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.