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42. Brazing Process Overview
Definitions Brazing is the joining of two dissimilar materials using a third material. Soldering and brazing are the same thing except the soldering takes place below 840 F and brazing takes place above 840F.
Pretinning is the application of braze alloy to one part, usually the tungsten carbide part, which is then brazed onto the steel part. It is done this way because the tungsten carbide is harder to braze, it is much smaller and most shops are not set up to do this kind of small work. It is also much harder to get a good join to the tungsten carbide than it is to the steel.
Process Tungsten Carbide is brazed to steel to make saws, other tools and a whole variety of machine components called wear parts.
Saws and tools were originally steel and a lot of steel saws and drills are still sold.
Tungsten carbide is put on the tips or points of tools because it is very hard and stays sharper, longer than steel. It is also used in areas where abrasion is a problem. Tungsten carbide plates are used to line hoppers in cotton gins because the raw cotton will wear steel.
The tungsten carbide tip can be attached to the steel tool by brazing. It is also possible to weld the materials and mechanical fastening is used where suitable. In saws and drills the tips are brazed where stability of the tip is needed and room is limited. Mechanical fastening is used in machining operations to allow for changing of inserts. Welding is used on band saws where a very small piece of carbide is attached to a large piece of steel.
Brazing can have real advantages over other forms of material joining. It used to be thought that brazing a piece of carbide to a steel holder or saw body could damage the carbide. Originally braze alloy was seen as just something to join the materials. Research we did in the 1980’s showed that the alloy also has a definite effect in preventing breakage.
Good Braze Joints Form a Composite All three components were important. All three parts (steel, alloy and carbide) actually form a composite material that has different properties than the individual components. This composite could create a lot more strength and impact resistance in the final assembly than was possible before. In this case a saw tip on a properly brazed saw will survive a great deal more impact force than a saw tip on an anvil will.
Four parts to brazing Brazing carbide has four parts: the carbide, the braze alloy, the steel tool body and the process of putting it together.
The Carbide Carbide is a hard thing to make well. Some companies produce top quality parts batch after batch after batch. Other companies sometimes get it right and sometimes have problems.
Poor quality carbide may have contaminants that make it weak. It can be stressed which makes it brittle. It can have surface contaminants, which can make it hard to braze. The size may also vary which causes problems with grinding.
Preparing the carbide An essential step in brazing carbide is to make sure the carbide is clean and that the surface is prepared properly. You need a clean, prepared surface. You can put braze alloy on untreated carbide the same way you can paint over rust or dirt. It works better is you have a clean surface. The surface must be cleaned, prepared and protected to make it suitable for use in today’s top quality tools.
The Braze Alloy Brazing is like soldering. It uses a material to join two different dissimilar materials. Brazing is a high temperature process. At these temperatures and with good alloys you actually create new metallic alloys that join the parts together.
There are many braze alloys. In carbide brazing the alloys are typically high silver with copper, zinc, nickel and some other metal in them. The standard alloy used to be 50% silver with Cadmium. There are two problems with that. Cadmium will kill you and it does not take very much to do it. The government is coming down harder all the time on Cadmium contamination.
There are various Cadmium-free braze alloys. There is a 50% silver alloy without Cadmium that works pretty well but not as well as the Cadmium alloy. There is an alloy that is 56% silver with tin that seems to work well for some people but not for others. The best available alloy now seems to be 49% silver with Manganese.
It can make a tremendous difference which alloy you use even though some suppliers will tell you it does not.
The steel body The steel body must be very clean. The big problems are oil and greases. The residue from the gumming wheel can also cause a problem. The best way to do this is to buy plate from a good supplier or to make sure that your plate shop is producing clean plate.
The process Brazing is a complex operation. Here is a partial list of things that have caused problems over the last fifteen years.
Cleanliness of the plate Cleanliness of the tip Flux clean and stirred Flux on the sides of the plate Lots of flux inside the joint Acetylene or gas Oxygen Regulators Hoses Handles Handle controls Torch tips Tightness of joints Temperature of anvils Temperature of the shop New furnaces New fans New brazing locations New doors New brazer Proper pretinning Right kind of solder Temperature of the tips Kind of flux Condition of flux Amount of flux used Flux on plate Flux on tips Brazing temperature Brazing time Color of braze joint Sound of braze joint Feel of braze joint Where is heat being applied How is heat being applied Time of day Kind of plate Tip manufacturer Dirty carbide Bad carbide treatment
Brazing Tungsten Carbide for the First Time The surface condition of tungsten carbide can make the difference between parts that are joined with a strength of 100,000 psi. and parts that fall off by themselves. If it is a big part such as for a snow plow blade then you can probably use whatever you buy. If it is a small part such as a saw tip then you need to make sure the surface is clean and ready for brazing. The best way to determine this is to use a tungsten carbide supplier that can tell you about the surface condition of your parts. Often good tungsten carbide comes ready to braze from the manufacturer.
Do not get the tungsten carbide oily or greasy. Make sure the steel is clean also. Do not heat either part without a protective flux coating.
Flux Braze alloy joins to metal. Metal oxidizes faster if it is hot. If you heat metal without flux you will form an oxide layer that is similar to rust. Brazing over oxide is like painting over rust.
Braze alloy Lots of choices. The safest to use is a pretinned tip or trimetal also called plymetal or sandwich. It comes as ribbon. It is flat and has unique properties that relieve stress caused by overheating. It is a tremendous aid when heating large parts or working with a torch. It is more expensive than wire. If you are starting and doing just a few parts this is safer and probably easier. 0.15" (15 thousandths) is about as thick a ribbon as you will need.
Coat the steel with a light layer of Black Flux. (Black flux has more Boron than White Flux and works longer at higher temperatures.)
Put down your piece of braze alloy cut to shape and coat with a light layer of flux.
Put the clean tungsten carbide on top of the flux layer and coat the outside with a thick layer of flux.
Heat the whole part as evenly as possible until the whole joint is at the proper temperature. This is usually a dark cherry red. Remember that the part has to be hot enough all the way through. This can be a problem even on the parts as small as saw tips.
Once the parts are hot enough you will see the alloy flow out of the joint. You may also feel the part move just a bit as the alloy turns liquid. Once the part is the right color (dark cherry red to cherry red) and the alloy is coming out just a bit (and it may feel mushy), then wiggle the part just a little bit to allow any flux or fumes to escape. If you work the part too hard you can force all the braze alloy out of the joint and ruin it.
The flux cleans off with warm water. You can use mild brushing if you wish to speed the process.
The Brazing Process
How the heat enters the joint can be very important.
Brazing tungsten carbides can be chemically tricky but the process of brazing pretinned tips to steel is relatively simple. If a few basic steps are followed it will be possible to eliminate tip loss forever.
You must start with top quality pretinned tips. The braze alloy should flow evenly from corner to corner. It should cover the top without running over the sides. Depending on the material there may be a bit of bleed or color showing on the sides. A good pretinned tip should be a uniform color. If there is any sort of trace of any other color the tips should be rejected. Other colors come from the solder being burnt during the pre-tinning and can weaken the solder joint. Each tip should have an identical amount of solder. An average solder depth of .010” seems to work the best.
Flux protects and cleans the surfaces to be brazed. The flux that we have found to be best is called Black Flux. Use the flux properly. There is generally a tendency to use too little rather than too much. As a general rule of thumb there should be enough flux so that you can’t see the steel plate showing through the flux layer.
The torch flame is important both as a source of heat and as a way to protect the material. The flame needs to be a reducing flame, which means it, is a little oxygen starved.
An oxygen/acetylene ratio where the acetylene is higher is the best for several reasons. First, all the oxygen is consumed. Secondly, if the ratio should slip because of regulator or other pressure problems there is still a safety margin. Third, if there is a breeze in the room there is protection from excess oxygen. This problem with room oxygen can occur if the flame used is a relatively small flame and if there are air currents in the room such as might occur from heating and ventilating units or doors and windows. The suggestion has been made that the ratio of acetylene to oxygen might be as high as 6 acetylene to 1 (6:1) oxygen although this is generally considered extreme. A ratio of one to one (1:1) or one point two to one (1.2: 1) is much more common. It can be important to have a vigorous torch flame so that the flame is as much as eight inches long with an inner flame of up to three inches. Brazing at the tip of the white part of the flame is suggested.
The brazing temperature and how it is reached are both extremely important. The solder flows over a definite range. If the solder is either too cold or too hot then it will not form a good bond. This is such a critical point that I have devoted a separate article to it. The article is called “How to Tell Braze Temperature by the Feel of the Solder” and immediately follows this.
The way that the heat is supplied to the brazing operation can also be extremely important. It has been suggested that instead of pre-heating the plate, the pre-tinned tip should be pre-heated. The tip to be inserted is placed approximately 1/2 inch from the plate. The torch is held so that the hand holding the torch is over the plate and the flame is pointing out, away from, and over the plate. The tip is pre-heated until the solder starts to flow. Then the tip is slid into the notch and smoothly slid up the notch with the torch. The torch is centered in the middle of the tip and the operator’s hand is still over the plate so the torch is pointed out and away from the plate. Once the tip has been slid about 2/3 of the way up then it should be repositioned (slid back down) in the notch.
An experienced operator will feel the tip sliding into place on its own due to the effects of capillary action. Once the tip is in place pressure should be kept on it so as to keep it in place while the torch is pulled away in a direction that is down and over the gullet. Keep pressure on the tip until it cools but only enough pressure so as to keep the tip in place. The pointer or positioning stick or sharpened file should exert only enough pressure to keep the tip from moving. It should not push the tip in but only keep the tip from being pushed out.
Be very gentle. Pre-heat the tip or the alloy but not the plate. Pre-heat the tip until the solder starts to flow. Be very gentle when inserting the tip so that all the solder is not forced out the sides.
WARNING: There can be a tendency to want to check the quality of the braze by testing the tip before it is fully cool. The inside of the solder joint can stay somewhat fluid or “plastic” for quite a while as the joint cools. Do not test for braze strength in any way until the joint is fully cool.
Quality checkpoint: The thickness of the shoulder and the tip after brazing should measure from .003 to .005 inches greater than the thickness of the shoulder and the tip before brazing. The tip must not be pushed in so hard that it is directly contacting the steel. There must be a layer of .003 to .005 inches of solder in between the steel and the tungsten carbide. If this layer is too thick or too thin then the joint loses a lot of strength. In addition a too-thin joint will not provide enough cushion to provide shock absorption during any impact.
The final consideration is the pressure that is used to seat the tip. A well-trained brazer will make sure the tip is seated firmly and the torch is drawn away slowly enough to do a very good job of brazing. The amount of pressure needed to properly seat tips becomes smaller as the tips become narrower (thinner kerf). There can be problems caused if the amount of pressure used to seat the thinner tips is the same as with the wider tips. When the same pressure is used on all width tips the psi. (Pounds per square inch) stays the same as the square inch becomes smaller so there is greater pressure forcing the solder out the side. Also in the smaller kerf tips the solder doesn’t have as far to go to be squeezed out the side. It is easier to force the solder out from under the smaller kerf tip so it is more likely that there won’t be enough solder left to give a good braze tip. In addition, the smaller kerf tip cools more rapidly than a larger kerf tip just because it is smaller in size.
The single greatest cause of braze failure from brazing is the fact that the saw tip is pushed in too hard and all the silver solder is forced out the sides. There needs to be .003 to .005 inches of silver solder between the tungsten carbide tip and the saw plate. Anything greater or thinner will drastically weaken the braze joint.
The brazer applies heat until the solder hits the flow point. The brazer removes the heat as soon as the solder has flowed out of the joint and onto the saw.
In good operations, the torch brazer is well trained and extremely conscientious about the amount of heat he is supplying. The torch heat travels very rapidly in the steel saw body and will affect the character of the saw plate. Any excess heat will dramatically increase the chances of a soft shoulder that will rip loose when the saws are run in the mill.
Once the torch is removed the heating stops immediately and the plate begins to cool very quickly. This process is sensitive enough that the flow is distinctly different on the top of the plate where the heat is applied from the bottom of the plate.
The point to be made here is that the brazing process is a manual process but it is extremely precise and the time at full brazing temperature is very short. Considerably less than a second. This is important because part of successful brazing is to hold the materials at temperature long enough to form an intermediate compound in a layer between the two materials and the braze alloys.
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