Braze alloy and what it does…

What Does Braze Alloy Do?

 The immediate topic is brazing tungsten carbide to steel specifically for use in tools such as saw blades, router bits, etc.

 

Bonding Properties

30 years ago I was taught that braze alloy was sort of like a hot melt glue in that the bonding strength was based on purely physical bonding.  The next step was to treat braze alloy in terms of chemical bonding. This seems to be much more accurate in that a physically rough surface which is not chemically clean will bond very weakly if at all.

 

Handling Stress

After considering braze alloy as a bonding agent the next step was to consider braze alloy in terms of its ability to reduce stress in the braze joint.

When brazing tungsten carbide to steel it is necessary to heat braze alloy up to roughly 1350 Fahrenheit or a bit higher.  Most braze alloys used in this application are rated for service up to around 1500 Fahrenheit.

The difference in coefficients of expansion between tungsten carbide and steel are approximately 3 :1.  Other figures are given and, of course, it depends greatly on which steel and which tungsten carbide.   However the ratio seems to always be 3 : 1, 5 : 1 or higher.  

When the braze alloy has reached full flow and the heat is removed then the two parts are locked together.  As the parts cool and shrink the steel shrinks much more than the carbide which causes stress on the carbide. 

This stress can be thought of as analogous to a drawn bow or perhaps an animal trap. Both of them contain a great deal of energy waiting to be released.

 

In the case of the tungsten carbide tool tip this energy can be released through impact in use.  So a piece of tungsten carbide it may be strong enough to survive an impact on its own can fail if it is already highly stressed or stress loaded.  The failure mode is a crack in the carbide.  Typically it is a crack running through the carbide to relieve the stress.  You can even see this crack occur during grinding of a saw tip, for example.  I believe the removal of mass during grinding significantly weakens the tip.  I also believe that the vibration and other energy loading it that occurs during grinding can contribute. I would guess that the mass removal is the more significant of the two but have no data to back that up.

 

(I understand there are a great number of definitions and ways of thinking of stress.  I believe the way I’m using it here is fairly common and should make my point.  As there is no absolutely authoritative source for defining these terms I have absolutely no interest in getting involved in such a discussion.  I have been there, done that and it wasn’t any fun.)

 

It seems that a thicker layer of braze alloy or tri-metal piece of braze alloy can greatly reduce or eliminate so-called “Heat Stress“ failure.  So it appears that braze alloy has a second function in making tungsten carbide tips more survivable than impact situations.

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