks by closing the cylinder valve. If the regulator pressure drops, check the hose and the connections at the power source, flow meter, and cylinder for leaks.
- Set the amperage control to the maximum setting required for the job.
- Set the high-frequency switch to start (automatic) for DC welding, or to continuous for conventional square wave AC.
- Adjust the high frequency intensity control.
- Run a few test welds on scrap material to fine tune the settings.
Shut the system down when the job is completed.
- Close the valve on the gas cylinder.
- Tap the foot pedal to bleed off the shielding gas.
- Close the valve on the flow meter.
- Turn off the power source.
- Clean up your work area.
- As a safety precaution, turn the power switches on the wire feeder and the power source to the off position before checking electrical connections.
- Check all electrical connections to make sure they are tight, and check cables for cracks and exposed wire.
- To Table of Contents -
The electrodes used for this process are non-consumable, so a tungsten electrode is needed as well as a filler rod if any filler metal is to be added. The shielding gas is an important consumable of gas tungsten arc welding because its main purpose is to shield the electrode and molten weld puddle from the atmosphere. Filler metal may or may not be added, depending on the specific welding application.
TIG uses a non-consumable or nearly non-consumable electrode made of tungsten or tungsten alloys that melt at 6170 degrees Fahrenheit (3410 degrees Celsius), which is the highest melting point of all metals. It is virtually impossible to vaporize a tungsten electrode during welding, provided you use the electrode within the current-carrying capacity range for its specific type and diameter, with sufficient inert shielding gas. Tungsten retains its hardness, even at red heat.
There are several types of electrodes for gas tungsten arc welding. These are made of pure tungsten or alloyed with thoria, zirconia, ceria, lanthana, or a combination of oxides (Table 9-2). Welding electrodes are classified by chemical composition and are identifiable by colored markings in the form of bands, dots, etc. on the surface of the electrode. The AWS classification uses letters to distinguish differences in the electrodes. The first two letters of a tungsten electrode are E for electrode and W for tungsten the next letter represents the material the electrode is made of.
Table 9-2 — Chemical composition requirements for electrodes (AWS A5.12).
|AWS Classification||UNS Numberb||W Min. (difference)c||CeO2||La2O3||ThO2||ZrO2||Other Oxides or Elements Total|
|Notes for Table 2 (above)
Tungsten electrodes usually come in lengths of 3 to 24 inches (76-610 mm) in diameters from .01 to 1/4 inch (.25 to 6.4 mm). Table 9-3 shows the types of tungsten electrodes used for welding different metals. Table 9-4 shows the welding current ranges for tungsten electrodes.
Table 3 — Types of tungsten electrodes and shielding gasses
|Type of Metal||Thickness||Type of Current||Electrode||Shielding Gas|
+Low Alloy Steels
|Stainless Steel||All||DCEN||Thoriated||Argon, Argon-helium|
Table 9-4 — Typical current ranges for tungsten electrodes.
|0.010||0.30||Up to 15||nab||Up to 15||Up to 15||Up to 15||Up to 15|
Generally, you will use pure tungsten electrodes (green marking) on the less critical applications with alternating current; they have a relatively low current-carrying capacity and a low contamination resistance, but they give good arc stability.
The tungsten electrodes alloyed with 1% (yellow marking) or 2% (red marking) thoria have several advantages over pure tungsten electrodes. These electrodes have higher current-carrying capacities, longer life, higher electron emissivity, and greater contamination resistance. Thoriated tungsten electrodes also give easier arc starting and a more stable arc.
Ceriated tungsten electrodes (orange marking) contain cerium oxide and have a reduced rate of vaporization or burn-off, as compared with pure tungsten electrodes.
The EWLa (black marking) electrodes contain lanthanum oxide and are very similar to the ceriated tungsten electrodes. EWZr (brown marking) electrodes contain a small amount of zirconium oxide. Their welding characteristics generally fall between those of pure and thoriated tungsten, but they have a higher resistance to contamination. The EWG (gray marking) electrodes contain an unspecified addition of oxides (rare earth or others) which affect the characteristics of the arc.
Argon and helium or mixtures of the two gases are the most widely used shielding gases for gas tungsten arc welding. The characteristics most desirable for shielding purposes are chemical inertness and an ability to produce smooth arc action at high current densities. Argon and helium are both inert, which means that they do not form compounds with other elements. Inert shielding gas is used because it will protect the tungsten electrode as well as the molten weld metal from contamination. Special applications may call for the addition of hydrogen and nitrogen as well. In addition to showing the types of tungsten electrodes used for welding different metals, Table 9-3 (above) shows the type of shielding gas recommended when welding different metals.
Gas purity can have a considerable effect on welding. Metals such as carbon steel, stainless steel, copper, and aluminum will usually tolerate very small amounts of impurities. For the best results, the purity rating should be 99.99+%. Titanium and zirconium have a very low tolerance to impurities, and you should use only the very purest shielding gas.
Argon is a heavy gas obtained from the atmosphere by the liquefaction of air, and is available as a compressed gas or a liquid, depending on the volume of use. It is obtained at much lower prices in the bulk liquid form compared to the compressed gas form, and it is the most widely used type of shielding gas for gas tungsten arc welding. Argon has several advantages over helium:
- Quieter and smoother arc action.
- Easier arc starting.
- Lower arc voltage for current settings and arc lengths. This is good on thin metals.
- Good cleaning action, which is preferred for the welding of aluminum and magnesium
- Lower flow rates are required for good shielding. Argon is heavier than air.
- Lower cost and more availability.
- Better resistance to cross-drafts.
- Better for welding dissimilar metals.
- Better weld puddle control in the overhead and vertical positions.
Helium is a light gas obtained by separation from natural gas. It is available as a liquid but used more often as compressed gas in cylinders. Since helium is lighter than air, it leaves the welding area quicker and therefore requires higher flow rates. Another disadvantage is that it is more expensive and is less available than argon. Helium does have several advantages over argon shielding gas:
- Gives a smaller heat affected zone.
- Produces higher arc voltages for given current settings and arc lengths. This is good on thicker metals and metals with high conductivity.
- Is better for welding at higher speeds.
- Gives better coverage in vertical and overhead positions.
- Provides deeper penetration because of more heat input.
- Tends to flatten out the root pass of the weld bead when used as a backing gas.
The argon-helium mixtures provide the better control of argon and the deeper penetration of helium. Common mixtures of these gases by volume are 75% helium- 25% argon, or 80% helium-20% argon. A wide variety of mixtures is available, particularly for their wide usage in automatic welding.
Welders use mixtures of argon and hydrogen when welding stainless steel, Inconel, Monel, and when porosity is a problem; in some cases, no other shielding gas can prevent porosity.
Argon-hydrogen mixtures increase the welding heat, help control the weld bead profile, and give the weld puddle better wetting action and a more uniform weld bead. This gas mixture is not completely inert.
Do not use argon-hydrogen mixtures for welding plain carbon or low alloy steels, but you can use it for stainless steel with the hydrogen percentage up to 15%. A typical argon-hydrogen mixture is 95% argon and 5% hydrogen.
You can use nitrogen as a shielding gas to obtain higher voltage and produce higher current, but it is rarely done. The efficiency of heat transfer is higher than for either helium or argon, which makes nitrogen good for welding copper and copper alloys. However, nitrogen will reduce arc stability and contaminate the electrodes because it is not an inert gas. If you use thoriated electrodes, there is negligible contamination by the nitrogen.
Since the TIG process can weld a wide variety of metals, it generates a need for various filler metals. Table 9-5 lists the American Welding Society specifications covering the different filler metals used for gas tungsten arc welding. The selection of the proper filler metal is primarily dependent on the chemical composition of the base metal; filler metals are often similar to the base metal, although not necessarily identical.
Manufacturers produce filler metals with closer control on chemistry, purity, and quality than for base metals. The choice of a filler metal for a given application depends on the suitability for the intended operation, the cost, and the metallurgical compatibility. The required tensile strength, impact toughness, electrical conductivity, thermal conductivity, corrosion resistance, and weld appearance of a weldment are also important considerations. Deoxidizers added to the filler metals can give better weld soundness as well.
Table 9-5 — American Welding Society filler metal specifications that cover the different metals welded by the gas tungsten arc welding process.
AWS Filler Metal
Aluminum and Aluminum Alloys
Surfacing Welding Rods and Electrodes
Nickel and Nickel Alloys
Titanium and Titanium Alloys
Composite Surfacing Welding Rods and Electrodes
Zirconium and Zirconium Alloys
Copper and Copper Alloy Gas Welding Rods
Low Alloy Steels
The American Welding Society devised the classification system for filler metal used with gas tungsten arc welding. In this system, designations for filler metal rods consist of the letters ER (for electrode or rod) and an alloy number in most cases. The difference between an electrode and a rod is that an electrode carries welding current and the metal transfers across the arc, but a filler rod is added directly to the weld puddle without electricity running through it.
Because gas tungsten arc welding filler rods are generally chosen based on chemical composition, they are also classified according to their chemical composition. This is not true of the specification for carbon and low alloy steel welding rods, which are classified according to mechanical properties and chemical compositions.
An example of a classification is an ER4043 aluminum welding rod. The ER indicates that the wire is usable as either an electrode or a filler wire, and the 4043 indicates the chemical composition as shown in Table 9-6.
Table 9-6 —Aluminum filler metal classifications (AWS
-- Table 9-6 above
The classification of other non-ferrous metals and stainless steels are similar; Table 9-7 shows manganese classifications, Table 9-8 the copper and copper alloys, Table 9-9 the stainless steels, and Table 9-10 the nickel and nickel alloys.
—Magnesium filler metal classifications (AWS A5.19).
-- Table 9-7 above
Table 9-8 —Copper filler metal classification
Table 9-9 — Chemical compositions of bare
stainless steel filler wire and rods (AWS A5.9).
Table 9-10 — Chemical compositions of filler wire and rods used for welding nickel and nickel alloys (AWS A5.14)
-- Table 9-10 above
Filler metals come either in straight cut lengths that are 36 inches (914mm) long for manual welding or in continuous spooled wire for mechanized welding. The diameter of the filler wire ranges from about .020 inches (.50mm) for delicate or fine work, to about 1/4 inch (6.4mm) for high current welding and surfacing.
The type of base metal and the specific mechanical and chemical properties desired are the major factors in determining the choice of a filler metal. You must be able to identify the base metal to select the proper filler metal. If you do not know the base metal’s composition, you need to test it based on appearance and weight with magnetic checks, chisel tests, flame tests, fracture tests, spark tests, and chemistry tests. The selection of the proper filler metal for specific job applications is quite involved, but you should base it on the following factors:
- Base metal strength properties — This is done by choosing a filler metal to match the tensile strength of the base metal. This is usually most important with steel.
- Base metal composition — The chemical composition of the base metal must be known. Matching the chemical composition is not as important for mild steel as it is for stainless steels and non-ferrous metals. Closely matching the filler metal to the base metal is needed when corrosion resistance and color match are important considerations.
- Thickness and shape of base metal weldments — Thick sections or complex shapes may require maximum ductility to avoid weld cracking. Filler metal types that give best ductility should be used.
- Service conditions and/or specifications — When weldments are subjected to severe service conditions, such as low temperatures, high temperatures, or shock loading, a filler metal that closely matches the base metal composition, ductility, and impact resistance properties should be used.
Filler metals must conform to written specifications for many applications of gas tungsten arc welding. The three major code-making organizations that issue filler metal specifications are the American Welding Society (AWS), the American Society for Mechanical Engineers (ASME), and the military. The ASME recognizes the AWS specifications or makes its own specifications. The filler wire must meet particular requirements in order to conform to filler metal specifications.
|Test Your Knowledge
5. What should be the purity rating of the shielding gas?
6. SAE devised the filler metal classifications.
- To Table of Contents -
Gas tungsten arc welding is widely used because of its versatility. When weld purity is important, this process welds stainless steel, low alloy steel, maraging (mahr-ey-jing) steel, nickel, cobalt, titanium, aluminum, copper, magnesium, and most other metals in all positions and produces clean weld deposits. The clean weld deposits TIG produces usually avoids the need of grinding and finishing, and all methods are usable: manual, semiautomatic, mechanized, and fully automatic.
Welding pipe or nuclear power components are typical examples of the wide variety of TIG applications. This process can also weld thin metals and small objects such as transistor cases, instrument diaphragms, and other delicate parts.
TIG is appropriate for welding pipe and tubing in all positions. The excellent control of heat input gives maximum penetration while preventing meltthrough on the root pass. Welders use TIG in both the manual and automatic methods to weld industrial piping made of various metals and thicknesses, from 1/32 inch (.8 mm) and up (Figure 22).
Figure 22 — Industrial pipe welding.
The maximum thickness welded depends on the equipment available and the type of metal. In some critical welds with metal thicknesses greater than 1/4-3/8 inch (6.4-9.5 mm), the root pass of the pipe is deposited by TIG and then completed with SMAW, GMAW, or FCAW. Sometimes, pipe welders will use consumable inserts in critical service applications. These inserts reduce porosity when alloyed with deoxidizers, improve the contour of the underside of the weld, and minimize cracking in the weld. In thin pipe wall (depending on the base metal), complete fusion is obtainable without using filler metal, but of course filler metals are used with thicker sections to fill the joint.
Thus, the different ways of depositing the first layer on a pipe or tube are the following:
- Ends abutted and fused.
- Ends abutted or slightly separated with filler metal added to the arc area.
- Ends abutted against a filler ring and then completely fused.
If deep penetration with controlled heat input is necessary, then pulsed current may be used.
Automatic circumferential or orbital TIG is another option to weld tube and pipe. The programmed procedure can produce a quantity of identical welds with a high degree of quality and efficiency. Industries with high quality control requirements and those that demand accessibility to the joint use this method extensively.
Power piping, air piping, refrigeration piping, chemical industry process piping, and nuclear power piping are some of the different industries that apply the gas tungsten arc welding process for welding piping and tubing. Vacuum jacketed piping and pressure piping are a couple cases where critical welding is required.
The construction and repair of nuclear power facilities requires critical welding. . Many nuclear applications use both the manual and automatic methods because of their precise control of the welding.
Gas tungsten arc welding performs the welding for end closure caps and plugs to fuel rods, and the airtight sealing of the end closures on fuel rods.
This process is also a primary welding method for rod type fuel elements. It is used to close a backfilling hole that was used to pressure the fuel rods after welding the end closures.
TIG applies also to the shipbuilding industry because it uses different materials like aluminum, stainless steel, and molybdenum.
On hydrofoils, which are primarily made of aluminum, light gauge material and root passes of heavier sections are welded by this process, with GMAW usually completing the weld on the heavier sections. Stainless steel hydrofoils and struts are virtually all welded by the TIG process. Liquefied natural gas (LNG) tanks have a stainless steel liner inside the vessel that is completely TIG welded.
The gas tungsten arc welding process is the major welding process used in the aerospace industry. This industry includes the welding of aircraft, spacecraft, and launch vehicles. Some of the materials welded include aluminum, titanium, low alloy steel, maraging steel, magnesium, nickel, stainless steel, and super alloys in both the manual and automatic methods.
In the aircraft industry, examples of the many different welded parts and assemblies include the fuselage, wing and tail assemblies, landing wheels, engine parts, engine motor cases, and conventional aircraft assemblies such as ducts, fittings, accumulators, check valves, exhaust mufflers, and fairing and cowling components.
Launch vehicles and spacecraft are other major applications of the TIG process. Most aluminum tank fabricators use TIG for the critical pressure vessel butt welds. Titanium alloys used in the liquid propellant tanks, high pressure gas storage tanks, and solid rocket motor cases are almost exclusively TIG welded. From landing gears and re-entry capsules, to large diameter rocket booster cases made of high strength, high carbon, low alloy steel, with thicknesses ranging 0.04-2.0 inches (1.0-5.1 mm), all are welded by this process.
Maraging steels used to make solid rocket motor chambers are fabricated reliably using TIG, but additional sufficient inert gas shielding must protect the face and root of the weld from oxidation. Often, manufacturers accomplish this by welding within inert gas chambers or by using a backing gas to protect the root of the joint and a trailing gas to protect the cooling weld metal behind the torch.
The automotive and railroad industries only use TIG to a small extent, mainly for welding nonferrous metals, for maintenance, and for small components. Fabrication of aluminum radiators 3/32-1/8 inch (2.4-3.2 mm) thick is one application these industries (Figure 23).
Figure 23 — TIG on aluminum radiator.
In the railroad industry, several of the interior components made of aluminum, Monel, stainless steel, and copper are sometimes welded by this process, and there are some maintenance and repair of passenger trains with TIG.
Gas tungsten arc welding has wide applications in the pipe and tube industry for welding pressure vessels, boilers, and heat exchangers. This industry uses it for full fusion welding from one side without the use of permanent backing rings, and on girth butt welds with a smooth internal contour. By choosing the correct filler metal and welding conditions, you can obtain adequate mechanical strength and corrosion resistance for a particular service. Virtually all tube-to-tube sheet welding of heat exchangers is done by the automatic method.
TIG can provide maintenance and repair by both the manual and automatic methods. Several industries use this process because its versatility and weldability permit quality welding for various applications.
Figure 24 — Repair of a roll bar.
Figure 25 — TIG welding of a brace.
The TIG process repairs cast aluminum engine blocks and heads. The area of a defect is puddle melted, scraped out with a steel rod, and finally filled. This process also repairs stainless valves and copper heat-sealing dies for heat exchangers. Repairing the part instead of buying a new one saves money and time.
There are many other possible applications for gas tungsten arc welding in maintenance and repair.
There are numerous general applications for TIG throughout industry. The TIG process welds all the following:
• stainless aeration parts for pollution control equipment
• thin steel brackets on lift trucks
• low alloy steel stop rings to the accumulator on shock absorbers
• small-sized pressure sensing cells
• stainless steel jackets around the coil of superconducting magnets
• stainless steel adapter bushings to stainless steel bulbs for self- actuated thermostatic regulators
• aluminum frame for elevating platforms
• hospital equipment
• cold rooms made from stainless steel
Welders often use TIG for a wide variety of applications where the parts are made out of non-ferrous metals, and there are other applications too numerous to discuss in this course.
Obtained through the melting and fusion of the metal joint, gas tungsten arc spot welding is a method used for making small localized fusion welds from one side of a lap joint. Welding thick metals tends to cause depressions and surface cracking in the center of the weld, so gas tungsten arc spot welding is limited to welding metal about 16 gauge (1 .5 mm) thick or less.
Operators may or may not add filler metal depending on the metal thickness and size of the weld puddle. The equipment used is similar to that used for TIG except that TIG spot welding uses a timing device and a specially designed torch and nozzle (Figure 26).
Figure 26 — Gas tungsten arc spot welding torch.
Primarily used on mild steel, low alloy steel, stainless steel, and aluminum, this method of spot welding can replace resistance spot welding and riveting for many applications, including garage doors, radar cabins, electrical fittings, cable sheaths, and domestic hardware.
The advantages of this process are the high production rates and low costs obtained; the cost of the equipment is low compared to resistance welding equipment. In addition, when the equipment for gas tungsten arc spot welding uses the proper settings, visual inspection is more reliable than when resistance spot welding is done.
|Test Your Knowledge
7. What industry uses TIG almost exclusively?
8 What is the maximum metal thickness limitation on TIG spot welding?
- To Table of Contents -
Knowing the basics of welding metallurgy will provide a firm foundation for understanding the chemical and physical changes that occur on metal when using the TIG process.
The properties of the weld are items such as the chemical composition, the mechanical strength and ductility, and the microstructure. These items will determine the quality of the weld. The types of materials used affect the chemical properties. The mechanical properties and microstructure of the weld are determined by the heat input of welding as well as the chemical composition of the materials.
The chemical and physical properties such as the chemical composition, melting point, and thermal conductivity have a great influence on the weldability. These three items have an influence on the amount of preheating and postheating used, as well as the welding parameters, because preheating and postheating are used to prevent the area from becoming brittle and weak.
In the welding of steel, the carbon and other alloy content influence the hardness and hardenability of the weld metal, which in turn influences the amount of preheat needed. The two terms, hardness and hardenability, are not the same. The maximum hardness of a steel is the resistance to indentation. Hardenability is a measure of how easily a martensite structure forms when the steel is quenched.
Martensite is the phase or metallurgical structure in steel where the maximum hardness of the steel can be obtained. Steels with low hardenability must have very high cooling rates after welding to form martensite. Steels with high hardenability will form martensite even when they are slow-cooled in air. The hardenability will determine to what extent a steel will harden during welding. The carbon equivalent formula is one of the best methods of determining the weldability of steels. This value is determined by the amounts of some of the alloying elements used. There are several different formulas used. One of these is:
|Carbon Equivalent = %C +||%Cr||+||%Mn||+||%Mo||+||%Ni||+||%Cu|
Steels with lower carbon equivalents generally are readily weldable and require fewer precautions such as the use of preheat and postheat.
Steels with higher carbon equivalents are usually more difficult to weld. In the welding of many of the steels, matching the chemical composition of the filler metal to the base metal is not as important as matching the mechanical properties. Often, filler metal with a lower carbon content than the base metal is used because the weld metal absorbs carbon from the base metal during solidification. The carbon content is kept low to minimize the tendency toward weld cracking. Alloys are used in the filler metal to maintain weld strength. In the welding of stainless steels and non-ferrous metals, the chemical composition of the weld is often the most important property. The chemical composition of the weld must match the composition of the base metal when corrosion resistance, thermal and electrical conductivity, and appearance are major considerations.
Preheating helps reduce the cooling rate of the weld to prevent cracking. The amount of preheat needed depends on the type of metal being welded, the metal thickness, and the amount of joint restraint. In steels, those with higher carbon equivalents generally need more preheating than those with lower carbon equivalents. For the non-ferrous metals, this will often depend on the melting points and thermal conductivity of the metal. Table 9-11 shows typical preheat values for various metals welded by this process.
Table 9-11 — Preheats for various metals.
|Note for Table 9-11 above
The actual preheat needed may depend on several other factors such as the thickness of the base metal, the amount of joint restraint, and whether or not low-hydrogen types of electrodes are used. This chart is intended as general information; the specifications of the job should be checked for the specific preheat temperature used
Another major factor that also determines the amount of preheat needed is the thickness of the base metal. Thicker base metals usually need higher preheat temperatures than thinner base metals because of the larger heat sinks that thicker metals provide. Thick metal draws the heat away from the welding zone quicker because there is a large mass of metal to absorb the heat. It would increase the cooling rate of the weld if the same preheat temperature were used on thick base metals as is used on thinner base metals.
The third major factor for determining the amount of preheating needed is the amount of joint restraint. Joint restraint is the resistance of a joint configuration to moving or relieving the stresses due to welding during the heating and cooling of the weld zone. Where there is high resistance to moving or high joint restraint, large amounts of internal stress build up, and higher preheat temperatures are needed as the amount of joint restraint increases. Slower cooling rates reduce the amount of internal stress that build up as the weld cools.
The melting point of the base metal is a major consideration in determining the weldability of a metal. Metals with very low melting points are difficult to weld because the intense heat of the welding arc will melt them too quickly to join them easily. These metals must be brazed because welding is not practical.
Another property that affects the weldability is the thermal conductivity. The thermal conductivity is the rate at which heat is conducted by the metal, and it determines the rate at which heat will leave the welding area. Metals that have a high thermal conductivity often require higher preheats and welding currents to avoid cracking. Metals that have very low thermal conductivity may require no preheat and lower welding currents to prevent overheating an area, which can cause distortion, warpage, and changes in mechanical properties.
The mechanical properties that are most important in the weld are the tensile strength, yield strength, elongation, reduction of area, and impact strength. The first two are measures of the strength of the material, the next two are a measure of the ductility, and the last is a measure of the impact toughness. These properties are important in gas tungsten arc welding, especially for welding steel and the non-ferrous alloys that have been developed to give maximum strength, ductility, and toughness.
The yield strength, ultimate tensile strength, elongation, and reduction of area are all measured from a .505 in. (12.B mm) diameter machined testing bar. The metal is tested by pulling it in a tensile testing machine. Figure 27 shows a tensile bar before and a tensile bar after testing.
Figure 27 — Tensile strength testing bars.
The yield strength of the metal is the stress at which the material is pulled beyond the point where it will return to its original length. The tensile strength is the maximum load that can be carried by the metal. This is also measured in psi (MPa). Elongation is a measure of ductility that is also measured on the tensile bar. Two points are marked on the bar 2 in. (51 mm) apart before testing. After testing, the distance between the two points is measured again and the percent of change in the distance between them, or percent elongation, is measured.
Reduction of area is another method of measuring ductility. The original area of the cross section of the testing bar is .505 sq. in (104 sq. mm).
During the testing the diameter of the bar reduces as it elongates. When the bar finally breaks, the diameter of the bar at the breaking point is measured, which is then used to determine the area. The percent reduction of this cross-sectional area is called the reduction of area.
Impact tests are used to measure the toughness of a metal. The toughness of a metal is the ability of a metal to absorb mechanical energy by deforming before breaking. The Charpy V-notch test is the most commonly used method of making impact toughness tests. Figure 28 shows some typical Charpy V-notch test bars. These bars are usually 10 mm square and have v-shaped notches ground or machined in them. They are put in a machine where they are struck by a hammer attached to the end of a pendulum. The energy that it takes to break these bars is known as the impact strength and it is measured in foot-pounds (Joules).
Figure 28 — Charpy V-notch bars.
There are three basic microstructural areas within a weldment. These are the weld metal, the heat affected zone, and the base metal. The weld metal is the area that was molten during welding. This is bounded by the fusion line which is the maximum limit of melting. The heat affected zone is the area where the heat from welding had an effect
on the microstructure of the base metal. The limit of visible heat affect is the outer limit of this area. The base metal zone is the area that was not affected by the welding. Figure 29 shows a cross section of a weld showing the different areas.
Figure 29 — Cross section of a weld.
The extent of change of the microstructure is dependent on four factors:
- The maximum temperature that the weld metal reached.
- The time that the weld spent at that temperature.
- The chemical composition of the base metal.
- The cooling rate of the weld.
The weld metal zone, which is the area that is melted, generally has the coarsest grain structure of the three areas. Generally, a fairly fine grain size is produced in most metals on cooling, but in some metals, especially refractory metals, rapid grain growth in the weld metal can become a problem.
Large grain size is undesirable because it gives the weld poor toughness and poor cracking resistance. The solidification of the weld metal starts at the edge of the weld puddle next to the base metal. The grains that form at the edge, called dendrites, grow toward the molten center of the weld. Figure 30 shows the solidification pattern of a weld. These dendrites give the weld metal its characteristic columnar grain structure. The grains that form in the weld zone are similar to the grains that form in castings.
Figure 30 — Solidification pattern of a weld.
Deoxidizers and scavengers are often added to filler metal to help refine the grain size in the weld. The greater the heat input to the weld and the longer that it is held at high temperatures, the larger the grain size. A fast cooling rate will produce a smaller grain size than a slower cooling rate. Preheating will give larger grain sizes, but is often necessary to prevent the formation of a hard, brittle microstructure.
The heat affected zone is the area where changes occur in the microstructure of the base metal; the area that is closest to the weld metal usually undergoes grain growth. Other parts of the heat affected zone will go through grain refinement, while still other areas may be annealed and considerably softened. Because of the changes due to the heat input, areas of the heat affected zone can become embrittled and become the source of cracking. A large heat input during welding will cause a larger heat affected zone, which is often not desirable, so the welding parameters used can help influence the size of the heat affected zone.
TIG is used to weld most metals and their alloys. Some of the most common metals welded by this process are aluminum, copper, magnesium, nickel, mild steel, low alloy steel, titanium, zirconium, and the refractory metals. Lead and zinc are difficult to weld because of their low melting points and tendency to contaminate the tungsten electrode, but TIG is widely used for welding lead.
The gas tungsten arc welding process is one of the most widely used processes for welding aluminum and its alloys. The major alloying elements used in aluminum are copper, manganese, silicon, magnesium, and zinc. Table 9-12 shows how aluminum alloys are classified according to their alloy content.
Aluminum alloys are also classified into heat treatable and non-heat treatable categories. Alloys of the 2XXX, 6XXX and 7XXX series are heat treatable. Alloys of the 1 XXX, 3XXX, 4XXX, and 5XXX series are non-heat treatable, so they derive their strength from working.
Table 9-12 — Aluminum
Generally, you would use TIG to weld the thinner materials, with manual welding done on thicknesses ranging from .030 inch (1 mm) to 3/8 inch (9.5 mm), and automatic welding performed on metal ranging in thickness from .01 0 inch (.25 mm) to 1 inch (25.4 mm). You can use either alternating current or direct current welding power, but alternating current is the most popular for almost all manual and automatic welding applications.
Direct current electrode positive is used only for some very thin metal applications. Direct current electrode negative is used sometimes for high current automatic welding applications.
Pure or zirconium tungsten electrodes are the most commonly used types for aluminum. The thoriated tungsten electrodes have a tendency to spit and cause inclusions when used with alternating current, and are not very popular for welding aluminum. Argon shielding gas is normally used, but argon-helium mixtures are used sometimes to give deeper penetration and allow faster travel speeds. When direct current electrode negative is used, mixtures of argon and helium are preferred.
Depending upon the joint and the application, you may or may not use a filler metal; often, thin metal is welded without a filler metal. The filler metal used for welding aluminum is generally of the non-heat treatable type. Consequently, when welding some of the higher strength heat treatable alloys, the weld deposit will be weaker than the base metal.
Choosing the type of filler metal to use for welding a specific aluminum alloy is based on ease of welding, corrosion resistance, strength, ductility, elevated temperature service, and color match with the base metal after welding. You should not use aluminum filler metal with magnesium contents greater than 3% at service temperatures greater than 1500 F because they become sensitive to stress corrosion cracking. Table 9-13 shows a filler metal selection chart based on the specific properties desired. Table 9-14 shows a filler metal selection chart for welding different aluminums together.
The oxide layer on the surface of the aluminum is what makes aluminum more difficult to weld than many other metals. This oxide layer has a very high melting point compared to the melting temperature of the aluminum itself. Direct current electrode positive gives the welding arc an oxide-cleaning action which breaks the oxide layer so that welding can take place. This type of current can be used only at very low current levels because the heat buildup on the tungsten electrode can cause it to melt. Direct current electrode negative can be used at high current levels, but it has difficulty removing the oxide layer. For these reasons, alternating current is the most popular for the welding of aluminum.
During the electrode positive portion of the cycle, the oxide layer is broken down, and during the electrode negative portion of the cycle, penetration is obtained. Alternating current prevents the electrode from overheating and permits the use of enough welding current to give good penetration. Remove the oxide chemically or mechanically before welding.
Table 9-13 — Aluminum Filler Metal Selection Based on
A preheat is used on aluminum only when the temperature of the parts is below 15° F (- 10° C), or when a large mass of metal is being welded, which will draw the heat away very quickly. Aluminum has high thermal conductivity, so heat is drawn away from the welding area. Because aluminum has a relatively low melting point and a high thermal conductivity, overheating can be a problem, especially on thin metal, so preheating is seldom used. The maximum preheat normally used on aluminum is 300° F (150° C). It is usually preferable to increase the voltage and current levels to obtain adequate heat input rather than use preheating. However, a preheat of 200-300° F (93-15° C) is used often when using alternating current on metal thicknesses greater than 3/16 inch (4.8 mm). Some alloys such as 5083, 5086, and 5456 should not be preheated to between 200 and 300° F (95-150° C) because their resistance to stress corrosion cracking will be reduced due to high magnesium contents.
Table 9-14 — Aluminum filler metal selection chart.
|Notes for Table 14 above
Gas tungsten arc welding is well suited for welding copper and copper alloys because of the intense arc generated by this process. This is advantageous because copper has very high thermal conductivity and the heat is conducted away from the weld zone quite rapidly. An intense arc is important in completing the fusion with minimum heating of the surrounding base metal.
The main alloying elements used in copper are zinc (brasses), phosphorous (phosphor bronzes), aluminum (aluminum bronzes), beryllium (beryllium coppers), nickel (nickel silvers), silicon (silicon bronzes), tin and zinc (tin brasses), and nickel and zinc (nickel silvers). All are TIG weldable, but some are easier to weld than others.
The most weldable are the deoxidized coppers, the silicon bronzes, and the copper nickels. The most difficult alloys to weld are those with the highest zinc content, which have a high cracking tendency, and electrolytic tough pitch copper, which causes problems with porosity. Table 9-15 shows the relative ease of welding copper and copper alloys.
TIG welding copper and copper alloys is usually done with direct current electrode negative because of the high current capacity. Exceptions to this include welding beryllium coppers and aluminum bronzes, where you should use alternating current to prevent the buildup of oxides. You must take care when welding beryllium coppers; the fumes given off are dangerous to your health, so you need to wear a gas mask.
Thoriated or zirconium tungsten electrodes are recommended with the 2% thoriated type being the most popular for welding copper and copper alloys. Generally, argon shielding gas is used on the thinner sections while helium and mixtures of argon and helium are used more commonly on the thicker sections. Preheating is not necessary on the thinner sections, but frequently it is required on sections thicker than 1/8 inch (3.2 mm) so the heat does not leave the weld area too quickly. A temperature of 500-800 F (260-4250 C) is typical for preheating copper and copper alloys.
Gas tungsten arc welding is primarily used for welding metal thicknesses up to 1/8 inch (3.2 mm) and for repairing welding or castings. Welding currents used for copper are 50-75% higher than for aluminum because of the high thermal conductivity of copper.
Filler metal is frequently eliminated for welding thinner material, but for thicknesses greater than 1/8 inch (3.2 mm), filler metal is usually used. About ½ inch (12.7 mm) is the maximum practical thickness for TIG welding copper, above this thickness, you should use MIG.
Table 9-15 — Weldability ratings of coppers and copper alloys (1=excellent, 2=good, 3=fair)
|Oxygen free copper||2|
|Electrolytic tough pitch copper||3|
|A Silicon bronzes||1|
When filler metal is used, it is usually selected so the chemical composition of the filler rod closely matches the base metal. This is often necessary to obtain a strong weld joint in some of the copper alloys.
However, a filler metal with a different chemical composition than the base metal may be selected when welding some of the weaker alloys to give the weld joint added strength. The best choice of filler metal depends primarily on the type of copper alloy the base metal is, with consideration for the metal’s application as well.
TIG is the most popular process for welding magnesium and magnesium alloys. The major alloying elements used with magnesium are aluminum, zinc, and thorium. Most magnesium alloys are weldable with this process, but the weldability will vary with the alloy. Table 9-16 shows the main alloying elements used and the relative weldability of the alloys. The rating is based mainly on the susceptibility to cracking. Aluminum content up to about 10% helps the weldability because it promotes grain size refinement, and zinc content above about 1% will increase the tendency towards hot cracking. Alloys that have a high zinc content are very susceptible to cracking and have poorer weldability. Thorium alloys generally have excellent weldability.
Table 9-16 — Magnesium alloy classification, weldability and filler selection (1=excellent, 2=good, 3=fair, 4 =poor).
Magnesium forms an oxide similar to aluminum oxide, which gives these two metals similar welding characteristics. Alternating current is used for most magnesium and magnesium alloy welding applications because of its good oxide cleaning action, which allows higher welding speeds. Direct current electrode positive is often used for welding metal thicknesses from less than 3/16 inch (4.8 mm) up to 3/8 inch (4.8 mm). Above this thickness, gas metal arc welding is often used.
Inert gases such as argon, argon-helium mixtures, and helium are required for shielding because magnesium will react chemically with an active gas. Preheating is often used on thin sections and on highly restrained joints to prevent weld cracking. Thicker sections generally do not require preheating unless there is a high degree of joint restraint. All of the different types of tungsten electrodes are used, especially the pure and zirconium tungsten electrodes.
Filler metal for the gas tungsten arc welding of magnesium and magnesium alloys generally is one of four different types. Filler metals with lower melting points and wider freezing ranges than the base metal are often used to avoid cracking. Table 9-17 also shows a filler metal selection chart. The type of filler metal used is governed by the chemical composition of the base metal.
Table 9-17 — Magnesium filler metal selection chart.
-- Table 17 above
Gas tungsten arc welding is one of the major processes used for welding nickel and nickel alloys. The major alloying elements used in nickel are iron, chromium, copper, molybdenum, and silicon. The classification system for nickel and nickel alloys is shown in Table 9-18. TIG is used for welding both the solid solution strengthened alloys and the precipitation-hardenable alloys, but it is especially the preferred method for precipitation-hardenable alloys because of the difficult of transferring hardening elements across the arc in the other welding processes. Many of the cast alloys, especially ones with high silicon contents, are more difficult to weld.
Table 9-18 — Classifications of nickel and nickel alloys.
|200||Nickel, solid solution|
|400||Nickel-copper, solid solution (Monel)|
|500||Nickel-copper, precipitation-hardenable (Monel)|
|600||Nickel-chromium, solid solution (Inconel)|
|700||Nickel chromium, precipitation-hardenable (Inconel)|
|800||Nickel-iron-chromium solid solution (Incoloy)|
|900||Nickel-iron-chromium, precipitation-hardenable (Incoloy)|
One of the most important factors in welding nickel and nickel alloys is the cleanliness of the base metal. These metals are susceptible to embrittlement caused by sulfur, phosphorous, and lead. Therefore, the surface of the metal to be welded should be cleaned of any grease, oil, paint, dirt, and processing chemicals. Another welding characteristic of nickel is that the weld puddle is not very fluid; therefore, it is more difficult to get complete fusion. Direct current electrode negative (DCEN) is usually recommended for both manual and mechanized welding, with argon, argon-helium mixtures, and helium for shielding. Generally, helium is better for welding if you will not be adding a filler metal. When porosity is a problem for single pass welding of nickels, you should use argon-hydrogen mixtures. All of the different types of tungsten electrodes are used, but the alloyed tungsten electrodes are the most common. A filler metal is usually used when welding nickel and nickel alloys. The filler metals used for welding of these metals are generally similar in composition to the base metal being welded. The filler metals are alloyed to resist hot cracking and porosity in the weld metal.
TIG can weld steel, but because the process is relatively slow and expensive, it is not as popular for welding the plain carbon and alloy steels as it is for welding stainless steel and the non-ferrous metals. Its best usage is for critical applications and for stainless steel.
Functionally, you can use the gas tungsten arc welding process to weld all of the different kinds of steel that can be welded by the other arc welding processes, such as mild, low alloy, heat treatable, and chromium-molybdenum steels. The major alloying elements in these steels are carbon, manganese, silicon, nickel, chromium, and molybdenum. The weldability of the steel depends largely on the carbon content. The higher the carbon content of the steel, the more susceptible to cracking it becomes and the need for preheating and postheating increases.
Low carbon steels have carbon contents up to .14%; mild steels have carbon contents ranging from .15 to .29%. These are generally the easiest to weld and usually do not require preheat and postheat.
Alloy steels with carbon contents greater than .20% generally require preheating and postheating due to the increased alloy content.
Medium carbon steels have carbon contents ranging from .30% to .59%, and high carbon steels have carbon contents ranging from .60% to 1.00%. Many of the very high carbon steels are not welded, except for repair work, because they are very susceptible to cold cracking.
Generally, TIG is more sensitive to sulfur, phosphorous, and oxygen in the steel because there are forms to help remove these elements from the weld puddle. Silicon in the base metal and filler metal helps the weld puddle to wet out better at the edges, and it improves the bead shape.
An extremely low silicon content in the base metal will make welding difficult, so a filler metal is required to provide the silicon for the weld bead. Conversely, an excessively high amount of silicon in the base metal can promote cracking.
Direct current electrode negative is the most commonly used type of welding current, but sometimes alternating current is used for welding thin sheets.
All of the different types of shielding gases used for TIG may be used for welding steel. Argon is the most common with argon-hydrogen mixtures used when you need better weld puddle wetting and bead shape. The thoriated tungsten electrodes are the most popular for welding steel.
You should select the filler metal for the low carbon and low alloy steels by matching the tensile strength of the filler metal to that of the base metal. For welding heat treatable and chromium-molybdenum steels, base your selection by approximately matching the chemical composition to achieve similar hardenability, corrosion, and/or heat resistant properties.
You can make sound welds using the TIG welding process in three principal grades of cast iron: gray, white, and malleable, but you must always preheat cast-iron parts before welding. Preheat gray cast iron to a temperature ranging between 500°F to 1250°F; the required temperature depends on the size and shape of the workpiece.
In either TIG or MIG welding, you should allow the workpiece to cool slowly after welding. You can accomplish this by covering the workpiece in a bed of lime or ashes. This slow cooling prevents cracking and residual stresses.
Free machining steels are steels that have additions of sulfur, phosphorous, selenium, or lead in them to make them easier to machine. Except for the high sulfur, lead, or phosphorous, these steels have chemical compositions similar to mild, low alloy, and stainless steels. The addition of these elements makes these steels nearly unweldable because lead, phosphorous, and sulfur have melting points much lower than the melting point of the steel. As the weld solidifies, these elements remain liquid much longer than the steel so they coat the grain boundaries, which cause hot cracking in the weld. Hot cracking is cracking that occurs before the weld has had a chance to cool. Because of this hot cracking problem, free machining steels cannot be welded easily. If you must weld free machining steel, high manganese filler metal and low base metal dilution will help give the best results possible.
Most types of stainless steels can be TIG welded. The types that are very difficult to weld are types such as 303, 416, 416 Se, 430 F, and 430 FSe, which have high sulfur and selenium contents, and Type 440, which has a high carbon content.
Chromium is the major alloying element that distinguishes stainless steels from the other types of steel. . Steels with chromium contents greater than 11% are considered stainless steels. The high chromium content gives these steels very good corrosion and oxidation resistance. The three major groups of stainless steels that are welded are the austenitic, martensitic and ferritic types.
The austenitic types of stainless steels are generally the easiest to weld. In addition to the high chromium content of about 16-26%, these types have high nickel contents ranging from 6-22%. These steels are designated by the AISI as the 300 series. The 200 series, which have high manganese contents to replace some of the nickel, are also austenitic. Nickel and manganese are strong austenite formers and maintain an austenitic structure at all temperatures. This structure gives these steels good toughness and ductility but also makes them non-hardenable.
A major problem when welding these types of steels is carbide precipitation or sensitization, which occurs only in the austenitic structure. This occurs when the temperature of the steel is between approximately 1000-1600° F (540-870° C) and can greatly reduce the corrosion resistance. There are several methods for preventing this problem:
- A fast cooling rate after welding through this temperature range. This is a major reason why preheating is usually not used and why these steels require a relatively low maximum interpass temperature on multiple pass welds.
- The use of extra low carbon base and filler metal (.03% carbon max). Examples are 304L and 316L.
- The use of a stabilized alloy containing columbium, tantalum (tan-tl-uh m) or titanium. Examples are 347 and 321.
- The use of a solution heat treatment to redissolve the carbides after welding.
Martensitic stainless steels are not as easy to weld as the austenitic stainless steels. These stainless steels have approximately 11-18% chromium, which is the major alloying element, and are designated by the AISI as the 400 series. Some examples are 403,410, 420 and 440. These types of stainless steel are heat treatable because they generally contain higher carbon contents and a martensitic structure. Stainless steels with higher carbon contents are more susceptible to cracking and some, such as Type 440, have carbon contents so high that they are often considered unweldable.
A stainless steel with a carbon content greater than .10% will often need preheating, usually in the range of 400-600° F (205-315°C) to avoid cracking. For steels containing carbon contents greater than .20%, a postweld heat treatment such as annealing is often required to improve the toughness of the weld produced.
Ferritic stainless steels are also more difficult to weld than austenitic stainless steels because they produce welds having lower toughness than the base metal. These stainless steels form a ferritic grain structure and are also designated by the AISI as the 400 series. Some examples are types 405, 430, 442 and 446. These types are generally less corrosion resistant than austenitic stainless steel. To avoid a brittle structure in the weld, preheating and postheating are often required. Typical preheat temperatures range from 300-500° F (150-260° C). Annealing is often used after heat treatment welding to increase the toughness of the weld.
TIG is especially well suited for welding stainless steel because the filler metal does not cross the arc and therefore change the composition. The process provides an inert atmosphere and leaves no slag to react with the base metal. Lower current levels may be desirable for welding stainless steel compared to welding mild steel because of the higher thermal expansion, lower thermal conductivity, and generally lower melting points of stainless steel. The lower thermal conductivity and higher thermal expansion cause more distortion and warpage for a given heat input.
Use direct current electrode negative (DCEN) for most applications, and the most widely used tungsten electrode is the 2% thoriated type, with argon, argon-helium mixtures, and helium shielding gases. Argon is the preferred shielding gas, but argon-hydrogen mixtures are sometimes used to improve the bead shape and the wetting.
The filler metal for welding stainless steel is generally chosen to match the chemical composition of the base metal. For the 200 series austenitic stainless steels, a 300 series austenitic filler metal is usually used, due to lack of an available 200 series filler metal. This weld joint will generally be weaker than the surrounding base metal.
The Type 410 and 420 electrodes are the only martensitic stainless steel types recognized by the AWS. This limitation is often the reason why austenitic stainless steel filler metal is often used when welding martensitic stainless steel. Austenitic filler metal provides a weld with lower strength but higher toughness and eliminates the need for preheating and postheating. For welding ferritic stainless steels, both ferritic and austenitic filler metal may be used. Ferritic filler metal is used when higher strength and an annealing postheat are required. Austenitic filler metal is used when higher ductility is required. Table 9-19 shows filler metal selection for stainless steels
Table 9-19 — Filler metal selection for welding stainless steel.
You can TIG weld titanium and many of the titanium alloys. The major alloying elements contained in titanium alloys are aluminum, tin, zirconium, vanadium, and molybdenum. There are four basic groups of this metal:
- Unalloyed titanium
- Alpha alloys
- Alpha-beta alloys
- Beta alloys
The unalloyed titanium and alpha alloys are all weldable. The weakly beta-stabilized alpha-beta alloys are weldable, but the strongly beta-stabilized alpha-beta alloys are embrittled by welding. Most beta alloys can be welded, but proper heat treatment must be used to prevent the welds from becoming brittle.
In general, titanium requires the same welding techniques used for welding stainless steel with two exceptions: titanium requires greater cleanliness and an auxiliary shielding gas. The molten weld puddle reacts with most materials, and contamination from the atmosphere or from material on the surface of the metal can cause embrittlement in the weld zone and a loss of corrosion resistance. The surface of the metal to be welded must be cleaned thoroughly to avoid these problems. Argon or helium shielding gases are almost exclusively used for welding titanium. The only other shielding gas used is an argon-helium mixture. Welding titanium requires a shielding gas on the backside of the root pass. For out of chamber welding, a trailing shielding gas is used behind the torch to protect the hot metal until it cools below about 600°F (315° C), but in many cases, welding is done in an inert gas-filled chamber.
Thoriated tungsten electrodes are the best types for welding these metals with the 2% thoriated type being the most widely used with direct current electrode negative. Preheating is used rarely except when removing moisture from the surface of the metal.
For welding thicknesses greater than .10 in. (2.5 mm), filler metal is required, usually of the same chemical composition as the base metal. However, to improve the joint ductility, you can use a filler metal with a lower yield point than the base metal.
You can also use TIG to weld the reactive and refractory metals. Reactive metals include zirconium and beryllium. Refractory metals are metals such as tungsten, molybdenum, columbium, and tantalum. The weldability of zirconium is similar to that of titanium. Because this metal, when hot, is highly reactive with the atmosphere, welding must be protected by adequate shielding and is frequently done in vacuum chambers using direct current electrode negative and an argon or helium shielding gas.
Occasionally, beryllium is welded using TIG, but welders must closely control the heat input to prevent very large grains from being formed and to avoid cracking caused by its inherent low ductility. In addition, beryllium is very toxic, and you must take strict safety measures such as wearing special safety clothes and gas masks to prevent contact with the fumes. Usually, alternating current with an argon shielding gas is used, and a low heat input is essential when welding beryllium.
TIG is used commonly to weld tungsten and molybdenum. In the welding f these metals, good cleaning is necessary. Usually, welding is performed using direct current electrode negative, often in a vacuum chamber, with required preheating. Columbium and tantalum have good weldability, and TIG is the most popular process for welding these metals with direct current electrode negative, often in a vacuum chamber. A vacuum chamber is recommended for welding tantalum, but columbium can be welded without one.
|Test Your Knowledge
9. What term is used for the grains that form on the edge of a weld?
10. Why is preheating used when welding titanium?
- To Table of Contents -
The weld joint design used for gas tungsten arc welding is determined by the design of the weldment, metallurgical considerations, and codes or specifications. Good joint designs are those that provide accessibility and economy during construction to help reduce the cost and generally raise the quality of the weld joint. A weld joint consists of a specific weld made in a specific joint. A joint is defined as being the junction of members that are to be joined or have been joined. Figure 31 shows the five basic joint types. Different types of welds can join each of the different joint types. In Figure 32, the most common types of welds are shown. The type of weld made is governed by the joint configuration. Figure 33 lists the nomenclature used for groove and fillet welds.
Figure 31 — Five basic weld joints.
Note: Many other variations are possible.
Figure 32 — Types of welds.
Figure 33 — Weld nomenclature.
Several factors influence the joint design to be used:
- Metal composition
- Strength required
- Welding position
- Metal thickness
- Joint accessibility
The purpose of any joint design is to produce a sound weld deposit with the desired properties as economically as possible. The edge and joint preparation are important because they will affect both the quality and the cost of welding. The exactness of the joint and edge preparation is dependent on the method of welding. Manual welding applications can tolerate greater irregularities in joint fitup than machine and automatic applications.
Of the five basic types of joints, the butt and T are the most commonly used. Since TIG is often used on thinner material, proper fitup can eliminate the need for filler metal when welding square groove butt joints. Lap joints have the advantage of not requiring much preparation other than squaring the edges and making sure the metal is in close contact.
Lap joints in thinner metals do not always require filler metal, nor do edge joints, which are used often on thin material. For example, on tubing, the end of the tubes are often flared or flanged so that the edges may be melted and provide the filler metal for the weld (Figure 34). Corner joints use edge preparations similar to those used for T-joints and usually require a filler metal.
Figure 34 — Edge joint without use of filler metal.
Due to the variety of base metals and their individual characteristics such as surface tension, fluidity, melting temperature, etc., joint designs should be developed to use optimum welding conditions.
For a given joint design, the type of metal influences the maximum base metal thickness that can be sensibly welded. The maximum thickness for a full penetration square-groove butt joint is about 5/16 inch (7.9 mm) in stainless steel, and about 3/16 inch (4.8 mm) in aluminum and magnesium. The differences are in the current used; you weld stainless steel using direct current electrode negative, which gives better penetration than the alternating current used on aluminum and magnesium.
In aluminum, the weld puddle will become larger and form quicker, making it more difficult for the welder to control. This is due to the higher thermal conductivity, the wider, shallower bead produced by alternating current, and the narrower melting temperature range of aluminum. For example, on 1/4-inch (6.4 mm) thick metal, a V-groove would be used in aluminum while a square-groove would allow full penetration in stainless steel. This difference between the metals will also affect the size of the root face used. In general, larger root faces can be used in mild, low alloy and stainless steel than can be used in aluminum and magnesium because of the difference in the penetration capability.
In nickel and high nickel alloys, the weld puddle is very sluggish when molten. The puddle does not spread or wet out very well, so you must place the filler metal at the proper location in the joint. As a result, to permit enough space for manipulation, you need to use larger root openings for nickel than the root openings you would use in carbon and low alloy steels.
The strength required of a weld joint is a major factor governing weld joint design. Weld joints may be either full or partial penetration, depending on the strength required of the joint. Full or complete penetrating welds are those that have weld metal through the full cross section of the joint; partial penetrating welds are those that have an unfused area in the joint. Welds subject to cyclic, impact, or dynamic loading require complete penetration welds. This is even more important for applications that require low temperature service.
Partial penetration welds may be adequate for joints where loading is static only, and they are easier to prepare and require less filler metal than full penetration joints.
The amount of penetration obtained will be affected by the root opening and root face used. A root opening is used to allow good access to the root of the joint and is usually used in full penetrating weld joints. A root opening is usually not used in partial penetration weld joints because access to the root is not necessary and parts are easier to fit together without a root opening. The size of the root face is also affected. A larger root face is used for partial penetration welds than for complete penetration welds because less penetration is required.
TIG can be used in all welding positions. The welding position selected often affects the shape of the joint. A diagram of the welding position capabilities is shown in Figure 35. Good quality welding in flat, horizontal, vertical, and overhead positions depends on the skill of the welder. **64 Welding positions are classified by a set of numbers and letters. The four basic welding
Figure 35 — Welding test positions.
Welding positions are classified by a set of numbers and letters. The four basic welding positions are designated by the numbers 1 for flat, 2 for horizontal, 3 for vertical, and 4 for overhead. F designations are used for fillet welds, and G designations are used for groove welds. The 5G and 6G positions are used in pipe welding.
The groove angle is often varied for different positions. Wider groove angles are often used when welding in the vertical and horizontal positions. Some groove joints welded in the horizontal position have unsymmetrical groove angles. Usually the lower groove face is horizontal or nearly horizontal and the upper groove face is raised accordingly (Figure 36).
Figure 36 — V-groove joint in the horizontal position.
The thickness of the base metal has a large influence on the type of groove that gives the best weld joint possible. The thickness of the base metal welded by this process is not limited, but gas tungsten arc welding is particularly well adapted for welding thin metal. Thicknesses down to .005 inch (.1 mm) can be welded.
The TIG process, because of its relatively low deposition rate, does generally not weld thick metal sections. GMAW is used on many of the thicker applications, especially on the non-ferrous metals.
The most common groove preparations used on butt joints are the square-, V-, J-, U-, bevel-, and combination-grooves. The square-, J-, bevel-, and combination-groove configurations are also used for T-joints; these preparations are used to make it possible to get full or adequate penetration. Square-groove welds are the most commonly used weld joints for TIG because most applications of this process are on thin metal.
The square-groove joint design is the easiest to prepare and requires the least addition of filler metal, and in many cases, filler metal is not used at all. Thicknesses up to 3/16 inch (4.8 mm) or 5/16 inch (7.9 mm) can be welded with full penetration, depending on the type of metal. Many square-groove joints are welded in one pass. A backing strip may be used so that the root can be opened to ensure adequate penetration.
V-grooves for groove welds on butt joints and bevel-grooves for T-joints are commonly used for thicker metal up to about ½ inch (12.7 mm), but these joints are more difficult to prepare, which increases the cost of preparation, and filler metal must be used for V-grooves and bevel-grooves. The included angle for a V-groove is usually up to 90 degrees, the wider angles providing better accessibility to the root. Root faces usually range from 1/8 inch (3.2 mm) to ¼ inch (6.4 mm) depending on the thickness and type of metal.
U- and J-grooves are generally used in metal thicknesses over ½ inch (12.7 mm) to reduce the filler metal required for thicker sections. These joint configurations are also the most difficult and expensive to prepare, but greatly add to the ease of depositing the root pass. When possible in thick sections, the fill passes in this type of joint are deposited by the higher deposition welding processes.
A major consideration in TIG welding joint design is the provision for proper accessibility. Since TIG typically applies to thinner metals, often welds can be made from either one side or both sides of the joint. On thicker metals, when both sides of the joint are accessible, double-grooves are usually made. Double-grooves have less area to fill than single-grooves, therefore requiring less filler metal and developing less distortion with proper weld bead sequencing. When double-grooves are used, the roots of the welds are usually near the center of the base metal.
Welding from both sides of a square-groove usually ensures complete penetration, and on thicker metal is better than complete penetration welding from one side. Also, smaller root openings may be used, which will require less filler metal.
When the joints are accessible only from one side, you can use backing strips and consumable inserts for wider root openings to provide better accessibility to the root of the joint.
Often, on thick metal accessible from only one side, V-, U-, and J-grooves are used, although U- and J-grooves are preferred because they provide better accessibility to the root of the joint and require less filler metal than V-grooves. However, U- and J-grooves are more difficult to prepare, thus increasing time and costs.
Consumable inserts are widely used in welding tube and pipe, and have an effect on joint design. They are used when the joint is accessible from only one side and a uniform, high quality root pass is required. Consumable inserts also provide full penetration to the root of the weld as long as enough heat is available to melt the insert to the root of the joint.
Consumable inserts can help line up the joint during the fitup procedure. For best results, joints with consumable inserts should be precisely prepared and closely fitted, but often they are used when there is joint misalignment.
Consumable inserts also serve as a type of backing. Inserts usually require the use of a different joint design, depending on the type used. Consumable inserts are available in various shapes and sizes (Figure 37).
Figure 37 — Consumable inserts.
When an insert is used, the dimensions of the joint must be compatible with the particular insert, and an insert may require the use of a different size root face and root opening than a normal joint. On square-groove joints, wider root openings are often used so the insert will fit. An example of this is shown in Figure 38, where a V-groove weld joint of the same type and thickness is shown with and without a consumable insert. In this case, a smaller root face and root opening are used with a consumable insert because the insert reduces the danger of melt-through.
Figure 38 — A V-groove joint with and without a consumable insert.
The weld joint designs shown in the rest of the course are those typically used for TIG. Thickness limitations on the weld joints are approximate numbers and vary, depending on the type of base metal. While the thickness limits are generally smaller than what can be used for steels and silicon bronze, they may be slightly large for aluminum and magnesium applications.
These joint designs are generally for thinner material. Thick material is not included because it is rarely TIG welded. Several joint designs used with consumable inserts are included. The following charts and figures show "Standard Welding Symbols" of the American Welding Society. Some of these are shown in the weld joint designs.
AWS welding symbols are the shorthand of welding. They enable the engineer and draftsman to convey complete welding instructions to the welder on blueprints and drawings.
Using welding symbols promotes standardization and a common understanding of design intent. It also eliminates unnecessary details on drawings and mistakes caused by lack of information or misunderstanding.
Figure 39 — Welding symbols
Figure 39 — Welding symbols (Cont'd.)
These charts are intended only as shop aids. The only complete and official presentation of the standard welding symbols is in A2.4.
Figure 40 — Welding symbols.
As it is with other welding processes, in TIG welding the proper positions of the welding torch and weldment are important. The position of the torch and filler metal (if used) in relation to the plate is called the work and travel angle. Work and travel angles are shown in Figure 41. If the parts are equal in thickness, the work angle should normally be on the center line of the joint; however, if the pieces are unequal in thickness, the torch should angle toward the thicker piece.
Figure 41 — Travel angle and work angle for TIG.
The travel angle refers to the angle in which welding takes place. This angle should be between 5 and 25 degrees. The travel angle may be either a push angle or a drag angle, depending on the position of the torch. When the torch is angled ahead of the weld, it is known as pulling (dragging) the weld or backhand welding. When the torch is angled behind (over) the weld, it is referred to as pushing the metal or forehand welding (Figure 49).
Figure 49 — Pulling and pushing travel angle techniques.
The pulling or drag technique is for heavy-gauge metals, and thus not as applicable to TIG. Usually the drag technique produces greater penetration than the pushing technique. Also, since the welder can see the weld crater more easily, better quality welds can consistently be made. Typically, TIG uses the pushing technique for light-gauge metals. Welds made with this technique are less penetrating and wider because the welding speed is faster.
For the best results, you should position the weldment in the flat position. This position improves the molten metal flow and bead contour, and gives better shielding gas protection.
After you have learned to weld in the flat position, you should be able to use your acquired skill and knowledge to weld out of position. These positions include horizontal, vertical-up, vertical-down, and overhead welds. The only difference in welding out of position from the flat position is a 10-percent reduction in amperage.
If you must weld a heavier thickness of metal with the TIG welding process, you should use the multi-pass technique (buildup sequence discussed in Chapter 3). This is accomplished by overlapping single small beads or making larger beads, using the weaving technique. Various multipass welding sequences are shown in Figure 50. The numbers refer to the sequences in which you make the passes.
Figure 50 — Multi-pass welding.
As presented earlier with gas tungsten arc welding, the maximum thickness for a full penetration square-groove butt joint is about 5/16 inch (7.9 mm) in stainless steel, and about 3/16 inch (4.8 mm) in aluminum and magnesium. The following sections on welding positions will include greater thicknesses in the examples, which will have more application for shielded metal arc welding (SMAW or stick), gas metal arc welding (MIG or MAG), and flux core arc welding (FCAW), each with greater deposition rates.
However, the topics are included in this course on gas tungsten arc welding (TIG) as well, because often a precision root pass with TIG may be the best process before applying one of the alternate, higher deposition processes.
Welding can be done in any position, but it is much simpler when done in the flat position. In this position, the work is less tiring, welding speed is faster, the molten puddle is not as likely to run, and better penetration can be achieved. Whenever possible, try to position the work so you can weld in the flat position. In the flat position, the face of the weld is approximately horizontal.
Butt joints— After you strike the arc, hold the torch at a 90-degree angle to the workpiece surface, and with small circular motions, as shown in Figure 51, form a molten puddle. After you form the molten puddle, hold the torch at a 75-degree angle to the work surface and move it slowly and steadily along the joint at a speed that produces a bead of uniform width. Move the torch slowly enough to keep the puddle bright and fluid. No oscillating or other movement of the torch is necessary except the steady forward movement.
When you must use a filler metal, form the molten puddle as described previously. When the puddle becomes bright and fluid, you should move the arc to the rear of the puddle and add the filler metal by quickly touching the rod to the front edge of the puddle. Hold the rod at about a 15-degree angle from the work. Because the electrode is pointing toward the filler metal or pushing it, it is known as the push angle. Remove the filler rod and bring the arc back to the front edge of the puddle. When the puddle becomes bright and fluid again, you should repeat the steps as described before. Figure 52 shows the correct procedures for adding filler metal. Continue this sequence until the weld joint has been completed. The width and height of the weld bead are determined by the speed of travel, by the movement of the torch, and by the amount of filler metal added.
In welding practice, it is again stressed that good TIG welding depends on following this definite procedure— form the molten pool and then feed filler rod intermittently to the leading edge of the pool as you move the torch forward. DO NOT feed the filler rod into the arc. You should practice making single-pass butt welds until you can produce satisfactory welds.
Butt joints are the primary type of joints used in the flat position of welding; however, flat-position welding can be made on just about any type of joint providing you can rotate the section you are welding to the appropriate position. Techniques that are useful in making butt joints in the flat position, with and without the use of backing strips, are described below.
Butt joints without backing strips — A butt joint is used to join two plates having surfaces in about the same plane. Several forms of butt joints are shown in Figure 51. Plates up to 1/8 inch thick can be welded in one pass with no special edge preparation.
Figure 51 — Butt joints in the flat position.
Plates from 1/8 to 3/16 inch thick also can be welded with no special edge preparation by welding on both sides of the joint. Tack welds should be used to keep the plates aligned for welding. The torch motion is the same as that used in making a bead weld. In welding 1/4-inch plate or heavier, you should prepare the edges of the plates by beveling or by J-, U-, or V-grooving, whichever is the most applicable. You should use single or double bevels or grooves when the specifications and/or the plate thickness require it. The first bead is deposited to seal the space between the two plates and to weld the root of the joint. This bead or layer of weld metal must be thoroughly cleaned to remove all slag and dirt before the second layer of metal is deposited.
Figure 52 — Butt welds with multi-pass beads.
In making multi-pass welds, the second, third, and fourth layers of weld metal are made with a weaving motion of the torch (Figure 52). Clean each layer of metal before laying additional beads. You may use one of the weaving motions shown in Figure 53, depending upon the type of joint.
Figure 53 — Weave motions.
In the weaving motion, oscillate or move the torch uniformly from side to side, with a slight hesitation at the end of each oscillation. Incline the torch 5 to 15 degrees in the direction of welding as in bead welding. When the weaving motion is not done properly, undercutting can occur at the joint (Figure 54). Excessive welding speed also can cause undercutting and poor fusion at the edges of the weld bead.
Figure 54 — Undercutting in butt joint welds.
Butt joints with backing strips — Welding 3/16-inch plate or thicker requires backing strips to ensure complete fusion in the weld root pass and to provide better control of the arc and the weld metal. Prepare the edges of the plates in the same manner as required for welding without backing strips. For plates up to 3/8 inch thick, the backing strips should be approximately 1 inch wide and 3/16 inch thick. For plates more than 1/2 inch thick, the backing strips should be 1 1/2 inches wide and 1/4 inch thick. Tack-weld the backing strip to the base of the joint (Figure 55). The backing strip acts as a cushion for the root pass. Complete the joint by welding additional layers of metal. After you complete the joint, you may “wash” off or cut the backing strip away with a cutting torch. When specified, place a seal bead along the root of the joint.
Figure 55 — Use of back strips in welding butt joints.
Bear in mind that many times it will not always be possible to use a backing strip; therefore, the welder must be able to run the root pass and get good penetration without the formation of icicles.
You will discover that it is impossible to weld all pieces in the flat position. Often the work must be done in the horizontal position. The horizontal position has two basic forms, depending upon whether it is used with a groove weld or a fillet weld. In a groove weld, the axis of the weld lies in a relative horizontal plane and the face of the weld is in a vertical plane (Figure 56). In a fillet weld, the welding is performed on the upper side of a relatively horizontal surface and against an approximately vertical plane (Figure 57).
Figure 56 — Horizontal groove weld.
Figure 57 — Horizontal fillet weld.
An inexperienced welder usually finds the horizontal position of arc welding difficult, at least until he has developed a fair degree of skill in applying the proper technique. The primary difficulty is that in this position you have no “shoulder” of previously deposited weld metal to hold the molten metal. When welding in the horizontal position, start the arc on the edge of the joint. Then hold the torch at a work angle of 15 degrees and a push angle of 15 degrees. After you establish the puddle, dip the rod into the front edge of the puddle on the high side as you move the torch along the joint (Figure 58). Maintain an arc length as close as possible to the diameter of the electrode. Correct arc length coupled with the correct speed of travel helps prevent undercutting and permits complete penetration.
Figure 58 — Horizontal welding angles.
Horizontal-position welding can be used on most types of joints. The most common types of joints it is used on are tee joints, lap joints, and butt joints.
Tee joints — When you make tee joints in the horizontal position, the two plates are at right angles to each other in the form of an inverted T. The edge of the vertical plate may be tack-welded to the surface of the horizontal plate (Figure 59).
Figure 59 — Tack-weld to hold the tee joint elements in place.
Figure 60 — Position of electrode on a fillet weld.
A fillet weld is used in making the tee joint, and a short arc is necessary to provide good fusion at the root and along the legs of the weld (Figure 60, View A). Hold the torch at an angle of 45 degrees to the two plate surfaces (Figure 60, View B) with an incline of approximately 15 degrees in the direction of welding.
When practical, weld light plates with a fillet weld in one pass with little or no weaving of the torch. Welding of heavier plates may require two or more passes in which the second pass or layer is made with a semicircular weaving motion (Figure 61). To ensure good fusion and to prevent undercutting, you should make a slight pause at the end of each weave or oscillation.
Figure 61 — Weave motion for multi-pass fillet weld.
For fillet-welded tee joints on 1/2-inch plate or heavier, deposit stringer beads in the sequence shown in Figure 62.
Figure 62 — Order of string beads for tee joint on heavy
Figure 63 — Intermittent fillet welds.
Chain-intermittent or staggered-intermittent fillet welds are used on long tee joints (Figure 63). Fillet welds of these types are for joints where high weld strength is not required; however, the short welds are arranged so the finished joint is equal in strength to that of a joint that has a fillet weld along the entire length of one side. Intermittent welds also have the advantage of reduced warpage and distortion.
Lap joints — When you make a lap joint, two overlapping plates are tack-welded in place (Figure 64), and a fillet weld is deposited along the joint.
The procedure for making this fillet weld is similar to that used for making fillet welds in tee joints. You should hold the torch so it forms an angle of about 30 degrees from the vertical and is inclined 15 degrees in the direction of welding. The position of the torch in relation to the plates is shown in Figure 65.
Figure 64 — Tack welding a lap joint.
Figure 65 — Position of electrode on a lap joint.
The weaving motion is the same as that used for tee joints, except that the pause at the edge of the top plate is long enough to ensure good fusion without undercut. Lap joints on 1/2-inch plate or heavier are made by depositing a sequence of stringer beads (Figure 65),
In making lap joints on plates of different thickness, you should hold the torch so that it forms an angle of between 20 and 30 degrees from the vertical (Figure 66). Be careful not to overheat or undercut the thinner plate edge.
Figure 66 — Lap joints on plates of different thickness.
Figure 67 — Horizontal butt joint.
Butt joints— Most butt joints designed for horizontal welding have the beveled plate positioned on the top. The plate that is not beveled is on the bottom, and the flat edge of this plate provides a shelf for the molten metal so that it does not run out of the joint (Figure 67). Often both edges are beveled to form a 60-degree included angle. When this type of joint is used, more skill is required because you do not have the retaining shelf to hold the molten puddle.
The number of passes required for a joint depends on the diameter of the torch and the thickness of the metal. When multiple passes are required, place the first bead deep in the root of the joint (Figure 68). The torch should be inclined about 5 degrees downward. Clean and remove all slag before applying each following bead. The second bead should be placed with the torch held about 10 degrees upward. For the third pass, hold the torch 10 to 15 degrees downward from the horizontal. Use a slight weaving motion and ensure that each bead penetrates the base metal.
Figure 68 — Multiple passes.
A vertical weld is a weld that is applied to a vertical surface or one that is inclined 45 degrees or less (Figure 69). Erecting structures such as buildings, pontoons, tanks, and pipelines require welding in this position. Welding on a vertical surface is much more difficult than welding in the flat or horizontal position due to the force of gravity. Gravity pulls the molten metal down.
Figure 69 — Vertical weld plate positions.
When welding thin material with the TIG welding process, you should weld from the top, moving downward. This helps you produce an adequate weld without burning through the metal. Filler material is not normally needed for welding downward.
On heavier materials, you should weld from the bottom, upwards. This enables you to achieve adequate penetration. When welding upward, you normally need to use a filler rod.
Current Settings and Torch Movement
In vertical arc welding, the current settings should be less than those used for the same torch in the flat position. Another difference is that the current used for welding upward on a vertical plate is slightly higher than the current used for welding downward on the same plate.
To produce good welds, you must maintain the proper angle between the torch and the base metal. In welding upward, you should hold the torch at 90 degrees to the vertical (Figure 70, View A). When weaving is necessary, oscillate the torch as shown in Figure 70, View B.
9-70 — Bead welds in the vertical position.
In vertical down welding, incline the outer end of the torch downward about 15 degrees from the horizontal while keeping the arc pointing upward toward the deposited molten metal (Figure 70, View C). When vertical down welding requires a weave bead, you should oscillate the torch as shown in Figure 70, View D.
Vertical welding is used on most types of joints. The types of joints you will most often use it on are tee joints, lap joints, and butt joints.
Hold the torch at 90 degrees to the plates or not more than 15 degrees off the horizontal for proper molten metal control when making fillet welds in either tee or lap joints in the vertical position. Keep the arc short to obtain good fusion and penetration.
Tee joints — To weld tee joints in the vertical position, start the joint at the bottom and weld upward. Move the torch in a triangular weaving motion as shown in Figure 71, View A. A slight pause in the weave, at the points indicated, improves the sidewall penetration and provides good fusion at the root of the joint.
When the weld metal overheats, you should quickly shift the torch away from the crater without breaking the arc, as shown in Figure 71, View B. This permits the molten metal to solidify without running downward. Return the torch immediately to the crater of the weld in order to maintain the desired size of the weld.
When more than one pass is necessary to make a tee weld, you may use either of the weaving motions shown in Figure 71, Views C and D. A slight pause at the end of the weave will ensure fusion without undercutting the edges of the plates.
Lap joints — To make welds on lap joints in the vertical position, you should move the torch in a triangular weaving motion as shown in Figure 71, View E. Use the same procedure as outlined above for the tee joint, except direct the torch more toward the vertical plate marked G. Hold the arc short, and pause slightly at the surface of plate G. Try not to undercut the plates or allow the molten metal to overlap at the edges of the weave.
Figure 71 — Fillet welds in the vertical position.
Lap joints on heavier plate may require more than one bead. If so, clean the initial bead thoroughly and place all subsequent beads as shown in Figure 71, View F. The precautions to ensure good fusion and uniform weld deposits that were previously outlined for tee joints also apply to lap joints. Butt joints — Prepare the plates used in vertical welding identically to those prepared for welding in the flat position. To obtain good fusion and penetration with no undercutting, you should hold a short arc and carefully control its motion. Butt joints on beveled plates 1/4 inch thick can be welded in one pass by using a triangular weave motion, as shown in Figure 72, View A. Welds made on 1/2-inch plate or heavier should be done in several passes, as shown in Figure 72, View B. Deposit the last pass with a semicircular weaving motion with a slight “whip-up” and pause of the torch at the edge of the bead. This produces a good cover pass with no undercutting. Welds made on plates with a backup strip should be done in the same manner.
Figure 72 — Butt joint welding in the vertical position.
Overhead welding is the most difficult position in welding. Not only do you have to contend with the force of gravity, but the majority of the time you also have to assume an awkward stance. Nevertheless, with practice it is possible to make welds equal to those made in the other positions.
Current Settings and Torch Movement
When TIG welding in the overhead position, you should lower the welding current by 5 to 10 percent of what normally is used for flat welding. This reduced welding current enables you to maintain better control of the welding puddle. Conversely, you need a higher flow of shielding gas. Hold the torch and the rod as you do for flat welding. You should try to maintain a small weld puddle to avoid the effects of gravity. Most inexperienced welders find overhead welding awkward; therefore, try to get in as comfortable and relaxed a position as possible when welding. This helps you to maintain steady, even torch and filler rod manipulation.
One of the problems encountered in overhead welding is the weight of the cable. To reduce arm and wrist fatigue, drape the cable over your shoulder when welding in the standing position. When sitting, place the cable over your knee. With experience, cable placement will become second nature.
Because of the possibility of falling molten metal, use a protective garment with a tight fitting collar that buttons or zips up to the neck. Roll down your sleeves and wear a cap and appropriate shoes.
Type of Welds
Techniques used in making bead welds, butt joints, and fillet welds in the overhead position are discussed in the following paragraphs.
Bead welds — For bead welds, the work angle of the torch is 90 degrees to the base metal (Figure 73, View A). The travel angle should be 9 to 15 degrees in the direction of welding (Figure 73, View B).
Figure 73 — Position of electrode and weave motion in the overhead position.
Weave beads can be made by using the motion shown in Figure 73, View C. A rather rapid motion is necessary at the end of each semicircular weave to control the molten metal deposit. Avoid excessive weaving because this can cause overheating of the weld deposit and the formation of a large, uncontrollable pool.
Butt Joint — Prepare the plates for overhead butt-welding in the same manner as required for the flat position. The best results are obtained when backing strips are used; however, you must remember that you will not always be able to use a backing strip. When you bevel the plates with a featheredge and do not use a backing strip, the weld will repeatedly burn through unless the operator takes extreme care.
For overhead butt-welding, bead welds are preferred over weave welds. Clean each bead and chip out the rough areas before placing the next pass. The torch position and the order of deposition of the weld beads when welding on 1/4- or 1/2-inch plate are shown in Figure 74,
Figure 74 — Multi-pass butt joint in the overhead position.
Figure 75 — Fillet welds in the overhead position.
Views B and C. Make the first pass with the torch held at 90 degrees to the plate, as shown in Figure 74, View A. When you use a torch that is too large, you cannot hold a short arc in the root area. This results in insufficient root penetration and inferior joints.
Fillet welds — In making fillet welds in either tee or lap joints in the overhead position, maintain a short arc and refrain from weaving of the torch. Hold the torch at approximately 30 degrees to the vertical plate and move it uniformly in the direction of welding, as shown in Figure 75, View B. Control the arc motion to secure good penetration in the root of the weld and good fusion with the sidewalls of the vertical and horizontal plates. When the molten metal becomes too fluid and tends to sag, whip the torch quickly away from the crater and ahead of the weld to lengthen the arc and allow the metal to solidify. Immediately return the torch to the crater and continue welding.
Overhead fillet welds for either tee or lap joints on heavy plate require several passes or beads to complete the joint. One example of an order of bead deposition is shown in Figure 75, View A. The root pass is a string bead made with no weaving motion of the torch. Tilt the torch about 15 degrees in the direction of welding, as shown in Figure 75, View C, and with a slight circular motion make the second, third, and fourth pass. This motion of the torch permits greater control and better distribution of the weld metal. Remove all slag and oxides from the surface of each pass by chipping or wire brushing before applying additional beads to the joint.
Welding is the simplest and easiest way to join sections of pipe. The need for complicated joint designs and special threading equipment is eliminated. Welded pipe has reduced flow restrictions compared to mechanical connections, and the overall installation costs are less. The most popular method for welding pipe is the shielded metal arc process; however, gas shielded arc methods have made big inroads as a result of new advances in welding technology.
Pipe welding has become recognized as a profession in itself. Even though many of the skills are comparable to other types of welding, pipe welders develop skills that are unique only to pipe welding. Because of the hazardous materials that most pipelines carry, pipe welders are required to pass specific tests before they can be certified
In the following paragraphs, pipe welding positions, pipe welding procedures, definitions, and related information are discussed.
You may recall from Figure 35 that there are four positions used in pipe welding. They are known as the horizontal rolled position (1G), the horizontal fixed position (5G), the pipe inclined fixed (6G), and the vertical position (2G). Remember, these terms refer to the position of the pipe and not to the weld
Pipe Welding Procedures
Welds that you cannot make in a single pass should be made in interlocked multiple layers, with at least one layer for each 1/8 inch of pipe thickness. Deposit each layer with a weaving or oscillating motion. To prevent entrapping slag in the weld metal, you should clean each layer thoroughly before depositing the next layer.
Butt joints are commonly used between pipes and between pipes and welded fittings. They are also used for butt welding of flanges and welding stubs. In making a butt joint, place two pieces of pipe end to end, align them, and then weld them (Figure 76).
Figure 76 — Butt joints and socket fitting joints.
When the wall thickness of the pipe is 3/4 inch or less, you can use either the single V or single U type of butt joint; however, when the wall thickness is more than 3/4 inch, only the single U type should be used. Fillet welds are used for welding slip-on and threaded flanges to pipe. Depending on the flange and type of service, fillet welds may be required on both sides of the flange or in combination with a bevel weld (Figure 77).
Figure 77 — Flange connections.
Fillet welds are also used in welding screw or socket couplings to pipe, using a single fillet weld (Figure 76). Sometimes flanges require alignment. Figure 78 shows one type of flange square and its use in vertical and horizontal alignment. Another form of fillet weld used in pipefitting is a seal weld. A seal weld is used primarily to obtain tightness and prevent leakage. Seal welds should not be considered as adding strength to the joint.
Figure 78 — Flange alignment.
Joint Preparation and Fitup
You must carefully prepare pipe joints for welding if you want good results. Clean the weld edges or surfaces of all loose scale, slag, rust, paint, oil, and other foreign matter. Ensure that the joint surfaces are smooth and uniform. Remove the slag from flamecut edges; however, it is not necessary to remove the temper color.
When you prepare joints for welding, remember that bevels must be cut accurately. Bevels can be made by machining, grinding, or using a gas cutting torch. In fieldwork, the welding operator usually must make the bevel cuts with a gas torch. When you are beveling, cut away as little metal as possible to allow for complete fusion and penetration. Proper beveling reduces the amount of filler metal required, which in turn reduces time and expense. In addition, it also means less strain in the weld and a better job of design and welding.
Align the piping before welding and maintain it in alignment during the welding operation. The maximum alignment tolerance is 20 percent of the pipe thickness. To ensure proper initial alignment, you should use clamps or jigs as holding devices. A piece of angle iron makes a good jig for a small-diameter pipe (Figure 79), while a section of channel or I-beam is more suitable for larger diameter pipe.
Figure 79 — Angle iron jig.
When welding material solidly, you may use tack welds to hold it in place temporarily. Tack welding is one of the most important steps in pipe welding or any other type of welding. The number of tack welds required depends upon the diameter of the pipe. For 1/2-inch pipe, you need two tacks; place them directly opposite each other. As a rule, four tacks are adequate for standard size of pipe. The size of a tack weld is determined by the wall thickness of the pipe. Be sure that a tack weld is not more than twice the pipe thickness in length or two thirds of the pipe thickness in depth. Tack welds should be the same quality as the final weld. Ensure that the tack welds have good fusion and are thoroughly cleaned before proceeding with the weld.
In addition to tack welds, spacers sometimes are required to maintain proper joint alignment. Spacers are accurately machined pieces of metal that conform to the dimensions of the joint design used. Spacers are sometimes referred to as chill rings or backing rings, and they serve a number of purposes, for example, they provide a means for maintaining the specified root opening, provide a convenient location for tack welds, and aid in the pipe alignment. In addition, spacers can prevent weld spatter and the formation of slag or icicles inside the pipe.
Do not assign a welder to a job under any of the following conditions listed below unless the welder and the work area are properly protected:
- When the atmospheric temperature is less than 0°F
- When the surfaces are wet
- When rain or snow is falling, or moisture is condensing on the weld surfaces
- During periods of high wind
At temperatures between 0°F and 32°F, heat the weld area within 3 inches of the joint with a torch to a temperature warm to the hand before beginning to weld.
|Test Your Knowledge
11. How many basic types of pipe weld joints are there?
12. In addition to tack welds, what is also used for proper pipe alignment?
- To Table of Contents -
Welding procedure variables control the welding process and the quality of the welds that are produced. The selection of the welding variables is done after the base metal, filler metal, and joint design are selected. The selection of the filler metal and joint design have been discussed in previous chapters.
A proper selection of welding variables will make the welding easier for the welder, increasing the chance of producing the weld properties required. The three major types of welding variables are the fixed or preselected, the primary adjustable, and the secondary adjustable.
The fixed or preselected variables are set before the actual welding takes place. These are items such as the electrode type and size, the type of current, the type of shielding gas, and the electrode taper angle. These variables cannot be easily changed once the welding starts.
The primary adjustable variables are used to control the welding process after the fixed variables have been selected. They control the formation of the weld bead by affecting the bead width and height, joint penetration, arc stability, and weld soundness (Figure 80). The primary adjustable variables for gas tungsten arc welding are the welding current, arc length, and travel speed.
The secondary adjustable variables are used to control the welding process. These are usually more difficult to measure and their effects may not be as obvious. In TIG welding, secondary adjustable variables are things such as the work and travel angles of the electrode and the electrode extension.
The different variables affect the characteristics of the weld including the joint penetration of the weld, the bead height and width, and the deposition rate. The joint penetration is the distance the weld metal extends from its face into a joint, exclusive of
Figure 80 — Bead height, width, and penetration.
weld reinforcement. The bead height is the height of the weld metal above the surface of the base metal. The bead width is the width of the weld bead. The deposition rate is the weight of material deposited in a unit of time.
The welding variables presented in this section focus on joint penetration, bead shape, and the effect they have on the other welding variables. The deposition rate is a lesser issue with TIG. It will vary widely because the filler metal does not cross the arc and is not as dependent on variables such as the type and amount of welding current used, and of course there is no deposition rate when you do not use a filler metal.
With gas tungsten arc welding, fixed variables include the type, size, and taper of the electrode, and the types of welding current and shielding gas.
The type of tungsten electrode used in gas tungsten arc welding depends on the type of metal and the specific application. Refer to Table 3 for the correct type of electrode to weld various base metals.
For less critical applications, you should use the pure tungsten electrodes rather than the thoriated or zirconium tungsten electrodes. Pure tungsten electrodes have a lower current carrying capacity and a lower resistance to contamination, and tend to leave more tungsten inclusions in the weld metal. However, pure tungsten electrodes are widely used for AC welding of aluminum and magnesium because they do not disintegrate as fast with alternating current, and are the least expensive.
The thoriated tungsten electrodes are more expensive but are preferred for many applications because of the higher current carrying capacity, longer life, easier starting, more stable arc, and greater resistance to contamination.
Zirconium tungsten electrodes generally have properties that fall somewhere in the middle. Zirconium electrodes often give the best characteristics with alternating current and are used to give x-ray quality welds in aluminum and magnesium.
The size of the electrode used will depend on the intended welding current range. Refer again to an earlier table, Table 9-4, which shows the current ranges for various types and sizes of tungsten electrodes. This is not the only determining factor though. For all types of tungsten electrodes, in addition to the electrode diameter, the current-carrying capacity is affected by the electrode extension, type of electrode holder, type of shielding gas, and type of welding current.
Larger electrodes will allow you to use higher welding currents. For a given welding current setting, you will need to use a larger electrode when using direct current electrode positive because of the high heat buildup that occurs in the electrode. Also, for a given size of electrode, direct current electrode negative will be able to carry the largest amount of current. Although larger electrodes are generally used for welding thicker metal, very small electrodes may be used for welding very thin sheet metal.
The type of welding current used depends primarily on the type of metal to be welded, the current levels required, and the availability of a machine that produces that type of welding current. Figure 81 describes some of the characteristics of different polarity electrodes; also, refer once again to Table 3 for the type of recommended current for different base metals.
Figure 81 — Characteristics of current types for gas tungsten arc welding.
Direct current electrode positive is often used for welding thin aluminum and magnesium parts. It is popular for these applications because the cathodic cleaning action created at the surface of the workpiece removes the refractory oxide surface that inhibits wetting of the weldment. DCEP also provides shallow penetration and has a low current-carrying capacity because of the high amount of heat that builds up on the electrode. Since this heat buildup can cause electrode melting, using DCEP is limited to welding thin materials at low current levels.
Direct current electrode negative is used to obtain deep penetrating welds and is the most common type of current used for welding metals other than aluminum and magnesium. For aluminum and magnesium, alternating current with a superimposed high frequency current is most commonly used. This type of current provides good oxide cleaning when the electrode is positive and good penetration when the electrode is negative. Overall, alternating current gives moderate penetration and is the second choice of current type on most other metals.
Shielding gas is directed by the torch to the arc and weld pool to protect the electrode and the molten weld metal from atmospheric contamination. The inert shielding gas used will affect the penetration of the weld, the heat input, and the cost of the welding operation.
Argon is the most common type of shielding gas used in TIG and can be used for most applications. Argon will give less penetration and heat input than helium but is less expensive to use because it requires lower flow rates, produces the least spatter, and costs less. It provides a smoother, quieter, arc action, better cross-draft resistance, and an easier starting arc. Argon is used exclusively on thin metals because the high heat input of helium causes melt-through.
Helium gives a hotter arc and more heat input into the base metal, which produces deeper penetration and allows faster travel speeds. It is used especially for welding thick sections, for metals with high heat conductivity, and for high-speed mechanized applications.
Mixtures of argon are used to obtain a balance between the characteristics of these two gases. Using helium instead of argon allows you to use lower welding currents and produces higher arc voltages for a given arc length.
Electrode taper angle is the angle ground on the end of the tungsten electrode (Figure 82). This variable applies only to thoriated tungsten electrodes. These are ground to a tip to give better arc starting with high frequency ignition and a more stable arc. The grinding wheel should be reserved for grinding only tungsten to eliminate possible contamination of the tungsten tip with foreign matter during the grinding operation. When grinding thoriated electrodes, you should use exhaust hoods to remove the grinding dust from the work area.
Figure 82 — Electrode taper angle.
You can taper thoriated tungsten electrodes because of their higher current-carrying capacity. The most common taper angle is approximately 22°. This means that the electrode is tapered about 2 1/2 electrode diameters. The degree of taper also affects the bead shape and penetration. Increasing the taper angle tends to reduce the bead width and increase the weld penetration.
The disadvantage of the smaller taper angles is that they tend to wear away quicker, especially on starts where the tip of the electrode is touched to the work. To reduce the erosion and the number of times you will need to regrind the electrode, you should use a larger taper angle because it does not wear away as quickly.
Regardless of the electrode tip geometry selected, it is important that you use a consistent taper angle once a welding procedure is established. Changes in the electrode angle can significantly influence the weld bead shape and size. Therefore, the electrode tip configuration is a variable that you need to study during the welding procedure development.
As with any other type of welding, the TIG welding procedure consists of certain variables that you must understand and follow. Many of the variables have been discussed. This section applies some of these variables to the actual welding procedure.
Once you have chosen the fixed or preselected variables, the amount of welding current you use will have the greatest effect on the characteristics of the weld bead. A knob or handle on the front of the welding machine, or a foot pedal rheostat controls the welding current. On some automatic applications, weld programmers may control the weld current. All of the following help determine the welding current:
- type of electrode
- size of the electrode
- type of welding current
- joint design
- metal thickness
- current range of the machine
The welding current is the best variable for controlling the depth of penetration and the volume of weld metal.
As the other factors remain constant, when you increase the welding current, the penetration and size of the weld bead increases. An excessive weld current can produce undercutting, excessive penetration, and an irregular weld deposit.
While the other factors remain constant, lowering the welding current will reduce the penetration and size of the weld bead. An extremely low weld current can cause piling up of the weld metal, poor penetration, and overlapping at the edges of the weld bead. Figure 83 shows the effects of different welding currents and speeds.
Figure 83 — Effects of different primary variables.
The travel speed is the rate that the arc travels along the workpiece. For a given welding current and voltage, the travel speed determines the amount of heat that is delivered for a given length of weld. Changes in the travel speed have a strong effect on the shape of the weld bead and the amount of penetration. In manual TIG welding, the welder controls the rate that the arc travels along the work. In mechanized and automatic welding operations, the travel speed is controlled by the equipment.
While the other variables remain constant, increasing the travel speed will reduce the size of the weld bead and decrease the amount of penetration. Conversely, decreasing the travel speed will increase the size of the weld bead and increase the penetration.
If the welding current and travel speed are increased or decreased proportionally together, the weld will maintain the same penetration and width.
An excessive travel speed will produce a weld bead that is too small, has poor penetration, and is irregular in shape. A travel speed that is too slow will give a weld bead with excessive penetration, size, and piling up of the weld metal when filler metal is added.
The welding or arc voltage is dependent on the shielding gas and the distance between the tip of the electrode and the work. In the case of manual TIG welding, the welder controls the distance from the tip of the electrode to the adjacent surface of the weld pool, called arc length.
In mechanized and automatic welding, the arc length is pre-set by the distance from the electrode tip to the work. In automatic welding, arc voltage controllers may be used to move the electrode tip up and down to maintain the desired arc length. The arc voltage controller compares the measured and desired arc voltages to determine which direction and at what speed the welding electrode should be moved. This determination, expressed as a voltage error signal, is amplified to drive motors in a slide that supports the torch. The changing voltage that results from the motion of the welding electrode is detected and the cycle repeats to maintain the desired arc voltage.
The shielding gas has an effect on the arc voltage. Helium will give higher arc voltages for a given arc length than argon, which accounts for the greater penetrating ability of helium.
The arc length has a direct effect on the welding voltage. Increasing the arc length will increase the arc voltage, and decreasing the arc length will decrease the arc voltage. A welding voltage that is too high indicates that the arc is too long. An excessive arc length will produce an irregular welding bead with poor penetration. When the arc length is extremely long, the shielding gas may not provide enough protection, which could cause porosity and a discolored weld bead. Figure 83 also shows the effect of an excessive arc length. Too short an arc can also cause problems. It increases the danger of electrode contamination because the welder is more likely to dip the end of the electrode in the weld puddle. Another problem is a higher heat buildup on the tungsten electrode and the torch nozzle because they are closer to the weld puddle. This reduces the service life of the electrode.
Before starting the arc, you should form a ball on the end of the electrode for AC welding. To do this, simply set the current to DCRP and strike an arc for a moment on a piece of carbon or a piece of copper. The ball diameter should be only slightly larger than the original diameter of the tungsten electrode.
When starting the arc with an AC high-frequency current, you do not have to bring the electrode into contact with the workpiece. To strike the arc, you must hold the torch in a horizontal position about 2 inches above the work surface, as shown in Figure 84. Then rapidly swing the electrode end of the torch down to within 1/8 of an inch of the work surface. The high-frequency arc will then jump the gap between the electrode and the plate, establishing the arc. Figure 85 shows the torch position at the time the arc strikes.
Figure 84 — Torch position for the starting swing to strike the arc.
Figure 85 — Torch position at the end of the swing.
If you are using a DC machine, hold the torch in the same position, but touch the plate to start the arc. When the arc is struck, withdraw the electrode so it is about 1/8 of an inch above the plate.
To stop the arc, quickly swing the electrode back to the horizontal position. If the machine has a foot pedal, gradually decrease the current before stopping the arc.
Secondary variables for TIG include the travel and work angles of the electrode, and electrode extension.
The angular position of the electrode in relation to the work may have an effect on the quality of the weld deposit. The position of the electrode may determine the ease at which you can add the filler metal (if used), the quality of the weld bead, and the uniformity of the bead.
The electrode angles are called the travel angle and the work angle. The travel angle of the electrode is the angle between the joint and the electrode in the longitudinal plane. The work angle is the angle between the electrode and the perpendicular plane to the direction of travel. These are shown in Figure 86. The welder manually controls the electrode angles, and the angles used may vary slightly from welder to welder.
Figure 86 — Travel angle and work angle.
An incorrect work angle can cause undercutting and an inadequate weld bead. An example of this is in the case of making a fillet weld. If the welder favors or directs the arc more toward one plate, undercutting or lack of fusion may result on the other plate, and the bead may have an irregular shape. The travel angle used will have an effect on the penetration and the bead height. Increasing the travel angle in the direction of welding will generally build up the height of the bead. Increasing the travel angle in the opposite direction of welding will decrease the amount of penetration and give a wider bead.
The distance that the tip of the electrode extends beyond the end of the gas nozzle is known as the electrode extension. Usually, the amount of extension is equal to one or two electrode diameters, as shown in Figure 87. There are cases where the electrode extension used will be greater or less.
Figure 87 — Electrode extension.
The longer the electrode extension, the greater the chance of contamination by striking the base metal or the filler rod to the tip of the electrode, or by inadequate gas coverage.
Alternatively, the farther the electrode tip is withdrawn into the gas nozzle, the less current the electrode will be able to withstand because some of the heat is reflected back to the electrode from the gas nozzle. Often, longer electrode extensions are used on fillet welds so the electrode may approach the root of the joint and the arc will be visible to the welder.
In some cases, the end of the electrode is withdrawn into the gas nozzle, making it very difficult to contaminate the electrode. This hinders visibility and requires a high degree of welder skill. For welds requiring a very short arc length, a longer than normal extension is used so the welder has better vision. Longer electrode extensions require higher gas flow rates and will not cool as efficiently. The electrode extension should not be longer than absolutely necessary because of the added gas flow rates needed and the added danger of electrode contamination.
- To Table of Contents -
The welding procedure schedules in this section give typical welding conditions that can be used to obtain high quality welds under normal welding conditions. The gas tungsten arc welding process can use a wide variety of operating conditions for welding various base metals. The schedules presented here provide only a few examples of the many different welding procedures that can be used. The tables given here are not the only conditions that could be used because factors such as weld appearances, welder skill, method of application, and the specific application often require variations from the schedules.
For example, when automatic gas tungsten arc welding is used, the travel speeds are often higher than if the welding was performed manually. As the particular requirements of the application become known, the settings may be adjusted to obtain the optimum welding conditions. Qualifying tests or trials should be made in the shop or field prior to actual use.
When adjusting or changing the variables for welding, the effect of the variables on each other must be considered. In order to obtain a stable arc and good overall welding conditions, one variable cannot usually be changed very much without adjusting or changing the other variables.
The following schedules are based on welding specific metals and their alloys such as aluminum, magnesium, copper, nickel, and titanium as well as steel. The tables have the type of weld, base metal thickness, number of passes, tungsten electrode size, gas nozzle size, filler rod size, gas flow rate, welding current, and travel speed as the variables that can be changed. The arc voltage is not included because the arc length will vary depending on the welder. Gas tungsten arc welding is done using constant current types of power sources, which allow the welding voltage to vary, while keeping the welding current at approximately the same level. In automatic gas tungsten arc welding, the voltage is easily measured because the machine can hold a constant arc length.
The tables presented in this course are the conditions for manual TIG welding. The main emphasis of these schedules is on the welding conditions used for welding thin materials, especially for non-ferrous metals. The type of current, shielding gas, and tungsten electrode used are those recommended for welding these different metals, and will not be considered as variables here.
Because of the wide variety of applications TIG welding is capable of performing, the procedure schedules presented here are not a complete guide to the procedures for TIG. They are not the only conditions that may be used to obtain a specific weld. You should make qualifying tests under actual conditions before using this process or these schedules for production welding. Figures 9-88 through 9-93 are representative of some of the configurations you will encounter when welding.
Figure 88 — Square groove welds.
Table 9-20 — Square groove welds in various types of base metal.
Aluminum and Aluminum Alloys
AC, Argon Shield, Pure or Zirconium Tungsten Electrode
|3/64 (1.2)||1||1/16 (1.6)||1/4 (6.4)||1/16 (1.6)||19 (9.0)||20-60||12 (5.1)|
|1/16(1.6)||1||3/32 (2.4)||5/16 (7.9)||3/32 (2.4)||19 (9.0)||40-90||10 (4.2)|
|3/32 (2.4)||1||3/32 (2.4)||5/16 (7.9)||3/32 (2.4)||19 (9.0)||50-110||10 (4.2)|
|1/8 (3.2)||1||1/8 (3.2)||3/8 (9.5 )||1/8 (3.2)||20 (9.4)||100-150||10 (4.2)|
Copper and Copper
Alloys (Except Silicon Bronze)
DCEN, Argon Shield for Thicknesses less than 3/16” (4.8) — Helium for all others
|1/16 (1.6)||1||1/16 (1.6)||1/4 (6.4)||1/16 (1.6)||18 (8.5)||100-150||12 (5.1)|
|1/8 (3.2)||1||3/32 (2.4)||5/16 (7.9)||3/32 (2.4)||18 (8.5)||150-230||10 (4.2)|
|3/16 (4.8)||1||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||36 (17)||175-250||10 (4.2)|
DCEN, Argon Shield, Thoriated Tungsten Electrode
|1/16(1.6)||1||1/16 (1.6)||1/4 (6.4)||1/16 (1.6)||15 (7.1)||60-125||12 (5.1)|
|1/8 (3.2)||1||1/16 (1.6)||1/4 (6.4)||3/32 (2.4)||20 (9.4)||80-150||12 (5.1)|
|3/16 (4.8)||1||3/32 (2.4)||5/16 (7.9)||3/32 (2.4)||20 (9.4)||100-195||10 (4.2)|
|1/4 (6.4)||2||3/32 (2.4)||5/16 (7.9)||1/8 (3.2)||25 (11.8)||150-225||10 (4.2)|
AC, Argon Shield, Pure or Zirconium Tungsten Electrode
|20 ga (.9)||1||1/16 (1.6)||1/4 (6.4)||3/32 (2.4)||15 (7.1)||25-40||15 (6.3)|
|16 ga (1.5)||1||1/16 (1.6)||1/4 (6.4)||3/32 (2.4)||15 (7.1)||35-70||15 (6.3)|
|14ga (1.9)||1||1/16 (1.6)||1/4 (6.4)||3/32 (2.4)||15 (7.1)||40-75||13 (5.5)|
|12 ga (2.7)||1||3/32 (2.4)||5/16 (7.9)||1/8 (3.2)||15 (7.1)||50-100||13 (5.5)|
|11 ga (3.0)||1||3/32 (2.4)||5/16 (7.9)||1/8 (3.2)||25 (11.8)||65-125||13 (5.5)|
| Nickel and
DCEN, Argon Shield, Thoriated Tungsten electrode
|24 ga (.6)||1||1/16 (1.6)||3/8 (9.5)||None||15 (7.1)||8-10||8 (3.4)|
|16 ga (1.5)||1||3/32 (2.4)||1/2 (12.7)||1/16(1.6)||18 (8.5)||40-70||8 (3.4)|
|1/8 (3.2)||1||1/8 (3.2)||1/2 (12.7)||3/32 (2.4)||25 (11.8)||75-140||11 (4.7)|
|1/4 (6.4)||2||1/8 (3.2)||1/2 (12.7)||1/8 (3.2)||30 (14.2)||100-175||8 (3.4)|
Carbon and Low Alloy Steel
DCEN, Argon Shield, Thoriated Tungsten Electrode
|24 ga (.6)||1||1/16 (1.6)||1/4 (6.4)||1/16 (1.6)||10 (4.7)||15-35||13 (5.5)|
|20 ga (.9)||1||1/16 (1.6)||1/4 (6.4)||1/16 (1.6)||10 (4.7)||20-45||13 (5.5)|
|18 ga (1.2)||1||1/16(1.6)||1/4 (6.4)||1/16 (1.6)||10 (4.7)||25-55||12 (5.1)|
|16ga(1.5)||1||1/16 (1.6)||1/4 (6.4)||1/16(1.6)||10 (4.7)||35-65||12 (5.1)|
|14 ga (1.9)||1||1/16(1.6)||1/4 (6.4)||1/16 (1.6)||10 (4.7)||35-70||12 (5.1)|
|3/32 (2.4)||1||3/32 (2.4)||5/16 (7.9)||3/32 (2.4)||10 (4.7)||35-80||12 (5.1)|
|1/8 (3.2)||1||3/32 (2.4)||5/16 (7.9)||3/32 (2.4)||12 (5.7)||45-100||11 (4.7)|
|3/16 (4.8)||1||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||15(7.1)||65-140||10 (4.2)|
|1/4 (6.4)||1||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||18 (8.5)||85-175||10 (4.2)|
DCEN, Argon Shield, Thoriated Tungsten Electrode
|1/16 (1.6)||1||1/16(1.6)||1/4 (6.4)||1/16 (1.6)||12 (5.7)||35-60||12(5.1)|
|3/32 (2.4)||1||1/16 (1.6)||1/4 (6.4)||3/32 (2.4)||12 (5.7)||45-85||12 (5.1)|
|1/8 (3.2)||1||1/16 (1.6)||5/16 (7.9)||3/32 (2.4)||12 (5.7)||55-100||12 (5.1)|
|3/16 (4.8)||1||3/32 (2.4)||5/16 (7.9)||1/8 (3.2)||15 (7.1)||65-130||10 (4.2)|
DCEN, Argon Shield, Thoriated Tungsten Electrode
|24 ga (.6)||1||1/16(1.6)||3/8 (9.5)||None||18 (8.5)||20-35||6 (2.5)|
|16 ga (1.5)||1||1/16(1.6)||5/8 (15.9)||None||18 (8.5)||45-85||6 (2.5)|
|3/32 (2.4)||1||3/32 (2.4)||5/8 (15.9)||1/16 (1.6)||25 (11.8)||60-90||8 (3.4)|
|1/8 (3/2)||1||3/32 (2.4)||5/8 (15.9)||1/16 (1.6)||25 (11.8)||80-125||8 (3.4)|
|3/16 (4.8)||2||3/32 (2.4)||5/8 (15.9)||1/8 (3.2)||25 (11.8)||90-140||8 (3.4)|
Figure 89 — V-groove welds.
Table 9-21 — V-groove welds in various types of base metal.
Aluminum and Aluminum Alloys
AC Argon Shield Pure or Zirconium Tungsten Electrode
|3/16 (4.8)||2||5/32 (4.0)||7/16(11.1)||5/32 (4.0)||25 (11.8)||160-180||11 (4.7)|
|1/4 (6.4)||2||5/32 (4.0)||1/2 (12.7)||3/16 (4.8)||30 (14.2)||200-220||9 (3.8)|
|3/8 (9.5)||2||3/16 (4.8)||1/2 (12.7)||3/16 (4.8)||30 (14.2)||240-300||8 (3.4)|
|1/2 (12.7)||2||3 3/16(4.8)||1/2 (12.7)||3/16 (4.8)||35 (16.5)||300-350||8 (3.4)|
Copper and Copper Alloys (Except Silicon Bronze)
DC EN Helium Shield Thoriated Tungsten Electrode
|1/4 (6.4)||2||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||36 (17.0)||220-275||7 (3.0)|
|3/8 (9.5)||2||3/16 (4.8)||1/2 (12.7)||3/16 (4.8)||45 (21.2)||275-325||7 (3.0)|
|1/2 (12.7)||2||1/4 (6.4)||5/8 (15.9)||1/4 (6.4)||45 (21.2)||370-500||6 (2.5)|
DC EN Argon Shield Thoriated Tungsten Electrode
|3/8 (9.5)||3||1/8 (3.2)||3/8 (9.5)||3/16 (4.8)||25 (11.8)||295-355||8 (3.4)|
|1/2 (12.7)||4||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||25 (11.8)||245-295||8 (3.4)|
|3/4(19.1)||9||1/8 (3.2)||3/8 (9.5)||3/16 (4.8)||25 (11.8)||295-355||8 (3.4)|
AC Argon Shield Pure or Zirconium Tungsten Electrode
|3/16 (4.8)||1||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||25 (11.8)||95-115||24 (10.2)|
|1/4 (6.4)||2||3/16 (4.8)||1/2 (12.7)||5/32 (4.0)||25 (11.8)||110-130||20 (8.5)|
|3/8 (9.5)||2||3/16 (4.8)||1/2 (12.7)||3/16 (4.8)||30 (14.2)||135-165||18 (7.6)|
|1/2 (12.7)||2||3 1/4 (6.4)||5/8 (15.9)||3/16(4.8)||35 (16.5)||280-320||10 (4.2)|
|3/4 (19.1)||3||1/4 (6.4)||3/4 (19.1)||3/16 (4.8)||40 (18.9)||340-380||10 (4.2)|
| Stainless Steel
DC EN, Argon Shield, Thoriated Tungsten Electrode
|1/16 (1.6)||1||1/16(1.6)||1/4 (6.4)||1/16 (1.6)||10 (4.7)||45-75||10 (4.2)|
|3/32 (2.4)||1||1/16 (1.6)||1/4 (6.4)||3/32 (2.4)||10 (4.7)||65-85||10 (4.2)|
|1/8 (3.2)||1||1/16 (1.6)||5/16 (7.9)||3/32 (2.4)||10 (4.7)||75-125||10 (4.2)|
|3/16 (4.8)||1||1/8 (3.2)||3/8 (9.5)||1/8 (3.2)||15 (7.1)||100-175||8 (3.4)|
|1/4 (6.4)||2||1/8 (3.2)||3/8 (9.5)||3/16 (4.8)||18(8.5)||125-225||10 (4.2)|
|3/8 (9.5)||2-3||3/16 (4.8)||1/2 (12.7)||3/16 (4.8)||25 (11.8)||175-300||10 (4.2)|
|1/2 (12.7)||3||3/16 (4.8)||1/2 (12.7)||1/4 (6.4)||25 (11.8)||200-325||10 (4.2)|
|Titanium DC EN, Argon Shield, Thoriated Tungsten Electrode|
|24 ga (.6)||1||1/16(1.6)||3/8 (9.5)||None||18 (8.5)||20-35||6 (2.5)|
|16 ga (1.5)||1||1/16(1.6)||5/8 (15.9)||None||18 (8.5)||45-85||6 (2.5)|
|3/32 (2.4)||1||3/32 (2.4)||5/8 (15.9)||1/16 (1.6)||25 (11.8)||60-90||8 (3.4)|
Figure 90 — Fillet welds.
Table 9-22 — Fillet welds in various types of base metals.
Figure 91 — Pulsed current parameters.
Table 9-23 — Pulsed current procedures for
Figure 92 — Pulsed current parameters (cont.).
Table 9-24 — Pulsed current for stainless steel.
Figure 93 — Gas tungsten arc spot welding - Flat and vertical position.
Table 9-25 — Gas Tungsten Arc Spot Welding.
- To Table of Contents -
Several steps must be taken before making a weld with the gas tungsten arc welding process. These include preparing the weld joint, preparing the electrode tip, fixturing the weldment, setting the variables, and in some cases, preheating. The amount of preweld preparation depends upon the size of the weld and weldment, the type of base metal, the ease of fitup, the quality requirements, the governing code or specification, and the welder.
There are different ways of preparing the edge of the joint for welding. For fillet or square-groove welds, the joints are prepared simply by squaring the edges of the members if the as-received edge is not suitable. In TIG welding, a large percentage of the joints are prepared this way because this process is widely used for welding thin materials.
The methods most often used for edge preparation are oxygen fuel cutting, plasma arc cutting, shearing, machining, air carbon arc gouging, grinding, and chipping. When they are available, with the exception of shearing, the thermal cutting methods such as oxy-fuel cutting, plasma arc cutting, or air carbon arc cutting are faster than the mechanical cutting methods.
Oxy-fuel cutting is used on carbon and low alloy steels, plasma arc cutting is used on ferrous and non-ferrous metals and is best for applications where high production rates are required, and air carbon arc cutting is used for most steels, including stainless steels. However, do not use air carbon arc on stainless steels involving critical corrosion applications because of the high carbon deposition. The surfaces cut by these thermal methods often have to be ground lightly to remove the scale or contamination.
Common types of prepared joints are the V-, U-, J-, bevel-, and combination grooves. The more complex types of bevels require longer joint preparation times, which makes the joint preparation more expensive.
Next to the square edge preparation, the V-groove and single-bevel grooves are used most often, and can be prepared easily by oxy-fuel cutting or plasma arc cutting. These two methods leave a smooth surface if properly done. The edges of U- and J-grooves can be prepared by using special oxy-fuel tips and techniques, air carbon arc cutting, or by machining, which will produce a more uniform groove. These joint preparations are not as common in TIG welding because they are joint preparations for thicker materials.
The welds made by TIG are very susceptible to contamination during the welding process. The surface of the base metal must be free of grease, oil, paint, plating, dirt, oxides, or any other foreign material. This is especially critical when welding aluminum and non-ferrous metals. Usually, extremely dirty workpieces, except titanium, are cleaned by using solvent cleaners followed by vapor degreasing, and simple degreasing is used for cleaning metals that have oxide-free surfaces. Generally, acid pickling is used for cleaning metals that have a light oxide coating, while heavier oxide coatings are removed mechanically by grinding and abrasive blasting.
The type of required cleaning operation will vary depending on the metal. Aluminum has a thick, refractory oxide coating which has a high electrical resistance. This coating is removed by de-oxidation with a hot alkaline cleaning solution, followed by rinsing in distilled water. Carbon and low alloy steels may be cleaned chemically in a hydrochloric acid solution. Nickel alloys and stainless steels may be cleaned by pickling, which removes iron, sand blast residue, and other contaminants. Titanium and titanium alloys may be cleaned in molten salt baths or by abrasive blasting. Chlorinated solvents used for degreasing operations should not be used on titanium because they will cause corrosion cracking. Chemical cleaning can be done by pickling with hydrofluoric acid.
You need to perform several tasks just before welding. One is to file the edges of the joint smooth so no burrs are present; burrs can cause physical pain and be a place to trap contaminants in a weld joint. Another is to wire brush the surfaces of the joint and surrounding area. Use mild steel brushes for cleaning mild and low alloy steel, and use stainless steel wire brushes for stainless steel, aluminum, and other non-ferrous metals. Following this procedure will help you avoid contaminating the stainless steel and non-carbon metals with a mild steel brush. You should do the welding as soon as possible after cleaning, especially on metals that form moderate or thick surface oxides such as stainless steel, aluminum, and magnesium. Wire brushing does not completely remove the oxide, but it reduces its thickness and makes the metals easier to weld. Wear gloves while cleaning to prevent oil or dirt from your fingers from getting on the joint surfaces, which can also cause contamination.
Contaminates on the workpiece can lead to arc instability and result in welds that contain pores, cracks, or inclusions.
The shape of the tungsten electrode tip is an important process variable in gas tungsten arc welding. The type of electrode tip preparation depends on the type of tungsten electrode; it may have a pointed, hemispherical, or balled profile. A pointed electrode tip is best for welding in restricted areas such as narrow joints, and it permits a high current density to be maintained. Pointed electrode tips are used on thoriated electrodes, while the hemispherical and balled tips are used for zirconium and pure tungsten electrodes.
The pure and zirconium types of electrodes form a hemispherical or balled tip and are used mainly for welding with alternating current. These two types of electrode tip preparations are shown in Figure 94. You produce a hemispherical electrode tip by starting an arc between the electrode and a piece of scrap metal or copper and maintaining it at a moderate current level until a hemispherical ball is formed on the end of the electrode.
Figure 94 — Hemispherical and balled tip.
You produce a balled tip the same way, except you use higher current levels. As you increase the current beyond the point where a hemispherical tip exists, the ball will increase in size proportionately. The diameter of the balled end should not exceed one and one-half times the electrode diameter because the excessive current will consume the electrode too quickly. The surface of the hemispherical and balled tips should always be perfectly clean, shiny, and highly reflective.
The pointed type of tip preparation is used on 1% and 2% thoriated tungsten electrodes, which are generally used for DCEN welding. Unless the thoriated electrodes are used for welding with AC, they are normally ground to a sharp point (Figure 95). The length of the ground surface of the electrode should be about two or three times the size of the electrode diameter.
Figure 95 — Point tip preparation
To produce optimum arc stability, grind the tungsten electrodes with the axis of the electrode perpendicular to the axis of the grinding wheel or along the length of the electrode and not across the diameter. This will produce a more stable arc. Slightly blunt the tip of the electrode before welding; when higher current levels are used, the tip of the electrode will melt back a bit and give a slightly wider tip. Reserve a grinding wheel for grinding tungsten only to eliminate possible contamination of the tungsten tip, and use exhaust hoods when grinding thoriated electrodes to remove the grinding dust from the work area.
Thoriated and zirconium electrodes will maintain a pointed edge preparation over a wide current range, but pure tungsten electrodes will change their tip profile according to the amount of current they are carrying. The surface of a pointed electrode should be kept clean at all times, but it will not be shiny.
Fixturing can affect the shape, size and uniformity of a weld bead. Fixtures are devices used to hold the parts in proper relation to each other until welded. When fixturing is not used, it usually indicates that the resulting weld distortion can either be tolerated or corrected by straightening operations. The following are primary functions of fixturing:
When you use a welding fixture, you can assemble and hold the components securely in place while you position the weldment and perform the weld. The need to use these devices depends on the specific application; they are used more often when large numbers of the same parts are produced. When you can use fixtures, your production time for the weldments can be reduced significantly. They are also good for applications where you must hold close tolerances.
Positioners are used to move the workpiece into a position so welding can be done more conveniently, which affects the appearance and quality of the weld bead. Sometimes you need a positioner simply to make the weld joint more accessible. The main objective of positioning is to put the joint in the flat or other more favorable position, which increases your efficiency because you can use higher welding speeds. This also allows you to use larger diameter wires with globular and high current spray transfer. These modes of metal transfer will produce the highest deposition rates. Flat position welding usually increases the quality of the weld because it makes the welding easier.
Weld backings are commonly used in TIG to provide support for the weld metal and to control the heat input. Copper, stainless steel, and consumable insert rings are the three most common methods. Copper is the most popular method of weld backing because it does not fuse to thin metals. It also provides a fast cooling rate; the high heat conductivity of copper makes this a good method of controlling the heat input. Stainless steel is good backing material for argon shielded TIG welding. Often, consumable inserts are used as weld backing for welding the root pass in pipe welding. They fit into place and are available in plain carbon steel, alloy steel, and stainless steel, as well as in copper and nickel alloys.
Preheating is sometimes required, but this depends on the type of metal being welded, the base metal thickness, and the amount of joint restraint. These factors were discussed in the section on Welding Metallurgy. The specific amount of preheat needed for a given application is often obtained from the welding procedure.
The preheat temperature of the metal should be carefully controlled. There are several good methods of performing this: furnace heating, electric induction coils, and electric resistance heating blankets. On thin materials, hot air blasts or radiant lamps may be used; with these methods, temperature indicators are attached to the parts being preheated.
Oxy-fuel torches are another method of preheating. This method gives a more localized heating than the previously mentioned methods. When you use oxy-fuel torches, you need to avoid localized overheating and keep deposits of incomplete combustion products from collecting on the surface of the parts to be welded. There are several methods of measuring the temperature of preheat such as colored crayons, pellets, and hand-held temperature indicators. The crayons and pellets melt at a specific predetermined temperature; the handheld temperature indicators give meter readings, digital readings, or recorder readings, depending on the type of temperature indicator.
|Test Your Knowledge
14. Which is NOT a major type of welding variable?
15. On a pointed tip electrode, what should the length of the ground surface be?
- To Table of Contents -
TIG, like the other processes, can have welding procedure problems resulting in weld defects. Some defects are caused by problems with the materials, including the use of improper base metal, filler metal, or shielding gas. Other welding problems may not be foreseeable, such as arc blow and electrode contamination, and may require immediate corrective action. A poor welding technique and an improper choice of welding parameters are other causes of welding defects.
Discontinuities that can occur when using TIG welding are tungsten inclusions, porosity, wormhole porosity, undercutting, incomplete fusion, melt-through, arc strikes, and craters. Problems with the welding technique or procedure weaken the weld and can cause cracking. The base metal and filler metal must be clean to avoid many of these problems. Other problems that can occur and reduce the quality of the weld are arc blow, lack of shielding gas, and drafts or air currents.
TIG welding does not have many problems with slag inclusions because a shielding gas, instead of a slag layer, protects the weld puddle. However, some filler metals, particularly those used for mild steel, will sometimes leave a small amount of slag, which may cause slag inclusions if it is not cleaned properly. However, this is rarely a problem. Welding spatter rarely occurs because the tungsten is a non-consumable electrode and the filler metal is added directly to the weld puddle, not transferred across the arc.
Tungsten inclusions are chunks or particles from the elctrode which are found in the weld metal (Figure 96).
Figure 96 — Inclusions.
These inclusions are the result of problems in the welding procedure such as:
This problem can be corrected by:
Oxide inclusions are particles of surface oxides which have not melted and mixed into the weld metal (Figure 97). These inclusions occur when welding those metals that have surface oxides with very high melting points. This problem is mainly associated with welding aluminum and magnesium, but some problems will also occur when welding stainless steel. Oxide inclusions weaken the weld and can serve as initiation points for cracking. The best method of preventing this problem is to wire brush the joint and weld area and clean the area thoroughly before welding.
Figure 97 — Oxide inclusion.
Porosity is the presence of gas pockets in the weld metal that may be scattered in small clusters or along the entire length of the weld (Figure 98). The voids left in the weld cause it to be weakened.
Figure 98 — Porosity.
One or more of the following cause porosity:
- Inadequate shielding gas flow.
- Excessive welding current.
- Rust, grease, oil, moisture, or dirt on the surface of the base metal or filler metal, including moisture trapped in aluminum oxide.
- Impurities in the base metal, such as sulfur and phosphorus.
- An excessive travel speed, which causes freezing of the weld puddle before gases can escape.
- Contaminated or wet shielding gas.
Porosity can be prevented or corrected by the following:
- Increasing the shielding gas flow.
- Lowering the welding current.
- Cleaning the surface of the base metal.
- Changing to a different base metal with a different composition.
- Lowering the travel speed.
- Replacing the shielding gas.
Wormhole porosity is the name given to elongated gas pockets and is usually caused by sulfur in the steel or moisture on the surface of the base metal that becomes trapped in the weld joint (Figure 99). Wormhole porosity can seriously reduce the strength of the weld.
Figure 99 — Wormhole porosity.
The best methods for preventing this are to clean the surfaces of the joint and preheat to remove moisture. If sulfur in the steel is the problem, a more weldable grade of steel should be selected.
Undercutting is a groove melted in the base metal next to the toe or root of a weld that is not filled by the weld metal (Figure 100). This is particularly a problem with fillet welds. Undercutting causes a weaker joint at the toe of the weld, which may result in cracking.
Figure 100 — Undercutting.
Undercutting is caused by one or more of the following:
- Excessive welding current.
- Arc voltage too high.
- Excessive travel speed
- Not enough filler metal added.
- Excessive weaving speed. On vertical and horizontal welds, undercutting may also be caused by incorrect electrode angles.
This discontinuity can be prevented by:
- Reducing the welding current.
- Holding a short arc length.
- Using a travel speed slow enough so the weld metal can completely fill all of the melted out areas of the base metal.
- Using more filler metal.
- Pausing at each side of the weld bead when a weaving technique is used.
Incomplete fusion occurs when the weld metal is not completely fused to the base metal (Figure 101). This can occur between the weld metal and the base metal or between passes in a multi-pass weld. Incomplete fusion between the weld metal and the base metal is usually due to inadequate penetration.
Figure 101 — Incomplete fusion.
Causes of this include:
- Excessive travel speed.
- Welding current too low.
- Poor joint preparation.
- Letting the weld metal get ahead of the arc.
Incomplete fusion can be prevented by:
- Reducing the travel speed.
- Increasing the welding current.
- Preparing the joint better.
- Using proper electrode angles.
Overlapping is the protrusion of the weld metal over the edge or toe of the weld bead (Figure 102). This defect can cause an area of incomplete fusion, which creates a notch and can lead to crack initiation. Although TIG is primarily for welding thin metals, if this occurs, you can grind off the excess weld metal after welding.
Figure 102 — Overlapping.
Overlapping is produced by one or more of the following:
- Too slow a travel speed which permits the weld puddle to get ahead of the electrode.
- Arc welding current that is too low.
- Addition of too much filler metal.
- Incorrect electrode angle that allows the force of the arc to push the molten weld metal over unfused sections of the base metal.
Overlapping can be prevented or corrected by the following:
- Using a higher travel speed.
- Using a higher welding current.
- Reducing the amount of filler metal added
- Using the correct electrode angles.
- Grinding off the excess weld metal
Melt-through occurs when the arc melts through the bottom of the weld and creates holes (Figure 103).
Figure 103 — Melt-through.
This can be caused by one or more of the following:
- Excessive welding current.
- Travel speed that is too slow.
- Root opening that is too wide or a root face that is too small.
This can be prevented by:
- Reducing the welding current.
- Increasing the travel speed.
- Reducing the width of the root opening, using a slight weaving motion, or increasing the electrode extension.
Many codes prohibit striking the arc on the surface of the workpiece. Striking the arc on the base metal outside of the weld joint can produce a hard spot on the base metal surface. Failures can then occur due to the notch effect. The arc strikes might create a small notch on the surface of the metal which can act as an initiating point for cracks.
Weld craters are depressions on the weld surface at the point where the arc was broken (Figure 104). These are caused by the solidification of the metal after the arc has been broken. The weld crater often cracks and can serve as an origin for linear cracking back into the weld metal or into the base metal.
Figure 104 — Weld crater.
These craters can usually be removed by chipping or grinding and the depression can be filled in with a small deposit of filler metal. For TIG welding, there are two common methods of preventing craters. The first is to reverse the travel of the electrode a little way back into the weld bead from the end of the weld bead, before breaking the arc. A second method is to use a foot rheostat to control the welding current. This is done by gradually reducing the welding current at the end of the weld, which gradually reduces the size of the molten weld puddle. For machine and automatic applications, a slope control on the machine will automatically reduce the welding current at the end of the weld, which will also gradually reduce the size of the molten weld puddle.
An improper welding procedure, welder technique, or materials can cause weldment cracking. All types of cracking can be classified as either hot or cold cracking. These cracks are transverse or longitudinal to the weld. Transverse cracks are perpendicular to the axis of the weld where longitudinal shrinkage strains acting on excessively hard and brittle weld metal. Longitudinal cracks are often caused by high joint restraint and high cooling rates. Although TIG is primarily for thin metals, preheating may be necessary to help reduce these problems. Hot cracking occurs at elevated temperatures and generally happens just after the weld metal starts to solidify. This type of cracking is often caused by excessive sulfur, phosphorous, and lead contents in the steel base metal. In non-ferrous metals, it is often caused by sulfur or zinc. It can also be caused by an improper method of breaking the arc or in a root pass when the cross-sectional area of the weld bead is small compared to the mass of the base metal. Hot cracking often occurs in deep penetrating welds and can continue through successive layers if it is not repaired. Hot cracking may be prevented or minimized by the following:
- Preheating to reduce shrinkage stresses in the weld.
- Using clean or uncontaminated shielding gas.
- Increasing the cross-sectional area of the weld bead.
- Changing the contour of the weld bead.
- Using base metal with very low contents of those elements that tend to cause hot cracking.
Crater cracks are shallow hot cracks that are caused by improperly breaking the arc. Crater cracks may be prevented the same way that craters are, by reversing the travel of the electrode back into the weld bead a little way, gradually reducing the welding current at the end of the weld, or by stopping the travel before breaking the arc.
Cold cracking occurs after the weld metal solidification is complete. Cold cracking may occur several days after welding and is generally caused by hydrogen embrittlement, excessive joint restraint, and rapid cooling. Preheating and using a dry high purity shielding gas help reduce this problem. Centerline cracks are cold cracks that often occur in single pass concave fillet welds. A centerline crack is a longitudinal crack that runs down the center of the weld (Figure 105).
Figure 105 — Centerline crack.
This problem may be caused by one or more of the following:
- Weld bead that is too small for the thickness of the base metal.
- Poor fitup.
- High joint restraint.
- Extension of a crater crack.
The best methods of preventing centerline cracks are the following:
- Increasing the bead size.
- Decreasing the width of the root opening.
- Preventing weld craters.
Base metal and underbead cracks are cold cracks that form in the heat affected zone of the base metal. Underbead cracks occur underneath the weld bead (Figure 106).
Figure 106 — Underbead cracks.
Base metal cracks are those cracks that originate in the heat affected zone of the weld. These types of cracking are caused by excessive joint restraint, entrapped hydrogen, and a brittle microstructure. A brittle microstructure is caused by rapid cooling or excessive heat input. Underbead and base metal cracking can be reduced or eliminated by using preheat.
Other problems that can occur with TIG and reduce the quality of the weld are arc blow, loss of shielding gas coverage, and electrode contamination.
The electric current that flows through the electrode, workpiece, and work cable sets up magnetic fields in a circular path perpendicular to the direction of the current. When the magnetic fields around the arc are unbalanced, it tends to bend away from the greatest concentration of the magnetic field. This deflection of the arc is called arc blow. Deflection is usually in the direction of travel or opposite to it, but it sometimes occurs to the side. Arc blow can result in an irregular weld bead and incomplete fusion.
Direct current is highly susceptible to arc blow, especially when welding is being done in corners and near the ends of joints. Arc blow occurs with direct current because the induced magnetic field is in one direction. Alternating current is rarely subject to arc blow because the magnetic field is building and collapsing continuously due to the reversing current. The problem also occurs when welding complex structures and massive structures with high currents and poor fitup. Forward arc blow is encountered when welding away from the ground connection or at the beginning of the weld joint. Backward arc blow occurs toward the grounding connection, into a corner, or toward the end of a welding joint. Several corrective methods that can be used to correct the arc blow problem include:
- Changing to alternating current.
- Welding toward an existing weld or tack weld.
- Reducing the welding current and making the arc length as short as possible.
- Placing the work connection as far as possible from the weld, at the end of the weld, or at the start of the weld, and welding toward the heavy tack weld.
- Wrapping the work lead cable around the workpiece so that the magnetic field caused by the current in the work cable will neutralize the magnetic field causing the arc blow.
Many defects that occur in TIG welding are caused by an inadequate flow or blockage of shielding gas to the welding area. An inadequate gas supply can cause oxidation of both the tungsten electrode and the weld puddle, as well as porosity in the weld bead. This can be detected easily because the arc will change color, the weld bead will be discolored, and the arc will become unstable and difficult to control. The most common causes of this problem are the following:
- Blockage of gas flow in the torch or hoses.
- Leak in the gas system.
- Very high travel speed.
- Improper flow rate.
- Wind or drafts.
- Arc length or stickout too long.
There are several ways this problem can be corrected or prevented. Check the torch and hoses before welding to make sure the shielding gas can flow freely and is not leaking. A very high travel speed may leave the weld puddle, or a portion of it, exposed to the atmosphere. This may be corrected, in some cases, by inclining the torch in the direction of travel, using a nozzle that directs shielding gas back over the heated area, or by increasing the gas flow rate. Increasing the gas flow rate will increase the expense of the welding.
When welding some of the reactive metals, you may have to use an inert atmosphere chamber or trailing nozzles. An improper flow rate may occasionally be a problem. For example, when using argon and welding in the overhead position, you may have to use higher gas flow rates to provide adequate shielding. This is because argon is heavier than air and it will fall away from the weld area.
When winds or air drafts are present, you may take several corrective steps. Setting up screens around the operation is the best method of solving this problem. Increasing the gas flow rate is another method but, again, this will increase the cost of welding. An excessive arc length or stickout will also create a problem in providing adequate shielding because the distance between the end of the nozzle and the molten weld puddle is very long. This can be corrected by shortening the arc length or stickout.
Contamination of the tungsten electrode can cause discontinuities in the weld as well as a hard to control arc and loss of several minutes of welding time to clean the electrode. The electrode can become contaminated by several means, such as contact of the weld puddle with the electrode, contact of the electrode with the filler metal, inadequate shielding gas flow, or post welding gas flow time that is too short. Figure 107 shows the effects of different causes of electrode contamination.
Figure 107 — Electrode contamination.
When the electrode becomes contaminated by contact with the filler or weld metal, it produces a wild and unstable arc. When a lack of shielding gas is the cause of the contamination, it greatly reduces the life of the electrode.
There are two major methods of correcting this problem. The first is to break off the contaminated section and then prepare the clean section for welding. This is usually done by using a pair of pliers or by putting the contaminated section over the end of a workbench and breaking it off by striking it with a hammer. The second method is to hold the arc on a section of copper or other metal until the electrode has been cleared of contaminating metal through its vaporization. The first method is more commonly used when the electrode is very contaminated.
- To Table of Contents -
Several operations may be required after welding, such as cleaning, inspecting, repairing or straightening the welds, and postheating. These operations may or may not be part of the procedure, and those performed will depend on the governing code or specification, type of metal, and the quality of the weld deposit.
One of the major advantages of gas tungsten arc welding is that it produces a very smooth, clean weld bead with very little or no spatter, so there is no slag to be chipped off the weld bead. Because of this, postweld cleaning may be omitted and only wire brushing or buffing may be required to remove the discoloration around the weld bead.
Inspection and testing the weld to determine the quality of the weld joint is done after cleaning. There are many different methods of inspection and testing which were covered in previous chapters. The uses of these methods wiII often depend on the code or specification that covered the welding. Testing of a weldment may be done nondestructively or destructively.
Nondestructive testing is used to locate defects in the weld and base metal. Of the many different nondestructive testing methods, some of the most widely used methods are visual, magnetic particle, liquid penetrant, ultrasonic, and radiographic. Visual, magnetic particle, and liquid penetrant inspection are used to locate surface defects, whereas ultrasonic and radiographic inspections are used to locate internal defects. Destructive testing is used to determine the mechanical properties of the weld, such as the strength, ductility, and toughness.
Destructive testing is also done by several methods, depending on the mechanical properties being tested for. Some of the most common types of destructive testing are tensile bar tests, impact tests, and bend tests.
Repairing the weld is sometimes necessary when defects are found during inspection. When a defect is found, it can be gouged, ground, chipped, or machined out depending on the type of material being welded.
For steels, grinding and air carbon arc gouging are commonly used. It is not used on the non-ferrous metals because it causes contamination in the form of carbon deposits.
For the stainless steels and the non-ferrous metals, chipping is a common method for removing defects. Air carbon arc gouging is preferred for many applications because it is usually the quickest method. Grinding is popular for removing surface defects and shallow lying defects. Once you have removed the defects, you can reweld the low areas created by the grinding and gouging using gas metal arc welding or some other welding process. You should then reinspect the welds to make sure the defects have been properly repaired.
Postheating is the heat treatment applied to the weld or weldment after welding. Postheating is often required after the weld has been completed, but this depends upon the type of metal being welded, the specific application, and the governing code or specifications. Many of the low carbon steels and non-ferrous metals are rarely post-heated.
Various types of post-heating are used to obtain specific properties. Some of the most commonly used postheats are annealing stress relieving, normalizing, and quenching and tempering. Stress relieving is the most widely used heat treatment after welding.
Postheating is accomplished by most of the same methods used for preheating, such as furnaces, induction coils, and electric resistance heating blankets. One method used for stress relieving that does not involve the reheating of the weldments is called vibratory stress relief. This method vibrates the weldment during or after welding to relieve the residual stresses during or after solidification.
Annealing is a process involving heating and cooling that is usually applied to induce softening. This process is widely used on metals that become very hard and brittle because of welding. There are several different kinds, and when used on ferrous metals it is called full annealing. Annealing is the heating up of a material to cause recrystallization of the grain structure which causes softening. Full annealing is a softening process in which a ferrous alloy is heated to a temperature above the transformation range and is slowly cooled to a temperature below this range. This process is usually done in a furnace to provide a controlled cooling rate. Normalizing is a heat treatment that is applied only to ferrous metals.
Normalizing occurs when the metal is heated to a temperature above the transformation range and is cooled in still air to a temperature below this range. The main difference between normalizing and annealing is that a normalized weldment is cooled in still air, which produces a quicker cooling rate than an annealed weldment which is slowly cooled in a furnace. A normalizing heat treatment will refine the metal grain size and yield a tougher weld, whereas an annealing heat treatment will result in a softer weld.
Stress relieving is the uniform heating of a weldment to a high enough temperature (below the critical range) to relieve most of the residual stresses due to welding. This is followed by uniform cooling. This operation is performed on the ferrous metals and some of the non-ferrous metals. This process also reduces warpage during machining that may occur with a high residual stress buildup. Stress relieving is performed on nonferrous metal when stress buildup is a problem; however, in the case of aluminum alloys, for example, this heat treatment also will reduce the mechanical properties of the base metal. In the case of magnesium alloyed with aluminum, stress relieving is performed to avoid problems with stress corrosion.
On parts and metals that are likely to crack due to the internal stress created by welding, the parts should be put into stress relief immediately after welding without being allowed to cool to room temperature. The terms normalizing and annealing are misnomers for this heat treatment. Quenching and tempering is another postweld heat treatment that is commonly used where the metal is heated up and then quenched to form a hard and brittle metallurgical structure. The weldment is then tempered by reheating to a particular temperature dependent on the degree of ductility, strength, toughness, and hardness desired. Tempering reduces the hardness of the part as it increases the strength, toughness, and ductility of the weld.
|Test Your Knowledge
15. Why is spatter rarely a problem when using the TIG process?
16. What is the annealing process used for?
- To Table of Contents -
Gas tungsten arc welding requires a high degree of welder skill to produce good quality welds. This process requires the use of two hands when filler metal is added. A welder that is skilled in this process will generally have less trouble learning to weld with the other arc welding processes.
The exact content of a training program will vary depending on the specific application of the process. The program should be flexible enough so that it can be adapted to changing needs and applications. The complexity of the parts to be welded, the governing codes and specifications, and the type of metal to be welded all need to be taken into consideration.
A pipe welding course would take more training than a course on welding of plate. A course concerning the welding of stainless steel might cover the use of pulsed current and a different type of tungsten electrode preparation than a course covering the welding of aluminum. The welding characteristics of the metals would also be different.
The basic gas tungsten arc welding training program is used to teach the students the basic skills necessary for using the process to weld plate. Such a course would provide training on how to strike the arc, run weld beads, and make good quality fillet and groove welds. It would also include the welding of mild steel, stainless steel, and aluminum. Because of this, the course shown in the sample outline below has been split into three sections covering each of the three metals. The proper cleaning techniques are also covered for the three metals.
The training obtained by the student should give him or her enough skill to perform a job welding plate material. This course should also provide the background skill and knowledge required to take a course on gas tungsten arc welding of pipe and tubing. The following outline is for a course approximately seventy hours long.
- Lecture/Discussion -"Introduction to Gas Tungsten Arc Welding"
- Lecture/Discussion -"The Safety and Health of Welders"
- Lecture/Discussion -"Preparation for Welding Starting, Equipment Adjustment, and Shutdown"
This part of the course covers welding fillet and square groove welds in the flat, horizontal, and vertical positions on mild steel using direct current. This includes techniques used with and without filler metal.
- Stringer Bead, Flat Position, without and with Filler Metal
- Fillet Weld, Lap Joint, Horizontal Position, without and with Filler Metal
- Lecture/Discussion -"Weld Properties and Weld Quality, Mild Steel"
- Fillet Weld, Outside Corner Joint, Flat Position, without and with Filler Metal
- Fillet Weld, T-Joint, Horizontal and Vertical Position, with Filler Metal
- Square-Groove Weld, Butt Joint, Flat Position with Filler Metal
- Single-V-Groove Weld, Butt Joint, Guided Bend Test
- Square-Groove Weld, Butt Joint, Overhead Position, with Filler Metal
This part of the course covers the welding of stainless steel and the use of pulsed direct current. Groove and fillet welds are made in the flat, horizontal, and vertical positions with and without the use of pulsed current and filler metal.
- Lecture/Discussion -"Introduction to Gas Tungsten Arc Welding Using Pulsed Current”
- Square-Groove Weld, Butt Joint, Flat Position, with Filler Metal, without and with Pulsation
- Fillet Weld, Lap Joint, Horizontal Position, without and with Filler Metal
- Lecture/Discussion -"Weld Properties and Qualities, Stainless Steel"
- Filler Weld, Outside Corner Joint, Flat Position, without and with Filler Metal
- Visual Inspection Test, Stainless Steel
- Fillet Weld, T-Joint, Horizontal and Vertical Position Up, with Filler Metal
- Stringer Bead, Flat Position, with Filler Metal
The last part of the course covers welding of fillet and square-groove welds in the flat, horizontal, and vertical positions on aluminum using alternating current.
- Lecture/Discussion -"Equipment Adjustments and Their Effects on the Welding Arc Electrode, Current Amperage Chart"
- Square-Groove Weld, Butt Joint, Flat Position, with Filler Metal
- Lecture/Discussion. "Weld Properties and Qualities, Aluminum"
- Fillet Weld, Lap Joint, Horizontal Position, with Filler Metal
- Fillet Weld, Outside Corner Joint, Flat Position, with Filler Metal
- Fillet Weld, T-Joint, Horizontal and Vertical Position Up, with Filler Metal
- Visual Inspection Test, Aluminum
- Square-Groove Weld, Butt Joint, Vertical Position Up, with Filler Metal
- Square-Groove Weld, Butt Joint, Overhead Position. with Filler Metal
The training program for gas tungsten arc welding of tubing and pipe is used to teach students basic skills and provides additional training to students who previously learned to weld plate material. This course covers the welding of mild steel, small diameter pipe, tube, and larger diameter pipe. It is divided into two sections.
The first part of the course includes the welding of 3-inch mild steel pipe. All passes are welded using gas tungsten arc welding. Also included in this section of the course is the welding of 4-inch diameter, Schedule 10 tubing, which is welded in one pass.
The second part of the course covers the welding of 8-inch diameter, mild steel pipe. Since gas tungsten arc welding is only used for welding the root and hot passes on the large diameter pipe, the course includes filling out the remainder of the joint with shielded metal arc welding. The student should be skilled in shielded metal arc pipe welding before taking this portion of the training program. The following outline is for a course that is approximately 210 hours in length.
1. Lecture/Discussion -"Introduction to Gas Tungsten Arc Welding of Pipe" 2. Lecture/Discussion -"Safety and Health of Welders" 3. Lecture/Discussion -"Preparation for Welding"
This part of the course covers the welding of 3-inch diameter, Schedule 40 piping in the 2G and 5G positions, and 4-inch diameter tubing in the 2G, 5G, and 6G positions. This portion of the course is approximately 70 hours in length.
- Set-up, Tack Welding of Pipe
- Single-V-Groove Weld, Butt Joint, Vertical Fixed Position (2G), with Filler Metal, 3-inch Pipe
- Single-V-Groove Weld, Butt Joint, Horizontal Fixed Position (5G) with Filler Metal, 3-inch Pipe
- Single-V-Groove Weld, Butt Joint, Vertical Fixed Position (2G) and Horizontal Fixed Position (2G) and Horizontal Fixed Position (5G). Visual and Guided Bend Tests, 3-inch Pipe
- Single-V-Groove Weld, 45 Degrees Inclined Position (6G)
- Lecture/Discussion -"Pipe Weld Quality"
- Square-Groove Weld, Butt Joint, 45 Degrees Inclined Position (6G), 4-inch Tubing
This part of the course covers the welding of 8-inch diameter, Schedule 40, mild steel piping in the 2G, 5G, and 6G positions. The root and hot passes are welded using gas tungsten arc welding. A section on the use of pulsed current is also included.
The fill and cover passes are welded using shielded metal arc welding and E7018 electrodes. This part of the course also includes the use of consumable inserts put in the root of the joint, and the welding of stainless steel pipe. This portion of the course is approximately 140 hours in length.
- Single-V-Groove, Butt Joint, Rolled Flat Position (1G)
- Single-V-Groove, Butt Joint, Horizontal Fixed Position (5G)
- Single-V-Groove, Butt Joint, Vertical Fixed Position (2G)
- Single-V-Groove, Butt Joint, Horizontal Fixed Position (5G) and Vertical Fixed Position (2G) Visual and Guided Bend Tests
- Single-V-Groove, Butt Joint, 45 Degrees Inclined Position (6G) Visual Test
- Lecture/Discussion -''Variations of the GTAW Process for Pipe"
- Single-V-Groove, Butt Weld, 45 Degrees Inclined Position, Using Pulsed Current
- Lecture/Discussion -"Stainless Steel Pipe Welding”
- Single-V-Groove Weld, Butt Joint, 45 Degrees Inclined Position (6G) Stainless Steel Pipe
- Lecture/Discussion -"Joint Designs for Gas Tungsten Arc Welding"
- Tack Weld, Butt Joint (with consumable insert) Vertical Fixed Position (2G)
- Single-V-Groove, Butt Joint (with consumable insert) 45 Degrees Inclined Position
Before a welder can begin work on any job covered by a welding code or specification, the welder must become certified under the code that applies. Many different codes are in use today and it is extremely important that the specific code is referred to when taking qualification tests.
In general, the following types of work are covered by codes: pressure vessels and piping, bridges, public buildings, storage tanks and containers that will hold flammable or explosive materials, cross-country pipelines, aircraft, ordnance material, ships and boats, and nuclear power facilities. Certification is obtained differently under the various codes.
Certification under one code will not necessarily qualify a welder to work under a different code. In most cases, certification for one employer will not allow the welder to work for another employer.
Also, if the welder uses a different process or if the welding procedure is altered drastically, recertification is required. In most codes, if the welder is continually employed, welding recertification is not required, providing the work performed meets the quality requirements. An exception is the military aircraft code, which requires requalification every six months.
Responsible manufacturers or contractors may give qualification tests. On pressure vessel work, the welding procedure must also be qualified, and this must be done before the welders can be qualified. Under other codes, this is not necessary.
To become qualified, the welder must make specified welds using the required process, base metal, thickness, electrode type, position, and joint design. Test specimens must be made according to standardized sizes and under the observation of a qualified person. In most government specifications, a government inspector must witness the making of weld specimens. Specimens must be properly identified and prepared for testing.
The most common test is the guided bend test. However, in some cases, x-ray examinations, fracture tests, or other tests are used. Satisfactory completion of test specimens, providing that they meet acceptability standards, will qualify the welder for specific types of welding. The welding that will be allowed depends on the particular code. In general, the code indicates the range of thicknesses that may be welded, the positions that may be used, and the alloys which may be welded.
Welder qualification is a highly technical subject and cannot be fully covered here. You should obtain and study the actual code prior to taking any tests. Some frequently used codes for welder qualification are the following:
ASME Boiler and Pressure Vessel Code
Section IX AWS Structural Welding Code D1
Military Specifications and Standards
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Safety is an important consideration when welding. Every welding shop should have a safety program and take adequate safety precautions to protect welders. Every welder should be made aware of safety precautions and procedures. Employees who fail to follow adequate safety precautions can cause physical injury to themselves and others as well as damage to property. Failure to take safety precautions can result in physical discomfort and loss of property, time, and money.
Welding is a safe occupation when safety rules and common sense are followed. A set of safety rules that should be followed is presented in the American National Standard Z49.1, "Safety in Welding and Cutting," published by the American Welding Society.
There are a number of hazards associated with gas metal arc welding. These do not necessarily result in serious injuries; they can also be of a minor nature which can cause discomforts that irritate and reduce the efficiency of the welders. These hazards are:
- Electrical shock
- Arc radiation
- Air contamination
- Compressed gases
- Fire and explosion
- Weld cleaning and other hazards
- Other hazards related to other projects
Several precautions should be taken to prevent an electrical shock hazard. First, make sure that the arc welding equipment is properly installed, grounded, and in good working **131 condition. Maintain and install the electrical equipment in accordance with the National Electrical Code and any state and local codes that apply. Operate equipment within NEMA Standards’ usual operating conditions for proper safety and equipment life. Connect the case or frame of the power supply to an adequate electrical ground such as an approved building ground, cold water pipe, or ground rod. Welding cables with frayed or cracked insulation and faulty or badly worn connections can cause electrical short circuits and shocks. An improperly insulated welding cable is both an electrical shock hazard and a fire hazard. Keep the welding area dry and free of any standing water. When it is necessary to weld in a damp or wet area, wear rubber boots and stand on a dry, insulated platform.
The gas tungsten arc emits invisible ultraviolet and infrared rays. Skin exposed to the arc for a short time can suffer serious ultraviolet and infrared burns that are essentially the same as sunburn, but the burn caused by welding can take place in a much shorter time and can be very painful. Prolonged and repeated exposure to ultraviolet rays may cause skin cancer in some skin types. You should always wear protective clothing suitable for the welding to be done.
Since there is no spatter in this process, general precautions include wearing long sleeve shirts or cloth lab coats to protect your arms, shoulders, chest, and stomach from the arc radiation. Wear leather gloves, but wear lighter ones than those worn for shielded metal arc welding. Wear cloth gloves for light duty work.
Your eyes must be protected from the radiation emitted by the welding arc. Arc burn can result if your eyes are not protected. Arc burn to the eye is similar to sunburn to the skin and it is extremely painful for about 24 to 48 hours. Usually, arc burn does not permanently injure the eyes but it can cause intense pain. There are several commercial solutions available to soothe the skin and eyes during the period of suffering.
Infrared arc rays can cause fatigue of the retina of the eye. Ultraviolet radiation is the only known cause of cataracts at this time. Impaired vision can be the result.
Gas tungsten arc welding produces a brighter arc than shielded metal arc welding because there is no smoke and it is often used on bright and shiny metals such as aluminum and stainless steel. Protect your eyes and face with a head shield that has a window with a filter lens set in it. Helmets with large windows are popular for welding with this process. Head shields are generally made of fiberglass or pressed fiber material and are lightweight. The filter lens is made of a dark glass capable of absorbing infrared rays, ultraviolet rays, and most visible light coming from the arc.
The lens shade used varies for different welders, different metals, and different current levels, but it should be dark enough so that you can view the arc without discomfort but not so dark that you cannot see the arc and puddle clearly. A number 12 filter lens is recommended for use in gas tungsten arc welding because of its brighter arc, but the project’s variables may dictate a darker lens. Table 9-26 shows the different lenses commonly recommended for use in GTAW. The higher the lens numbers the darker the lens. A clear glass should be put on the outside of the welding lens to protect it from spatter and breakage. Welding should never be done with a broken filter lens or with cracks in the head shield.
Table 9-26 — Recommended filter lens shades used in gas tungsten arc welding (ANSI/AWS Z49.1).
|Electrode Diameter-In. (mm)||Lens Shade Number|
|1/16 (1.6), 3/32 (2.4), 1/8 (3.2), 5/32 (4.0)||10|
|3/16 (4.8), 7/32 (5.6), 1/4 (6.4)||12|
|5/16 (7.9), 3/8 (9.5)||14|
Welding fumes are generated by the arc. The welding area should be adequately ventilated because the vaporized metals are potentially hazardous for the welder. When welding is done in confined areas, adequate mechanical ventilation or protection for the welder is required. This may be furnished by the use of a gas mask or on a special helmet. A second person should stand just outside the confined area to lend assistance to the welder if necessary.
Another method to use is a mechanical exhaust system to remove the welding fumes. The argon or helium shielding gas may displace the air that the welder needs for breathing. Welding should never be done near degreasing and other similar operations. When they are exposed to an arc, the fumes from chlorinated cleaning solvents form a very toxic gas, called phosgene, so welding should never be done near cleaning chemicals. In addition, a mechanical exhaust should be used when welding metals such as lead, copper, beryllium, cadmium, zinc, brass, bronze, chromium, cobalt, manganese, nickel, and vanadium.
When grinding tungsten electrodes, which are mildly radioactive, it is advisable to use a dust collector on the grinder to prevent inhalation of the dust.
The shielding gases used for TIG, typically argon and helium, are compressed and stored in cylinders. Only use compressed gases for their intended purpose. Cylinders containing oxygen should be stored separately from cylinders containing fuel gases. Cylinders in use or in stores or cargo should be securely fastened to prevent their shifting or falling under any weather conditions. The welder should open the valve of the cylinder slowly and stand away from the face of the regulator when doing this. The welding arc should never be struck on a compressed gas cylinder. When not in use, gas cylinders should be stored with their caps on; caps should also be on when the cylinders are moved. If the valve should get knocked off, the cylinder acts like a missile because of the escaping gas and can cause injury and damage. When compressed gas cylinders are empty, the valve should be closed and they should be marked empty. This is done by marking the letters "MT" or "EMPTY" on the cylinder.
Move cylinders by tilting and rolling them on their bottom edges. Avoid dragging and sliding cylinders. When cylinders are transported by vehicle, secure them in position. Cylinders should not be dropped, struck, or permitted to strike each other violently. Discontinue the use of any cylinder before the pressure falls to zero. In particular, do not use oxygen cylinders in welding or cutting operations after the pressure falls below approximately 25 psi.
Fires and explosions are hazards that can exist in a welding area if the proper precautions are not taken. The TIG process may produce sparks which can start a fire or explosion in the welding area if it is not kept free of flammable, volatile, or explosive materials. Welding should never be done near degreasing and other similar operations.
Although TIG welding does not produce spatter and long sleeve shirts or cloth lab coats are used sometimes for skin protection, welders should wear leather clothing to protect from burns; the leather is fireproof. Fires can also be started by an electrical short or by overheated, worn cables. In case of a fire that is started by a flammable liquid or an electrical fire, use a CO2 or dry chemical type of fire extinguisher. Fire extinguishers should be kept at handy spots around the shop, and the welders should make a mental note of where they are located. Welders should not have disposable butane or propane lighters when welding. Sparks or weld spatter hitting them can cause an explosion which may cause injury.
Other precautions that have to do with explosions are also important. Do not weld on containers that have held combustibles unless it is absolutely certain that there are no fumes or residue left. Do not welding on sealed containers without providing vents and taking special precautions. Never strike the welding arc on a compressed gas cylinder. When the welding torch is set down or not in use, it should never be allowed to touch a compressed gas cylinder.
Hazards can also be encountered during the weld cleaning process. Precautions must be taken to protect your skin and eyes from hot slag particles. Wear safety glasses, gloves, and heavy clothing during chipping and grinding operations. Set screens up if there are other people in the area to protect them from arc burn.
- Make sure your arc welding equipment is installed properly, grounded, and in good working condition.
- Always wear protective clothing suitable for the welding to be done.
- Always wear proper eye protection when welding, grinding or cutting.
- Keep your work area clean and free of hazards. Make sure no flammable, volatile, or explosive materials are in or near the work area.
- Handle all compressed gas cylinders with extreme care. Keep caps on when not in use.
- Make sure compressed gas cylinders are secured to the wall or other structural supports.
- When compressed gas cylinders are empty, close the valve and mark the cylinder “Empty” or “MT.”
- Do not weld in a confined space without extra special precautions.
- Do not weld on containers that have held combustibles without taking extra special precaution.
- Do not weld on sealed containers or compartments without providing vents and taking special precautions.
- Use mechanical exhaust at the point of welding.
- When it is necessary to weld in a damp or wet area, wear rubber boots and stand on a dry, insulated platform.
- Shield others from the light rays produced by your welding arc.
- Do not weld near degreasing operations.
- When the welding gun is not in use, do not hang it on a compressed gas cylinder.
- Follow guidelines and standards set forth by the American Welding Society, the Occupational Safety and Health Administration, the American National Standards Institute, the National Electrical Manufacturers Association, the Compressed Gas Association, and the Material Safety Data Sheets provided by U.S. manufacturers.
This course introduced you to the gas tungsten arc welding (GTAW or TIG) process, from the types of power sources, controls, and welding torches to the types of training and qualifications needed. It described the industries that use the TIG process and its applications. Welding metallurgy, weld and joint design, and welding procedure variables were also discussed. The course concluded with a description of possible weld defects and how to identify them, and safety precautions used for the TIG process. As always, refer to the manufacturer’s operator manuals for the specific setup and safety procedures of the welding machine you will be using.
1. A tungsten electrode has what type of characteristic?
2. What does Wolfgram mean?
3. The TIG process uses a _____.
4. In the AWS classification for tungsten electrodes, what is the letter designation for tungsten?
5. Which is NOT an advantage of TIG?
6. For AC welding with a conventional square wave power source, the High Frequency should be set to what position?
7. Gas tungsten arc welding uses all of these items except which item?
8. As a general rule, what should the inside diameter of the gas nozzle be?
9. For AC welding with a conventional square wave power source, how should the electrode tip be shaped?
10. For DCEN welding, how should the electrode tip be shaped?
11. What condition is caused by filler metal or base metal on the electrode?
12. On conventional sine wave and conventional square wave power sources, why is high frequency added to alternating current?
13. A good rule of thumb for setting post flow time is _____.
14. What parameters are set when the power source is set to DC?
15. What type of electrode could you use with ac?
16. What does a 60% duty cycle mean with regard to power source operation?
17. In TIG, what type of current produces the deepest weld penetration?
18. In the AWS electrode classification ER4043, the 4043 means _____
19. What should be done with the torch when the torch is not in use?
20. The type of heat treatment where the weldment is held above the transformation temperature and allowed to cool in still air is called _____
21. What is the type of heat treatment that reduces warpage?
22. What is the type of heat treatment that produces the highest ductility in carbon steel?
23. When, if ever, is a transformer welding machine used for TIG welding?
24. What power supply was developed to overcome the arc-extinguishing – restriking problem?
25. What is the most common torch head angle for TIG welding?
26. Between 6% and 22% _______ by weight is contained in Austenitic stainless steel.
27. What is the major alloying element that distinguishes stainless steels from other types of steel?
28. Between what temperatures does carbide precipitation occur?
29. The best way to prevent carbide precipitation is to use base metals and filler rods with extremely low carbon content; what other elements also prevent carbide precipitation?
30. What type of bristle brush should you use when brushing stainless steel?
31. A stainless steel with a carbon content greater than ____% will often need preheating?
32. What is the maximum preheat temperature used on aluminum?
33. What layer on the surface of aluminum makes it difficult to weld?
34. What type of current is the pulsed current method of welding commonly used with?
35. What is the maximum welding current of an air cooled torch?
36. Why is pulsed current useful for welding stainless steel?
37. How many types of nozzles are available for TIG welding torches?
38. What is the popular type of nozzle used?
39. Where is the gas orifice located on a TIG torch?
40. Orbital welding head torch oscillation speed and width are _____ adjusted?
41. What is the most common type of gas flow control?
42. What is the constant outlet pressure from the regulator to the flowmeter?
43. A welding cable AWG no. 8 has what maximum amperage rating?
44. What group of stainless steels is included in the 200 and 300 series?
45. What is not a basic group of titanium and titanium alloys?
46. What is a refractory metal?
47. What does the number 3 refer to when describing welding positions?
48. What is the purpose of using helium on thick sections of base metal?
49. Which of the following characteristics help determine welding current?
50. What lens shade number is recommended for 1/16 “ diameter electrodes
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