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Carbide Surface Treatment

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Carbide Surface Treatment

From the book Building Superior Brazed Tools   Buy the Book

 

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Graph shows that the ECP surface treatment doubled joint strength on the average and dramatically increased process reliability. 

 

The Problem

Tungsten carbide is made to be wear and corrosion resistant.  Yet, in order to braze it successfully, you must create a physical and chemical joining.  More information on Tungsten carbide and other tool materials can be found in our carbide tooling index. 

In addition carbide made with a shortage of carbon is a poor quality product.  If there is a bit too much carbon then the carbide is good and the excess carbon deposits on the surface.   This free carbon deposits as graphite and interferes with brazing. 

Time in a sintering oven is expensive and there is a strong motivation to remove the parts before they are dully cool.   The figure of 1,000 F is given as the point for the beginning of significant oxide formation on tungsten carbide in air.  However the question arises as to what level of formation is significant.   

 

Tools are steadily becoming thinner

The material in the width of the saw cut becomes sawdust which is less valuable than lumber. Thus saws have been getting constantly thinner.  Now there are saw blades in mills that maybe three feet across and yet somewhere between 0.060” and 0.090” thick.  State of the art seems to be about .060” kerf on a 12” saw blade with a blade change every sixteen hours.  The thinnest we have gone with carbide is 0.025” in a butt braze.  We know a filer in a mill who runs .053” kerf. 

The carbide surface must be properly prepared in order to get he necessary strength from a braze join over a very small area.

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This is a reciprocating knife.  To make it we butt brazed a piece of carbide 0.5” by 0.025” to a piece of spring steel.  Two of the knives are shown next to a dime.  On the right is a knife in vise.  The vise was then tightened to get a bend and test for strength.  You can see by the bend how much pressure is on it.    I think it would have taken more bending but I don’t know how much more.  

In any case this shows what can be done with good surface preparation and brazing techniques.  

 

Intermetallic

A good braze joint creates a chemical bond and an intermetallic compound.  A bad braze creates three distinct layers.   Think of plywood with and without glue.

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Bad surfaces  (Three distinct layers)

 Microphotograph; bad

Good surfaces (Three interlocked layers)

Microphotograph; Good

  

  

  

  

  

  

Pretinning

Pretinning or tinning is the application of the brazing alloy to one part first.  It is very important here because it clearly reveals the surface condition of the carbide.  Below are two views of curved tungsten carbide parts.  These were designed for a machine that would duplicate the look of an adze on wood.  The parts need to be pretinned. 

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This is the customer’s attempt on an untreated surface using a torch and a braze alloy rod. 

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This is the same part surface treated and pretinned.  This was pretinned in an oven.  The cut alloy was laid on the part and then it was heated.  Remember we are looking down into the curve.  You can see where the molten alloy puddled in the center however the capillary attraction was so strong that the alloy largely stayed in place on the curves in spite of gravity.

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Braze Treating Options

Sandblasting

Washing

Plating

Toney salt bath Process

ARS

ECP

Tuffco

Sandblasting

Sandblasting works somewhat.  Carbide is typically sintered in sand and then the parts are sandblasted to clean them up.  However you have the usual sandblasting problems.  The sand (or whatever media) can become contaminated with carbon.  In addition the sandblasting process can smear the carbon in an even layer rather than removing it.   

There is a study that shows that using alumina for sandblasting carbide results in the alumina shattering and becoming embedded into the carbide.  This happens but I have never seen where it is a significant problem. 

Washing

Carbide parts are often washed after sandblasting however this is rarely meant to do more than remove loose dust.   This process provides an excellent opportunity for simple chemical wash that will remove oxides and free carbon however this is almost never done. 

Plating

This is standard plating technology.  Sometimes there is a layer of cobalt or nickel plated directly to the part and sometimes it is plated over an intermediate layer of copper or something similar.  Copper plates well to tungsten carbide.  Nickel and cobalt both plate well to copper.  The problem is that the coating or plating is only as good as the bond to the underlying surface.  If the surface is not prepared properly the plating will peel off. 

Toney Salt Bath Process

L.B. Toney was granted a patent to treat tungsten carbide in a salt bath.   # 2,979,811

Patented Apr. 18, 1961.  The illustrations on the front page of the patent show the kind of wetting one would expect after a salt bath cleaning.  My guess is that L.B. Toney was at a company that had a salt bath process and decided to put carbide tips into the molten salt to see what would happen.  It looks like the 2200 F salt thoroughly cleaned the carbide.   

This press was once described as making the cobalt migrate to the surface. There have been other explanations.  Currently I believe the explanation is that they remove material from the surface leaving it Cobalt rich. 

Maybe a decade ago Ajax Electric sold equipment that would allow people to do the Toney process.  We looked at it but, in our opinion, the process was 40 years old, expensive and hard to control.  It involves molten salt at 2200 degrees F.  Besides heating carbide to 2200 F isn’t particularly good for it.  Cobalt is unusual in that it goes through a phase change form cobalt II to cobalt III.  I know of two companies that did buy and install the salt bath equipment and then later abandoned it.  The Toney process by any name leaves holes in the surface of the material.  This is generally explained as being good because the braze alloy flows into the holes and that’s how it bonds.  The holes are microscopic in size and much too small for the described process to be effective.  

The following photographs were all taken at 1,000x magnification.  The left-hand photos are standard photos from a Scanning Electron Microscope (SEM).  The right hand photos were taken using Backscatter Electron Imagining (BSE) which identifies the elements by shades of gray.  Cobalt is darker and tungsten carbide is lighter.

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Untreated Tungsten carbide.  Fairly flat, impervious surface in the SEM on the left.  Scattered cobalt and tungsten carbide in the BSE on the right.  Cobalt is the light material.

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This is tungsten carbide from a high temperature braze treating described as a “cobalt enhanced surface".  Apparently a 2200 F salt bath is used to clean the parts and them they are electroplated using molten salts as a medium.  Finally there is an acid rinse.  The molten salts and the acid rinse account for the rounded “melted” effect.  It is definitely a high cobalt surface with a line of tungsten carbide running through it.

 

  

  

  

  

  

  

  

 

 

 

 

 

 

 

 

 

 

 

 

There can be flow problems such as those below. In this case the surface did not wet well for whatever reason. This showed up in the pretinning and did not cause failure problems.

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Left pictures below show that the carbide ripped which means defective carbide. The middle and right pictures show that the braze just did not stick to the surface which is also defective carbide.

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In the second instance the tips were supplied to us as “Ready to Braze.” They wet well and no alloy fell off during tumbling. However the tips fell off the saw because the surface treatment peeled off.

 

6 bad tips in the left hand photograph below.

3 on left – the alloy just peeled off

3 on right – the alloy peeled off in spots (especially visible on rt. side of bottom rt. tip) and the carbide ripped where the alloy did hold.

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ARS Process

1985 We developed a process to thoroughly clean tungsten carbide parts.  It worked well but left the tips dark black so was not deemed suitable for sale because the Toney process left tips a dull gray.  We had already developed and licensed our ARS process about 1984.

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Tungsten carbide at 4,000 (untreated left) and 5,000 (treated right).  The different shapes on the left are different kinds of oxides over an oxide layer that looks like velvet.  The part on the right has been ARS treated and it has a clean, porous surface to promote wettability and bonding. 

ECP Process

Our Patent 5,624,626, Apr. 29, 1997 – a purely electrical method of improving surface wettability.    The ECP treatment takes advantage of the difference in electrical resistivity of tungsten carbide grains and the cobalt binder.  It selectively removes the cobalt and leaves exposed tungsten carbide grains.  This process is inexpensive and works very well.  The major drawback seems to be that the surface of the treated part is the same dark gray color as pure tungsten carbide. 

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The ECP treated tips grind faster than the high cobalt surface tips because the wheels do not gum up as fast.  Braze alloy on ECP treated tips melts more readily than braze alloy on high cobalt tips.  One possible explanation is that the darker ECP tips absorb radiant energy more readily and thus heat up faster.  This difference is about 5 seconds in a cycle of two minutes and 10 seconds.  The high cobalt tips are shiny and reflective. 

Push off Test Results

Black & Decker ran these tests on Carbide Processor’s ECP process.  The ECP treated parts were compared with untreated parts.  The minimum acceptable level of performance was 200.  Untreated parts gave a wide range of results including a low of 77.  All the treated parts performed within acceptable limits and gave much more consistent and much higher readings.  

Analysis of results

                    Untreated    ECP # 7     ECP #5

High               262            336            392

Low                  77            227            208

Mean              173.5         277.4         283.1

Range             185            109            184

 

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Tuffco

Tuffco is a room temperature surface process that leaves the surface of the tungsten carbide equivalent to the high heat process.  It does this at a much lower cost and there is no danger of thermal stress.  To get a shiny surface we created our Tuffco process. (United States Patent 6,322,871; November 27, 2001  METHOD OF TREATING CERAMICS FOR USE AS TIPS IN SAWS AND OTHER TOOLS OR OTHER STRUCTURES)   This works extremely well.  Currently we are using it on cermets and ceramics in house for our own products. 


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