Other courses in this welding series, Introduction to Welding and Gas Welding, address two processes for joining metals by fusion welding, the first by electricity and the second by heated gases. This course also presents procedures for joining metals, but without fusion. These procedures, which include soldering, brazing, braze welding, and wearfacing, allow you to join dissimilar metals and produce high-strength joints while not affecting the heat treatment of the base metal or warping it as much as conventional welding may do with the requisite high temperatures. A welder functioning at the journeyman level is expected to be capable, if not proficient, in each of these four methods of non-fusion metal joining. They are part of the total package of skills you need to develop for your professional skills tool kit. Working with metal, whether stock material or parts, you will encounter situations where you must determine whether the tasking is a permanent or expedient effort, and what is the best method of fabrication or repair given the tools and assets available. With the capability to join metals by both fusion and non-fusion methods in your skills inventory, you increase your value to yourself and coworkers while upholding the “Can Do” spirit.
When you have completed this course, you will be able to do the following:
Soldering is a simple and fast means for joining sheet metal, making electrical connections, and sealing seams against leakage. Like welding, soldering uses a filler metal (commonly called solder) to join two metals. However, unlike welding, soldering joins the two metals without heating them to their melting points. In addition, you can also use soldering to join dissimilar metals such as iron, nickel, lead, tin, copper, zinc, aluminum, and many other alloys.
800°F is a key determining temperature. Because solder’s melting temperature is below 800°F, it is not classified as a welding or brazing process. Welding and brazing usually take place above 800°F, the one exception being lead welding, which occurs at 621°F.
Do not confuse “silver soldering” with soldering. The silver soldering process is a form of brazing because it uses a temperature above 800°F.
Soldering requires very little equipment. Typically, you only need a heat source, a soldering copper or iron, solder, and flux.
Heat sources can vary according to the available equipment and the method you need to use. Some common sources are welding torches, blowtorches, forges, and furnaces, all of which heat the soldering coppers that secondarily supply the direct heat to the metal surfaces, thus melting the solder. Occasionally you may opt to use a heat source directly to heat the metal, but if or when you do this, you must be careful not to damage the metal or the surrounding material.
Externally heated soldering coppers (soldering irons) consist of a forged copper head, an iron rod, and a handle, usually wood or fiber either screwed or forced on. Other soldering irons are electrically heated (Figure 1).
Soldering heads are available in various shapes. Figure 2 shows three of the more commonly used types.
Pointed copper — for general soldering work.
Stub copper — for soldering flat seams needing a considerable amount of heat.
Bottom copper — for soldering hard to reach seams, such as those in pails, pans, trays, and similar objects.
Nonelectrical coppers come in pairs so you can use one copper as the other is heating. When coppers are referred to by size designation, they are referred to by weight (in pounds) of the pair, so a reference to a pair of 4-pound coppers means each copper head weighs 2 pounds.
Pairs of coppers are usually available in 1-pound, 1 1/2-pound, 3-pound, 4-pound, and 6-pound sizes. As you would expect, because of their differing heat transfer ranges, heavy coppers are designed for soldering heavy gauge metals, while light coppers are for thinner metals, and using the incorrect size of copper usually results in poorly soldered joints from problems caused by not enough steady heat or by overheating.
Before you can use new soldering coppers, you must tin them (coat with solder). In addition, you must file and re-tin them if they overheat or lose their solder coating for any reason.
Figure 3 — Filing and tinning a soldering copper head with solder on a cake of sal ammoniac.
Use the following procedure for filing and tinning a copper (Figure 3).
Remember, the copper is hot! Do not touch it with your bare hands!
If sal ammoniac is unavailable, you can use powdered rosin (Figure 4).
Commercially prepared soldering salts (in powder form) are also used to tin soldering coppers.
Dissolve the powder in water according to the directions, dip the soldering copper into the solution, and apply the solder.
When nonelectric soldering coppers become blunt or deformed, you can reshape them by a forging process. Use the following procedures (Figure 5).
Electric soldering coppers (usually called soldering irons) use internal heating coils to heat the head, and the heads are removable and interchangeable.
Tinning is the same with the exception that the tip usually does not become cherry red as the internal coils have limiting resistors.
Forging or reshaping is not necessary since the heads are easily replaced.
Electric soldering irons are especially suited for, and usually used for, electrical work or other small jobs (Figure 6).
They do not require auxiliary heating and can be as small as a pencil.
You can use a gas torch in combination with soldering head attachments (Figure 7, View A) or as a direct heat source.
A Prest-O-Lite heating unit (Figure 7, View B) delivers a small controllable flame and is ideal for soft soldering, or you can use it effectively to heat soldering coppers. The unit includes a fuel tank regulator, hose, torch, interchangeable tips, and burns either acetylene or MAPP gas in the presence of oxygen.
Commercial industry uses many different types of solders, and they are available in various forms including bar, ingot, powder, and wire, which is available with or without a flux core. Because there are so many types of solders available, this course will cover only solders that welders would most commonly use.
The atomic symbol for tin is Sn; the symbol for lead is Pb. Occasionally, you may see these symbols used as Sn-Pb instead of the term tin-lead but they have equal meaning.
The tin-lead alloy group of solders is the largest group used. You can use them for joining most metals, they have good corrosion resistance, and they have excellent compatibility with soldering processes, most types of flux, and cleaning.
Industry custom is to state the tin content first when describing solders, so a 40/60 solder has a content of 40% tin and 60% lead.
The melting characteristics of any tin-lead alloy will depend on the ratio of tin to lead; the higher the tin content, the lower the melting temperature. Tin also increases the wetting ability and lowers the solder’s cracking potential
Figure 8 shows the behavior of a 63/37 tin-lead solder.
Note that 100% lead melts at 621°F and 100% tin melts at 450°F.
Also, solders containing 19.5% to 97.5% tin remain a solid until they exceed 360°F.
The eutectic composition for tin-lead solder is about 63% tin and 37% lead.
The eutectic point is the point in an alloy system when all the elements of the alloy melt at the same but lower temperature than any other composition. A 63/37 solder becomes liquid at 361°F.
Other compositions do not. Instead, they remain in the pasty stage until the temperature increases to the melting point of the other alloy. For an example, refer to Figure 8 again.
The less expensive solders with lower tin content are used primarily for sheet metal products and other high-volume solder requirements. The solders with higher tin content are used extensively in electrical work, and the solders with 60% tin or more (fine solders) are used in instrument soldering where temperatures are critical.
Antimony (AN-tuh-moh-nee), symbol Sb, is added up to 6% to a tin-lead solder as a substitute for some of the tin. It increases the solder’s strength and mechanical properties.
Do not use solders with high antimony content on aluminum, zinc, or zinc-coated materials. They form an intermetallic compound of zinc and antimony that causes the solder to become very brittle.
There are several tin-zinc solders available for joining aluminum alloys. The 91/9 and 60/40 are used for higher temperature ranges (above 300°F), and normally the 80/20 and 70/30 are used as precoating solders.
Tin-antimony solders are used for refrigeration work or for joining copper to cast iron joints. The 95/5 is the most common.
Tin-silver solder (96/4) is used for food or beverage containers that must be cadmium and lead-free. The 95/5 tin-silver can also be used as a replacement for the 95/5 tin-antimony solder for refrigeration work.
Lead-silver solders are useful where the requirement is for strength at moderately high temperatures.
Lead by itself cannot be used since it does not normally wet steel, cast iron, or copper and its alloys, so adding silver results in alloys that wet steel and copper more readily.
However, flow characteristics for straight lead-silver solders are rather poor, and they are susceptible to humidity and corrosion during storage. By adding a tin content of 1%, manufacturers enhance the wetting and flow characteristics, and increase resistance to corrosion.
Lead-silver solders require higher soldering temperatures and special fluxing techniques such as using a zinc-chloride-based flux (an acid flux) on uncoated metals because rosin-based fluxes decompose rapidly at high temperatures.
Most metal surfaces form scale, rust, and oxides when exposed to air, and heating accelerates their formation.
Solder will not adhere to or wet metal with these pollutants.
Fluxes are chemical compounds you use to clean and maintain the metal surfaces during the soldering process (Figure 9).
They also decrease the surface tension of the solder, making it a better wetting agent.
Fluxes are available in cake, paste, liquid, or powder form and are classified as either noncorrosive or corrosive for situational application with specific metals.
Table 1 shows fluxes you would normally use for soldering common metals.
|Aluminum||Stearine, special flux|
|Brass, copper, tin||Rosin|
|Galvanized iron||Zinc chloride|
|Iron, steel||Borax sal ammoniac|
|Stainless steel and other nickel alloys||Phosphenic acid|
For soldering electrical connections or other work that must be free of any trace of corrosive residue, you need to use a noncorrosive flux. Rosin is the most commonly used noncorrosive flux. In the solid state, it is inactive and noncorrosive; when heated, it melts and provides some fluxing action.
Available in powder, paste, or liquid form, rosin fluxes frequently leave a nonconductive brown residue that is sometimes difficult to remove since it is made of purified pine sap.
You can reduce the removal problem by adding a small amount of turpentine to the (pine sap) rosin, and you can add glycerin to it to make it more effective.
Corrosive fluxes provide the most effective cleaning action, but since any trace of corrosive flux remaining on the work can cause corrosion later, do not use corrosive fluxes on electrical connections or other work where corrosion would cause a problem.
Sal ammoniac (ammonium chloride) and zinc chloride, in either solution or paste form, are the most common and frequently used corrosive fluxes.
If present, any solvent evaporates as the work heats, leaving a layer of solid flux on the metal. When the metal heats further to soldering temperature, this layer of solid flux melts, partially decomposes, and liberates hydrochloric acid. Then the hydrochloric acid dissolves the oxides from the work surfaces and filler metal (solder) if applied, thus providing a clean surface for the solder process to bond (refer again to Figure 9, above).
You can make zinc chloride (also called cut acid or killed acid) in the shop as long as you follow specific safety precautions.
You must use rubber gloves, a full-face visor, and an apron. Fumes given off by muriatic acid or the mixture of muriatic acid and zinc are explosive and a health hazard as well.
Prepare zinc chloride under a ventilation hood, out in the open, or near openings to the outside to reduce the danger of explosion or inhalation, and take precaution to prevent flames or sparks from coming in contact with the liberated hydrogen gas.
To prepare zinc chloride:
Always add acid to water when diluting. Adding water to acid can result in an explosive reaction, resulting in serious injuries.
Make only enough as required and strain it before use; store any leftover in a tightly sealed glass container.
Soldering salts are another type of corrosive flux. Commercial soldering salts are normally manufactured in a water-soluble, powder form that allows you to mix only the amount needed.
If you use a corrosive flux for soldering, upon completion, remove as much of the residue as possible. Most corrosive fluxes are water-soluble, so you can wash the work with soap and water and rinse it thoroughly with clear water to remove the corrosive residue. To minimize potential damage, clean the work immediately after soldering.
Soldering with coppers and torch soldering are the two most common methods of soldering, and the same considerations apply to both methods.
Use sweat soldering when you need to make a joint but do not want the solder exposed.
You can use this process on electrical and pipe connections (Figure 10).
To make a sweated joint:
Seam soldering involves running a layer of solder along the edges of a joint (Figure 11), on the inside whenever possible. Soldering with coppers is the best method for seam soldering; they provide better heat control and cause less distortion.
To seam solder:
Riveted seams are often soldered to make them watertight. Figure 13 shows the procedure for soldering a riveted seam.
Solder beads, or solder shots, are sometimes used for soldering the bottom of square, rectangular, or cylindrical vessels.
To make solder beads, simply hold the solder against a hot copper and allow the melted beads to drop onto a clean surface (Figure 14).
To solder a bottom seam with solder beads:
To heat and use an electric soldering copper (electric iron), you merely plug it in; otherwise, the procedure is much the same as that just described. Although electric irons have built-in resistors to prevent it, be careful not to let an electric unit overheat. Overheating can burn out the electrical element as well as damage the copper and tinning.
Soldering is more difficult on aluminum alloys than on many other metals because of the layer of oxide that always covers them, and the thickness of the layer will depend on the type of alloy and the exposure conditions. Wrought aluminum alloys are usually easier to solder than cast aluminum alloys, while heat-treated aluminum alloys are extremely difficult to solder, as are aluminum alloys containing more than 1% magnesium. However, you can still successfully solder many aluminum alloys by using proper techniques. Aluminum alloys usually require tin-zinc or tin-cadmium solder alloys, generally called the aluminum solders. Most of these solders have higher melting points than the tin-lead solders used for ordinary soldering, and both corrosive and noncorrosive fluxes are used for soldering aluminum depending on a given situation.
To solder an aluminum alloy:
• Clean surfaces and remove layer of oxide.
o Thick layer — remove mechanically by filing, scraping, sanding, or wire brushing.
o Thin layer —remove by using corrosive flux.
• Apply flux to work and solder.
• Tin surfaces with aluminum solder.
o Use either a soldering copper or torch.
If you use a torch, do not apply heat directly to the work surfaces, solder, or flux. Instead, play the torch on a nearby unsoldered part of the work and let the heat conduct through the metal to the work area.
Do not use any more heat than necessary to melt the solder and tin the surfaces.
• Work aluminum solder well onto surfaces.
• Sweat parts together. For an alternate procedure to solder an aluminum alloy:
• Tin surfaces with aluminum solder.
• Use regular tin-lead solder to join aluminum solder-tinned surfaces. o No need to use flux when using tin-lead solder with aluminum solder.
You can use this procedure when the shape of the parts prevents you from using the sweat method or the task demands a large amount of solder. For both methods, after you complete the soldering, clean with a wire brush, soap and water, or emery cloth to ensure you remove all the flux from the joint; any flux left will cause corrosion.
|Test Your Knowledge
1. Like welding, soldering joins two metals by heating them to their melting points.
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Do you remember the key determining temperature of 800°F? Brazing is the process of joining metal by heating the base metal to a temperature above 800°F and adding a nonferrous filler metal that melts below the base metal’s temperature. Sometimes brazing is called hard soldering or silver soldering because the filler metals are either hard solders or silver-based alloys. Do not confuse brazing with braze welding, though the two terms are often interchanged. In brazing, the filler metal is drawn into the joint by capillary action; in braze welding the filler metal is distributed by tinning. Both processes require distinct joint designs. Like soldering, brazing offers important advantages over some other metal-joining processes such as oxygas welding. It does not affect the heat treatment of the original metal as much as welding, does not warp the metal as much, and allows you to join dissimilar metals.
Brazing requires three basic items: a heat source, filler metal, and flux.
The source of heat depends on the type and amount of brazing required. If you were doing production work and the pieces were small enough, you could put them into a furnace and braze them all at once. Alternatively, you could mount individual torches in groups for assembly line work, or you could use an individual oxyacetylene or MAPPoxygen torch to braze an individual item.
Brazing filler metals are nonferrous metals or alloys with a melting temperature below the base metal, but above 800°F. They must have the ability to wet and bond with the base metal, be stable, and not be excessively volatile.
The most commonly used filler metals for brazing are the silver-based alloys available in rod, wire, powder, and preformed form.
Brazing filler metals include the following groups:
Copper-zinc (brass) alloys
Brazing requires flux to stop any oxides or similar contaminants from forming during the process, and flux increases both the flow of the filler metal and its ability to stick to the base metal. Flux helps form a strong joint by bringing the filler metal into immediate contact with the adjoining base metals and permitting the filler to penetrate the pores of the metal.
Carefully select the flux for each brazing operation; read the manufacturer’s label for the type of metal than can be brazed with the flux. Consider the following three factors:
— Base metal or metals — Brazing filler metal — Source of heat
Flux is available in powder, liquid, and paste form. You can apply the powdered form of flux by dipping the heated end of the brazing rod into the container, allowing the flux to stick to it. Alternatively, you can heat the base metal slightly and sprinkle the powdered flux over the joint, allowing the flux to partly melt and stick. Sometimes you may find it desirable to mix the powdered flux with distilled water to form a paste.
You can apply flux with a brush in either the paste or liquid form, but in either case, you will achieve better results if you give the filler metal a coat also.
The most common type of flux for brazing is borax or a mixture of borax with other chemicals, while some commercial fluxes contain small amounts of phosphorus and halogen salts of iodine, bromine, fluorine, chlorine, or astatine.
When a prepared flux is not available, you can use a mixture of 12 parts borax and 1 part boric acid.
Nearly all fluxes give off fumes that may be toxic. Use them only in WELL VENTILATED spaces.
In brazing, the filler metal is distributed by capillary action. Therefore, the joints must have close tolerances and a good fit prior to brazing in order to produce a strong bond.
Brazing has three basic joint designs: lap, butt, and scarf (Figure 16), but they can be found in flat, round, tubular, or irregular shapes.
The lap joint is one of the strongest and most frequently used joint in brazing, especially in pipe work. Its primary disadvantage is the increased thickness of the final product. For maximum strength, the overlap should be at least three times the thickness of the metal. A 0.001-inch to 0.003-inch clearance between joint members provides the greatest strength with a silver-based filler metal. With such close tolerances for pipe fittings, you need to take precautions to prevent heat expansion from closing joints before the capillary action.
The size of a butt joint is limited to the thinnest section, so maximum joint strength is impossible, but you can maximize the available butt joint strength by maintaining a clearance of 0.001 to 0.003 of an inch in the finished braze. The edges of the joint must be perfectly square to maintain that uniform clearance between all parts of the joint. Butt joints are usually used where it is undesirable to have double thickness.
When double metal thickness is objectionable but you still need more strength, the scarf joint is a good choice. A scarf joint provides an increased bond area without increasing the thickness of the joint. The amount of bond area depends on the angle the scarf is cut; usually, an area two to three times the butt joint area is desirable. A 30° scarf angle gives a bond area twice that of a 90° butt joint, and a 19½° scarf angle increases the bond area three times. Figure 17 shows some variations of butt and lap joints designed to produce good brazing results.
Figure 18 compares some good and bad brazing joint designs and preparations.
The procedure for brazing is very similar to braze welding and oxyacetylene welding. You must clean the metal mechanically, chemically, or with a combination of both to ensure good bonding, fit the two pieces properly, and support them to prevent voids in the joint or accidental movement during your brazing and cooling operations.
Clean the work. The metal surfaces must be clean for capillary action to take place. When necessary and practical, you can chemically clean the surface by dipping it in acid, then remove the acid by washing the surface with warm water. You can use steel wool, a file, or abrasive paper for mechanical cleaning, but do not use an emery wheel or emery cloth; abrasive particles or oil might become embedded in the metal.
Support the work. If the joint moves during the brazing process, the finished bond will be weak and subject to failure, so mount the work in position on firebricks or other suitable means of support, and if necessary, clamp it.
Flux the work (and filler rod). Flux application varies depending on the form of flux you are using and the type of metal you are brazing, but the flux must be suitable for the job. Refer to the previously described material on fluxes and always refer to the manufacturer’s information.
Heat the work. The next step is to heat the parts to the correct brazing temperature. Use a neutral flame; it gives the best results under normal conditions. A reducing flame produces an exceptionally neat-looking joint, but you sacrifice strength; an oxidizing flame produces a strong joint, but you get a rough-looking surface.
Watch the behavior of the flux as you heat it to determine the temperature of the joint. First, the flux dries out as the moisture (water) boils off at 212°F, then it turns milky in color and starts to bubble at about 600°F, and finally it turns into a clear liquid at about 1100°F, just short of brazing temperature.
When the flux appears clear, it is time to start adding the filler metal with the heat of the joint, not the flame, melting the filler metal.
If you have properly aligned the parts and applied the temperature, the filler metal will spread over the metal surface and into the joint by capillary attraction. For good bonding, ensure the filler metal penetrates the complete thickness of the metal.
Figure 19 shows a good position for the torch and filler metal when brazing a butt joint. Note the position is the forehand method, so you are heating the metal ahead of applying the filler metal to the joint.
Stop heating the work. As soon as the filler metal has completely covered the surface of the joint, turn off the torch and let the joint cool slowly. Do not remove the supports or clamps or move the joint in any way until the surface is cool and the filler metal has solidified completely.
Clean the work. Finally, after the joint has cooled sufficiently, clean it; you can do this with hot water. Be sure you remove all traces of flux since it can corrode the metal, and you can file off any excess metal left on the joint.
The procedure described is a general one, but it applies to the three major types of brazing: silver, copper alloy, and aluminum, where the differences lay in the type of base metal, composition of filler metal, and appropriate flux, not in the procedure.
You may be called upon often to do a silver brazing job. For many years, the primary reference standard for silver solders was the American Society for Testing and Materials’ standard ASTM B73-29 Specification for Silver Solders. In 1952, that standard was withdrawn and replaced by ASTM B260-62 Specification for Brazing Filler Metal. However, in 1968 the B260-62 standard was once again withdrawn, this time with no replacement.
Currently, the primary source to access standards for silver-based brazing alloys is the American Welding Society standard AWS 5.8 (Tables 6-2 and 6-3)
Figure 20 shows a common and popular way to apply silver brazing metal on tubing, by using silver alloy rings. This is a practical and economical way to add silver alloy when using a production line system.
Figure 21 shows another method of brazing, by using preplaced brazing shims.
Jobs will vary according to the metal and the dictates of the task, but the experiences will help you become capable of selecting the proper procedure to produce quality brazing.
|Test Your Knowledge
2. Brazing is the process of joining metal by heating the base metal to a temperature below 800°F and adding a nonferrous filler metal that melts below the base metal’s temperature.
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Braze welding (also called bronze welding) is another procedure you can use to join two pieces of metal. It is very similar to fusion welding except you do not melt the base metal and you distribute the filler metal onto the metal surfaces by tinning. Braze welding can produce bonds comparable to those made by fusion welding without the destruction of the base metal characteristics. Advantages of braze welding over fusion welding:
Disadvantages of braze welding compared to fusion welding:
The equipment you need for braze welding is essentially identical to the equipment you need for brazing. However, braze welding usually requires more heat than brazing, so you should definitely use an oxyacetylene or oxy-MAPP torch for braze welding.
Copper and zinc are the primary elements of a braze-welding rod; they provide ductility and high strength. Iron, tin, aluminum, manganese, chromium, lead, nickel, and silicon are also added in small amounts to improve the rod’s welding characteristics. These elements aid in deoxidizing the weld metal, increasing flow action, and decreasing the chances of fuming. Table 4 lists some copper alloy brazing filler metals and their uses. Brass brazing alloy and naval brass are the most commonly used filler rods, but the selection of the proper brazing filler metal always depends on the types of base metals you need to join.
Proper fluxing is as essential in braze welding as it is in the other processes; if the surface of the metal is not clean, the filler metal will not flow smoothly and evenly over the weld area. Even after you have mechanically cleaned the workpiece, certain oxides often remain and interfere with the flow of the filler metal, so always use the correct flux to eliminate them. You can apply flux directly to the weld area, or you can apply it by dipping the heated end of the rod into the flux; once the flux sticks to the rod, you can transfer it to the weld area. Some braze welding rod is also available in a prefluxed form; this eliminates the need to add flux during welding.
Edge preparation is essential in braze welding. You can bevel the edges of thick parts by grinding, machining, or filing, but it is not necessary to bevel thin parts (1/4-inch or less). You need to make the piece bright and clean on the underside as well as on the top of the joint. If you clean with a file, steel wool, or abrasive paper, it will remove most of the foreign matter such as oils, greases, and oxides, and using the proper flux will complete the process to permit the tinning to bond.
After you prepare the work’s edges, use the following steps to braze weld:
More flux during the tinning process produces stronger welds.
See Figure 22 for an example of tinning and welding with the backhand method.
|Test Your Knowledge
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Wearfacing (also called hardfacing, hard-surfacing, resurfacing, or surfacing) is the process you use to apply an overlay to the surface of new or old parts to increase their resistance to abrasion, impact, corrosion, and erosion, or to obtain other properties. It can be used also to build up undersized parts. The goal of wearfacing is to provide an additional means of maintaining sharp cutting edges and reduce wear between metal parts. It is an excellent means for reducing maintenance costs and downtime, thus improving productivity, profitability, efficiency, and longevity of equipment (Figure 23).
Various types of construction equipment use repair and maintenance hardfacing on their leading or wearing edges, and as a Steelworker and one of the Battalion’s metal experts, there will be times when you need to build up and wearface some of that equipment. It could be the cutting edges of scraper or dozer blades, sprocket gears, or shovel and clamshell teeth. You may even get an opportunity to wearface new blades or shovel teeth before they are put into service for the first time.
You can wearface using several different methods (typically it is done by arc welding), but this presentation will cover only the oxygas process of wearfacing. Wearfacing with an oxygas flame is, in many respects, similar to braze welding. The wearfacing metals generally consist of high-carbon filler rods, such as high chromium (Cr) or a chromium-cobalt-tungsten (Cr-Co-W) alloy, but in some instances you may need to use special surfacing alloys. In any of the methods, wearfacing is a process in which a layer of metal of one composition is bonded to the surface of a metal of another composition.
Hardfacing is suitable for all low-carbon alloys and stainless steels as well as Monel and cast iron, although it is not appropriate for aluminum, copper, brass, or bronze, as their lower melting points prohibit using the hard-surfacing process.
You can increase the hardness of aluminum by applying a zinc-aluminum solder to the surface, and you can improve the wear strength of copper, brass, and bronze with an overlay of work-hardening bronze.
You can surface-hardened carbon and alloy tool steels also, but with difficulty due to the frequent development of shrinkage and strain cracks. If you do surface these materials, do so when they are in an annealed condition, not a hardened condition. When necessary, you can heat treat and harden after the surfacing operation, but quench the part in oil, not water.
Using a copper-base alloy filler metal will produce a relatively soft surface. Work hardening bronzes are soft when applied and give excellent resistance against frictional wear. Other types of alloys produce a corrosion- and wear-resistant surface at high temperatures. Many different manufacturers produce wearfacing materials, so be sure the filler alloys you select for a particular hardfacing job meet Navy specs. Two general types of hard-surfacing materials are iron-base alloys and tungsten carbide.
Iron-base alloys are used for a number of applications requiring varying degrees of hardness. They contain nickel, chromium, manganese, carbon, and other hardening elements. welders frequently work with iron-base alloys when building up and resurfacing parts of construction equipment.
Tungsten carbide is one of the hardest substances known to man. You use it to build up wear-resistant surfaces on steel parts. You can apply tungsten carbide in the form of inserts or composite rod. When applied as inserts, they are not melted; instead, they are welded or brazed to the base metal as you saw in Figure 21 with the brazing shims. When you apply it as a rod, you use the same surfacing technique as you use for oxygas welding, but with a slightly carburizing flame.
Like all work with metal, proper surface preparation is an important part of wearfacing operations.
If wear is extensive, you may need to use a buildup rod before adding the wearfacing material. Check with your leading petty officer if you are in doubt about when to use a buildup rod.
Most parts that require wearfacing can be preheated with a neutral welding flame of about 800°F before surfacing. Do not preheat to a temperature higher than the critical temperature of the metal, or to a temperature that can cause scale to form.
In general, for wearfacing you manipulate the torch similar to the technique for brazing but you need higher temperatures (about 2200°F) for wearfacing, so use tips one to two sizes larger than normal and adjust the torch to a carburizing flame.
When you heat steel with a carburizing flame, it turns red first, but as you continue to add heat, the color becomes lighter and lighter until the metal attains a bright whiteness. Sweating occurs when you heat the steel with a carburizing flame to this white heat temperature. It carburizes an extremely thin layer of the base metal, approximately 0.001 inch thick.
The carburized layer has a lower melting point than the base metal, and as a result, it becomes a liquid, while the underlying metal remains a solid. This thin liquid film provides the medium to flow the filler metal over the surface of the base metal. It is similar to, and serves the same purpose as, a tinned surface in soldering and braze welding.
Surfacing alloy added at this time flows over the sweated surface and absorbs the film of carburized metal. This surface condition is not difficult to recognize, but you should make several practice passes before you try your first wearfacing.
If you use an oxygas torch for surfacing with chromium cobalt (Cr-Co), you need to adjust the torch flame to have an excess fuel-gas feather (carburizing flame) about three times as long as the inner cone. If you do not use a carburizing flame, you will not be able to develop the base metal surface properly to a condition that will allow the surfacing alloy to spread over the surface of the part.
\Hardfacing, whether applied by oxygas or arc welding, can include a number of configurations depending on the environmental conditions the equipment is expected to work in. Figure 24 shows a few standard patterns along with their intended working environments.
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This course presented information on four processes for joining metals without fusion: soldering, brazing, braze welding, and wearfacing. Each method has its own unique application depending on the metal and the task. Your responsibility as one of the unit’s metal experts will be to know which process will best accomplish the given task, and then be able to apply the process. Take whatever opportunities you can to practice these (as well as other) processes to develop your “hands-on” skills; a well rounded welder is a valued asset in the labor force.
1. The soldering, brazing, braze welding, and wearfacing processes allow the joining of dissimilar metals, produce high strength joints, and do not affect heat treatment or warp the original metal as much as conventional fusion welding.
2. Below what temperature does the soldering process join metals by melting filler metal?
3. Which type of soldering coppers (irons) is used for soldering flat seams requiring considerable heat?
4. The size designation of soldering coppers refers to the weight of two copper heads in _____.
5. A pair of coppers has a weight designation of 3 pounds. This designation indicates that each individual copper weighs _____.
6. What type of file should you use to file a soldering copper head during the filing and tinning process?
7. What should you do to carry out the preliminary steps in filing a cold, but once overheated soldering copper head?
8. What technique should you use to manipulate a file?
9. In the forging process of reshaping a copper, you should ensure a sharp point and a long taper are created.
10. Most solder alloys consist of _____.
11. Which solder has the lowest melting point?
12. What term describes the point in an alloy system when all the elements of the alloy melt at the same temperature?
13. What solder composition is best for joining aluminum alloys?
14. Which purpose does flux serve?
15. Which flux is used to solder galvanized iron?
16. What is the most commonly used noncorrosive flux?
17. Which action should you make a practice when heating solder or surfaces to be soldered?
18. Heating solder to a temperature higher than its working temperature increases oxidation and changes the proportions of tin and _____.
19. What action should you take immediately after finishing the soldering when you use a corrosive flux?
20. How should you manipulate the copper when soldering seams that are held together by rivets or other fasteners?
21. Which action should you take when soldering a bottom seam using solder beads?
22. Which action should you take before soldering an aluminum alloy?
23. A thick layer of oxides is present on the piece of aluminum you are going to solder. Which cleaning method can you use to remove the oxides?
24. Toward what location should you direct the torch flame when soldering aluminum with a torch?
25. What type of solder does NOT require the use of flux when you are using it in combination with aluminum solder?
26. What type of solder is recommended for food containers?
27. What process is used to join two base metals together by using a filler metal such as a hard solder?
28. What action in brazing distributes the filler metal to the joint?
29. One of the advantages that brazing or braze welding has over oxygas welding is that you can use it to join dissimilar metals.
30. Which function is NOT served by the use of flux in brazing operations?
31. What type of application should be used with paste or solution fluxes to ensure a uniform coating on metals to be brazed?
32. What chemical mixture should you use for brazing when a prepared flux is not available?
33. For which reason should you maintain a clearance of 0.001 inch to 0.003 inch when lap joining two base metals with silver-based brazing filler metal?
34. A scarf of 19 1/2° produces a bond area _____ times greater than that of a 90° butt joint.
35. What condition will result if there is any movement of the base metal while you are brazing?
36. To what temperature do you heat two pieces of base metal before adding the filler metal when brazing or braze welding?
37. With what kind of flame from an oxygas torch should you obtain the heat needed to braze or braze weld?
38. Which tool should NOT be used to clean base metals mechanically before brazing or braze welding?
39. What is the primary reason you must remove all traces of flux after brazing?
40. Braze welding often produces bonds that are comparable to those made by fusion welding without the destruction of the base metal characteristics.
41. What type of welding has the disadvantages of loss of strength when subjected to high temperatures and an inability to withstand high stresses?
42. You can braze tubing by using a filler-metal rod or what type of rings?
43. What is the next step in braze welding after you have cleaned, aligned, clamped, or tack-welded the base metals?
44. When using a pre-fluxed braze welding rod, you do NOT have to add flux during welding.
45. What condition has developed in braze welding if the filler metal forms little balls and runs off the metal?
46. Which is NOT a purpose of wearfacing?
47. The two types of hard-surfacing materials in general use by the Navy are _____.
48. Before commencing wearfacing procedures, you must remove scale, rust, and foreign matter from the metal surfaces.
49. With (a) what type of flame and (b) at what temperature do you preheat parts that require wearfacing?
50. What type of flame do you use in wearfacing to heat the steel to a white heat temperature for sweating?
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