Common Tungsten Carbide Defects

From the book Building Superior Brazed Tools    Buy the Book

Internal

The most common and most serious problems in Carbide are internal.  Left is a well made tip that suffered impact and had only minor chipping in one corner.  Right is tip that was poorly made and failed along a fault line.  Refer to our Carbide Material Section for more information on Carbide used as Tool Tipping Material.

Why This is Important

The customer reported some chipping and breaking during grinding.  It wasn't all the time and some times it was a lot worse than others.  The carbide supplier had examined the parts and said that there was nothing wrong with them.  We received the parts and sent them to an outside source.  They polished the surface off and took pictures of the interior.  The most visible features were internal cracks.  The crack in the middle picture runs though a pinhole.  There were also pinholes without cracks.

The tips also had internal voids or holes. Finally the carbide grains were not evenly distributed in the cobalt matrix so there is a puddle of cobalt with no carbide in it. 

Explanations

The first series of photos supplied by Valenite Die and Wear Walmet Copyright © 1999-2002 and used with permission.  The remainder are from our labs. 

Clusters (Cls)- Clusters are  defined as groups of three or more WC grains that are significantly larger than the average grain size. A WC cluster can be a weak spot in the carbide microstructure. Clusters are thought to form during the cooling cycle through WC crystallization. It is not completely clear what controls this crystallization but small impurities in the powder and non-uniform carbon distribution have been implicated. It is difficult, if not impossible, to avoid clusters completely. Low levels are not considered harmful to the integrity of cemented carbide parts. Large numbers of these clusters can adversely affect performance, especially where shock is involved.

Binder Lakes (Blk) - Binder lakes are pools of cobalt binder in the microstructure. They are formed when melted cobalt or nickel binder flows into open pores during sintering (1345°C+). Sinter HIP'ing eliminates binder lakes by sintering at high temperatures under an inert gas pressure of approximately 700psi. The pressure forces carbide grains, along with the binder, into the open pores. Low levels of binder lakes are not considered harmful to performance, but a large number of lakes may structurally weaken a cemented carbide part.

Eta Phase (Eta-1, Eta-2, Eta-3)  - Valenite's internal rating system for eta phase. Eta phase is a carbon deficient form of tungsten carbide that results in a harder, more brittle cemented carbide part. Insufficient carbon levels are generally the result of improper formulation of the carbide powder, long term exposure of unsintered parts to the atmosphere, or poor control of sintering conditions. We rate eta phase on a scale of 0 to 3. Zero indicates that no eta phase is present, eta -3 indicates the most severe level. Eta phase is generally considered to be harmful to the performance of cemented carbide parts.  With our typical 5 minute etch, the eta phase is rapidly etched leaving a void with characteristic geometric patterns.

Grade Contamination: An area of a distinctly different grade in the microstructure, round to oval in shape, whose longest axis exceeds 25 microns. Cross Grade Contamination is generally the result of ineffective cleaning of powder processing equipment.  Low levels of grade contamination are not considered harmful to performance, but a large number of these areas may adversely alter the physical properties of the cemented carbide part.

Porosity

A Porosity: Pores in the microstructure less than 10 microns in diameter. Rated from A01 to A08. 

B Porosity: Pores in the microstructure 10-25 microns in diameter. Rated from B00 to B08.

C Porosity:  Not true porosity. Rather, carbon porosity consists of discrete areas of graphite in the microstructure resulting from an overabundance of carbon. Rated from C00 to C08.

Free Carbon:  A term used to describe C Porosity in excess of C00

Microphotographs of Bad Material

500 x magnification

Top right shows a very large porosity.  This material looked good until it was ground.  These pores were hidden under the surface.  During grinding pores like this opened up.  The bottom two photos show cracks in the material.  These cracks were in the material as supplied.  In this case about 30% of the material broke during the initial sharpening

This is at 1500x magnification – This is a photo of the surface and the white is binder.  The small black specks in the top right photo are areas of porosity.  The bottom right photo shows a very large ‘A’ porosity about ten microns in diameter.  The bottom left photo shows uneven distribution and contamination by foreign materials. 

1500x magnification – The top left photo shows the average quality of this material.   There are at least two oversize grains of material.  The top right photo shows a very large grain of the material. The bottom two photos show ‘B’ porosity.

How Bad Carbide Breaks & Wears Out Faster

External Problems 

Green State Problems

As part of the manufacturing process carbide has a stage where it is very soft.  This is where chipping, bending, cracking and similar damage occurs most often.  

Cracking

Any piece of carbide showing a crack should not be used.  This is also true if there is a corner knocked off.   There are two arguments.  One is that the part is acceptable if the material will be ground down anyway.  The other argument is that the force that caused the crack or loss of a corner may have created smaller cracks that are not visible and the part should not be used.   

Chips, edge radius and parallelism

These all looked good to naked eye inspection.  We used 30x  magnification for inspection with standard  light and black and white high contrast.  These are picture of 5 separate tips fixtured between the jaws of dial calipers.   

The gap on the right side of this tip is about 0.005” with the largest chip about 0.002”.  On the left side the gap is about 0.002” The gap is a combination of the flatness of the side, the parallelism of the sides and the radius of the edges. 

Chipping, Corner and Edge Radius Standards

Saw Tips Need To Be Flat And Square.  A radius of 0.001” to 0.002” is common on competitively priced production tips from good suppliers.  

Establishing the Standards

We used the figures of Top grind  0.015”, Face grind 0.005”and Side grind = 0.005” assuming grinding in that order.  Face view - tip is 0.150” wide.  Grind top & sides

Finding the maximum allowable radius ends up being a matter of adding the two grinding tolerances to get 0.020”. 

Chips On Stump Grinder Tips

These routinely come in with some chipping. I would suggest you specify these when you order them and send us a copy of the specification.  We will be happy to inspect them for you.  

Measuring Cracks and Chips in Carbide

Crack Detection and Inspection Techniques 

Visual - Naked eye, assisted by magnifying glass, low-power microscope, lamps, mirrors.  Only at places easily accessible. Detection of small cracks requires much experience.

Due to space limitations this is an incomplete list.