54. Pretinning

Reducing Breakage - Pretinning can improve impact resistance.

This is a weight drop test. We just drop a weight and measured the height at which the parts break. We laid the pretinned parts on their side. The solder wasn’t underneath the tips or on top of the tips. We did this so the softer solder would not absorb the impact force. 

This test seems to show two things. Proper use of braze alloy really increases the impact resistance of the carbide. Also proper pretinning creates much more uniform performance at a much higher level than untinned tips have.

Quality

Not all tips work well every time.  (See Flow Problems below.)  We have several different treatments we developed and a couple we patented )US patents   5,624,626 and 6,322,871) to make sure every tip works.

Untreated – patchy silver solder adhesion      Treated – full strong, silver solder adhesion

The contact angle between the braze alloy and the underlying material is a rather important means of measuring wettability and hence the success of the flow and the bond. 

Inspecting Tungsten Carbide Pretinned Tips

The top row is how pretinned tips should look.  The bottom row is bad.  The alloy did not

stick to the carbide and the carbide won’t stick to the saw.  This does not necessarily

mean that it is bad carbide.  It could just be bad pretinning.  If the surface has organic

matter such as oil, grease, wax or similar on it then the braze alloy will not stick well. 

There can also be problems due to oxides and free carbon from the manufacturing process.  

All tips are supposed to flow well.  This doesn’t always happen.  If the silver solder

doesn’t stick to the tips then the tips don’t stick to the saw blade well.  

Average Alloy Depth

We prefer to specify alloy depth by a term called “Average Alloy Depth.”   This treats

the part as though the alloy would form a flat topped, straight sided layer.   This makes it

very easy to calculate from either volume of weight.   

The humped line is the way the alloy actually forms.  The flat line is the mathematical model. 

Bad Pretinning and Other Overheating Damages the Finished Tool

This is a case where a little more heat was used to make the parts look good. 

This is the tip behind the ripped shoulder.  Here you can see that the braze alloy did not bond well to the carbide.  The tips did not have adequate preparation and the pretinning was poorly done.  You can get flow that looks good if you heat it hot enough but you seriously weaken the steel.  With this kind of problem you would expect to see tip loss.  It looks like the shoulders were so much weaker from overheating and embitterment of the steel that they ripped off before this tip could come out. 

When you overheat the braze alloy to make it look good in Pretinning then you boil the zinc.  As the alloy gets hotter it gets runnier and flows better.  As the zinc boils it forces the alloy to spread out and flow.  However once you have overheated the alloy in Pretinning than you have to overheat it again to braze it on the saw and this causes a heat affected zone, which in turn means brittle and ripped shoulders. 

Saw 1, side 1 - 10x. 60x & 200 x the braze alloy was overheated to the point where the zinc boiled out of it.   The round pits are left from the boiling.   Zinc boils at 1,664 F.  This weakens the braze alloy and is an excellent indicator of other problems such as a brittle heat-affected zone in the steel.

Our work – smooth, even flow, undamaged alloy.

No pits or voids.  Greater intermetallic zone for

better strength.  Brazes at lower temperature.

Not ours – uneven flow, extra alloy to hide bad

surface conditions.  Gas bubbles from boiling

Zinc.