Split Tip Breakage

Problem:  Tips are highly susceptible to breakage even during routine handling. 

Executive Summary:   The carbide is not strong enough for the application because it is not a suitable grade, because it was not manufactured properly or a combination of both.   It also appears that improper brazing was a contributing problem.  

I cannot see any evidence of mishandling.  If the carbide had been strong enough you would much more surface roughness instead of the huge, smooth fracture planes.   You would also expect to see a great deal of chipping and spalling on the edges and there is no evidence of any of that.  

Five pieces were submitted for analysis.   The two right parts are from the same tip. 

 Analyzing Carbide Breakage

There are many ways of analyzing carbide breakage.   The first step is always an observation of how the parts broke.   Carbide breaks in essentially two ways.  The first ways is that it splits.  This leaves a nice, smooth, even surface.  The second way carbide breaks is that it gets beat apart.  This leaves a very rough surface.  Carbide crack initiation and propagation are controlled by the carbide mixture formulation and the manufacturing process. 

Split Carbide (left below) and broken carbide (right below).

Carbide Should be Tough

Some carbide grades are much tougher than others and thus more likely to break.  Modern carbide grades have solved many of these problems but they are not as widely used as they could be because they cost a bit more.  To give an idea of toughness I have seen a customer bounce a carbide tipped saw on a concrete floor with no damage.  Carbide should be hard to break even when beaten with a steel hammer.   As a demonstration I fired carbide out of a shotgun and into a concrete block wall with no damage.  

Brazing Stress or Heat Stress

The brazing process can stress the carbide so that it is more susceptible to breakage.   (See Heat Stress paper)  

Essentially braze alloy does three things.  1.  It holds then tip on the saw.  2.  it provides a soft layer between the carbide and the steel to relieve heat stress.  3.  It provides a layer which acts with the steel and the carbide to form a composite structure that makes each part much stronger and better able to resist impact forces.  

In this case it appears that there may not be a thick enough layer of braze alloy between the carbide and the steel.   The saw was not provided for examination but the pictures seem to show that a great deal of braze alloy was forced out of the joint during brazing. 

Tip 1.   This break shows what is essentially one smooth fracture plane.   The carbide was not beaten apart but ruptured nicely.   This part is also speckled with what might be holes or foreign material.   This could be contamination from handling after fracture.

Tip 2. 

This tip was a little harder to break.  It shows three separate fracture planes.   However the tip essentially split very readily.  On the lower right you can see a rough section showing the other way carbide breaks.  Big smooth areas means the carbide ruptured or split like splitting wood.    The rough are is more like trying to split plywood.   Plywood won’t split.  You have to sort of beat or chew your way though it.

Tip 3.  is very interesting because there is no way that brazing could be a contributing factor due to the location of the breaks.  There are three smooth fracture planes where the carbide just split. 

Tip 4. 

We have two mating parts of tip 4.   The bottom part is much darker because it was removed with a torch from the steel saw body after the fracture occurred.  Again we see the smooth fracture planes.

Tip 4 also shows the smooth curve which is the classic heat stress curve.  The curve appears to break at two points so the cause was not all heat stress. 

Above we see tip 4 from two sides.  Both sides exhibit signs characteristic of rupturing or splitting caused by heat stress.

At right we see the tip from the top with the two parts in place.   There is also large piece missing which is not indicative of heat stress.