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Pretin

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Pretin

Pretin, Pretinning, or Tinning

 

Tinning, Pretinning or Pretin is applying brazing alloy to one or both parts separately then putting the parts together and heating them to complete the brazing.   This allows for easier handling of the final assembly.   It also ensures that the parts wet well and that brazing alloy covers all the surfaces being joined.     For more articles on  Pretinning and Brazing visit our Brazing Services article page.

 

Reducing Breakage - Pretinning can improve impact resistance.

This was done with a weight drop test. A weight was dropped on the parts and a measurement was taken of the height at which the parts broke. 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.  Get a Free Quote on our Brazing and Pretin Services

 

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.

 

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Tinned part with bad flow

Untreated – patchy silver solder adhesion  

Tinned part with very good flow

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. 

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                 Good Contact angle        |        Bad angle           |        Terrible (virtually no bonding)

 

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.   For more information on Solving Brazing Problems and how to Braze see our Solutions to Brazing Problems Article

  

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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. 

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Bad Pretinning and Other Overheating Damages the Finished Tool

 

On the right is the tip behind the ripped shoulder.  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.

 

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 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.  To see other ways that overheating can damage the finished tool, visit our Truly Horrible Saw Blade article.

In the picture on the right you can see classic bubbles from over heating. 

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Our work – smooth, even flow, undamaged alloy. No pits or voids.  Greater intermetallic zone for better strength.  Brazes at lower temperature.

 

 

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Not ours – uneven flow, extra alloy to hide bad surface conditions.  Gas bubbles from boiling Zinc.

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