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Tungsten Carbide Explanation and Manufacturing

 

Tungsten carbide saw tips are not solid tungsten carbide.   They are made of grains of tungsten and carbon held together in a matrix of metal such as cobalt or nickel. 

 

Tungsten carbide can be called a powdered metal because tungsten is a metal.  It is also classified as a ceramic.  Ceramics are often defined as a class of materials that are not anything else.  It can actually make the most sense to think of tungsten carbide as a ceramic when you are using it and a powdered metal when you make it.   

 

Tungsten and most metals arrange themselves in a lattice which is like a 3 D version of a chain link fence.  Under great heat and pressure the carbon atoms are actually packed inside the tungsten atoms instead of being joined side by side as with ordinary compounds. 

 

Interstitial carbides, such as tungsten carbide (WC), form when carbon combines with a metal that has an intermediate electronegativity and a relatively large atomic radius. In these compounds, the carbon atoms pack in the holes (interstices) between planes of metal atoms. The interstitial carbides, which include TiC, ZrC, and MoC retain the properties of metals.

 

 

How tungsten carbide is made

1.  Mix Carbon black, Tungsten metal and metal oxides

2.  Then heat the mixture until the carbon bonds with the tungsten (carburises)     

3.  You get tungsten carbide powder

4.  Mix the tungsten carbide powder with wax and cobalt

5.  Take this and mix very thoroughly using a ball mill

6.  This gives you a final powder

7.  Put the final powder in a mold and press it to the desired shape

8.  Heat (presinter) the pressed, final powder enough so that is sticks together like soft chalk

9.  Take the soft chalk and do your final machining / shaping       

10.  Put the soft chalk pieces in a very hot, high pressure, special atmosphere oven and do the final sinter.

11.  The powder cooks, shrinks and gets very hard.

12.  Now you have the final piece of tungsten carbide

 

Making tungsten carbide is very difficult.  First, you need to pack the carbon atoms into the tungsten lattice.   In a piece of Carbide 1 inch by ½ inch by 1/8 inch you need to pack about 975,000,000,000,000,000,000,000 (975 septillion) atoms in to that many holes.  Second, the tungsten is trying to grow into a single big crystal but you want millions of small crystals.  

 

These grains are combined with Cobalt powder and mixed in a ball mill.  Tungsten carbide balls are mixed with grains allowed to run for several days to get even dispersal of the grains and the cobalt powder.  This powder is then dried and wax is added as a binder.  The wax holds the powder together and makes it somewhat slippery so it presses into shapes well.  The shapes are presintered in an atmosphere-controlled furnace at temperatures of 1,000 - 1,500F.  The wax melts out and leaves the pieces sort of like a soft chalk.  These chalk pieces can be easily machined although they are also easy to break and can be chipped here if handled improperly. 

 

Sidewalk Chalk has a rupture strength of 4 # to 6 # per square inch.  Tungsten carbide is supposed to have a rupure strength of 200,000 to 400,000 pounds per square inch.  

(Chalk figures from Binney & Smith for dustless chalk both U.S. & French manufacture.) 

 

The final step is another sintering step that can take place in a special atmosphere, a vacuum or both.  The temperature is typically 2,500 - 2,700 f.  During final sintering the parts will shrink up to 15% in any dimension and up to 35% in volume.  Typically 15 to 30 tons of pressure is used to form the tungsten carbide into a tool shape such as a saw tip. 

 

Forming Carbide Shapes

1.  Molding - lowest part cost but figure at least $3,000 - $5,000 for the mold.  Good shape and edge definition.  The parts are typically pressed one of three ways.  They are rammed in a mold before sintering.  They are isostatically pressed.  Isostatic pressing means they are surrounded by a liquid or a gas and the pressure is applied to the liquid.  This transfers the pressures to the surface of the parts uniformly.  The third pressing method is hot pressing during sintering.

2.  Green state machining – high labor cost.  Shape and edge definition depend on design.  If shape and edge definition are critical this is usually followed by grinding after the final sintering.  

3.  Grinding – diamond is necessary.  The speeds can be very good and shape and edge definition can be excellent.  

4.  Brazing carbide to carbide - uncommon as the whole, purpose is to use as little expensive carbide as possible and to braze it to steel or similar.

 

It starts with at least four powders and can have eight or more powders.  These are extremely fine and hard to work with.  If you have ever had to work with toner powder you have some idea.  It wants to stick to itself and everything else.  

 

 

 

 

 

 Once the powder is mixed then it is pressed into shape. The wax was added to keep the powder together for pressing.  After pressing the wax is melted out. 

 

 

If you make carbide right you get a nice even distribution of the same sized grains (left).  If you are sloppy and / or use cheap materials then you get carbide like that on the right which has odd bits of the basic materials sort of like lumps in gravy.  

 

Why Cobalt Is Used As a Binder In Tungsten Carbide

Cobalt was the first material used as a binder because it was the first one that worked.  Cobalt is easiest to use as a binder because it has a high melting point 1493°C  (2719F), is strong at high temperatures and forms a liquid phase with tungsten carbide grains at 1275°C which draws tungsten carbide in on itself by surface tension and helps eliminate voids and porosity.  In addition Cobalt dissolves tungsten carbide and the tungsten carbide precipitates back out as the material cools.  

 

Nickel binders are used but the manufacturing process makes the finished parts expensive and the nickel oxides make the parts very hard to braze or to treat for brazing. 

 

Iron, Nickel, Chrome and Cobalt are all used as binders.  When used as pure or elemental materials they all have a susceptibility to chemical attack.   This has been very successfully handled two ways.  First is the use of an alloy binder such as Nickel / Chrome which alloys and forms a material with corrosion resistance similar to Stellite and similar alloys.  Second is the use of a post sintering process that creates a Boron metalloid and greatly reduces susceptibility to corrosion. 

 

Properties of Tungsten Carbide

1.  Extremely high compressive strength.

2.  Rigidity – approximately 2.5 times steel and 5 times cast iron and brass. 

3.  High Heat Properties Retention

4.  Impact Resistant about equal to hard tool steels

5.  Oxidation resistance - app. 1,000°F in oxygen and 1500°F in vacuum or protective atmospheres. 

6.  Cryogenic toughness and strength to app. – 450 F

7.  Twice as Thermally Conductive as steel.

8.  Electrical Conductivity about that of steel.

9.  Hot Hardness – hardness retention up to 1300F

10.  Lubricity – tungsten carbide can be polished to a relatively low friction surface

11.  Wear Resistance about 100 times that of steel

12.  Dimensional Stability

 

Tungsten carbide, or WC, has a number of unique and impressive characteristics, the most significant being the ability to resist abrasion. It is the hardest metal known to man. Sintered and finished carbide has a combination of compressive strength, extreme hot hardness at high temperatures, and resistance to abrasion, corrosion and thermal shock.

 

Tungsten carbide has a compressive strength greater than any other metal or alloy and is three times more rigid than steel. Abrasion resistance is up to 100 times greater than steel. Thermal expansion is less than one-half that of steel, and tungsten carbide resists thermal shock and oxidation temperatures up to 1200°F (648.89°C). Tungsten carbide compositions have exceptional resistance to galling and welding at the surface and can withstand cryogenic temperatures to -453°F (-269.44°C) while retaining their toughness and abrasive qualities. Since carbides are nearly chemically inert, they are ideally suited for wear applications in corrosive environments.

 

Catalytic chemistry

The metals that go into an alloy are only part of what determines the quality of the alloy. 

Time, temperature, number of steps, kind of steps, quality of ingredients also determine the quality.  There are also "secret ingredients" that can be added to considerably improve the quality of the alloy.   In chemistry some of those secret ingredients are called catalysts.      

 

Catalysts speed up or slow down chemical reactions without being part of the reaction.   Talonite is superior because it is made using more sophisticated chemistry.    A catalytic additive can give an alloy smaller tungsten carbide grains, which makes it more wear resistant.  A catalyst can alter the structure of the cobalt bonding mechanisms so they grow more slowly and more evenly which gives a more structure that is both softer (more impact resistant) and tougher (more resistant to tear or rupture).                           

 

You never see these in the end product because they go into the reaction and then come back out.   Heat is a catalyst.   You take chemicals, heat them and then let them cool and they are different.   There are also chemical catalysts that do many things such as: retard grain growth, promote different intermolecular bonding mechanisms, speed up or slow down reactions, purify reactions and do other important things.    

 

You do not see the catalysts in the end product.   This is why metals, such as Talonite, can be chemically identical but have considerably superior performance to other alloys that are not so carefully made. 

 

Carbide properties can be changed by adjusting:

     1.  Ratio of binder

     2.  Grain size and

     3.  Grain distribution.

 

In general

 

More cobalt means it is harder to break but does not wear as well. 

Smaller grains mean more wear resistance.

More wear resistance means less toughness, which is the ability to withstand fracture.

Toughness increases with an increase in cobalt and with an increase in grain size.

Hardness increases with a decrease in cobalt content and a decrease in grain size.

Transverse rupture strength (T.R.S.) increases with an increase in cobalt content.

 

These are general rules and they are pretty good.  However there are lots of techniques used making carbide that can make tremendous differences.  Different materials can be combined to make greatly different things.  Pigs and people are both made from the same chemicals.  The arrangement is different.