|1.0.0 Introduction to the Process||9.0.0 Welding Procedure Schedules|
Shielded Metal Arc Welding (SMAW) is an arc welding process in which the fusing of metals is produced by heat from an electric arc that is maintained between the tip of a consumable covered electrode and the surface of the base metal in the joint being welded. Shielded Metal Arc welding is one of the most widely used welding processes, particularly for short welds in production, maintenance, and repair work and for field construction.
This course gives you an understanding of the safety precautions for SMAW and an awareness of the importance of safety in welding. You will also get a basic understanding of the SMAW process and equipment along with the key variables that affect the quality of welds such as electrode selection, polarity and amperage, arc length, travel speed, and electrode angles. We will also cover core competencies such as setting up welding equipment, preparing weld materials, fitting up weld materials, welding carbon steel plates, and repairing welds.
Always refer to the manufacturer’s manuals for specific operating and maintenance instructions
When you complete this course, you will be able to:
Shielded Metal Arc Welding (SMAW) is an electric arc welding process in which an electric arc between a covered metal electrode and the work generates the heat for welding. The filler metal is deposited from the electrode, and the electrode covering provides the shielding. Some slang names for this process are ''stick welding" or ''stick electrode welding': Figure 1 shows a diagram of this process.
Figure 1 — Shielded metal arc welding.
The shielded metal arc welding process is one of the simplest and most versatile arc welding processes. It can be used to weld both ferrous and non-ferrous metals, and it can weld thicknesses above approximately 18 gauge in all positions. The arc is under the control of the welder and is visible. The welding process leaves slag on the surface of the weld bead which must be removed.
The most common use for this process is welding mild and low alloy steels. The equipment is extremely rugged and simple, and the process is flexible in that the welder needs to take only the electrode holder and work lead to the point of welding.
Most sources give credit for the invention of the electric arc to Sir Humphrey Davy of England in 180l. For the most part, the electric arc remained a scientific novelty until 1881, when the carbon arc street lamp was invented and the first attempts to weld using the carbon arc process were made. The metal arc welding process came into being when metal rods replaced the carbon electrodes in 1889. Coverings for the bare wire electrodes were first developed in the early 1900's. The first major use occurred during World War I, especially in the shipbuilding industry. After the war, there was a period of slow growth until the early 1930's when shielded metal arc welding became a major manufacturing method and a dominant welding process. Today, the shielded metal arc welding process is widely used, even though its relative importance has been declining slowly in recent years.
The shielded metal arc welding process is basically a manually operated process. The electrode is clamped in an electrode holder and the welder manipulates the tip of the electrode in relation to the metal being welded. The welder strikes, maintains, and stops the arc manually.
Several variations of this process are done automatically rather than manually. These are: gravity welding, firecracker welding, and massive electrode welding. These methods comprise only a very small percentage of welding done by the shielded metal arc welding process.
Shielded metal arc welding is widely used because of its versatility, portability, and comparatively simple and inexpensive equipment. In addition, it does not require auxiliary gas shielding or granular flux.
Welders can use the shielded metal arc welding process for making welds in any position they can reach with an electrode. Electrodes can be bent so they can be used to weld blind areas. Long leads can be used to weld in many locations at great distances from the power source.
Shielded metal arc welding can be used in the field because the equipment is relatively light and portable. This process is also less sensitive to wind and draft than gas shielded arc welding processes.
Shielded metal arc welding can be used to weld a wide variety of metal thicknesses. This process is more useful than other welding processes for welding complex structural assemblies because it is easier to use in difficult locations and for multi-position welding. Shielded metal arc welding is also a popular process for pipe welding because it can create weld joints with high quality and strength. However, the shielded metal arc welding process has several limitations. Operator duty cycle and overall deposition rates for covered electrodes are usually less than those of a continuous electrode process. This is because electrodes have a fixed length and welding must stop after each electrode has been consumed to discard the remaining portion of the used electrode clamped into the holder and reapply another. Another limitation is that the slag must be removed from the weld after every pass. Finally, the shielded metal arc welding process cannot be used to weld some of the non-ferrous metals.
- To Table of Contents -
The shielded metal arc welding process uses the heat of the electric arc to melt the consumable electrode and the work being welded. The welding circuit includes a power source, welding cables, an electrode holder, a work clamp and a welding electrode. One of the welding cables connects the power source to the electrode holder and the other cable connects to the workpiece.
The welding begins when the welder initiates the arc by momentarily touching the electrode to the base metal, which completes the electrical circuit. The welder guides the electrode manually, controlling both the travel speed and the direction of travel. The welder maintains the arc by controlling the distance between the work material and the tip of the electrode (length of the arc). Some types of electrodes can be dragged along the surface of the work so that the coating thickness controls the arc length, which controls the voltage.
The heat of the arc melts the surface of the base metal and forms a molten weld puddle. The melted electrode metal is transferred across the arc and becomes the deposited weld metal. The deposit is covered by a slag produced by components in the electrode coating. The arc is enveloped in a gas shield provided by the disintegration of some of the ingredients of the electrode coating. Most of the electrode core wire is transferred across the arc, but small particles escape from the weld area as spatter, and a very small portion leaves the welding area as smoke.
The constant current type of power source is best for shielded metal arc welding. The constant current welding machines provide a nearly constant welding current for the arc. The constant current output is obtained with a drooping volt ampere characteristic, which means that the voltage reduces 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.
Under certain conditions, there is a need for variations in the volt-ampere slope. A steep volt-ampere characteristic is desirable when the welder wants to achieve maximum welding speed on some welding jobs. The steeper slope gives less current variation with changing arc length, and it gives a softer arc. The types of machines that have this kind of curve are especially useful on sheet metals. Machines with this characteristic are typically used with large diameter electrodes and high amperages. On some applications, such as welding over rust, or a position pipe welding where better arc control with high penetration capability is desired, a less steep volt-ampere characteristic is more desirable. Machines with the less steep volt-ampere curve are also easier to use for depositing the root passes on joints with varying fit-up. This type of power source characteristic allows the welder to control the welding current in a specific range by changing the arc length and producing a more driving arc. Differences in the basic power source designs cause these variations in the power sources. Figure 2 shows volt-ampere curves for different performance characteristics. This shows several slopes, all of which can provide the same normal voltage and current. The flatter slopes give a greater current variation for a given voltage change or arc length change. Machines that have a higher short circuit current give more positive starting.
Figure 2 — Typical volt-ampere curves for constant current types of power sources
Many terms are associated with arc welding. The following basic terms are especially important.
Alternating current — Alternating current is an electrical current that has alternating negative and positive values. In the first half-cycle, the current flows in one direction and then reverses itself for the next half-cycle. In one complete cycle, the current spends 50 percent of the time flowing one way and the other 50 percent flowing the other way. The rate of change in direction is called frequency, and it is indicated by cycles per second. In the United States, the alternating current is set at 60 cycles per second. The number of cycles per second is expressed in units of Hertz (Hz).
Ampere — Amperes, sometimes called “amps,” refers to the amount of current that flows through a circuit. It is measured by an “amp” meter. .
Conductor — Conductor means any material that allows the passage of an electrical current.
Current — Current is the movement or flow of an electrical charge through a conductor.
Direct current — Direct current is an electrical current that flows in one direction only.
Electrical circuit — Electrical circuit is the path taken by an electrical current flowing through a conductor from one terminal of the source to the load and returning to the other terminal of the source.
Polarity — Polarity is the direction of the flow of current in a circuit. Since current flows in one direction only in a dc welder, the polarity becomes an important factor in welding operations.
Resistance — Resistance is the opposition of the conductor to the flow of current. Resistance causes electrical energy to be changed into heat.
Volt — A volt is the force required to make the current flow in an electrical circuit. It can be compared to pressure in a hydraulic system. Volts are measured with a voltmeter.
The intense heat of the welding arc melts the tip of the electrode and melts the surface of base metal. The temperature of the arc is about 9000°F (5000°C) which causes almost instantaneous melting of the surface of the work. Globules form on the tip of the electrode and transfer through the arc to the molten weld puddle on the surface of the work. When the detaching globules are small during the transfer, this is known as spray type metal transfer. When the globules are relatively large during transfer, it is known as globular type metal transfer. Surface tension sometimes causes a globule of metal to connect the tip of the electrode to the weld puddle. This causes an electrical short and makes the arc go out. Usually this is a momentary occurrence, but occasionally the electrode will stick to the weld puddle. When the short circuit occurs, the current builds up to a short circuit value and the increased current usually melts the connecting metal and reestablishes the arc. A welding machine with a flatter volt-ampere curve will give a higher short circuit current than one with a steeper volt-ampere curve. The electrode sticking problem will be slightly less with a machine that has a flatter volt-ampere curve. A softer arc, produced by a steeper slope, will decrease the amount of weld spatter. A more driving arc, produced by a flatter slope, causes a more violent transfer of metal into the weld puddle, which will cause a greater splashing effect. This greater splashing effect will generate more spattering from the weld puddle. When the welds are made in the flat or horizontal positions, the forces of gravity, magnetism, and surface tension induce the transfer of the metal. When the welds are made in the vertical or overhead positions, the forces of magnetism and surface tension induce the metal transfer, while the force of gravity opposes metal transfer. Lower currents are used for vertical and overhead welding to allow shorter arc lengths and promote a smaller metal droplet size less affected by gravity.
|Test Your Knowledge
1. What does the welding process leave on the surface of the weld bead which must be removed?
2. When is a steep volt-ampere characteristic desirable?
- To Table of Contents -
The equipment for the shielded metal arc welding process consists of a power source, welding cable, electrode holder, and work clamp or attachment. Figure 3 shows a diagram of the equipment.
Figure 3 — Equipment for shielded metal 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. Many different sizes and types of power sources are designed for shielded metal arc welding. Most power sources operate on 230 or 460 volt input electric power, but power sources that operate on 200 or 575 volt input power are also available.
Shielded metal arc welding can use either direct current (DC) or alternating current (AC). Electrode negative (straight polarity) or electrode positive (reverse polarity) can be used with direct current. Each type of current has distinct advantages, but selection of the type of welding current used, usually depends on the availability of equipment and the type of electrode selected. Direct current flows in one direction continuously through the welding circuit. The advantages it has over alternating current are:
Polarity or direction of current flow is important in the use of direct current. Electrode negative (straight polarity) is often used when shallower penetration is required. Electrode positive (reverse polarity) is generally used where deep penetration is needed. Normally, electrode negative provides higher deposition rates than electrode positive. The type of electrode often governs the polarity to be used .
Alternating current is a combination of both polarities that alternates 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 again decays to zero. The polarity of the alternating current changes 120 times during the 60 Hz cycle used in the United States. Depths of penetration and deposition rates for alternating current are generally intermediate between those for DC electrode positive and DC electrode negative. Some advantages of alternating current are:
Arc blow is rarely a problem with alternating current.
Alternating current is well suited for welding thick sections using large diameter electrodes.
Duty cycle is the ratio of arc time to total time. For a welding machine, a 10 minute time period is used. Thus, for a 60% duty cycle machine, the welding load would be applied continuously for 6 minutes and would be off for 4 minutes. Most industrial type constant current machines are rated at 60% duty cycle. The formula for determining the duty cycle of a welding machine for a given load current 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 350 amperes would be.:
|% Duty Cycle =||(300)2||x 60 = 44%|
Figure 4 represents the ratio of the square of the rated current to the square of the load current multiplied by the rated duty cycle. A line is drawn parallel to the sloping lines through the intersection of the subject machines rated current output and rated duty cycle. For example, a question might arise whether a 400 amp 60% duty cycle machine could be used for a fully automatic requirement of 300 amps for a 10-minute welding job. It shows that the machine can be used at slightly over 300 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 short period. This illustration can be used to compare various machines. Relate all machines to the same duty cycle for a time comparison.
Figure 4 — Duty cycle vs. current load.
The output characteristics of the power source must be of the constant-current (CC) type. The normal current range is 25 to 500 amps using conventional size electrodes. The arc voltage is 15 to 35 volts.
The generator can be powered by an electric motor for shop use or by an internal combustion engine (gasoline, gas, or diesel) for field use. Engine driven welders can have either water or air cooled engines, and many of them provide auxiliary power for emergency lighting, power tools, etc. Generator welding machines can provide both AC and DC power, See Figures 8-5 and 8-6.
Figure 5 — Portable welder/generator.
Figure 6 — Diesel engine driven power source.
An alternator welding machine is an electric generator that produces AC power. This power source has a rotating assembly.
These machines are also called rotating or revolving field machines.
On dual control machines, normally a generator, the slope of the output curve can vary. The fine adjustment control knob controls open circuit, or "no load", voltage. This control is also the fine welding current adjustment during welding. The range switch provides coarse adjustment of the welding current. In this way, a soft or harsh arc can be obtained. With the flatter curve and its low open circuit voltage, a change in arc voltage will produce a greater change in output current. This produces the digging arc preferred for pipe welding. With a steeper curve and its high open circuit voltage, the same change in arc voltage will produce less of a change in output current. This is a soft or quiet arc, useful for sheet metal welding. This type of welding machine gives the smoothest operating arc because it produces less voltage ripple.
The transformer type welding machine is the least expensive, lightest, and smallest type of welder. It produces alternating current for welding. The transformer welder 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 welding current output of a transformer welder may be adjusted in many different ways. The simplest method of adjusting output current is to use a tapped secondary coil on the transformer. This is a popular method many of the limited input, small welding transformers employ. The leads to the electrode holder and the work are connected to plugs, which the welder may be insert in sockets on the front of the machine in various locations to provide the required welding current. Some machines employ a tap switch instead of the plug-in arrangement. In any case, exact current adjustment is not entirely possible.
Industrial types of transformer welders usually employ a continuous output current control. This can be obtained by mechanical means, or electrical means. The mechanical method usually involves moving the core of the transformer. Any method that involves mechanical movement of the transformer parts requires considerable movement for full range adjustment. The more advanced method of adjusting current output is by means of electrical circuits. In this method the core of the transformer or reactor is saturated by an auxiliary electric circuit which controls the amount of current delivered to the output terminals. By merely adjusting a small knob, to the welder can provide continuous current adjustment from the minimum to maximum of the output.
Although the transformer type of welder has many desirable characteristics, it also has some limitations. The power required for a transformer welder must be supplied by a single phase system, and this may create an unbalance of the power supply lines, which is objectionable to most power companies. In addition, transformer welders have a rather low power factor unless they are equipped with power factor correcting capacitors. The addition of capacitors corrects the power factor under load and produces a reasonable power factor that is not objectionable to electric power companies.
Transformer welders have the lowest initial cost. They require less space and are normally quiet in operation. In addition, alternating current welding power supplied by transformers reduces arc blow, which can be troublesome on many welding applications. They do not, however, have as much flexibility for the operator as the dual controlled generator.
The previously described transformer welders provide alternating current to the arc. Some types of electrodes operate successfully only with direct current power. A method of supplying direct current power to the arc without using a rotating generator is adding a rectifier, an electrical device which changes alternating current into direct current. Transformer-rectifier welding machines operate on single phase input power. These machines are used when both AC and DC current are needed. A single phase type of AC welder is connected to the rectifier which then produces DC current for the arc. By means of a switch that can change the output terminals to the transformer or the rectifier, the operator can select either AC or DC current for the welding requirement. Transformer-rectifier welding machines are available in different sizes. These machines are more efficient electrically than the generator welding machines, and they provide quieter operation. Figure 7 shows an AC/DC single phase power source.
Figure 7 — AC/DC single phase power source.
Three phase rectifier welding machines provide DC welding current to the arc. These machines operate on three phase input power. The three phase input helps overcome the line unbalance that occurs with single phase transformer-rectifier welding machines. In this type of machine, the transformers feed into a rectifier bridge, which then produces direct current for the arc. The three-phase rectifier unit is more efficient electrically than a generator and provides quiet operation. This type of machine also gives the least voltage ripple and produces the smoothest arc of the static type welding machines. Figure 8 shows a three phase solid state constant voltage power source. It automatically monitors output voltage and makes required changes to compensate for line voltage fluctuation.
Figure 8 — Three-phase constant voltage power source.
A multiple operator welding system uses a heavy duty, high current, and relatively high voltage power source which feeds a number of individual operator welding stations. At each welding station, a variable resistance is adjusted to drop the current to the proper welding range. Based on the duty cycle of the welding equipment, one welding machine can supply welding power simultaneously to a number of welding operators. The current supplied at the individual station has a drooping characteristic similar to the single operator welding machines described above. The power source, however, has a constant voltage Output. Constant voltage power sources are those that maintain a constant voltage for a given current setting. The volt-ampere curve for this type of power source is nearly flat. The welding machine size and the number and size of the individual welding current control stations must be carefully matched for an efficient multiple operator system. The formula for determining the number of arcs that can be operated off of one power source is:
|Available Power||= Number of Arcs|
|Average Arc Amperes x Duty Cycle|
Figure 9 — Inverter power source.
In this type of power source, which utilizes the inverter, the power from the line is first rectified to pulsing direct current (Figure 9). This current then goes to a high frequency oscillator or chopper, which changes the DC into high-voltage, high-frequency AC in the range 5 to 30 kHz. The output of the chopper circuit is controlled in accordance with welding procedure requirements. The high frequency AC is then transformed down to the operating welding voltage. The advantage of the inverter is the use of a small lightweight transformer, since transformers become smaller as frequency increases. The high frequency AC current is then rectified with silicon diodes to provide direct current output at normal welding current and voltage. The inverter power source has become economically feasible due to the availability of high current, high speed solid state electronic components at a reasonable cost. Inverter power sources are about 25% the weight of a conventional rectifier of the same power capacity and about 33% of the size. They provide higher electrical efficiency, a higher power factor, and a faster response time. Several variations of the inverter power source are available.
Selecting a welding machine is based on:
The size of the machine is based on the welding current and duty cycle required. Welding current, duty cycle, and voltage are determined by considering weld joints, weld sizes, and welding procedures. The incoming power available dictates this fact. Finally, the job situation, personal preference, and economic considerations narrow the field to the final selection. Consult the local welding equipment supplier to help make your selection. Know the following data when selecting a welding power source:
The controls are usually located on the front panel of the welding machine. These usually consist of a knob or tap switch to set the rough current range and a knob to adjust the current within the set range. On DC welding machines there is usually a switch to change polarity, and on combination AC-DC machines, there is usually a switch to select the polarity or AC current. An On-Off switch is also located on the front of the machine.
Arc Force Control is a function of amperage triggered by a preset (internal module) voltage. The preset trigger voltage is 18 volts. What this means is that anytime the arc voltage drops from normal welding voltage to 18 volts or less, the drop triggers the arc force current, which gives the arc a surge of current to keep the arc from going out.
When an arc is struck, the electrode is scratched against the work. At that point, the voltage goes to -0- which triggers the arc force current and the arc is initiated quickly. On a standard machine without arc force control, arc striking is difficult and electrode sticking may occur.
After the arc is established, a steady burn-off is desired. As the electrode burns and droplets of metal are transferred from the end of the electrode to the work piece, there is a time period when the droplet is still connected to the end of the electrode but is also touching the work piece. When this occurs, the machine is, in effect, in a "dead-short" - the voltage drops, the arc force is triggered and the droplet is transferred. On machines without arc force, the burn-off is the same; however, without the arc force to help, an arc outage may occur, and the electrode will stick in the puddle.
In tight joints, such as pipe welding, the arc length is very short and with standard machines, it is difficult to maintain the arc since it wants to "short-out" against the sidewalls or bottom of the joint. The arc force control can be adjusted on this type application to prevent electrode sticking; whenever the voltage drops, the drop triggers the arc force current and the sticking doesn't happen because the current surge occurs.
In many applications, there is a need for a very forceful arc to obtain deeper penetration, or in the case of arc gouging, the forceful arc is essential in helping to force the metal out of the groove being gouged. With arc force control, this type application is made much easier than with conventional machines, with which arc length becomes critical and arc outages can occur.
When welding with a given size electrode, there is always an optimum amperage setting. When using arc force control, the optimum amperage setting is continually working to maintain the arc, which means that although we can't see it on the meters, there is usually some added amperage to assist in rod burn-off. This in turn means we really get a slightly faster burn-off than with a conventional rectifier.
When working out-of-position, a forceful arc is needed to help put metal in place. Each individual operator can adjust the arc force control to provide just the amount needed. Arc force can also be of assistance when welding rusty or scaly material, since the more forceful arc will help to break up these deposits.
An electrode holder, commonly called a stinger, is a clamping device for holding the electrode securely in any position. The welding cable attaches to the holder through the hollow insulated handle. The design of the electrode holder permits quick and easy electrode exchange. Two general types of electrode holders are in use: insulated and non-insulated (Figure 10).
Figure 10 — Insulated pincher and collet types of electrode holders.
Non-insulated holders are not recommended because they are subject to accidental short circuiting if bumped against the workpiece during welding. For safety reasons, try to ensure the use of only insulated stingers on the jobsite.
Electrode holders are made in different sizes, and each manufacturer has its own system of designation. Each holder is designed for use within a specified range of electrode diameters and welding current. Welding with a machine having a 300-ampere rating requires a larger holder than welding with a 100-ampere machine. If the holder is too small, it will overheat.
The welding cables and connectors connect the power source to the electrode holder and to the work. These cables are normally made of copper or aluminum. The cable that connects the work to the power source is called the work lead. The work leads are usually connected to the work by pincher clamps or a bolt. The cable that connects the electrode holder to the power source is called the electrode lead.
The welding cables must be flexible, durable, well insulated, and large enough to carry the required current. Use only cable specifically designed for welding. Always use a highly flexible cable for the electrode holder connection. This is necessary so the operator can easily maneuver the electrode holder during the welding process. The work lead cable need not be so flexible because once it is connected, it does not move.
Two factors determine the size of welding cable to use: the amperage rating of the machine and the distance between the work and the machine. If either amperage or distance increases, the cable size must also increase. Cable sizes range from the smallest at AWG No.8 to AWG No. 4/0 with amperage ratings of 75 amperes and upward. Table 8-1 shows recommended cable sizes for use with different welding currents and cable lengths. The best size cable is one that meets the amperage demand but is small enough to manipulate easily.
As a rule, the cable between the machine and the work should be as short as possible. Use one continuous length of cable if the distance is less than 35 feet. If you must use more than one length of cable, join the sections with insulated lock-type cable connectors. Joints in the cable should be at least 10 feet away from the operator.
Table 8-1 — Suggested copper welding cable sizes for SMAW.
A good ground clamp is essential to produce quality welds. Without proper grounding, the circuit voltage fails to produce enough heat for proper welding, and there is the possibility of damage to the welding machine and cables. Three basic methods are used to ground a welding machine. You can fasten the ground cable to the workbench with a C-clamp, attach a spring-loaded clamp directly onto the workpiece, or bolt or tack-weld the end of the ground cable to the welding bench. The third way creates a permanent common ground.
Accessory equipment used for shielded metal arc welding consists of items used for removing slag and cleaning the weld bead. Chipping hammers are often used to remove the slag. Wire brushes or grinders are the most common methods for cleaning the weld.
Manufacturers offer various options and accessories also, depending on the type of power source and the procedure recommendations.
Learning to arc weld requires many skills. Among these are the abilities to set up, operate, and maintain your welding equipment.
In most factory environments, the work is brought to the welder, but you will be called to the field for welding on buildings, earthmoving equipment, well drilling pipe, ship to shore fuel lines, pontoon causeways ... and the list goes on. To accomplish these tasks, you have to become familiar with your equipment and be able to maintain it in the field. It would be impossible to give detailed maintenance information here because of the many different types of equipment found in the field; therefore, we will only cover the highlights.
Become familiar with the welding machine you will be using. Study the manufacturer’s literature and check with your senior petty officer or chief on items you do not understand. Machine setup involves selecting current type, polarity, and current settings. The current selection depends on the size and type of electrode used, position of the weld, and the properties of the base metal.
Cable size and connections are determined by the distance required to reach the work, the size of the machine, and the amperage needed for the weld.
Operator maintenance depends on the type of welding machine used. Transformers and rectifiers require little maintenance compared to engine-driven welding machines. Transformer welders require only to be kept dry and need a minimal amount of cleaning. Only electricians should perform internal maintenance due to the possibility of electrical shock. Engine-driven machines require daily maintenance of the motors. In most places you will be required to fill out and turn in a daily inspection form called a “hard card” before starting the engine. This form is a list of items, such as oil level, water level, visible leaks, and other things, that affect the operation of the machine.
After checking all of the above items, you are now ready to start welding. Here are some additional welding rules you must follow:
- To Table of Contents -
The covered electrode provides both the filler metal and the shielding for the shielded metal arc welding process. Covered electrodes have different compositions of core wire and a wide variety of types of flux coverings that perform one or all of the following functions, depending upon the type of electrode:
- Forming a slag blanket over the molten puddle and solidified weld
- Providing shielding gas to prevent atmospheric contamination of both the arc stream and the weld metal
- Providing ionizing elements for smoother arc operation
- Provides deoxidizers and scavengers to refine the grain structure of the weld metal
- Providing alloying elements such as nickel and chromium for stainless steel
- Providing metal such as iron powder for higher deposition rates
The first two functions listed prevent the pickup of nitrogen and oxygen into the weld puddle and the red hot solidified weld metal. The nitrogen and oxygen form nitrides and oxides which cause the weld metal to become brittle.
The classification system for covered electrodes used throughout industry in the United States was devised by the American Welding Society. In this system, designations for covered electrodes consist of the letter E (for electrode) and four (or five) digits for carbon steel and low-alloy steel covered electrodes. Sometimes a suffix appears on the end as well. These digits have specific meanings, which are:
Table 8-2 — Digit position indicating
tensile and yield strength.
Table 8-3 — Digit indicating position an electrode can be used.
|EXX1X||Flat, Horizontal, Vertical, Overhead|
|EXX2X||Flat, Horizontal - Fillet|
|EXX4X||Flat, Horizontal, Vertical - Down, Overhead|
Table 8-4 — Digits indicating electrode arc and coating characteristics.
For example, the E8018-B1 designation indicates an electrode that deposits metal that has a minimum tensile strength of 80,000 psi (550 MPa), can be used in all welding positions, has a low hydrogen iron powder classification, and has chemical composition in the weld deposit of .12 C, .90 Mn, .03 P, .04 S, .BO Si, .40-.65 Cr and .40-.65 Mo. (Table 8-5)
Table 8-5 — Chemical composition.
Other types of electrodes are classified in different ways. Table 8-6 lists the American Welding Society (AWS) specifications covering filler metals for shielded metal arc welding.
Table 8-6 — AWS filler metal specifications for
covered electrodes used in SMAW.
For example, stainless steel electrodes are classified according to the chemical analysis of the weld metal and the type of welding current they can use. An example of this is the E308-15 designation. The E stands for Electrode. The 308 indicates the chemical composition of the weld metal. Table 8-7 shows the different classifications.
Table 8-7 — AWS classifications.
The suffix indicates the positions and the type of welding current to be used. A suffix of 15 means to use direct current electrode positive and a 16 means that you may use alternating current or direct current electrode positive. All stainless steel electrode classifications that are now used have a one in the suffix that indicates that they are all position electrodes.
The size of the electrode is designated by the diameter of the core wire and the length of the electrode. Standard electrode diameters are 1/16 in. (1.6 mm) to 5/16 in. (7.9 mm). Lengths of the electrodes are from 9 in. (229 mm) to 18 in. (457 mm), although electrodes for special applications can be up to 36 in. (914 mm) long. The most common electrode length is 14 in. (346 mm). The bare uncoated end of the electrode, which is needed to make electrical contact with the electrode holder, is standardized at lengths ranging from 3/4 in. (19 mm) to 11/2 in. (38 mm).
The deposited weld metal should equal or exceed the mechanical properties of the base metal and have approximately the same composition and physical properties. Identification of the base metal is absolutely required to properly select the correct electrode. If you do not know the identification, you must make tests based on appearance, magnetic check, chisel test, flame test, fracture test, spark test, or chemistry test. The selection of welding electrodes for specific job applications is quite involved, but can be based on the following eight factors:
Identification of the base metal is required. In the cases of mild and low alloy steels, choose the electrodes to match (at least) the tensile strength of the base metal.
The chemical composition of the base metal must be known, but matching the chemical composition is not as important for mild steels as it is for stainless steels, low alloy steels, and nonferrous metals. For these metals, matching the chemical composition of the filler metal to the base metal is required.
Electrodes are designed to be used in specific positions. Choose the electrodes to match the positions of the welding you will encounter.
Covered electrodes are designed to operate on specific currents and polarity. The type of electrode you use might depend on the type of welding current available. Operate electrodes on their recommended current type.
Choose the electrodes according to their penetration characteristic. For joints with no beveling or tight fit-up, an electrode with a digging arc would be the best. For welding on thin material, a light penetrating electrode would be the best.
Weldments may include thick sections or complex shapes which require maximum ductility to avoid weld cracking. Use electrodes that give the best ductility.
For weldments subject to severe service conditions such as low temperature, high temperature, or shock loading, use the electrode that matches the base metal composition, ductility, and impact resistance properties. This usually indicates selecting low hydrogen types of electrodes.
Some electrodes are designed for high deposition rates but may be used under specific position requirements. If they can be used, the high deposition electrodes would be the best.
According to Section IX of the ASME Boiler and Pressure Vessel Code and the AWS Structural Welding Code, the covered electrodes for welding mild and low-alloy steel can be placed into four categories. The electrodes within each of these categories generally operate and run the same way.
F-1 — High Deposition Group (EXX20, EXX24, EXX27, EXX28)
F-2 — Mild Penetration Group (EXX12, EXX13, EXX14)
F-3 — Deep Penetration Group (EXX10, EXX11)
F-4 — Low-Hydrogen Group (EXX15, EXX16, EXX18)
The high deposition types of electrodes have additions of iron powder in their coatings. These additions of iron powder usually range from 40-55% of the weight of the coating. During welding, the large amounts of iron powder in the electrode coating go into the weld puddle which increases the deposition rates. These electrodes are usually selected when high deposition welding is desired.
The mild penetration types of electrodes are generally used for welding sheet metal, partial penetration welds when strength is not the governing factor, and other less critical applications. These electrodes have rutile as a main component in their coatings. The EXX14 electrodes have an addition of 25-40% iron powder in the coatings to give them a higher deposition rate than the EXX12 and EXX13 types.
The deep penetration types of electrodes are the EXX10 and the EXX11 electrodes. The electrodes are used on applications where the deep penetrating characteristics of the weld are needed and for full penetration welding. These electrodes have cellulose as the major component in their coatings. The cellulose is the material that gives these electrodes their deep penetrating characteristic.
The low hydrogen electrodes are those which have very low moisture content in their coatings. These electrodes are used for welding steels when hydrogen cracking can be a problem, such as in many of the low alloy steels. Much of the hydrogen in the weld metal comes from the electrode coating. The cellulose types of electrodes require higher moisture contents in their coatings to operate properly.
Table 8-8 shows the general characteristics of different types of electrodes on penetration, surface contour, and deposition rates.
Table 8-8 — Relative Comparison of different
characteristics for several mild steel electrodes.
The correct choice of electrode size involves consideration of a variety of factors such as:
Most classes of electrodes are designed for multiple pass welding. Each diameter electrode has its own specific limits on the current carrying capacity. The large diameter electrodes are also used to give the highest welding speed possible. When welding in the vertical and overhead positions, smaller diameter electrodes are preferred because gravity will affect a smaller weld puddle less than a larger one. The weld puddle small diameter electrodes create is easier for the welder to control. The type of weld joint also has a limiting effect on the size of the electrodes. Small diameter electrodes may have to be used to reach the root of the joint where larger electrodes would not fit. For example, in V groove joints, smaller diameter electrodes may have to be used to put in the root pass and possibly several more of the initial passes. The experience of the welder will also influence the size of the electrode used depending on the welder’s manipulative skill with the electrode. Use the largest possible electrode size to obtain the fastest welding speeds, providing that this does not cause overwelding. Overwelding can be harmful and wasteful. The proper electrode diameter to use is the one that, when used with the proper welding conditions, will result in a weld of the required quality and size at the greatest productivity.
Covered electrodes must conform to the specifications of or be approved by code making organizations for many applications of shielded metal arc welding. Some of the organizations that issue specifications or approvals are the American Welding Society (AWS), American Society of Mechanical Engineers (ASME), American Bureau of Shipping (ABS), Federal Bureau of Roads, U.S. Coast Guard, Canadian Welding Bureau, and the Military. The American Welding Society (AWS) provides specifications for covered electrodes. The electrodes manufactured must meet specific requirements in order to conform to a specific electrode classification. Most code-making organizations such as the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API) recognize and use the AWS Specifications. Some of the code making organizations such as the American Bureau of Shipping (ABS) and the Military must directly approve the electrodes before they can be used for welding on a project covered by that code. These organizations send inspectors to witness welding and testing and approve the classification of covered electrodes.
To conform to the AWS specifications for mild steel electrodes, the covered electrode must be able to produce a weld deposit that meets specified mechanical properties. The requirements vary depending on the class of electrode. Table 8-9 gives a list of mechanical properties different mild steel covered electrodes require.
Table 8-9 — Minimum mechanical properties required for the different mild steel covered electrodes (AWS A5.1).
|Test Your Knowledge
3. What is the normal current range of a power supply when using conventional size electrodes?
4. Electrodes are designed to be used in specific positions.
- To Table of Contents -
Shielded metal arc welding is widely used because of its versatility. Welding can be performed at a distance from the power source which makes it popular for welding in the field. The equipment for this process is relatively simple to operate, portable, and inexpensive. Shielded metal arc welding is a major process used for maintenance and repair work. It is popular in small production shops where limited capital is available and where the amount of welding done is minor compared to other manufacturing operations. Shielded metal arc welding is often used for tacking parts together which are then welded by another process.
Shielded metal arc welding is the welding process of choice in a number of industries because it is versatile and user friendly. It has been replaced in recent years by flux cored arc welding but remains competitive because of the low equipment costs and wide applicability.
Field welded storage tanks differ from pressure vessels because they are used to store petroleum, water or other liquids at atmospheric pressure. Shielded metal arc welding is widely used in the fabrication and erection of field welded storage tanks. These tanks are generally constructed of low-carbon and structural steels. Nickel steels are employed when the tanks require higher toughness. This process is used to weld longitudinal and circumferential seams on the tanks as well as the structural support members. Figure 11 shows field welding of a large circumference pipe. An engine driven generator power source is being used because there is no electricity available.
Figure 11 — Pipe welding.
Pressure vessels and boilers are also welded using this process. Shielded metal arc welding is primarily used for welding attachments to the vessel. This kind of welding commonly uses all sizes of electrodes. For applications where the vessels will be operating at low temperatures, smaller electrodes are used on multiple pass welds. This will produce smaller weld beads that build up the weld in relatively thin layers. The smaller weld beads give a stronger, tougher weld.
Shielded metal arc welding is widely used in the industrial piping industry which includes many types of pressure piping. The types of electrodes most often used are the E6010 and E7018 electrodes for welding low-carbon steel pipe. A common practice is the use of E6010 electrodes to weld in the root passes and the E7018 electrodes to weld in the fill and cover passes. Industrial piping is generally welded from the bottom to the top, except on small diameter pipe where it is done both ways. The reason that welding from bottom to top is most common is because slag is often trapped when welding in the opposite direction. For welding low-carbon steel pipe with a 70,000 psi (485 MPa) tensile strength, use E7010 and E7018 electrodes. Figure 12 shows shielded metal arc welding with E7018 electrodes to weld structural supports.
Figure 12 — E7018 electrode being used to weld structural supports.
Figure 13 —SMAW cylindrical support beams.
Another example of this process is shown in Figure 13, where pulsed shielded metal arc welding is being used to cylindrical support beams. Shielded metal arc welding is often used for welding on other types of industrial piping. EXX15, EXX16, and EXX18 electrodes are used for welding chromium-molybdenum alloy pipe. When welding stainless steel pipe, gas tungsten arc welding (TIG) is often used to put in the root pass, and shielded metal arc welding is used to weld in the fill and cover passes. Medium and high-carbon steel pipe are also welded by this process. For these, smaller diameter electrodes are used than on low-carbon steels, in order to reduce the heat effect on the pipe.
The shielded metal arc welding process is by far the major process for welding on transmission or cross-country pipelines. Welding is done in the field, usually from the outside of the pipe, but whenever possible, welding should be done from both sides of the pipe. E6010 and E7018 electrodes are the types used for welding transmission pipelines. Several common procedures are in use for welding transmission pipelines. One of these is to put in the root pass with E6010 electrodes and put in the fill and cover passes with E7018 electrodes. Another is to use E7018's to weld in all passes, and a third is to weld in the root pass with the gas metal arc welding process and put in the rest of the passes with E7018 electrodes.
The nuclear power industry employs this process for many applications. It is often used in the shop fabrication of low-carbon and low alloy steel heavy-walled pressure vessels and for welding longitudinal and circumferential weld seams. Shielded metal arc welding is the best method for welding nozzles and attachments to the vessels. A major application of this process is welding pressure piping for use in the nuclear power facilities. Nuclear power system pressure piping requires stronger quality control than normal pressure piping.
The construction industry is a major application for shielded metal arc welding. Most of the welding on buildings and bridges is done in the field at long distances from the power sources, which makes this process popular for these applications. Most types of covered electrodes are used in structural work because of the wide variety in the tensile strengths of the steels used. Figure 14 shows structural welding with the shielded metal arc welding process. Figure 15 shows a section of hinge being welded.
Figure 14 — Structural welding with the SMAW process.
Figure 15 — Hinge support welding.
In Figure 16, a worker is welding an I-beam; and another example of the use of shielded metal arc welding in the construction industry is shown in Figure 17, where this process is being used to weld armor plating.
Figure 16 — I beam welding.
Figure 17 — Welding armor plating.
Shielded metal arc welding is still the major process used in shipbuilding. It is used for many different applications including welding in areas where the other processes cannot reach. Most types of low-carbon steel covered electrodes are used except the EXX12, EXX13, and EXX14 types. These three types of electrodes are not approved for use on the main structural members in the ship because of the relatively low ductility obtained from the weld deposits of these electrodes. The electrodes with large amounts of iron powder in their coatings are popular for many shipbuilding applications because of the high deposition rates obtained. These types of electrodes are especially used on the many fillet welds that are made in a ship structure. Backing tape is often used for backing the weld metal when one side welding is done.
Another industry that widely uses this welding process is the railroad industry. It uses E60XX and E70XX electrodes to weld many parts of the underframe, cab, and engine of the locomotive. The underframe fabrication consists of mostly fillet welds. The frames and brackets for the diesel engines are also welded with these electrodes.
Railroad cars are commonly welded together by the shielded metal arc process. Underframes are often welded with E6020, E7016, and E7018 electrodes. The sills for the underframes are welded using E7024 electrodes because high deposition rates are desired for this application.
The automotive industry uses this process to a lesser extent. There, it is mainly used for welding low production components or on items where there are frequent model changes. This is because the fixtures and equipment for this process are less expensive.
The frames of many types of heavy industrial machinery are welded together using this process. It is the major process used for welding piping associated with this machinery. Shielded metal arc welding is used for welding areas the other processes cannot reach. Figure 18 shows a welder welding bars on a door.
Figure 18 — Welding bars on a door.
Another major application of this process is in the heavy equipment industry such as mining, agricultural, and earthmoving equipment. In these industries, shielded metal arc welding is used for welding structural steels, which are used for the frames, beams, and many other items in the assembly. Most types of covered electrodes are used depending on the type of steel being welded. Stainless steel and nonferrous metals are also used for some parts. Figure 19 shows a worker welding a plate for a backhoe bucket.
Figure 19 — Welding a structural plate.
One industry where shielded metal arc welding is and will probably always remain the major welding process is the maintenance and repair industry. This is especially true in small shops and general plant maintenance, where relatively inexpensive equipment, portability, and versatility are important considerations. This process is the major one for repair welding on railroad engines and cars as well as cast iron engine blocks and heads on automobiles. This kind of welding commonly employs nickel electrodes for repairing cast iron parts. Resurfacing worn parts and putting a hard surface on parts (wearfacing) are two other applications. Special surfacing and build-up electrodes are used for these purposes.
Gravity welding, which is seldom used today, was an automatic variation of the shielded metal arc welding process. Gravity welding was popular because one operator could operate several gravity feeders at the same time, increasing the production rate. The welder installed the electrode in the feeder and the electrode fed as it burned off, which gave a high quality horizontal fillet weld. The welders usually used 28 in. (710 mm) long electrodes of the drag type (E6027, E7024, E7028). These were used in diameters of 7/32 in. (5.6 mm) and in 1/4 in. (6.4 mm). This was possible in some shipbuilding work since the welds were often close together, which allowed the welding operator to quickly move from one holder to another to reload them, start them, and allow them to operate unattended.
Firecracker welding is a method of automatically making welds using a long electrode with an electrically nonconductive heavy coating. This method has been used very little in North America because of the popularity of semi-automatic processes. This method can be used for square groove butt welds and full fillet lap welds. To make a firecracker fillet weld, position the work flat. Place the welding electrode in the joint and place a retaining bar over it. Start the arc by shorting the end of the electrode to the work. The arc length depends on the thickness of the coating. As the arc travels along the electrode, the electrode melts and makes a deposit on the metal immediately underneath it. Once the arc is started, the process automatically proceeds to completion.
Another variation of shielded metal arc welding is the use of massive electrodes which have extremely large diameters and long lengths. These electrodes are so heavy that they require a manipulator to hold and feed them. Massive electrode welding is primarily used for repairing very large castings.
A welder can greatly extend the life of construction equipment by using wearfacing procedures. Wearfacing is the process of applying a layer of special composition metal onto the surface of another type of metal for the purpose of reducing wear. The selection of a wearfacing alloy for application is based on the ability of the alloy to withstand impact or abrasion. Impact refers to a blow or series of blows to a surface that results in a fracture or gradual deterioration. Abrasion is the grinding action that results when one surface slides, rolls, or rubs against another. Under high-compressive loads, this action can result in gouging.
Alloys that are abrasion resistant are poor in withstanding impact. Conversely, those that withstand impact well are poor in resisting abrasion; however, there are many alloys whose wearfacing properties fall between the two extremes. These alloys offer some protection against abrasion and withstand impact well.
Before you wearface a workpiece, remove all dirt, oil, rust, grease, and other foreign matter. If you do not, your finished product will be porous and subject to spalling. You also need a solid foundation; therefore, repair all cracks and remove any metal that is fatigued or rolled over.
Depending on the type of metal, sometimes it is necessary to preheat the base metal to lessen distortion, prevent spalling or cracking, and avoid thermal shock. The preheating temperature depends on the carbon and alloy content of the base metal. In general, as carbon content increases so does the preheating temperature. However, improper heating can adversely affect a metal by reducing its resistance to wear, making it hard and brittle or more prone to oxidation and scaling.
To preheat properly, you must know the composition of the base metal. You can use a magnet to determine if you are working with carbon steel or austenitic manganese steel. Carbon steel is magnetic, but be careful because work-hardened austenitic manganese steel is also magnetic. Make sure that you check for magnetism in a non-worked part of the austenitic manganese steel. There are other ways to tell the difference between metals such as cast iron and cast steel; cast iron chips or cracks, while cast steel shaves. Also, some metals give off telltale sparks when a chisel strikes them.
In preheating, raise the surface temperature of the workpiece to the desired point and soak it until the heat reaches its core. After wearfacing, cool the work places slowly.
Where possible, position the workpiece for downhand welding. This allows you to finish the job more quickly and at less cost.
Building up and wearfacing cast iron is not generally recommended because cast iron tends to crack. However, some cast-iron parts that are subject to straight abrasion can be wearfaced successfully. You must preheat these parts to temperatures of 1000°F to 1200°F and then allow them to cool slowly after wearfacing. Peening or hammering the deposits on cast iron helps to relieve stresses after welding.
Welding materials for building up worn parts differ from those used in wearfacing the same parts. Before wearfacing a badly worn part, you must first build it up to 3/16 to 3/8 of an inch of its finished size. The buildup material must be compatible with the base metal and the wearfacing overlay and strong enough to meet the structural requirements. Also, they must have the properties that enable them to resist cold flowing, mushing under high-compressive loads, and plastic deformation under heavy impact. Without these properties, the buildup materials cannot support the wearfacing overlay. When the overlay is not properly supported, it will spall.
Many times, high-alloy wearfacing materials are deposited on the parts before they are placed in service. The maximum allowable wear is usually no more than two layers deep (1/4-inch) before wearfacing. Try to deposit the wearfacing alloy in layers that are not too thick. Thick layers create more problems than no overlay at all. Usually you only need two layers. The first layer produces an admixture with the base metal; the second forms a wear-resistant surface.
In wearfacing built-up carbon-steel parts, maintain high interpass temperatures and use a weaving bead rather than a stringer bead. (Figure 20) Limit the thickness of a single pass bead to 3/16-inch. Use the same technique for each layer and avoid severe quenching.
Figure 20 — Wearfacing techniques.
Deposits made with high-alloy electrodes should check on the surface. Checking reduces residual (locked-in) stresses. Without checking, the combination of residual stresses and service stresses may exceed tensile strength and cause deep cracks or spalling (Figure 21). Be sure to induce checking if it does not occur naturally or if it is unlikely to occur, as in large parts where heat builds up. You can bring on checking by sponging the deposit with a wet cloth or by spraying it with a fine mist of water. Also you can speed up checking by occasionally striking the deposit with a hammer while it is cooling. When you require a check-free deposit, use a softer alloy and adjust preheating and post-heating requirements.
Figure 21 — Comparison between cross-checking and cracking.
Bulldozer blades are wear-faced by placing the end bits in the flat position and welding beads across the outer corners and along the edges. Be sure to preheat the high-carbon blades before wearfacing. On worn end bits, weld new corners and then wear-face (Figure 22).
Figure 22 — Wearfacing.
Shovel teeth should be wear-faced before being placed into service. The weld bead pattern used in wearfacing can have a marked effect on the service life of the teeth. Wear-face shovel teeth that work mainly in rock with beads running the length of each tooth (Figure 23). This allows the rock to ride on the hard metal beads. Wear-face teeth that are primarily used to work in dirt, clay, or sand with beads running across the width of each tooth, perpendicular to the direction of the material that flows past the teeth. This allows the material to fill the spaces between the beads and provides more protection to the base metal. Another effective pattern is the waffle or crosshatch (Figure 24). Lay the wearfacing on the top and sides of each tooth, 2 inches from its point. Stringer beads behind a solid deposit reduce wash (Figure 25).
Figure 23 — Wearfacing shovel teeth.
Figure 25 – Comparison of wearfacing patterns for shovel teeth.
Metals can be cut cleanly with a carbon electrode arc because it does not introduce foreign metals at the arc. The cutting current should be 25 to 50 amps above the welding current for the same thickness of metal. See Table 8-10 for more information.
Table 8-10 — Recommended Electrode Sizes, Current Settings, and Cutting Speeds for Carbon-Arc Cutting Different Thickness of Steel Plate
|Current Setting and Carbon Diameter|
Thickness of Plate
½ in. Diameter
5/8 in. Diameter
¾ in. Diameter
1 in. Diameter
Speed of Cutting in Minutes Per Foot
Grind the carbon electrode point so that it is very sharp. During the actual cutting, move the carbon electrode in a vertical elliptical movement to undercut the metal; this aids in removing the molten metal. As in oxygen cutting, a crescent motion is preferred. Figure 26 shows the relative positions of the electrode and the work in the cutting of cast iron.
Figure 26 — Carbon-arc cutting on cast iron.
The carbon-arc method of cutting is successful on cast iron because the arc temperature is high enough to melt the oxides formed. It is especially important to undercut the cast-iron kerf to produce an even cut. Position the electrode so the molten metal flows away from the gouge or cutting areas. Table 8-10 (above) is a list of cutting speeds, plate thicknesses, and current settings for carbon-arc cutting.
Because of the high currents required, the graphite form of carbon electrode is better. To reduce the heating effect on the electrode, do not let it extend more than 6 inches beyond the holder when cutting. If the carbon burns away too fast, shorten the length it extends out of the electrode holder to as little as 3 inches. Operating a carbon electrode at extremely high temperatures causes its surface to oxidize and burn away, resulting in a rapid reduction in the electrode diameter.
Carbon-arc cutting does not require special generators. Standard arc-welding generators and other items of arc-welding station equipment are suitable for use. Always use straight polarity direct current (DCSP). Because of the high temperature and the intensity of the arc, choose a shade of helmet lens darker than the normal shade you would use for welding on the same thickness of metal. A number 12 or 14 lens shade is recommended for carbon-arc welding or cutting.
Air carbon-arc cutting (ACC) is a process of cutting, piercing, or gouging metal by heating it to a molten state and then using compressed air to blow away the molten metal. Figure 27 shows the process. The equipment consists of a special holder, shown in Figure 28, that uses carbon or graphite electrodes and compressed air fed through jets built into the electrode holder. A push button or a hand valve on the electrode holder controls the air jet.
Figure 27 — Air carbon-arc cutting.
Figure 28 — Air carbon-arc electrode holder with carbon electrode installed.
The air jet blows the molten metal away and usually leaves a surface that needs no further preparation for welding. The electrode holder operates at air pressures between 60 and 100 psig.
During use, bare carbon or graphite electrodes become smaller due to oxidation caused by heat buildup. Copper coating these electrodes reduces the heat buildup and prolongs their use.
The operating procedures for air carbon-arc cutting and gouging are basically the same. The procedures are as follows:
Adjust the machine to the correct current for electrode diameter.
Start the air compressor and adjust the regulator to the correct air pressure. Use the lowest air pressure possible, just enough pressure to blow away the molten metal.
Insert the electrode in the holder. Extend the carbon electrode 6 inches beyond the holder. Ensure that the electrode point is properly shaped.
Strike the arc; then open the air-jet valve. The air-jet disc can swivel, and the V-groove in the disc automatically aligns the air jets along the electrode. Adjust the electrode relative to the holder.
Control the arc and the speed of travel according to the shape and the condition of the cut desired.
Always cut away from the operator as molten metal sprays some distance from the cutting action. You may use this process to cut or gouge metal in the flat, horizontal, vertical, or overhead positions.
Air carbon-arc gouging is useful in many metalworking applications, such as metal shaping and other welding preparations. For gouging, hold the electrode holder so the electrode slopes back from the direction of travel. The air blast is directed along the electrode toward the arc. The electrode angle and travel speed control the depth and contour of the groove. The diameter of the electrode governs the width of the groove.
When cutting or gouging a shallow groove on the surface of a piece of metal, position the electrode holder at a very flat angle in relation to the work. The speed of travel and the current setting also affect the depth of the groove. The slower the movement and the higher the current, the deeper you can cut the groove. Figure 29 shows an example of a V-groove cut made in a 2-inch-thick mild steel plate by a machine guided carbon-arc air-jet.
Figure 29 — V-groove gouged in 2-inch thick carbon steel.
You can remove metal with the standard electric arc, but for good gouging or cutting results, use special metal electrodes designed for this type of work,
Manufacturers have developed electrodes with special coatings that intensify the arc stream for rapid cutting. The covering disintegrates at a slower rate than the metallic center. This creates a deep recess that produces a jet action that blows the molten metal away (Figure 30). The main disadvantage of these electrodes is that the additional metal they produce must be removed.
Figure 30 — Steel electrode being used to cut plate
These electrodes are designed for cutting stainless steel, copper, aluminum, bronze, nickel, cast iron, manganese, steel, or alloy steels.
A typical gouge-cutting operation is shown in Figure 31. Notice that the angle between the electrode and plate is small (5 degrees or less). This makes it easy to remove the extra metal the electrode produces.
Figure 31 — Gouge-cutting operation using a solid core arc-cutting electrode.
The recommended current setting is as high as the electrode will take without becoming overheated to the point of cracking the covering. For 1/8-inch electrodes, the setting ranges from 125 to 300 amperes; for 5/32- inch electrodes, the setting ranges from 250 to 375 amperes; and for 3/16-inch electrodes, the setting ranges from 300 to 450 amperes. Use a very short arc, and when cutting takes place underwater, the coating must be waterproof.
|Test Your Knowledge
5. Which of the following electrodes is typically used to weld in the root passes in industrial piping?
6. What characteristic makes the carbon-arc method of cutting successful on cast iron?
- To Table of Contents -
Welding metallurgy concerns the chemical, physical, and atomic properties and structures of metals and the principles by which metals are combined to form alloys.
The properties of the weld are items such as chemical composition, mechanical strength and ductility, and microstructure. These items will determine the quality of the weld. The types of materials used affect the chemical properties. The heat input of welding and the chemical composition of the materials determine the mechanical properties and microstructure of the weld.
The chemical composition of the base metal is a major factor in determining the choice of the electrodes to use for welding. The chemical composition of the base metal influences the need for preheating and post heating, because preheating and post heating are used to prevent the weld area from becoming brittle and weak.
When welding steels, the carbon and other alloy content influences the hardness and hardenability of the weld metal, which in turn influences the amount of preheat and postheat needed. The two terms, hardness and hardenability, are not the same. The maximum hardness of the steel is primarily a function of the amount of carbon in the steel. Hardenability is a measure of how easily a martensite structure is formed 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 to form martensite; whereas, steels with high hardenability will form martensite even when slow cooled in air. The hardenability will determine the extent to which a steel will harden during welding. The carbon equivalent formula is one of the best methods of determining the weldability of steels. The amounts of some of the alloying elements used determine this. There are several different formulas used, one of them is:
|Carbon Equivalent = %C +||%Cr||+||%Mn||+||%Mo||+||%Ni||+||%Cu|
Steels with lower carbon equivalents generally are more readily weldable and require fewer precautions such as the use of preheat and postheat.
Steels with higher carbon equivalents are generally more difficult to weld. Matching the chemical properties of the filler metal is not as important as matching the mechanical properties when welding many of the steels. 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 amount of preheat needed depends on the type of metal being welded, the metal thickness, and the amount of joint restraint. Preheating helps reduce the cooling rate of the part being welded. This is important on many steels because a slower cooling rate will not allow as much of the hard and brittle martensite structure to form in the metal. Since martensite formation is the carbon equivalent, steels with high-carbon equivalents will generally require higher preheat temperatures than those with low-carbon equivalent values. Table 8-11 shows typical preheat values for different steels and cast iron.
Table 8-11 — Typical Recommended Preheats for Various Steels and Cast Iron Welded by the SMAW Process
|Type of Steel||Preheat|
|Low-Carbon Steel||Room Temperature or up to 200°F (93°C)|
|Medium-Carbon Steel||400-500°F (205-260°C)|
|High-Carbon Steel||500-600°F (260-315°C)|
Low Alloy Nickel Steel
Low Alloy Nickel-Chrome Steel
|Low Alloy Manganese Steel||400-600°F (205-315°C)|
|Low Alloy Chrome Steel||Up to 750°F (400°C)|
Low Alloy Molybdenum
|Low Alloy High Tensile Steel||150-300°F (66-150°C)|
|Austenitic Stainless Steel||Room Temperature|
|Ferritic Stainless Steel||300-500°F (150-260°C)|
|Martensitic Stainless Steel||400-600°F (205-315°C)|
|Cast Irons||700-900°F (370-480°C)|
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; check the specifications of the job for the specific preheat temperature to use.
Another major factor that determines the amount of preheat needed is the base metal thickness. Thicker base metals usually need higher preheat temperatures than thinner base metals because of the larger heat sinks that the thicker metals provide. Thick metals draw the heat away from the welding zone more quickly because there is a larger mass of metal. This increases the cooling rate of the weld if the same preheat temperature is used as with thinner base metals. The third major factor determining preheat is the amount of joint restraint. Joint restraint is the resistance of a joint configuration to moving during the heating and cooling of the weld zone. When there is high resistance to moving or high joint restraint, large amounts of internal stress builds up. Higher preheat temperatures are needed as the amount of joint restraint increases. Slower cooling rates reduce the amount of internal stresses that build up as the weld cools.
For welding nickel-base metals, copper base metals, and stainless steels, the chemical properties of the weld metal are often the most important properties of the weld. The chemical composition of the weld metal must closely match the chemical composition of the base metal to give the weldment good corrosion resistance and creep resistance. Creep resistance of a metal is the resistance to softening at high temperatures which can cause deformation if there is a load on the metal.
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 ductility, and the last is a measure of impact toughness. These properties are important in shielded metal arc welding.
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 32 shows a tensile bar before and after testing. 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 the metal can carry. 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.
Figure 32 — Tensile strength testing bars.
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 its ability to absorb mechanical energy by deforming before breaking. The Charpy Vee-notch test is the most commonly used method of testing impact toughness. Figure 33 shows some typical Charpy Vee-notch test bars. Bars with Vee notches are put in a machine where they are struck by a hammer attached to the end of a pendulum. The energy required to break these bars is known as the impact strength and is measured in foot-pounds (Newton-meters).
Figure 33 — Charpy V-notch bars
Figure 34 shows a cross section of a weld bead showing the weld metal zone, the heat affected zone, and the base metal zone. The weld metal zone is where the metal was molten during welding. The heat affected zone is the area where the heat from welding has had an effect on the microstructure of the base metal. The base metal zone is the area that was not affected by the welding.
Figure 34 — Cross section of weld bead showing in the three areas.
The extent of change of the microstructure depends on four factors:
The weld metal zone, the area heated above about 2800°F (1540°C) and melted, generally has the coarsest grain structure of the three areas. For the most part, welding with the shielded metal arc process produces a fairly fine grain size in steels, so a large grain size in the weld zone is not much of a problem when this process is used. Large grain size is undesirable because it gives the weld poor toughness and 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 are called dendrites and they grow toward the center of the weld into the area that is still molten. This gives 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. The grain size in the weld zone can be affected by the electrode covering as well as the factors mentioned before. One of the functions of the electrode coverings is to provide deoxidizers and scavengers to reduce the grain size of the metal. The greater the heat input to the weld and the longer it is held at high temperatures, the larger the grain size will be. A fast cooling rate will produce a smaller grain size than a slower cooling rate. Preheating will give larger grain sizes, but it is often necessary to prevent the formation of the hard and brittle martensite structure.
The heat affected zone is the area around the metal zone which has been affected by the heat of welding but did not become molten. For example, in mild steels, the area that reaches a temperature of 2200-2800°F (1220-1550°C) goes through a grain coarsening. The area of the heat affected zone that reaches 1700-2200°F (950-1220°C) will go through grain refinement. The area that is heated from 1400-1700°F (780-950°C) is annealed and made considerably softer.
The base metal zone is the area that is heated to 1300°F (720°C) or less and is basically unchanged.
Shielded metal arc welding may be used to weld a wide variety of base metals. The metals that are the most easily welded with the shielded metal arc welding process are the mild steels, low alloy steels, stainless steels and chromium-molybdenum steels. Cast irons, medium- and high-carbon steels, and hardenable types of steel may also be welded using this process, but special precautions must be taken. The selection and care of the electrodes is more critical when welding hardenable steels. Copper alloys and nickel are often welded by this process, but gas metal arc welding (MIG) and gas tungsten arc welding (TIG) are more widely used for welding these metals. Zinc, lead, and tin cannot be welded by shielded metal arc welding because of their low melting points. Aluminum can be welded by this process, but it is not very popular. Magnesium is not welded with the shielded metal arc welding process.
In general, steels are classified according to carbon content, such as low-carbon, medium-carbon, or high-carbon steels. Steels are also classified according to the types of alloy used such as chrome-moly, nickel-manganese, etc. For the purpose of discussion in this course, steels will be classified according to their welding characteristics.
Mild steels are generally those that have low carbon content and are most readily weldable. These steels are the most widely used type of metal for industrial fabrication. Included in this group are the low-carbon steels and the high strength structural steels.
Low-carbon steels have carbon content up to .30%. Mild steel electrodes of the E60XX series classification may be used for welding these steels, but E70XX series electrodes are used when higher strengths are required. The E70XX series electrodes are used, especially when the carbon content of the steels approach .30%. Preheating is often used, especially on thicker sections, highly restrained joints, or where codes require preheating. Other precautions such as controlled interpass temperature and post heating are often required. These heat controls help reduce the cooling rate of the weld metal and prevent large amounts of martensite from forming. On thicker sections, cracking may occur in the weld deposit or the heat affected zone. If preheating, interpass temperature control and post heating are not used. Use of these heat controls will help prevent the reduction of weld toughness, strength, and ductility.
The high strength structural steels are steels with yield strength between 45,000 psi (310 MPa) and 70,000 psi (485 MPa) and carbon content generally below .25%. These steels have relatively small amounts of alloying elements. Some common examples of these steels are ASTM designations A242, A441, A572, A588, A553 and A537. Preheating is generally not required when low hydrogen electrodes are used, except on thick sections or highly restrained joints where preheating is required. Preheating is required when the higher hydrogen types of electrode are used.
The low alloy steels discussed here will be those that are low-carbon and have alloy additions less than 5%. This includes the quenched and tempered steels, heat treated low alloy steels, and low nickel alloy steels. Elements such as nickel, chromium, manganese, and molybdenum are the main alloying elements these steels contain. These steels have a higher hardenability than mild steels and this factor is the principal **45 complication in welding. This higher hardenability permits martensite to form at lower cooling rates. As the alloy content and the carbon content increase, the hardenability also increases. In general, the weldability of the steel decreases as the hardenability increases. One of the best methods for determining the weldability of a low alloy steel is the carbon equivalent formula. Steels that have carbon equivalents below about .40% usually do not require preheating and post heating in the welding procedure and generally have the best weldability. Steels with carbon equivalents higher than .40% require more precautions for welding. Generally, the higher the carbon equivalent, the more difficult the steel is to weld. Low alloy steels are fairly weldable but not as easily weldable as the mild steels. Except in the case of the low nickel alloys, the selection of electrodes for welding these steels is based on the strength and mechanical properties desired of the weld rather than matching chemical compositions.
The quenched and tempered heat treated steels have yield strengths ranging from 50,000 psi (345 MPa) to very high yield strengths and have carbon content up to .25%. Some common examples of these types of steel are ASTM designations A533 Grade B, A537 Grade B, A514, A517, A543, and A553. The .25% carbon limit provides fairly good weldability. These steels provide high tensile and yield strength along with good ductility, notch toughness, corrosion resistance, fatigue strength and weldability. The presence of hydrogen is always bad in steel but it is even more critical in these types of steels compared to mild steels. Low hydrogen electrodes should be used when welding these steels. Preheat is generally not used on thinner sections, but it is used on thicker or highly restrained sections. Post-weld heat treatment is generally not used because the shielded metal arc welds made in these have good toughness. The steels are generally used in the welded or stress relieved conditions.
The nickel alloy steels in these low alloy steel groups are those with less than 5% nickel content. The 2 1/4% and 3 1/2% nickel steels are usually welded with covered electrodes that have the same general chemical composition as the base metal. Preheating is required with highly restrained joints.
The heat treatable steels are the medium- and high-carbon steels and medium-carbon steels that have been alloyed. This group includes steels quenched and tempered after welding, normalized or annealed steels, and medium- and high-carbon steels. These steels are more difficult to weld than the other types this course mentions. The most important factor for selecting the type of covered electrode to use is matching the chemical compositions of the base metal and the filler metal.
Medium-carbon steels have carbon content ranging from .30% to .60% and high-carbon steels have carbon content ranging from .6% to about 1.0%. When welding medium- and high-carbon steels, include precautions in the welding procedure because of the hardness that can occur in the weld joint. As the carbon content increases up to .60%, the hardness of the fully hardened structure (or martensite) increases to a maximum value. When the carbon content is above .60%, the hardness of the fully hardened structure does not increase, so these steels can be welded using about the same welding procedures as the medium-carbon steels. Martensite, which is the phase in which steel is at its fullest hardness, is harder and more brittle in high-carbon steel than in low-carbon steel. A high-carbon martensitic structure can have a tendency to crack in the weld metal and the heat affected zone during cooling. Welding procedures that lower the hardness of the heat affected zone and the weld metal reduce the cracking tendency. This can be done by using a procedure that reduces the carbon content in the filler metal and by slowing the cooling rate. The procedure includes the use of low hydrogen electrodes, preheat, interpass temperature control, and a postheat. The procedures for welding medium-carbon steels can be simpler than the one just mentioned, but that depends on the specific application. Medium-carbon steels can be welded with the low hydrogen electrodes of the E70XX, E80XX, or E90XX classifications. Weld high-carbon steels with the low hydrogen electrodes of the E80XX to the E120XX classes using the electrode of the proper tensile strength to match the tensile strength of the base metal. Generally, high-carbon steels are not used in welded production work. These steels are usually only welded in repair work. Mild steel electrodes may also be used, but the deposited weld metal absorbs carbon from the base metal and thus loses a considerable amount of ductility. Stainless steel electrodes of the austenitic type are sometimes used, but the fusion zone may still be hard and brittle. A preheat and/or postheat will help eliminate the brittle structure.
The steels quenched and tempered after welding have carbon content from about .25% to .45%, which distinguishes them from the steels that are quenched and tempered before welding. These steels also have small additions of alloying elements. Some common examples of these steels are the AISI designations 4130, 4140 and 4340. Because of the higher carbon contents, the steels in this group can be heat treated to extremely high levels of strength and hardness. Some of these steels have enough alloy content to give them high hardenability. Because of this combination of carbon and alloy content, the steels must be preheated before welding. The weldability of these steels is also influenced by the purity of the steels. High amounts of sulfur and phosphorous in the steel increase sensitivity to cracking and reduce ductility. The shielded metal arc welding process is often used for welding steels that are quenched and tempered after welding. The selection of the proper electrode to use is based on the chemical composition, strength required, and thickness of the base metal. The composition of the weld metal is usually similar to that of the base metal. Low hydrogen electrodes are the most commonly used for welding of steels.
The chromium-molybdenum steels in this section are those with alloy contents of 6% or less. These steels are in the low-carbon range, generally up to .15%, and are readily weldable. The chromium and molybdenum alloying elements provide these steels with good oxidation resistance and high temperature strength. The chromium is mainly responsible for the oxidation resistance, the molybdenum mainly responsible for the high temperature strength. All of the electrodes manufactured for welding stainless steels, recognized by the AWS, are of the low hydrogen type. Stainless steel electrodes of the EXXX-15 type have a lime based coating and the EXXX-16 types have a Titania based coating. The type EXXX-15 electrodes give greater penetration where the EXXX- 16 type gives a smoother surface finish.
The shielded metal arc welding process is one of the most common methods of welding the chromium-molybdenum steels. The stainless steel low hydrogen electrodes (EXXX- 15, EXXX-16.and EXXX-18) are used for welding except on the 1/2% Cr % Mo steels where the mild steel EXX10, EXX11, and EXX13 may be used. The cellulose coated electrodes for welding the other chromium-molybdenum steels are not used because they can cause underbead cracking. The use of the low hydrogen electrodes greatly reduces the amount of hydrogen present during welding.
The steels included in this group are the higher chrome-moly steels, martensitic stainless steels, ferritic stainless steels and austenitic stainless steels. The major **47 element that sets these steels apart from other steels is the high chromium content. Stainless steels have more than 11% chromium. The addition of chromium gives the steels a very high resistance to oxidation and increases the hardenability up to a point. If steel has too much chromium in it, it cannot be hardened at all. Stainless steels containing chromium above about 16% are generally non-hardenable. Chromium levels above about 25% give the stainless steel very good oxidation resistance at high temperatures. Normally, when you weld stainless steel, you should match the chemical composition of the filler metal and the base metal.
The higher chrome-moly steels contain about 6-10% chromium and .5-1% molybdenum. These steels are limited to a maximum carbon content of about .10% to limit the hardness because these steels are very sensitive to air hardening. For welding these steels, preheating, interpass temperature control, slow cooling, and post-weld heat treatment are required to make a weld with good mechanical properties.
The martensitic hardenable stainless steels generally have chromium contents between 11 and 13%. Some typical examples of these kinds of steels are the American Iron and Steel Institute (AISI) designations 403, 405, 410, 420, and 440. The most easily weldable are the steels with the lowest carbon contents. For applications requiring high hardness, such as cutlery, higher carbon contents, such as those types 420 and 440 contain, are desired. Types such as 420 and 440 are rarely welded. Electrodes with the same chemical compositions as the base metal are usually required for welding martensitic stainless steels. Sometimes, austenitic stainless steel or Inconel types of covered electrodes are used to weld martensitic stainless steel to avoid the use of preheat, but they give weld metal with lower strength than the base metal. When using martensitic stainless steel covered electrodes, preheating and post heating are required so that the weld metal will not be weaker than the base metal. The ferritic non-hardenable stainless steels have chromium contents greater than 13%.
As mentioned before, the higher chromium content makes these steels non-hardenable. Some typical examples of these types of stainless steels are the AISI designations 430, 436, 442 and 446. These stainless steels require preheating and post heating. Type 446 steel, which has 25% chromium, is very susceptible to rapid grain growth in the weld heat affected zone. Large grain sizes reduce the toughness and make the weld area more susceptible to cracking. Preheating and post heating minimize this grain growth. Austenitic stainless steel electrodes are often used, but this does not help reduce the grain size of the heat affected zone.
The austenitic non-hardenable stainless steels have at least 11 or 12% chromium and up to about 26% chromium with additions of nickel ranging from about 3.5 to 22%. Nickel is a strong austenite former, and it helps keep these kinds of steels in the austenitic phase at all temperatures, which also makes these steels non-hardenable. Austenitic stainless steels have very good oxidation resistance and high temperature strength. The austenitic stainless steels are designated by the AISI as the 200 and 300 series. In the 200 series of steels, manganese is used to replace some of the nickel. Some common examples of these steels are the Types 302, 304, 308, 310, 316, 321, 347, 201, and 202. Austenitic stainless steels have good toughness and ductility and are the most readily weldable of the stainless steels. When shielded metal arc welding is used to weld austenitic stainless steel, the filler metal composition is generally chosen to match the base metal.
The weld metal deposited by austenitic stainless steel electrodes generally has higher chromium and nickel contents than the base metal. Distortion is often a problem when welding these steels because they have a coefficient of expansion that is about 50% higher than for carbon steels, which creates residual stresses. Preheating and post heating are usually not required, but preheating may be used to remove the chill.
Free machining steels are steels that have additions of sulfur, phosphorous, or lead in them to make these steels easier to machine. Except for the lead, phosphorous, and sulfur contents, these steels often have chemical compositions of mild, low alloy, and stainless steels. This addition of these elements makes these steels unweldable. The reason for this is because the elements lead, phosphorous and sulfur have melting points that are much lower than the melting point of steel. As the weld solidifies, these elements remain liquid much longer than the steel so that they coat the grain boundary which causes 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 successfully.
Many types of cast irons may be welded using shielded metal arc welding. Cast irons have a carbon content higher than that of steel. Carbon is present in cast irons in two forms, as free carbon (graphite) and as combined carbon (as in steels). There are several types of cast irons: white, gray, malleable, modular, and austenitic. All of these are weldable except white cast iron, which is considered unweldable.
In gray cast irons, the graphite has a flake appearance. These flakes produce sharp notches and discontinuities which make gray cast iron brittle. The tensile strength of gray cast iron is usually between 30,000 psi (210 MPa) and 40,000 psi (280 MPa).
The success of shielded metal arc welding usually depends on the specific tensile strength, the form and distribution of graphite, the amount of sulfur and phosphorous, and the amount of joint restraint. Nickel base welding electrodes are widely used for welding gray cast iron, and preheating and interpass temperature controls are required except on minor repair jobs.
In malleable cast irons, the graphite has a nearly spheroidal appearance and in nodular iron, the graphite has a spheroidal appearance. The malleable and nodular cast irons do not have the brittleness that the gray cast irons have because of the shape of the graphite. Nickel-base covered electrodes are also used for welding malleable and nodular cast irons. After these cast irons have been welded, they should be annealed to obtain optimum ductility.
When it is used to weld copper and copper alloys, shielded metal arc welding is mainly used for minor repair jobs, difficult to reach fillets, or dissimilar metals. Shielded metal arc welding does not do as good a job as the gas metal arc welding (MIG) or the gas tungsten arc welding (TIG) process. The filler metal used for welding copper and copper alloys contains deoxidizers. Shielded metal arc welding of these metals is generally restricted to the flat position. Out of position welding can only be performed satisfactorily on phosphor bronzes and copper nickels. Shielded metal arc welding is usually not **49 recommended for welding many of the copper alloys because it produces poor mechanical properties and many unsound welds. The coppers and brasses are generally not welded using this process.
The shielded metal arc welding process can be used to weld nickel and nickel alloys in thicknesses ranging down to about .050" (1.3 mm). The covered electrodes used have chemical compositions similar to the base metals being welded. Elements such as manganese, columbium and titanium are contained by the electrodes and act as deoxidizers and prevent weld metal cracking. Direct current electrode positive welding current is used when welding nickel and nickel alloys. Flat position welding is used whenever possible because it produces a better quality weld. Molten nickel alloy weld metal does not flow as well as molten steel weld metal so the nickel alloy weld metal must be deposited where it is needed. Oscillating or weaving techniques are usually needed because of this. The heat of the welding arc usually does not have a negative effect on the nickel base metals. Preheat is usually not required for welding these metals, but the base metal should be warmed to at least 70°F (21°C) to avoid condensation of moisture, which could produce porosity in the weld metal.
|Test Your Knowledge
7. What primary property determines the maximum hardness of steel?
8. Which cast iron is considered unweldable?
- To Table of Contents -
The weld joint design used for shielded metal arc welding is determined by the design of the workpiece, metallurgical considerations, and codes or specifications. Joints are designed for accessibility and economy during construction. Good accessibility during construction also helps reduce the cost and generally raises the quality of the weld joint. A weld joint consists of a type of weld made in a type of joint. There are five basic types of joints, but these can be used in various combinations. A joint is the junction of members that are to be joined or have been joined. Figure 35 shows the five basic joint classifications. Each type of joint can be joined by many different types of welds.
There are only five basic types of joints. They can, however, be used in combinatinos.
Figure 35 — Types of joints.
Figure 36 shows the most common types of welds made. The type of weld made is governed by the joint configuration. Figure 37 shows the weld nomenclature for groove and fillet welds.
Note: Many other variations are possible.
Figure 36 — Types of welds.
Figure 37 — Weld nomenclature.
The strength required of the weld joint is a major consideration for determining the design of a welded joint. Weld joints are either full penetration or partial penetration, depending on the strength required for the weld joint. Full penetration weld joints have weld metal through the full cross section of the joint. Partial penetration weld joints have an unfused area in the joint. Welded joints subject to dynamic, cyclic, or impact loads usually require full penetration when full strength is required. These factors are even more important when the weld joints are used for low temperature service. Partial penetration welds may be adequate for joints that are statically loaded. These types of joints are easier to prepare and require less filler metal than full penetration joints
The shielded metal arc welding process can be used in all welding positions. The position in which the welding is to be done affects the design of the joint. The figures at the end of the section show some examples of this. A diagram of the welding position capabilities of shielded metal arc welding is shown in Figure 38. Welding in the horizontal, vertical, and overhead positions depends on the skill of the welder and the type of electrode the welder uses. The high deposition class of electrodes can normally only be used in the flat and horizontal positions because of the large weld puddles they produce.
Figure 38 — 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 for groove welds. The 5G and 6G positions are test positions used in pipe welding. Figure 38 also shows the number and letter designations for both plate and pipe.
The thickness of the metal that can be welded by the shielded metal arc welding process depends on welder skill, joint position, type of joint, fit-up, type of electrode, welding speed, arc length, welding current, and arc characteristics. The minimum thickness of metal that can be welded is dependent on the skill of the welder; a skilled welder can weld steel as thin as 1/16 in. (1.6 mm). Steel as thick as 1/4 in. (6.4 mm) can be welded without groove preparation if the width of the root opening is adequate to achieve full penetration welds. Partial penetration welds can be made in 1/2 in. (12.7 mm) thick metal without beveling. Thicker materials than those mentioned require joint preparation and multiple passes. Common beveled joint configurations for groove welds are the U, V, J, bevel, and combination grooves. The J, bevel, and combination groove configurations are also used for fillet welds. These configurations make it possible to get full penetration welds on thicker material. The thicker the material, the more passes it takes to fill the joint for a given joint design. Single bevel and V groove are the most often used types of edge preparation.
U grooves are the most common because they are the easiest to prepare. The bevels on the sides of the groove can be prepared by flame cutting; the joint faces of the J and U grooves are prepared by machining. Flame cutting is quicker than machining, so flame cutting reduces the preparation time.
U grooves generally require less filler metal than V grooves. Welding U grooves allows use of larger electrodes for the first pass than does welding V grooves because of the U grooves’ rounded bottom. However, spacer strips may be used in V grooves to provide easier access to the root.
Single bevel and J grooves are often used for corner and Tee-joints. Single V, single U, and single bevel grooves are the most common types of edge preparation for butt joints 3/16 in. (4.8 mm) or 1/4 in. (6.5 mm) thick to about 3/4 in. (19.1 mm) thick.
When the base metal is 3/4 in. (19.1 mm) or more, double V, double U, double bevel, and double J grooves are usually recommended if welding from both sides is possible. Joints welded with these grooves produce less distortion and require less filler metal than grooves that must be welded from one side. Groove angles of 45° to 60° are used for thinner base metals needing grooves, but are too large for use in thicker base metals. Smaller groove angles are used for the larger metal thicknesses because they require less welding time to fill than a 45° or 60° groove angle.
There are many variations of the basic joint designs. One design often used for the welding of thick walled pipe and thick plate over 3/4 in. (19.1 mm) is the variation of the single V groove joint shown in Figure 39. This is used when the joint is accessible from one side. It uses a steeper slope toward the top of the joint where the bevel angle has been reduced. The advantage of this type of joint design over a normal V groove design is that it is less expensive to weld because it requires less filler metal to fill the joint. The wider V groove toward the bottom gives good accessibility to the root of the joint. A disadvantage of this joint design is that it is more difficult to prepare the two different bevel angles.
Figure 39 — Variation of single V-groove joint design for unlimited thickness base metal.
Accessibility is another important factor in determining the joint design for shielded metal arc welding. Welds can be made either from one side or from both sides of the base metal. On thicker metals, when both sides of the joint are accessible, double bevels are usually made. The advantage of this is that the double bevels have less area to fill than single bevels and require less filler material. The roots of the welds are usually near the center of the base metal when double bevels are used. When the joints are only accessible from one side, U and J groove preparations are often used so that the root is more easily accessible, and on thick sections, less filler metal is required to fill the joint than with a standard V groove preparation. However, U and J grooves are harder and more expensive to prepare.
The weld joint designs in the rest of the course are those commonly used for shielded metal arc welding. Table 8-12 shows the minimum effective throat thicknesses for partial penetration welds, according to the AWS Structural Welding Code (AWS D1.1). The effective throat thickness is the minimum distance between the root of the weld and the surface less the reinforcement. Figures 8-40 and 8-41 show the American Welding Society’s "Standard Welding Symbols," some of which have been used in the weld joint designs.
Table 8-12 — Effective throat thickness for partial joint
penetration grove welds.
Figure 40 — Welding symbols.
Figure 41 — Welding symbols.
(Continued from Figure 40 above)
These charts are intended only as shop aids. The only complete and official presentation of the standard welding symbols is in A2.4.
Figures 42 through 52 show different welding position symbols.
Figure 42 — Basic joints.
Figure 43 — Applications of arrow and other side conventions
Figures 44, 45 — Applications of break in arrow of welding symbol.
Figures 46, 47 — Specification of location and extent of fillet welds.
Figure 48 — Specification of location and extent of fillet welds (cont.)
Figures 48, 49, and 50 — Specification of extent of
(A) Welds with Abrupt Changes in Direction
(B) Application of Weld-All-Around Symbol
(C) Weld in Several Planes , (D) Weld Around a Shaft
(E) Seal Weld
Figure 51 — Applications of “typical” welding symbols.
Figure 52 — Applications of melt-through symbol.
The details of a joint, which include both the geometry and the required dimensions, are called the joint design. Just what type of joint design is best suited for a particular job depends on many factors. Although welded joints are designed primarily to meet strength and safety requirements, other factors must be considered. A few of these factors are as follows:
Another consideration is the ratio of the strength of the joint compared to the strength of the base metal. This ratio is called joint efficiency. An efficient joint is one that is just as strong as the base metal.
Normally, the joint design is determined by a designer or engineer and included in the project plans and specifications. Even so, understanding the joint design for a weld enables you to produce better welds.
Earlier in this course, we discussed the five basic types of welded joints—butt, corner, tee, lap, and edge. Keep in mind that there are many different variations of the basic joint welds. If you want more information, refer to an earlier course in this welding sequence, “Introduction to Welding.”
The types of welds, joints, and welding positions used in shielded metal arc welding are very similar to those used in oxygas welding. Naturally, the techniques are somewhat different because the equipment involved is different.
Welding can be done in any position, but it is much simpler 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 you can achieve better penetration. 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 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 on to the appropriate position. Techniques 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. Figure 53 shows several forms of butt joints.
Figure 53 — Butt joints in the flat position.
Plates up to 1/8-inch thick can be welded in one pass with no special edge preparation. 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. Use tack welds to keep the plates aligned for welding. The electrode motion is the same as that used in making a bead weld.
In welding 1/4-inch plate or heavier, prepare the edges of the plates by beveling or by J- , U-, or V-grooving, whichever is the most applicable. Use single or double bevels or grooves when the specifications and/or the plate thickness require it. Deposit the first bead to seal the space between the two plates and to weld the root of the joint. Thoroughly clean this bead or layer of weld metal to remove all slag and dirt before depositing the second layer of metal.
In making multi pass welds, as shown in Figure 54, make the second, third, and fourth layers of weld metal with a weaving motion of the electrode. Clean each layer of metal before laying additional beads. Use one of the weaving motions shown in Figure 55, depending upon the type of joint and size of electrode.
Figure 54 — Butt welds with multipass beads.
Figure 55 — Weave motions used in SMAW.
In the weaving motion, oscillate or move the electrode uniformly from side to side, with a slight hesitation at the end of each oscillation. Incline the electrode 5 to 15 degrees in the direction of welding as in bead welding. Improper weaving motion could result in undercutting at the joint, as shown in Figure 56. Excessive welding speed also can cause undercutting and poor fusion at the edges of the weld bead.
Figure 56 — Undercutting in butt joint welds.
Butt joints with backing strips — Welding 3/16-inch or thicker plate 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. Tackweld the backing strip to the base of the joint, as shown in Figure 57. 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, the backing strip may be cut away with a cutting torch. When specified, place a seal bead along the root of the joint.
Bear in mind that many times using a backing strip will not be possible; therefore, the welder must be able to run the root pass and get good penetration without the formation of icicles.
Figure 57 — Use of back strips in welding butt joints.
You will discover that it is impossible to weld all pieces in the flat position. Often you must do the work 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 58). In a fillet weld, the welding is performed on the upper side of a relatively horizontal surface and against an approximately vertical plane (Figure 59).
Figure 58 — Horizontal groove weld.
Figure 59 — Horizontal fillet weld.
Inexperienced welders usually find the horizontal position of arc welding difficult, until they develop 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.
In horizontal welding, position the electrode so that it points upward at a 5- to 10-degree angle in conjunction with a 20-degree travel angle (Figure 60). Use a narrow weaving motion in laying the bead. This weaving motion distributes the heat evenly, reducing the tendency of the molten puddle to sag. Use the shortest arc length possible, and when the force of the arc undercuts the plate at the top of the bead, lower the electrode holder a little to increase the upward angle.
Figure 60 — Horizontal welding angles.
As you move in and out of the crater, pause slightly each time you return. This keeps the crater small, and the bead has fewer tendencies to sag.
Horizontal-position welding can be used on most types of joints, but it is most commonly used on tee joints, lap joints, and butt 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, as shown in Figure 61.
Figure 61 — Tack-weld to hold the tee joint elements in place.
Use a fillet weld in making the tee joint, and use a short arc to provide good fusion at the root and along the legs of the weld (Figure 62, view A). Hold the electrode at an angle of 45 degrees to the two plate surfaces (Figure 62, view B) with an incline of approximately 15 degrees in the direction of welding.
Figure 62 — Position of electrode on a fillet weld.
When practical, weld light plates with a fillet weld in one pass with little or no weaving of the electrode. Welding of heavier plates may require two or more passes in which the second pass or layer is made with a semicircular weaving motion, as shown in Figure 63. To ensure good fusion and the prevention of undercutting, make a slight pause at the end of each weave or oscillation.
Figure 63 — Weave motion for multipass fillet weld.
For fillet-welded tee joints on 1/2-inch plate or heavier, deposit stringer beads in the sequence shown in Figure 64.
Figure 64 — Order of string beads for tee joint on heavy
Chain-intermittent or staggered-intermittent fillet welds, as shown in Figure 65, are used on long tee joints. Fillet welds of these types are for joints that do not require high weld strength; however, the short welds are arranged so the finished joint is equal in strength to 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.
Figure 65 — Intermittent fillet welds.
Lap joints — To make a lap joint, tackweld two overlapping plates in place (Figure 66), and deposit a fillet weld along the joint.
Figure 66 — Tack welding a lap joint.
The procedure for making this fillet weld is similar to that used for making fillet welds in tee joints. Hold the electrode 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 electrode in relation to the plates is shown in Figure 67. 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, as shown in Figure 67.
Figure 67 — Position of electrode on a lap joint.
In making lap joints on plates of different thickness, hold the electrode so that it forms an angle of between 20 and 30 degrees from the vertical (Figure 68). Be careful not to overheat or undercut the thinner plate edge.
Figure 68 — Lap joints on plates of different thickness.
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 69). Often, both edges are beveled to form a 60-degree included angle. Using this type of joint requires more skill because there is no retaining shelf to hold the molten puddle.
Figure 69 — Horizontal butt.
The number of passes required for a joint depends on the diameter of the electrode and the thickness of the metal. When multiple passes are required (Figure 70), place the first bead deep in the root of the joint. Incline the electrode holder about 5 degrees downward. Clean and remove all slag before applying each following bead. The second bead should be placed with the electrode holder held about 10 degrees upward. For the third pass, hold the electrode holder 10 to 15 degrees downward from the horizontal. Use a slight weaving motion and ensure that each bead penetrates the base metal.
Figure 70 — Multiple passes. joint.
A “vertical weld” is a weld applied to a vertical surface or inclined 45 degrees or less (Figure 71). Erecting structures, such as buildings, pontoons, tanks, and pipelines, requires 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. To counteract this force, use fast-freeze or fill-freeze electrodes.
Figure 71 — Vertical weld plate positions.
Vertical welding is done in either an upward or downward position. The terms used for the direction of welding are vertical up or vertical down. Vertical down welding is suited for welding light gauge metal because the penetration is shallow and diminishes the possibility of burning through the metal. Furthermore, vertical down welding is faster, which is very important in production work.
Movement In vertical arc welding, the current settings should be less than those for the same electrode in the flat position. Another difference is that the current for welding upward on a vertical plate is slightly higher than the current for welding downward on the same plate.
Figure 72 — Bead welding in vertical position.
To produce good welds, maintain the proper angle between the electrode and the base metal. In welding upward, hold the electrode at 90 degrees to the vertical, as shown in Figure 72, view A. When weaving is necessary, oscillate the electrode, as shown in Figure 72, view B. In vertical down welding, incline the outer end of the electrode downward about 15 degrees from the horizontal while keeping the arc pointing upward toward the deposited molten metal (Figure 72, view C). When vertical down welding requires a weave bead, oscillate the electrode, as shown in Figure 72, view D.
Vertical welding is used on most types of joints, but the types you will most often use it on are tee joints, lap joints, and butt joints.
When making fillet welds in either tee or lap joints in the vertical position, hold the electrode at 90 degrees to the plates, or not more than 15 degrees off the horizontal, for proper molten metal control. 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 electrode in a triangular weaving motion, as shown in Figure 73, 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, quickly shift the electrode away from the crater without breaking the arc, as shown in Figure 73, View B. This permits the molten metal to solidify without running downward. Return the electrode immediately to the crater of the weld in order to maintain the desired size of the weld.
Figure 73 — Fillet welds in the vertical position.
When more than one pass is necessary to make a tee weld, use either of the weaving motions shown in Figure 73, 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, move the electrode in a triangular weaving motion, as shown in Figure 73 , View E. Use the same procedure outlined above for the tee joint except direct the electrode more toward the vertical plate marked “G.” Hold the arc short and pause slightly at the surface of plate G. Try not to undercut either of the plates or allow the molten metal to overlap at the edges of the weave.
A lap joint on heavier plate may require more than one bead. If it does, clean the initial bead thoroughly and place all subsequent beads as shown in Figure 73, View F. The precautions to ensure good fusion and uniform weld deposits, 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, hold a short arc and carefully control the motion of the arc.
You can weld butt joints on beveled plates 1/4-inch thick in one pass by using a triangular weave motion, as shown in Figure 74, View A.
Make welds on 1/2-inch plate or heavier should in several passes, as shown in Figure 74, View B. Deposit the last pass with a semicircular weaving motion and a slight “whip-up” and pause of the electrode at the edge of the bead. This produces a good cover pass with no undercutting. Make welds on plates with a backup strip in the same manner.
Figure 74 — Butt joint welding in the vertical position.
The previously described vertical welding techniques generally cover all types of electrodes; however, modify the procedure slightly when using E-7018 electrodes. When vertical down welding, drag the electrode lightly using a very short arc. Refrain from using a long arc since the weld depends on the molten slag for shielding. Small weaves and stringer beads are better than wide weave passes. Use higher amperage with ac than with dc. Point the electrode straight into the joint and tip it forward only a few degrees in the direction of travel.
On vertical up welding, a triangular weave motion produces the best results. Do not use a whipping motion or remove the electrode from the molten puddle. Point the electrode straight into the joint and slightly upward in order to allow the arc force to help control the puddle. Adjust the amperage in the lower level of the recommended range.
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.
To retain complete control of the molten puddle, use a very short arc and reduce the amperage as recommended. As in the vertical position of welding, gravity causes the molten metal to drop or sag from the plate. When you hold too long an arc, the transfer of metal from the electrode to the base metal becomes increasingly difficult and the chances of large globules of molten metal dropping from the electrode increase. When you routinely shorten and lengthen the arc, the dropping of molten metal can be prevented; however, you will defeat your purpose should you carry too large a pool of molten metal in the weld.
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 that has a tight fitting collar that buttons or zips up to the neck. Roll down your sleeves and wear a cap and appropriate shoes.
The following paragraphs discuss techniques used in making bead welds, butt joints, and fillet welds in the overhead position.
Bead welds — For bead welds, the work angle of the electrode is 90 degrees to the base metal (Figure 75, View A). The travel angle is 10 to 15 degrees in the direction of welding (Figure 75, View B).
Figure 75 — Position of electrode and weave motion in the overhead position.
Make weave beads using the motion shown in Figure 75, 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 using backing strips; 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 you take extreme care.
For overhead butt welding, bead welds are better than weave welds. Clean each bead and chip out the rough areas before placing the next pass. The electrode position and the order of deposition of the weld beads when welding on 1/4- or 1/2-inch plate are shown in Figure 76, views B and C. Make the first pass with the electrode held at 90 degrees to the plate, as shown in Figure 76, view A. When you use an electrode that is too large, you cannot hold a short arc in the root area. This results in insufficient root penetration and inferior joints.
Figure 76 — Multipass butt joint in the overhead position.
Fillet welds — In making fillet welds in either tee or lap joints in the overhead position, maintain a short arc and refrain from weaving the electrode. Hold the electrode at approximately 30 degrees to the vertical plate and move it uniformly in the direction of welding, as shown in Figure 77, 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 electrode quickly away from the crater and ahead of the weld to lengthen the arc and allow the metal to solidify. Immediately return the electrode 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 77, View A. The root pass is a string bead made with no weaving motion of the electrode. Tilt the electrode about 15 degrees in the direction of welding, as shown in Figure 77, View C, and with a slight circular motion make the second, third, and fourth pass. This motion of the electrode 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.
Figure 77 – Fillet welding in the overhead position.
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 less flow restriction 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 (TIG & MIG) have made big inroads as a result of 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 unique 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.
The following paragraphs, discuss pipe welding positions, pipe welding procedures, definitions, and related information.
You may recall that there are four positions used in pipe welding (Figure 38). They are known as the horizontal rolled position (1G), the horizontal fixed position (5G), pipe inclined fixed (6G), and the vertical position (2G). Remember, these terms refer to the position of the pipe and not to the weld.
Welds that you cannot make in a single pass, make in interlocked multiple layers, not less than 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, clean each layer thoroughly before depositing the next one.
Butt joints are commonly used between pipes and between pipes and welded fittings. They are also used for butt welding flanges and welding stubs. In making a butt joint, place two pieces of pipe end to end, align **82 them, and then weld them. (See Figure 78).
Figure 78 — Butt joints and socket fitting joints.
When the wall thickness of the pipe is 3/4-inch or less, use either the single V or single U type of butt joint; however, when the wall thickness is more than 3/4-inch, use only the single U type.
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 79). Single fillet welds are also used in welding screw or socket couplings to pipe (Figure 78).
Figure 79 — Flange connections.
Sometimes flanges require alignment. Figure 80 shows one type of flange square and its use in vertical and horizontal alignment.
Figure 80 — Flange alignment.
Another form of fillet weld used in pipe fitting is a seal weld. A seal weld is used primarily to obtain tightness and prevent leakage. Seal welds do not add strength to the joint.
Prepare pipe joints for welding carefully 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 flame-cut edges; however, it is not necessary to remove the temper color.
When you prepare joints for welding, remember to cut bevels accurately. You can make bevels by machining, grinding, or using a gas cutting torch. In fieldwork, you 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, use clamps or jigs as holding devices. A piece of angle iron makes a good jig for a small-diameter pipe (Figure 81), while a section of channel or I-beam is more suitable for larger diameter pipe.
Figure 81 — 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 pipe. The size of a tack weld is determined by the wall thickness of the pipe. Be sure the 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 of maintaining the specified root opening, provide a convenient location for tack welds, and aid in pipe alignment. In addition, spacers can prevent weld spatter and the formation of slag or icicles inside the pipe.
Select the electrode best suited for the position and type of welding you are doing. For the root pass of a multilayer weld, you need an electrode large enough, without exceeding 3/16-inch, to ensure complete fusion and penetration without undercutting and slag inclusions.
Make certain the welding current is within the range recommended by the manufacturers of the welding machines and electrodes.
Do not assign a welder to a job under any of the conditions listed below unless the welder and the work area are properly protected:
At temperatures between 0°F and 32°F, within 3 inches of the joint, heat the weld area with a torch to a temperature warm to the hand before beginning to weld.
|Test Your Knowledge
9. How many basic types of weld joints are there?
10. Which type of weld is used for welding slip-on and threaded flanges to pipe?
- To Table of Contents -
The welding procedure variables are those that control the welding process and the quality of the welds produced. There are three major types of welding variables. These are the fixed or preselected, primary adjustable, and the secondary adjustable variables.
The fixed or preselected welding variables are set before the actual welding takes place. These are items such as electrode type, electrode size, and type of current. These variables cannot be changed after welding starts. The primary adjustable variables are the major variables used to control the welding process once the fixed variables have been selected.
The primary variables control the formation of the weld bead by affecting the bead width, bead height, penetration, arc stability, and weld soundness. The primary welding variables are welding current, arc voltage, and travel speed. These can be easily adjusted and measured so they can be used effectively to control the welding process.
The secondary adjustable variables are the minor adjustable variables used to control the welding process. These variables are usually more difficult to measure. Secondary adjustable variables are the work angle and the travel angle of the electrode.
The penetration of the weld is the greatest depth below the surface of the base metal that the weld metal reaches. The bead height or reinforcement is the height of the weld metal above the surface of the base metal. The deposition rate is the weight of the metal that is deposited per unit of time. Figure 82 shows the definitions of bead height, bead width, and penetration.
Figure 82 — Bead height, bead width and penetration.
The welding variables are discussed with particular attention to the three major characteristics of penetration, deposition rate, and bead shape. Table 8-13 is a chart showing the effects of welding variables on the three major characteristics.
Table 8-13 — Effects of welding variables on the penetration, the bead size and shape, and the deposition rate.
Fixed variables include electrode size and type, welding current type, and polarity.
The type of electrode used has an effect on the penetration, deposition rate, and shape of the weld bead and usually depends upon the specific application. Some types of electrodes have a digging arc and produce welds with deep penetration. The deep penetrating effect of these electrodes results from the high amount of cellulose in their coatings. Some types of electrodes produce moderate penetration while others produce light penetration. The type of electrode used also greatly influences the deposition rate of the electrode. Iron powder is the main influence on deposition rate. Electrodes with high amounts of iron powder in their coatings can operate at higher welding currents than other types of electrodes. The iron powder goes into the weld metal and helps give the electrodes with iron powder in their coatings the highest deposition rates of any of the mild steel electrodes. The electrodes with the thinnest electrode coatings generally have the lowest deposition rates. The type of electrode used also influences the bead size and shape. The iron powder electrodes generally produce wider and flatter beads.
Larger diameter electrodes use higher welding currents, so the deposition rates and penetration depths increase as the electrode wire diameter increases. Lower deposition rates and penetration depths are obtained with smaller electrode wire diameters because they use lower welding currents. Table 8-14 shows typical deposition rates for different sizes of E6010, E7018, and E7024 electrodes.
Table 8-14 — Typical deposition rates of different sizes of E6010, E7018, and E7024 electrodes.
The melting rate in the arc zone is directly related to the electrical energy in the welding arc. Part of the energy is used to melt the electrode and coating, some is lost to the atmosphere, and the rest goes to melt the base metal. The electrical polarity determines the energy balance, and the constituents of the electrode covering the different types of current affect the deposition rate and depth of penetration.
Direct current electrode positive gives the most penetration at a given welding current setting, followed by alternating current, with direct current electrode negative giving the least penetration. However, direct current electrode negative gives the highest deposition rates at a given welding current setting, followed by alternating current, with direct current electrode positive giving the lowest deposition rates.
Primary variables include welding current, travel speed, and welding voltage.
The welding current is the most important factor in determining the characteristics of the weld. The welding current is controlled by a knob or handle on the welding machine. The amount of current used during a welding operation depends primarily upon the diameter of the electrode. As a rule, higher currents and larger diameter electrodes are better for welding in the flat position than the vertical or overhead position. Manufacturers of electrodes usually specify a current range for each type and size of electrode; this information is normally found on the face of the electrode container.
Since most recommended current settings are only approximate, final current settings and adjustments need to be made during the welding operation. For example, when the recommended current range for an electrode is 90-100 amperes, the usual practice is to set the controls midway between the two limits, or at 95 amperes. After starting the weld, make your final adjustments by either increasing or decreasing the current. When the current is too high, the electrode melts faster and the molten puddle will be excessively large and irregular. High current also leaves a groove in the base metal along both sides of the weld. This is called undercutting, and an example is shown in Figure 83.
Figure 83 — Effects of the welding variables on the weld bead.
With current that is too low, there is not enough heat to melt the base metal and the molten pool will be too small. The result is poor fusion and an irregular shaped deposit that piles up. This piling up of molten metal is called overlap. The molten metal from the electrode lays on the work without penetrating the base metal. Poor welds result from both undercutting and overlapping. When the electrode, current, and polarity are correct, a good arc produces a sharp, crackling sound. When any of these conditions are incorrect, the arc produces a steady, hissing sound, such as steam escaping.
Travel speed is another important factor in controlling the weld characteristics. The travel speed is determined by the welder, who manually controls the rate that the arc travels along the work. Increasing travel speed while the other variables remain constant reduces the width of the weld bead and increases the weld penetration. There is an optimum travel speed at which the penetration is at its maximum. Increasing the travel speed beyond this point will decrease the penetration. Excessive travel speed will produce a weld bead that is too small with an irregular contour. This can produce welds that have too small a cross section. A very slow travel speed can result in excessive piling up of weld metal and lack of fusion at the edges of the weld. The effects of travel speed are also shown in Figure 83.
The welding voltage is another important variable in shielded metal arc welding. The arc voltage is determined by the arc length between the end of the electrode and the base metal. The welder controls the arc voltage manually by moving the tip of the electrode close to or away from the surface of the base metal. Increasing the arc length increases the arc voltage; decreasing the arc length decreases the arc voltage. The welding voltage primarily affects the shape of weld bead cross-section and the general appearance of the weld. Increasing the welding voltage produces a wider and flatter weld bead and increases the susceptibility to arc blow. When the arc length is too long, which makes the welding voltage too high, the weld bead can look irregular with poor penetration and spatter. Also, the weld metal may be not be properly shielded by the gas from the decomposition of the electrode coating and much of the heat may be lost to the atmosphere. Decreasing the arc length will produce a stiffer and more easily controlled arc, but a very short arc length can cause the electrode to stick to the base metal. The effects of too long an arc length are also shown in Figure 83.
There are two basic methods for starting the arc: the striking or brushing method (Figure 84) and the tapping method (Figure 85). In either method, the arc is started by short circuiting the welding current between the electrode and the work surface. The surge of high current causes the end of the electrode and a small spot on the base metal beneath the electrode to melt instantly. In the striking or brushing method, bring the electrode down to the work with a lateral motion similar to striking a match. As soon as the electrode touches the work surface, raise it to establish the arc (Figure 84). The arc length or gap between the end of the electrode and the work should be equal to the diameter of the electrode. When you have obtained the proper arc length, it produces a sharp, crackling sound.
Figure 84 — Striking or brushing method of starting the arc.
Figure 85 — Tapping method of starting the arc.
In the tapping method, hold the electrode in a vertical position to the surface of the work. Start the arc by tapping or bouncing it on the work surface and then raising it to a distance equal to the diameter of the electrode (Figure 85). When you have established the proper length of arc, you will hear a sharp, crackling sound.
In either of the starting methods described above, withdrawing the electrode too slowly will cause it to stick or freeze to the plate or base metal. If this occurs, you can usually free the electrode by a quick sideways wrist motion to snap the end of the electrode from the plate. If this method fails, immediately release the electrode from the holder or shutoff the welding machine, and use a light blow with a chipping hammer or chisel to free the electrode from the base metal.
NEVER remove your helmet or the shield from your eyes as long as there is any possibility that the electrode could produce an arc.
After you strike the arc, the end of the electrode melts and flows into the molten crater of the base metal. To compensate for this loss of metal, you must adjust the length of the arc. Unless you keep moving the electrode closer to the base metal, the length of the arc will increase. An arc that is too long will emit a humming sound. One that is too short makes a popping noise. When the electrode is fed down to the plate and along the surface at a constant rate, a bead of metal is deposited or welded onto the surface of the base metal. After striking the arc, hold it for a short time at the starting point to ensure good fusion and crater deposition. Good arc welding depends upon controlling the motion of the electrode along the surface of the base metal.
The most commonly used method to break the arc is to hold the electrode stationary until the crater is filled and then slowly withdraw the electrode. This method reduces the possibilities of crater cracks.
To reestablish the arc (as in a long weld that requires the use of more than one electrode), clean the crater before striking the arc. Strike the tip of the new electrode at the forward (cold) end of the crater and establish an arc. Move the arc backward over the crater, and then move forward again and continue the weld. This procedure fills the crater and prevents porosity and slag inclusions.
Peening is a procedure that involves lightly hammering a weld as it cools. This process aids in relieving built-up stresses and preventing surface cracking in the joint area; however, peening should be done with care because excess hammering can work harden and increase stresses in the weld. This condition leads to weld embrittlement and early failure. Some welds are covered by specific codes that prohibit peening, so check the weld specification before peening.
Secondary variables include work and travel angles of the electrode,
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 determines the ease with which the filler metal is deposited, freedom from undercutting and slag inclusions, 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. Increasing the travel angle in the direction of welding generally builds up the bead height. A work angle that is too large may result in undercutting. Especially with the low-hydrogen types of electrodes, the electrode angles are important in maintaining weld quality.
Figure 86 — Travel angle and work angle.
- To Table of Contents -
The welding procedure schedules in this course give typical welding specifications that can be used to obtain high quality welds under normal welding conditions. These welding procedure schedules provide only a few examples of the many different welding procedures you can use. These tables are not the only conditions that can be used, because factors such as weld appearance, operator skiII, and the specific application often require variation from the schedules. As the particular requirements of the application become better known, the settings may be adjusted to obtain the optimum welding conditions. Make qualifying tests or trials prior to actual production. The following schedules are based on welding low-carbon mild steels with recommended types of mild steel, covered electrodes under normal welding conditions.
When adjusting or changing the variables for welding, you must consider the effect of one variable on the others. You cannot usually change one variable very much without adjusting or changing the other variables in order to maintain a stable arc and good overall welding conditions. Figures 8-87 through 8-90 show the type of weld, base metal thickness, welding position, number of passes, welding current, travel speed, electrode size, and type of covered electrode used. The arc voltage is not included because it depends on the arc length held by the welder; it is not constant and will vary from welder to welder.
Figure 87 — Square groove welds in plate 1/8- to ¼-in. thick.
First pass put back side and its root gouged or chipped out.
Figure 88 — Vee groove welds in plate 3/8- to 5/8-in. thick
First pass put
back side and its root gouged or chipped out.
Figure 89 — U-groove welds in plate greater than 1 inch (2.54 mm) thick.
Figure 90 — Filet Welds
- To Table of Contents -
Several operations may be required before making a weld. These include: preparing the weld joint, setting up or fixturing the weldment, setting the variables, and in some cases preheating. The amount of preweld preparation depends upon the size of the weld, the material to be welded, the ease of fit-up, the quality requirements, the governing code or specification, and the welder.
There are different ways of preparing the edge of the joint for welding. Joints for fillet or square groove welds are prepared simply by squaring off the edges of the members to be welded. Common types of machined bevels are V, U, J, bevel, and combination grooves. The more complex the type of bevel, the longer the edge preparation takes and the more expensive it becomes. The methods that are the most often used for edge preparation are oxygen-fuel gas cutting, shearing, machining, air carbon arc gouging, grinding, chipping, and plasma arc cutting. Plasma arc cutting is widely used for the cutting and beveling of stainless steels and nonferrous metals.
V groove and single bevel grooves are the types of grooves most often used because they can easily be prepared by oxygen-fuel cutting. If correctly done, this process leaves a smooth surface with a scale that can be easily removed. The edges of U grooves can be done by using special types and techniques, or by machining which will produce a more uniform groove.
To produce good quality welds, the surfaces of the weld joint should be clean of rust, scale, dirt, oil and grease. Grinding is useful for removing rust and scale including the scale left by oxyacetylene cutting and other related processes. Grease and oil must be removed from the joint surfaces by wiping or using degreasers. Scale, rust, dirt, oil, and grease can contaminate the weld metal and cause defects in the weld.
Fixtures and jigs are devices used to hold the parts to be welded in proper relation to each other. This alignment is called fit-up. Good fit-up is required for obtaining high quality welds. Poor fit-up increases welding time and causes many poor quality welds. The size of the root opening has an effect on the speed at which the welding of the root pass can be accomplished. Root openings are used so that full penetration welds can be made. Root passes in joints with a proper root opening can be welded much faster than joints that have excessive root opening. Fixtures and jigs are used for three major purposes:
When a welder employs a welding fixture or jig, the components of a weldment can be assembled and securely held in place while the weldment is positioned and welded. The use of those devices is dependent on the specific application. These devices are more often used when a large number of similar parts are produced. Using fixtures and jigs, when possible can greatly reduce the production time for the weldments.
Positioners are used to move the workpiece into a position so that welding can be done more conveniently. Positioning is sometimes needed simply to make the weld joint accessible. The main objective of positioning is to put the joint in the flat or other position that increases the efficiency of the welder because the welder can use higher welding speeds. Flat position welding usually increases the quality of the weld because it makes the welding easier.
Preheat is sometimes necessary, depending on the type of metal being welded, the base metal thickness, and the amount of joint restraint. The specific amount of preheat needed for a given application is often obtained from the welding procedure.
The preheat temperature of the base metal is often carefully controlled. Several good methods of doing this are furnace heating, electric induction coils, and electric resistance heating blankets. On thin metals, hot air blasts or radiant lamps may be used. With these methods, temperature indicators are connected to parts being preheated. Another method of preheating is using torches; these give more localized heating than the previously mentioned methods. However, when using torches for preheating, it is important to avoid localized overheating and prevent deposits of incomplete combustion products from collecting on the surface of the parts to be welded. Colored chalks and pellets that melt at a specific predetermined temperature are often used to measure the preheat temperature. Another method of measuring the temperature is by using a hand held temperature indicator. These can give meter readings, digital readings or recorder readings depending on the type of temperature indicator.
|Test Your Knowledge
11. Which of the following is NOT a major type of welding variable?
12. Fixtures and jigs are devices used to hold the parts to be welded in proper relation to each other. What is this alignment called?
- To Table of Contents -
Shielded metal arc welding, like other welding processes, may develop welding procedure problems that can cause defects in the weld. Some defects are caused by problems with the materials. Others may not be foreseeable and may require immediate corrective action.
A poor welding technique and improper choice of welding parameters can cause weld defects. Defects that can occur when using the shielded metal arc welding process are slag inclusions, wagon tracks, porosity, wormhole porosity, undercutting, lack of fusion, overlapping, burn through, arc strikes, craters, and excessive weld spatter. Many of these welding technique problems weaken the weld and can cause cracking. Other problems that can reduce the quality of the weld are arc blow, fingernailing, and improper electrode coating moisture contents.
Figure 91 — Slag inclusions.
Slag inclusions (Figure 91) occur when slag particles are trapped inside the weld metal, which produces a weaker weld.
These can be caused by:
- Erratic travel speed
- Too wide a weaving motion
- Slag left on the previous weld pass
- Use of too large an electrode
- Letting slag run ahead of the arc
This defect can be prevented by:
- A uniform travel speed
- A tighter weaving motion
- Complete slag removal before welding
- Using a smaller electrode
- Keeping the slag behind the arc by shortening the arc, increasing the travel speed, or changing the electrode angle
Figure 92 — Wagon tracks.
Wagon tracks (Figure 92) are linear slag inclusions that run the longitudinal axis of the weld. They result from allowing the slag to run ahead of the weld puddle and by slag left on the previous weld pass. These occur at the toe lines of the previous weld bead.
Figure 93 — Porosity.
Porosity (Figure 93) is gas pockets in the weld metal. They may be scattered in small clusters or along the entire length of the weld. Porosity weakens the weld in approximately the same way that slag inclusions do.
Porosity may be caused by:
- Excessive welding current
- Rust, grease, oil, or dirt on the surface of the base metal
- Excessive moisture in the electrode coatings
- Impurities, such as sulfur or phosphorous in the base metal
- Too short an arc length, except when using low-hydrogen or stainless steel electrodes
- Travel speed too high, which causes freezing of the weld puddle before gases can escape
Porosity can be prevented by:
- Lowering the welding current
- Cleaning the surface of the base metal
- Redrying electrodes
- Changing to a different base metal with a different composition
- Using a slightly longer arc length
- Lowering the travel speed to let the gases escape
- Preheating the base metal, using a different type of electrode, or both
Figure 94 — Wormhole.
Wormhole porosity (Figure 94) is the name given to elongated gas pockets and is usually caused by sulfur or moisture trapped in the weld joint. The best method of preventing this is to lower the travel speed to permit gases to escape before the weld metal freezes.
Figure 95 — Undercutting.
Undercutting (Figure 95) 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. Undercutting causes a weaker joint and can cause cracking. This defect is caused by:
- Excessive welding current
- Too long an arc length
- Excessive weaving speed
- Excessive travel speed
On vertical and horizontal welds, it can also be caused by too large an electrode size and incorrect electrode angles.
This defect can be prevented by:
- Choosing the proper welding current for the type and size of electrode and the welding position
- Holding the arc as short as possible
- Pausing at each side of the weld bead when using a weaving technique
- Using a travel speed slow enough so that the weld metal can completely fill all of the melted-out areas of the base metal
Figure 96 — Lack of fusion.
Lack of fusion (Figure 96) occurs when the weld metal is not fused to the base metal. This can happen between the weld metal and the base metal or between passes in a multiple pass weld.
Causes of this defect can be:
- Excessive travel speed
- Electrode size too large
- Welding current too low
- Poor joint preparation
- Letting the weld metal get ahead of the arc
Lack of fusion can usually be prevented by:
- Reducing the travel speed
- Using a smaller diameter electrode
- Increasing the welding current
- Better joint preparation
- Using a proper electrode angle
Figure 97 — Overlapping.
Overlapping (Figure 97) is the protrusion of the weld metal over the edge or toe of the weld bead. This defect can cause an area of lack of fusion and create a notch which can lead to crack initiation. Overlapping is often produced by: 1. Too slow a travel speed which permits the weld puddle to get ahead of the electrode 2. An incorrect electrode angle that allows the force of the arc to push the molten weld metal over unfused sections of the base metal 3. Welding away from the ground connection with large electrodes like the E6020, E6027, E7024, and E7028, which have very fluid weld puddles Overlapping can be prevented by or corrected by: 1. A higher travel speed 2. The electrode angle should be such that the force of the arc does not push the molten metal out of the weld puddle and over the cold base metal 3. Grinding off excess weld metal
Figure 98 — Burn through.
Burn-through (Figure 98) is when the arc burns through the bottom of the weld. This can be caused by: 1. Excessive welding current 2. Too slow a travel speed 3. Too wide a root gap This can be prevented by: 1. Reducing the welding current 2. Increasing the travel speed 3. Reducing the size of the root gap
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.
Figure 99 — Weld crater.
A weld crater (Figure 99) is a depression on the weld surface at the point where the arc was broken. 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. These craters can usually be removed by chipping or grinding and the depression can be filled in with a small deposit of filler metal. The best way of preventing weld craters 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.
Excessive weld spatter gives the weld a poor appearance, wastes electrodes, spreads difficult-to-remove slag, and can lead to lack of fusion in the case of multiple pass welds. If the spatter is coarse, it is usually produced by an arc length that is too long. If the spatter is fine, it is usually caused by too high a welding current. Spatter can be removed by grinding or prevented by using a shorter arc length and lower welding currents.
Cracking may be caused by an improper welding procedure, welder technique, or materials. All types of cracking can be classified as either hot cracking or cold cracking, and these cracks can be oriented transversely or longitudinally to the weld. Transverse cracks are perpendicular to the axis of the weld; longitudinal cracks are parallel to the axis of the weld. Transverse cracks are often the result of longitudinal shrinkage strains acting on excessively hard and brittle weld metal. Longitudinal cracks are often caused by high joint restraint and high cooling rates.
Hot cracking is a defect that occurs at higher 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 content in the base metal. It can also occur because of 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 it can continue through successive layers if it is not repaired.
Hot cracking may be prevented or minimized by:
- Using low-hydrogen electrodes
- Increasing the cross sectional area of the weld bead
- Changing the contour of the weld bead
- Using base metal with very low sulfur, phosphorous, and lead content
Crater cracks are shallow hot cracks caused by improperly breaking the arc. Several types of these are shown in Figure 100.
Figure 100 — Crater cracks.
Crater cracks may be prevented the same way that craters are, by reversing the travel of the electrode a little way back into the weld from the end of the weld 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 low hydrogen electrodes helps 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, as shown in Figure 101.
Figure 101 — Centerline crack.
This problem may be caused by:
- Too small a weld bead for the thickness of the base metal
- Poor fit-up
- High joint restraint
- Extension of a crater crack
The major methods of preventing centerline cracks are:
- Increasing the bead size
- Decreasing the gap width
- Positioning the joint slightly uphill
- 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 as shown in Figure 102.
Figure 102 — Underbead crack.
Base metal cracks originate in the heat affected zone of the weld. These types of cracking are caused by excessive joint restraint, 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 and low-hydrogen electrodes.
A number of other welding problems may occur, such as those caused by magnetic fields, improper moisture, or indirect electrode arc.
The electric current that flows through the electrode, workpiece, and ground 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 field. This deflection of the arc is called arc blow. Deflection is usually in the direction of travel or opposite it, but it sometimes occurs to the side. Arc blow can result in excessive weld spatter and lack of fusion.
Direct current is highly susceptible to arc blow, especially in welding corners and near the end of joints. Arc blow also occurs in welding complex structures and on massive structures with high currents and poor fit-up. 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 all the time due to the reversing current. 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 ground connection, into a corner, or toward the end of a weld joint.
There are several methods to correct the arc blow problem:
- Changing to alternating current
- Welding towards an existing weld or a heavy tack weld
- 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 a heavy tack weld
- Reducing the welding current and making the arc length as short as possible
- Wrapping the work lead around the workpiece so that the magnetic field caused by the current in the ground cable will neutralize the magnetic field causing the arc blow
The coatings of all covered electrodes contain a certain amount of moisture. Incorrect moisture content in an electrode coating can cause operating problems with the electrode. Some typical coating moisture contents of various mild steel electrode coatings are shown in Table 8-15.
Table 8-15 — Moisture Contents of Various Mild Steel Electrode Coverings
|Electrode Type||Moisture Content|
The E6010 and E6011 electrodes have relatively high moisture content in their coverings. These electrodes can operate fairly well when the moisture content is above the maximum limit, but an excessive amount of moisture in these electrode coatings can cause blistering of the coatings and poor arc operation. Low moisture content will cause the electrodes to give excessive amounts of spatter and possibly porosity. Too much moisture in the coatings of the other types of electrodes can cause blistering of the coating, poor arc operation, and underbead cracking.
Low hydrogen electrodes are called low hydrogen because of their very low moisture content. Covered electrodes should always be stored in dry places. High moisture content in low hydrogen electrode coatings will damage the quality of the weld deposit. Redrying is often done after a long storage period except on the cellulose electrodes for which it is generally not recommended.
Fingernailing is a problem that occurs when the arc does not come straight off the tip of the electrode, but moves over and comes more off the side of the electrode. This is usually because the electrode core wire is not concentric in the electrode coating. Fingernailing is shown in Figure 103. When the core wire is off center, a hard to control arc is produced because the electrode burns off more quickly on the side with the thinner coating. A cracked or damaged coating can also cause this problem.
Figure 103 — Fingernailing.
- To Table of Contents -
Several operations may be done after the weld. The first postweld procedure is to clean the slag off the weld bead. Other postweld procedures might call for cleaning, inspecting the weld for defects, straightening, and postheating.
After depositing the weld bead and breaking the arc broken, begin the clean up process. Remove the slag covering either by chipping or some other form of slag removal. This is particularly important when making multiple-pass welds. Complete removal of the slag for multiple pass welds prevents slag inclusions, porosity, and lack of fusion in the weld. After removal of the slag, a grinder is often used to grind the surface of the weld to give a more uniform surface. A wire brush is also often used to clean up the surface of the weld.
The weld is inspected and tested after cleaning to determine the quality of the weld joint. There are many different methods of inspection and testing which will not be covered in detail in this course. The use 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. There are 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; 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.
In the fabrication or repair of equipment, there are tests to determine the quality and soundness of welds. Many different tests have been designed for specific faults. The type of test used depends upon the requirements of the welds and the availability of testing equipment. This section will briefly discuss nondestructive and destructive testing.
Repair of the weld metal is sometimes necessary when testing reveals defects. The defects may be discovered by visual inspection and by other nondestructive testing methods. Where a defect is found, it is usually ground out or gouged out. Using a grinder is usually better for surface defects and for defects fairly near the surface of the weld metal. For deeper defects, an air carbon-arc gouging torch or some similar gouging method is often used for removal. Once the defects have been removed, the low areas created by the grinding and gouging can be filled in using the shielded metal arc process. The parts are then reinspected to make sure that the defects have been properly repaired.
Postheating is a heat treatment applied to the metal 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 codes or specifications. Various types of postheating are used to obtain specific properties. Types of postheating are annealing, stress relieving, normalizing, as well as quenching and tempering. Postheating is done with many of the same methods used for preheating, such as furnace heating, induction coils, and electric resistance heating blankets. One method used for stress relieving that does not involve heating is called vibratory stress relief. This method does not use heating because the part being stress relieved is vibrated mechanically to relieve the residual stresses.
Annealing is a process involving heating and cooling that is usually applied to induce softening. There are different kinds of annealing, but when it is applied to ferrous alloys, it is called full annealing. Full annealing is a softening process in which a ferrous alloy is heated, usually in a furnace, to a temperature above the transformation range and slowly cooled to a temperature below the transformation range.
Stress relieving is the uniform heating of a structure to a high enough temperature, but below the critical range, to relieve most of the residual stresses due to welding. This is followed by uniform cooling. The terms normalizing and annealing are often misnomers for this application.
Normalizing is a process in which a ferrous alloy is heated to a temperature above the transformation range and then cooled in still air to a temperature below the transformation range.
Quenching and tempering is another postweld heat treatment commonly used. The metal is heated and then quenched to provide a very hard and brittle metallurgical structure. The part is then tempered by reheating to a particular temperature dependent upon the degree of ductility, tensile strength, yield strength, and hardness required.
After welding with the shielded metal arc process, postheating is often required. For many applications, heat treating low-carbon steels after welding is unnecessary. The medium-carbon steels usually use postheating from 1100 to 1200°F (590 to 650°C) to remove the brittle microstructure that may have been caused by too rapid cooling. Highcarbon steels are often stress relieved at 1200°F (650°C). The various low alloy steels often require stress relieving from 1100-1250°F (590-680°C). Stainless steels are often post-weld heat treated to reduce the grain size and preserve good corrosion resistance. Annealing is used to reduce the grain size, which gives better ductility. The temperatures used depend on the specific stainless steel.
|Test Your Knowledge
13. What causes slag inclusions?
14. Which of the following is a nondestructive test?
- To Table of Contents -
To become a fully certified welder, you must know the requirements for training and qualifications. While these may differ somewhat from organization to organization, and you may need to demonstrate your skills to qualify for a particular project and specific welding task, the basic guidelines are the same for achieving the training and qualifications.
Shielded metal arc welding generally requires a high degree of welder skill to produce good quality welds. As a result, many training programs emphasize this process in their training schedule. A welder skilled in this process generally has much 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 training program should be flexible enough to be adapted to changing needs and applications. Emphasis may be placed on certain areas of the training schedule because of this. The complexity of the parts to be welded and the governing codes or specifications involved also dictate the length of such a training program.
The United States Department of Labor published a book entitled "Dictionary of Occupational Titles" that describes the duties of the different job titles for welders. The training programs used to develop the entry level skills for these job titles may vary depending on the amount of skill required. For instance, a pipe welder needs more skill than a tack welder, so the length of a training program for a pipe welder is greater than the length of a training program for a tack welder.
The job title of arc welder (DOT 810.384-014) describes a person who has the duty of welding together components of many products made of metal. This job includes setting up the machine and part to be welded, striking the arc and guiding it along the joint, and performing duties such as chipping, grinding, and slag removal. The welder should be able to weld in all positions, be able to pass employer performance tests, and meet certification standards of governmental agencies or professional and technical associations.
A tack welder (DOT 810.684-010) makes short beads at specified points to hold the parts in place for final welding. The tack welder also performs the duties of fitter helper. A production line welder (DOT 819.684-01 0) welds previously set up parts on a production line. The production line welder may also perform tack welding.
A combination welder (DOT 819.384-010) welds metal parts together to fabricate or repair the assembly. The combination welder uses both gas welding and any combination of arc welding processes. Other duties include seting up parts, cutting, grinding, and other related tasks. A combination welder may be required to pass employer performance tests to meet certification standards of governmental agencies or professional and technical associations.
The welder portion of the pipefitter course(DOT 862.381-018) is a person who welds the pipe together after it has been located and tacked in place.
The basic shielded metal arc welding training program is used to teach the student the basic entry level skills required for the job titles of arc welder, tack welder, production line welder, and the arc welder portion of combination welder. This course provides training on how to strike an arc, run weld beads, make good quality fillet welds, and an introduction to making groove welds. The training the student receives should impart enough skill to get a job as a tack welder, production line welder and enough skill for many of the simple arc welding jobs. This course should also provide the background skill required to take an advanced shielded metal arc welding course.
The following is an outline of topics for an approximately 140 hour course:
- Lecture/Discussion, "Arc Welding Introduction"
- Lecture/Discussion, "Safety and Health of Welders"
- Strike Arc and Run Bead, Surface Weld, Flat Position
- Pad of Beads, Surface Weld, Flat Position
- Fillet Weld, Lap Joint, Horizontal Position
- Lecture/Discussion, "Visual Inspection and Practical Weld Tests"
- Fillet Weld, Tee Joint, Horizontal Position
- Fillet Weld, Tee Joint, Flat Position
- Pad of Beads, Surface Weld, Horizontal Position
- Square Groove Weld, Butt Joint, Horizontal Position
- Lecture/Discussion, "Electrode Selection"
- Fillet Weld, Lap Joint, Vertical-Up Position
- Fillet Weld, Tee Joint, Vertical-Up Position
- Square Groove, Butt, Joint, Vertical-Up Position
- Lecture/Discussion "Power Sources for Welding"
- Fillet Weld, Lap Joint, Overhead Position
- Fillet Weld, Tee Joint, Overhead Position
- Square Groove Weld, Butt Joint, Overhead Position
- Lecture/Discussion, "Welding Distortion Control"
- Fillet Weld, Lap and Tee Joints, Flat and Vertical Down Positions
- String Beads, Flat, Horizontal and Vertical Positions
- Fillet and Square Groove Weld Lap-Butt and Corner Joints, Flat-Horizontal and Vertical Positions
- Fillet Weld, Lap Joint, Vertical-Down Position
- Square Groove Weld, Butt Joint, Flat Position
- Fillet Weld, Lap Joint, Horizontal Position
- Lecture/Discussion, "The Low Hydrogen Electrode and Its Use"
- Fillet Weld, Tee Joint, Vertical-Up Position
- Fillet Weld, Tee Joint, Overhead Position
The prerequisites for the advanced shielded metal arc welding course should be successful completion of the basic shielded metal arc welding course or equivalent welding training or experience. The purpose of this course is to develop the entry level skills for arc welder, production line welder, tack welder and the arc welding portion of combination welder. This course provides the skill training required for the student to make good quality fillet and multiple pass groove welds. This includes learning to use the proper weld bead sequence and welding grooved joints in all positions. A student who successfully completes this course should be able to do more complicated arc welding jobs on plate material. The following is approximately a 140 hour course outline for advanced shielded metal arc welding:
- Lecture/Discussion, "Introduction"
- Lecture/Discussion, "Safety and Health of Welders"
- Fillet Weld, Lap Joint, Horizontal Position
- Lecture/Discussion, "Air Arc Cutting and Gouging"
- Fillet Weld, Lap Joint, Overhead Position
- Lecture/Discussion, "Procedure and Welder Qualification"
- Fillet Weld, Lap Joint, Vertical Position, Up Hill Travel
- Lecture/Discussion, "Destructive Testing"
- Fillet Weld, Lap Joint, Cross Section Etch test
- Lecture/Discussion, "Non-Destructive Testing"
- Single Vee Groove Weld, Butt Joint, Horizontal Position
- Single Vee Groove Weld, Butt Joint, Overhead Position
- Single Vee Groove Weld, Butt Joint, Overhead Position, Guided Bend Test
- Lecture/Discussion," Metals Identification for Welding"
- Single Vee Groove Weld, Butt Joint, Vertical Position
- Single Vee Groove Weld, Butt Joint, Vertical Position, Guided Bend Test
- Lecture/Discussion, "Welding of Cast Iron and Surfacing of Steel"
- Single Vee Groove Weld, Butt Joint, Flat Position
- Fillet Weld, Lap Joint, All Positions
The prerequisites for the shielded metal arc pipe welding should be to have successfully completed the basic and advanced shielded metal arc welding courses or have equivalent welding training or experience. The pipe welding is divided into two categories, uphill and downhill pipe welding. The purpose of these courses is to develop the entry level skills required for the welder portion of pipefitter.
Since pipe welding is more difficult than plate welding, the student should be proficient in welding groove joints in plate in all positions before starting pipe welding.
Shielded metal arc pipe welding, uphill method is used on power plant, refinery, and chemical installation construction. This course covers pipe welding in the 2G, 5G, and 6 G positions on mild steel pipe. An outline for approximately a 210 hour course is as follows:
- Lecture/Discussion, "Introduction to Up Hill Pipe Welding"
- Lecture/Discussion, "Safety and Health of Welders"
- Prerequisite Skill Test, Single Groove Weld, Butt Joint, Vertical and Overhead Positions
- Lecture/Discussion, "How to Read and Apply Pipe Welding Procedures"
- Preparation and Assembly of a Pipe Joint
- Lecture/Discussion, "Weld Quality: Reading the Puddle"
- Single Groove Weld, Butt Joint, Horizontal Fixed Position (5G)
- Single V-Groove Weld, Butt Joint, Horizontal Fixed Position (5G), Visual Inspection
- Single V-Groove Weld, Butt Joint, Vertical Fixed Position (2G)
- Single V-Groove Weld, Butt Joint, Vertical Fixed Position (2G), Visual Inspection
- Lecture/Discussion, "Preheat and Interpass"
- Single V-Groove Weld, Butt Joint, 45° Fixed Position (6G)
- Single V-Groove Weld, Butt Joint, 45° Fixed Position (6G), Guided-Bend Test
In addition to the basic course outline, topics covering the welding of stainless steel pipe and the use of backing rings may be covered. Both of these are specialty items that are commonly welded by the uphill pipe welding method and are covered for use on special applications. Each of these items consists of approximately 70 hours additional training time.
The downhill pipe welding method is primarily used on cross country transmission pipelines. This course covers welding downhill in the 5G and 6G positions on mild steel pipe. The following is an outline for approximately a 140 hour course:
- Lecture/Discussion, "Introduction to Downhill Pipe Welding"
- Lecture/Discussion, "The Safety and Health of Welders"
- Prerequisite Skill Test, Single Vee Groove Weld, Butt Joint, Vertical and Overhead Positions
- Lecture/Discussion, "How to Read and Apply Pipe Welding Procedures"
- Preparation and Assembly of a Pipe Joint
- Lecture/Discussion, "Weld Quality-Reading the Puddle"
- Single Vee Groove Weld, Butt Joint, Horizontal Fixed Position (5G), Downhill
- Single Vee Groove Weld, Butt Joint, Horizontal Fixed Position (5G), Visual Inspection
- Lecture/Discussion, "Pipe Welding Fixtures and Line-up Clamps"
- Single Vee Groove Weld, Butt Joint, 45° Fixed Position (6G), Downhill
- Single Vee Groove Weld, Butt Joint, 45° Fixed Position, Guided-Bend Test
Before a welder can begin work on any job covered by a welding code or specification, he must become certified under the code that applies. Many different codes are in use today, and it is exceedingly important that the specific code is referred to when taking qualification tests. In general, the following type of work is covered by codes: pressure vessels and pressure piping, highway and railway bridges, public buildings, tanks and containers that hold flammable or explosive materials, cross country pipeline, aircraft, ordnance material, ships and boats, and nuclear power plants.
Certification is obtained differently under the various codes. Certification under one code will not necessarily qualify a welder to weld 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 requirement. 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 procedure, 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 welding 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 employed. Satisfactory completion of test specimens, providing they meet acceptability standards, will qualify the welder for specific types of welding. Again, the welding that will be allowed depends on the particular code. In general, however, the code indicates the range of thicknesses which may be welded, the positions which may be employed and the alloys which may be welded.
Qualification of welders is a highly technical subject and cannot be covered fully here. You should obtain the actual code, study it, and practice it prior to taking any qualification test.
Some often used codes are:
- To Table of Contents -
Safety is an important consideration when welding. Every welding shop should have a safety program and take adequate safety precautions to help protect welders. The welders should also be made aware of safety precautions and procedures. Employees who fail to follow adequate safety precautions can cause physical injury to themselves and others and damage property. Any of these conditions 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 is presented in the American National Standard Z49.1, "Safety in Welding and Cutting", published by the American Welding Society. Welders must follow these rules. There are several types of hazards associated with shielded metal arc welding. These do not necessarily result in serious injuries; they can also be of a minor nature. Even these minor injuries, however, can cause discomforts that irritate and reduce the efficiency of the welders. These hazards are:
Several precautions should be taken to prevent an electrical shock hazard. The first is to make sure before welding that the arc welding equipment is installed properly, grounded, and in good working condition. The electrical equipment should be maintained and installed in accordance with the National Electrical Code and any state and local codes that apply. Power supplies should be connected to an adequate electrical ground such as an approved building ground, cold water pipe, or ground rod. Power supplies are connected to ground through the cable that connects the power supply to the electrical system ground. Cables with frayed or cracked insulation and faulty or badly worn connections can cause electrical short circuits and shocks. If it is necessary to splice lengths of welding cable together, make sure the electrical connections are tight and insulated. Use the proper size welding cables also because constantly overloading a welding cable that is too small can destroy the insulation and create bare spots in the insulation. This occurs because excessive heat builds up in the cable and destroys the insulation. An improperly insulated welding cable is both an electrical shock hazard and a fire hazard. Be sure the welding area is dry and free of any standing water which could cause electrical shock. When it is necessary to weld in a damp or wet area, wear rubber boots and stand on a dry insulated platform
The welding arc of shielded metal arc welding emits large amounts of invisible ultraviolet and infrared rays. Skin that is exposed to the arc even for a short time can suffer serious ultraviolet and infrared burns, which are essentially the same as sunburn, but the burn caused by welding can take place in a much shorter time and be very painful. Because of this, always wear protective clothing suitable for welding. These clothes should be fairly heavy and not easily burned. Leather is often used to make jackets, capes and bibs, or other similar arrangements to shield the arms, shoulders, chest, and stomach from the arc radiation and arc spatter. Leather is also used to make gloves and gauntlets for the welder.
The eyes should also be protected from the radiation emitted by the welding arc. Arcburn can result if the eyes are not protected. Arc-burn of the eye is similar to sunburn of 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 as though several grains of sand were in your eyes. 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. The effects of infrared rays are not nearly as noticeable or immediate as the effects of ultraviolet rays.
Infrared rays are probably more dangerous in that their effects can be longer lasting and result in impaired vision. The best protection for the eyes and face is a headshield that has a window set in it with a filter lens in the window. Headshields are generally made of fiberglass or a pressed fiber material to be 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 type of lens used varies for different welders but it should be dark enough so the arc can be viewed without discomfort yet not so dark the welder cannot see what he or she is doing. Table 8-16 shows the different lenses commonly recommended for use in shielded metal arc welding. The higher the lens numbers the darker the lens. A clear, replaceable glass should be put on the outside of the welding lens to protect it from spatter and breakage.
Table 8-16 -Recommended Filter Lens Used in Shielded Metal 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|
The arc and the decomposition of the electrode coating during welding generate welding smoke and fumes. Because of this, the following warning is printed on the containers packaging the covered electrodes: "Welding may produce fumes and gases hazardous to health. Avoid breathing these fumes and gases. Use adequate ventilation. See American National Standard Z49.1, Safety in Welding and Cutting, published by the American Welding Society."
The welding area should be adequately ventilated because welding produces fumes and gases such as ozone, which is a hazardous gas for the welder to breathe. Welding in confined areas requires an external air supply. This is furnished by a gas mask on a special helmet. A second person should stand just outside the confined area to lend assistance to the welder if necessary. Use a mechanical exhaust when welding metals with toxic coatings such as lead, copper, cadmium, and zinc.
Fires and explosions are hazards that can exist in a welding area if the proper precautions are not taken. The shielded metal arc welding process produces sparks and spatters which can start a fire or explosion in the welding area if it is not free of flammable, volatile, or explosive materials. Never weld near degreasing and other, similar operations. Wear leather clothing for protection from burns, because leather is fireproof. Electrical shorts or overheated worn cables can also start fires. In case of a fire started by a flammable liquid or an electrical fire, a CO2 or dry chemical type of fire extinguisher is used. Fire extinguishers should be kept at handy spots around the shop and welders should make a mental note of where they are located.
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 there are no fumes or residue left. Do not weld on sealed containers without providing vents and taking special precautions. Never strike the welding arc on a compressed gas cylinder. When the electrode holder is set down or not in use never allow it to touch a compressed gas cylinder.
Welders can also encounter hazards during the weld cleaning process. Always take precautions to protect the skin and eyes from hot slag particles. The welding helmet, gloves, and heavy clothing protect the skin from slag chipping and grinding of the weld metal. Wear safety glasses underneath the welding helmet to protect the eyes from particles that could get inside the welding helmet.
The discarded stubs of the electrodes can also be a safety hazard. If these are dropped on the floor during electrode changes they can become a hazard because they roll or slide easily. If a welder steps on one, he or she could fall and possibly sustain injury, so it is necessary to keep the floor of the welding area clear of electrode stubs.
This course has introduced you to the SMAW process from the types of power sources, controls, and electrodes to the types of training and qualifications needed. It also described the industries that use the SMAW process and its applications. Welding metallurgy, weld and joint design as well as welding procedure variables were also discussed. The course finished up with a description of possible weld defects, and how to identify for them using multiple methods of destructive and nondestructive tests and inspections. As always, use the manufacturer’s operator manuals for the specific setup and safety procedures of the welder you will be using.
1. What type of current is used in shielded metal arc welding?
2. A constant flow of electrical current that travels in one direction only has what type of polarity?
3. What factors determine the size of a welding cable needed for a job?
4. The distance between an operator and any joint in the welding cable should be a minimum of how many feet?
5. When selecting an electrode holder for a specific task, you should base your selection on what criteria?
6. The use of a good ground clamp that provides proper grounding is essential to the production of quality welds. Which of the following conditions could develop without this proper grounding?
7. Which of the following safety devices should you use to protect other personnel in a welding work area from eye flash burns?
8. The coating on an arc-welding electrode provides which of the following advantages?
9. Electrodes manufactured in the U.S. must conform to what standards?
10. An electrode that has a minimum tensile strength of 80,000 psi for use in all positions for low alloy has what designation?
11. A welding electrode that has an AWS classification of E-7024 should be used for a metal-arc welding job in what position(s)?
12. When welding stainless steel, you must use what type of electrode?
13. Which of the following properties is the basic criterion for selecting an electrode for a job?
14. When the electrode is positive and the workpiece is negative, the electrons flow from the workpiece to the electrode. What polarity is being used?
15. Which of the following factors is a reason why reverse polarity is used in out-ofposition welding?
16. What kind of sound does improper polarity emit?
17. Which one of the following steps do you take to correct arc blow?
18. What is the first thing you should do to start an arc by the striking method?
19. Upon striking an arc; you immediately start the weld to ensure good fusion and penetration.
20. What ampere setting should you initially use when welding with a 5/32-inch diameter electrode?
21. What condition occurs when the welding current is too high?
22. What condition(s) can develop when the welding current is too low?
23. What kind of sound does a good arc produce when the electrode, current, and polarity are correct?
24. When shield metal arc welding, the distance between the electrode and the base metal, except in vertical and overhead welding, should be approximately equal to which of the following characteristic?
25. Of the following practices, which one is correct for breaking an arc with an electrode?
26. What is the maximum thickness a plate can be welded, in one pass, without edge preparation?
27. For what purpose do you use a backing strip when making a butt weld on 3/16- inch plate or heavier in the flat position?
28. What (a) width and (b) thickness of backing strip should be used on plate over 1/2-inch thick?
29. What angle should be maintained between the electrode and the vertical plate of a tee joint when 1/4-inch plate is used in the flat position?
30. What angle from the vertical should you hold the electrode when welding a lap joint on plates of varying thicknesses?
31. When vertical welding upwards, how many degrees do you hold the electrode to the vertical?
32. For which of the following reasons do you use relatively small electrodes for overhead butt welding?
33. What string bead do you deposit without the weaving motion of the electrodes when making a fillet weld of a lap or Tee-joint-in the overhead position?
34. Which of the following mistakes can cause undercutting in welds?
35. Which of the following mistakes can cause excessive spatter in welds?
36. Which of the following mistakes can cause cracked welds?
37. Which of the following mistakes can cause poor penetration?
38. Which of the following mistakes can cause brittle welds?
39. When pipe has _____ wall thickness, only the single U-type of butt joint should be used.
40. You do NOT need to do which of the following procedures when preparing a joint for welding?
41. What is the maximum size a tack weld should be when applied to a pipe with a wall thickness of 1/2-inch?
42. What maximum nominal diameter of electrode should you NOT exceed when making the root pass of a multilayer weld on pipe?
43. The root of a fillet weld is where the _____.
44. The face of a fillet weld is the _____.
45. The toe of a fillet weld is the _____.
46. The leg of the weld is the _____.
47. The throat of a fillet is the shortest distance from the _____.
48. Electrode holders should be _____.
49. Welding machine installations should be _____.
50. Welding machine frames should be _____.
51. The welding arc gives off ultra-violet rays which can cause eye injury. Injury can be prevented by _____.
52. Ultra-violet rays from the arc _____.
53. You only need ventilation when _____.
54. Vaporized metals, such as zinc, cadmium, lead, and beryllium _____.
55. Carbon dioxide produced by shielded metal arc welding is not considered harmful _____.
56. Before welding in a new area, _____.
57. Safety glasses with side shields _____.
58. When working in confined areas _____.
59. When you stop welding, you should _____.
60. When striking an arc, hold the arc length for a moment to _____.
61. When welding over a previously deposited bead _____.
62. At the completion of the weld, the crater should _____.
63. When welding in the overhead position the electrode should be_____.
D. 15° to the weld face
64. How is the melting rate related to the arc zone?
65. When restriking an arc to continue a bead (such as when changing electrodes), the arc should be restruck _____.
66. The axis of a weld is _____.
67. Flat position welding is done from the_____.
68. In the flat position welding, the face of the weld is approximately _____.
69. Horizontal position fillet welding is performed _____.
70. In a horizontal position groove weld, the axis of the weld lies in an approximately _____.
71. In vertical position welding, the axis of the weld is _____.
72. When making a horizontal fillet weld in a lap joint, the electrode should be positioned with a _____.
73. When lap welding base metal of different thickness the electrode should form an angle between _____.
74. Tack welds should be _____.
75. Compared to an E6012 electrode, an E6010 electrode _____.
76. When ending a butt joint on a multipass weld you should whip up and pause the electrode _____.
77. Before a welder can begin work on any job covered by a welding code or specification, he must become certified under the code that applies.
78. A combination welder welds metal parts together to fabricate or repair the assembly
79. The downhill pipe welding method is primarily used on cross country transmission pipelines
80. Using a filler metal not matching the base material may produce a faulty weldment.
81. A sound weld can be made over dirt, paint, and grease if the correct electrode is used.
82. Some of the most common types of destructive testing are _____.
83. Lay the wearfacing on the top and sides of each tooth _____.
84. The minimum tensile strength of an E11018 electrode is _____.
- To Table of Contents -
Copyright © David L.
All Rights Reserved