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. Brazing should not be confused with braze welding, even though these two terms are often interchanged. In brazing, the filler metal is drawn into the joint by capillary action and in braze welding it is distributed by tinning. Brazing is sometimes called hard soldering or silver soldering because the filler metals are either hard solders or silver-based alloys. Both processes require distinct joint designs.
Brazing offers important advantages over other metal-joining processes. It does not affect the heat treatment of the original metal as much as welding does, nor does it warp the metal as much. The primary advantage of brazing is that it allows you to join dissimilar metals.
Brazing requires three basic items. You need a source of heat, filler metals, and flux. In the following paragraphs these items are discussed.
Heating Devices T
he source of heat depends on the type and amount of brazing required. If you are doing production work and the pieces are small enough, they can be put into a furnace and brazed all at once. Individual torches can be mounted in groups for assembly line work, or you can use individual oxyacetylene or Mapp-oxygen torches to braze individual items.
Filler metals used in brazing are nonferrous metals or alloys that have a melting temperature below the adjoining base metal, but above 800°F. Filler metals must have the ability to wet and bond with the base metal, have stability, and not be excessively volatile.
The most commonly used filler metals are the silver-based alloys. Brazing filler metal is available in rod, wire, preformed, and powder form.
Brazing filler metals include the following eight groups:
- Silver-base alloys
- Aluminum-silicon alloys
- Copper-zinc (brass) alloys
- Copper-phosphorus alloys
- Gold alloys
- Nickel alloys
- Magnesium alloys
Brazing processes require the use of a flux. Flux is the substance added to the metal surface to stop the formation of any oxides or similar contaminants that are formed during the brazing process. The flux increases both the flow of the brazing filler metal and its ability to stick to the base metal. It forms a strong joint by bringing the brazing filler metal into immediate contact with the adjoining base metals and permits the filler to penetrate the pores of the metal.
You should carefully select the flux for each brazing operation. Usually the manufacturer’s label specifies the type of metal to be brazed with the flux. The following factors must be considered when you are using a flux:
- Base metal or metals used
- Brazing filler metal used
- Source of heat used
Flux is available in powder, liquid, and paste form. One method of applying the flux in powdered form is to dip the heated end of a brazing rod into the container of the powdered flux, allowing the flux to stick to the brazing rod. Another method is to heat the base metal slightly and sprinkle the powdered flux over the joint, allowing the flux to partly melt and stick to the base metal. Sometimes, it is desirable to mix powdered flux with clean water (distilled water) to form a paste.
Flux in either the paste or liquid form can be applied with a brush to the joint. Better results occur when the filler metal is also given a coat.
The most common type of flux used is borax or a mixture of borax with other chemicals. Some of the commercial fluxes contain small amounts of phosphorus and halogen salts of either iodine, bromine, fluorine, chlorine, or astatine. When a prepared flux is not available, a mixture of 12 parts of borax and 1 part boric acid may be used.
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. This requires the joints to have close tolerances and a good fit to produce a strong bond. Brazing has three basic joint designs (fig. 6-13): lap, butt, and scarf. These joints can be found in flat, round, tubular, or irregular shapes.
Figure 6-13.—Three types of common joint designs for brazing.
The lap joint is one of the strongest and most frequently used joint in brazing, especially in pipe work The primary disadvantage of the lap joint is the increase in 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 the joint members provides the greatest strength with silver-based brazing filler metals. You should take precautions to prevent heat expansion from closing joints that have initial close tolerances.
Butt joints are limited in size to that of the thinnest section so maximum joint strength is impossible. Butt joint strength can be maximized by maintaining a joint 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 a uniform clearance between all parts of the joint. Butt joints are usually used where the double thickness of a lap joint is undesirable. When double-metal thickness is objectionable and you need more strength, the scarf joint is a good choice.
A scarf joint provides an increased area of bond without increasing the thickness of the joint. The area of bond depends on the scarf angle cut for the joint. Usually, an area of bond two to three times that of a butt joint is desirable. A scarf angle of 30 degrees gives a bond area twice that of a 90-degree butt joint, and an angle of 19 1/2 degrees increases the bond area three times.
Figure 6-14 shows some variations of butt and lap joints designed to produce good brazing results. A comparison of good and bad designed joints is shown in figure 6-15.
Figure 6-14.—Joints designed to produce good brazing results.
Figure 6-15.—Some well-designed joints that have been prepared for brazing, and some poorly designed joints shown for comparison
The procedure for brazing is very similar to braze and oxyacetylene welding. The metal needs to be cleaned by either mechanical, chemical, or a combination of both methods to ensure good bonding. The two pieces must befitted properly and supported to prevent voids in the joint or accidental movement during brazing and cooling operations.
The surfaces of the metal must be cleaned for capillary action to take place. When necessary, chemically clean the surface by dipping it in acid. Remove the acid by washing the surface with warm water. For mechanical cleaning, you can use steel wool, a file, or abrasive paper. Do not use an emery wheel or emery cloth, because abrasive particles or oil might become embedded in the metal.
Mount the work in position on firebricks or other suitable means of support, and if necessary, clamp it. This is important because if the joint moves during the brazing process, the finished bond will be weak and subject to failure.
The method of application varies, depending upon the form of flux being used and the type of metal you are brazing. Refer to the material on fluxes previously described. It is extremely important that the flux is suitable for your job.
The next step is to heat the parts to the correct brazing temperature. Adjust the torch flame (oxygas) to a neutral flame because this flame gives the best results under normal conditions. A reducing flame produces an exceptionally neat-looking joint, but strength is sacrificed. An oxidizing flame will produce a strong joint but it has a rough-looking surface.
The best way to determine the temperature of the joint, as you heat it, is by watching the behavior of the flux. The flux first dries out as the moisture (water) boils off at 212°F. Then the flux turns milky in color and starts to bubble at about 600°F. Finally, it turns into a clear liquid at about 1100°F. That is just short of the brazing temperature. The clear appearance of the flux indicates that it is time to start adding the filler metal. The heat of the joint, not the flame, should melt the filler metal. When the temperature and alignment are proper, the filler metal spreads 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 6-16 shows a good position for the torch and filler metal when brazing a butt joint.
Figure 6-16.—Brazing a butt joint.
Stop heating as soon as the filler metal has completely covered the surface of the joint, 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 completely solidified.
Finally, clean the joint after it has cooled sufficiently. This can be done with hot water. Be sure to remove all traces of the flux because it can corrode the metal. Excess metal left on the joint can be filed smooth.
The above described procedure is a general one, but it applies to the three major types of brazing: silver, copper alloy, and aluminum. The differences being the base metals joined and the composition of the filler metals.
Often, you will be called on to do a silver brazing job. Table 6-2 lists different types of silver brazing alloys and their characteristics. A popular way to apply silver brazing metal on a tubing is to use silver alloy rings, as shown in figure 6-17. This is a practical and economical way to add silver alloy when using a production line system. Another method of brazing by using preplaced brazing shims is shown in figure 6-18. The requirements of each job varies; however, through experience you can become capable of selecting the proper procedure to produce quality brazing.
Table 6-2.—Silver Brazing Filler Metal Alloys
Figure 6-17.—Silver-brazed joints designed to use preplaced silver alloy rings. The alloy forms almost perfect fillets, and no further finishing is necessary.
Figure 6-18.—A machining tool bit showing how the carbide insert is brazed to the tool bit body using preplaced brazing filler metal shims.
Braze welding is a procedure used to join two pieces of metal. It is very similar to fusion welding with the exception that the base metal is not melted. The filler metal is distributed onto the metal surfaces by tinning. Braze welding often produces bonds that are comparable to those made by fusion welding without the destruction of the base metal characteristics. Braze welding is also called bronze welding.
Braze welding has many advantages over fusion welding. It allows you to join dissimilar metals, to minimize heat distortion, and to reduce extensive pre-heating. Another side effect of braze welding is the elimination of stored-up stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures and the inability to withstand high stresses.
The equipment needed for braze welding is basically identical to the equipment used in brazing. Since braze welding usually requires more heat than brazing, an oxyacetylene or oxy-mapp torch is recommended.
The primary elements of a braze welding rod are copper and zinc. These elements improve ductility and high strength. Small amounts of iron, tin, aluminum, manganese, chromium, lead, nickel, and silicon are also added to improve the welding characteristics of the rod. They aid in deoxidizing the weld metal, increasing flow action, and decreasing the chances of fuming. Table 6-3 lists some copper alloy brazing filler metals and their use. The most commonly used are brass brazing alloy and naval brass. The selection of the proper brazing filler metal depends on the types of base metals.
Table 6-3.—Copper Alloy Brazing Filler Metals
Proper fluxing is essential in braze welding. If the surface of the metal is not clean, the filler metal will not flow smoothly and evenly over the weld area. Even after mechanical cleaning, certain oxides often remain and interfere with the flow of the filler metal. The use of the correct flux eliminates these oxides.
Flux may be applied directly to the weld area, or it can be applied by dipping the heated end of the rod into the flux. Once the flux sticks to the rod, it then can be transferred to the weld area. A prefluxed braze welding rod is also available, and this eliminates the need to add flux during welding.
Braze Welding Procedures
Edge preparation is essential in braze welding. The edges of the thick parts can be beveled by grinding, machining, or filing. It is not necessary to bevel the thin parts (one-fourth inch or less). The metal must be bright and clean on the underside as well as on the top of the joint. Cleaning with a file, steel wool, or abrasive paper removes most foreign matter such as oil, greases, and oxides. The use of the proper flux completes the process and permits the tinning to occur.
After you prepare the edges, the parts need to be aligned and held in position for the braze welding process. This can be done with clamps, tack welds, or a combination of both. The next step is to preheat the assembly to reduce expansion and contraction of the metals during welding. The method you use depends upon the size of the casting or assembly.
Once preheating is completed, you can start the tinning process. Adjust the flame of the torch to a slightly oxidizing flame and flux the joint. Through experience, you will find that the use of more flux during the tinning process produces stronger welds. Apply heat to the base metal until the metal begins to turn red. Melt some of the brazing rod onto the surface and allow it to spread along the entire joint. You may have to add more filler metal to complete the tinning. Figure 6-19 shows an example of tinning being used with the backhand method of welding.
Figure 6-19.—Braze welding cast iron, using the backhand method.
Temperature control is very important. If the base metal is too hot, the filler metal bubbles or runs around like beads of water on a hot pan. If the filler metal forms little balls and runs off the metal, then the base metal is too cold.
After the base metal is tinned, you can start adding beads of filler metal to the joint. Use a slight circular motion with the torch and run the beads as you would in regular fusion welding. As you progress, keep adding flux to the weld. If the weld requires several passes, be sure that each layer is fused into the previous one.
After you have completed the braze welding operation, heat the area around the joint on both sides for several inches. This ensures an even rate of cooling. When the joint is cold, remove any excess flux or any other particles with a stiff wire brush or steel wool.
WEARFACING is the process you use to apply an overlay of special ferrous or nonferrous alloy to the surface of new or old parts. The purpose is to increase their resistance to abrasion, impact, corrosion, erosion, or to obtain other properties. Also, wearfacing also can be used to build up undersized parts. It is often called hard-surfacing, resurfacing, surfacing, or hardfacing.
As a welder, there are times when you are required to build up and wearface metal parts from various types of construction equipment. These parts include the cutting edges of scraper or dozer blades, sprocket gears, and shovel or clamshell teeth. You may even wearface new blades or shovel teeth before they are put into service for the first time. There are several different methods of wearfacing; however, in this discussion we only cover the oxygas process of wearfacing.
Wearfacing provides a means of maintaining sharp cutting edges and can reduce wear between metal parts. It is an excellent means for reducing maintenance costs and downtime. These and other advantages of wearfacing add up to increased service life and high efficiency of equipment.
Wearfacing with the oxygas flame is, in many respects, similar to braze welding. The wearfacing metals generally consist of high-carbon filler rods, such as high chromium or a Cr-Co-W alloy, but, in some instances, special surfacing alloys are required. In either event, wearfacing is a process in which a layer of metal of one composition is bonded to the surface of a metal of another composition.
The process of hard-surfacing is suitable to all low-carbon alloy and stainless steels as well as Monel and cast iron. It is not intended for aluminum, copper, brass, or bronze, as the melting point of these materials prohibits the use of the hard-surfacing process. It is possible to increase the hardness of aluminum by applying a zinc-aluminum solder to the surface. Copper, brass, and bronze can be improved in their wear ability by the overlay of work-hardening bronze. Carbon and alloy tool steels can be surface-hardened, but they offer difficulties due to the frequent development of shrinkage and strain cracks. If you do surface these materials, they should be in an annealed, and not a hardened condition. When necessary, heat treating and hardening can be accomplished after the surfacing operation. Quench the part in oil, not water.
A surfacing operation using a copper-base alloy filler metal produces a relatively soft surface. Work-hardening bronzes are soft when applied and give excellent resistance against frictional wear. Other types of alloys are available that produce a surface that is corrosion and wear resistant at high temperatures. Wearfacing materials are produced by many different manufacturers; therefore, be sure that the filler alloys you select for a particular surfacing job meet specifications.
Two types of hard-surfacing materials in general use are iron-base alloys and tungsten carbide.
These materials contain nickel, chromium, manganese, carbon, and other hardening elements. They are used for a number of applications requiring varying degrees of hardness. A welder frequently works with iron-base alloys when he builds up and resurfaces parts of construction equipment.
You use this for building up wear-resistant surfaces on steel parts. Tungsten carbide is one of the hardest substances known to man. Tungsten carbide can be applied in the form of inserts or of composite rod. Inserts are not melted but are welded or brazed to the base metal, as shown in figure 6-18. The rod is applied with the same surfacing technique as that used for oxygas welding; a slightly carburizing flame adjustment is necessary.
Proper preparation of the metal surfaces is an important part of wearfacing operations. Make sure that scale, rust, and foreign matter are removed from the metal surfaces. You can clean the metal surfaces by grinding, machining, or chipping. The edges of grooves, corners, or recesses should be well rounded to prevent base metal overheating and to provide a good cushion for the wearfacing material.
Wearfacing material is applied so it forms a thin layer over the base metal. The thickness of the deposit is usually from one sixteenth to one eighth of an inch and is seldom over one fourth of an inch. It is generally deposited in a single pass. Where wear is extensive, it may become necessary to use a buildup rod before wearfacing. If in doubt as to when to use a buildup rod, you should check with your supervisor..
Most parts that require wearfacing can be preheated with a neutral welding flame before surfacing. You should use a neutral flame of about 800°F. Do not preheat to a temperature higher than the critical temperature of the metal or to a temperature that can cause the formation of scale.
In general, the torch manipulations and the wearfacing procedures are similar to brazing techniques. However, higher temperatures (about 2200°F) are necessary for wearfacing, and tips of one or two sizes larger than normal are used.
To begin, you heat a small area of the part with a sweeping torch movement until the surface of the base metal takes on a sweating or wet appearance. When the surface of the base metal is in this condition, bring the end of the surfacing alloy into the flame and allow it to melt. Do not stir or puddle the alloy; let it flow. When the surface area has been properly sweated, the alloy flows freely over the surface of the base metal.
Being able to recognize a sweated surface is essential for surfacing. Sweating occurs when you heat the steel with a carburizing flame to a white heat temperature. This 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. As a result, it becomes a liquid, while the underlying metal remains a solid. This liquid film provides the medium for flowing the filler metal over the surface of the base metal. The liquid film is similar to and serves the same purpose as a tinned surface in soldering and braze welding.
When you heat steel with a carburizing flame, it first becomes red. As heating continues, the color becomes lighter and lighter until a bright whiteness is attained. At this point, a thin film of liquid, carburized metal appears on the surface. 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 wearfacing for the first time.
When you use an oxygas torch for surfacing with chromium cobalt, the torch flame should have an excess fuel-gas feather about three times as long as the inner cone. Unless the excess fuel-gas flame is used, the proper base metal surface condition cannot be developed. Without this condition, the surfacing alloy does not spread over the surface of the part.
Figure 6-20 shows a grader blade with a deposit of hardfacing material applied along the cutting edge. A grader blade is usually wearfaced by the electric arc process. If the electric arc process is not available, you may use the oxygas torch.
Figure 6-20.—Grader blade with hardfacing material applied to cutting edge.