Differences in Braze Alloys

Effects Of Different Braze Alloys

In the late 1980s the government really cracked down on cadmium in braze alloy.  The Bag-3 braze alloy had been the standard.  The industry switched to a 50% silver braze alloy without cadmium and immediately saw a dramatic increase in both tip loss and carbide tip breakage in sawmill saws and other tools.  Carbide Processors Inc. designed a series of experiments.   With the cooperation of Cascade Southern Saw Company and SystiMatic Saw Company a series of sharpie impact tests were run at the Weyerhaeuser Tech Center in Federal Way,Washington under the direction of Don Anderson who was then Head Of Cutting Tool Research And Development.  The incidence of tip loss and tip breakage, and / or the energy required to affect the same indicated that the Bag-24 braze alloy made braze joints that were about 30 to 40% more likely to fail than the Bag-3.  Bag-22 braze alloy was also tested and found to be equivalent to the Bag-3.  BAg-7 was found to be roughly about half as strong as Bag-3 or Bag-22.  We use Bag-22 Braze alloy as a standard for most of our Pretin services. 

 

In the mid-1990s, George Bellwoar of Engelhard did some work on braze alloy and tungsten carbide/braze alloy/steel joints considered as a single composite structure.

 

We did some testing on the effects of pretinning of tungsten carbide. We used a drop weight impact tester.  We tried tests three ways.  We took carbide parts with braze alloy on one side.  We dropped the weight directly on the side with the braze alloy and it was much harder to break the carbide parts that I was to break identical carbide parts with no braze alloy.  The same results occurred when we put the braze alloy side against the table and dropped the weight on the opposite side.  Surprisingly enough, we also saw similar results when the braze alloy was not directly involved in the impact path.  That is, we laid the tungsten carbide part with the braze alloy on one side so that the weight directly impacted the carbide and the carbide was resting against the steel with no cushioning braze alloy layer.

 

So there seems to be good evidence that braze alloy somehow affects the way that stress, strain, and force is handled in an assembly of tungsten carbide brazed to steel.

 

Why tungsten carbide fails

The first and most obvious consideration is the amount of energy trapped in the assembly by the differences in coefficients of thermal expansion between the tungsten carbide and the steel.

 

There is also, perhaps, a cushioning effect provided by the braze alloy. This cushion is thought to absorb impact energy.  The impact energy of the tool in use is transmitted through the tungsten carbide and absorbed in the softer braze alloy.

 

There is research that shows that the mode of failure for tungsten carbide is not compression but rather tension. In use this would mean that the carbide does not break when it hits something, but that the breakage occurs after the force is released and the carbide springs back.  It is a basic design principle when dealing with tungsten carbide that it is extremely strong in compression and very weak in tension.  I would hypothesize that a major part of this is the fact that tungsten carbide, as it is commonly used, actually applies to a composite of tungsten carbide grains cemented together with cobalt. Cobalt is a pretty brittle metal.

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