|Established in 1952
Sunday, May 19, 2013
Cemented Tungsten Carbide is an incredible material. It was originally developed for use as a cutting tool, in machine tools applications, where it still finds wide use today.
It is extremely hard at 91 HRA which is equivalent to 1500 Vickers 30. This material is very wear resistant, with some abrasion tests showing it at 30 times hard steel. It has good long-term dimensional stability, which makes a good material for gage applications.
Cemented Tungsten Carbide is extremely stiff, with a Young's modulus of elasticity of 98,000,000 pounds per square inch, compared with steel at 30,000,000 PSI. This high stiffness makes cemented tungsten carbide balls a good choice for ball-sizing and they provide premier results as components of kinematic couplings.
Its good performance at temperatures up to 800 degree Fahrenheit ( 427 centigrade ) make it a good choice in high temperature applications.
It has good corrosive resistance in many environments.
The material has a very low rate of thermal expansion, at 2 microinches per inch per degree Fahrenheit (4.9 X 10-4 per degree C). Steel is 6.4 and aluminum is 12 microinches per inch per degree Fahrenheit.
One of the very important features of this material is that it is electrically conductive, with a resistivity of two micro ohms per centimeter. This conductivity sets it apart from other hard stiff materials, such as ceramics and sapphire, because it allows it to be machined into complex forms by the Electrical Discharge Machining (EDM) process. Cemented Tungsten Carbide can be ground and lapped using diamond grit. The material is slightly magnetic. Small diameter balls can be picked up with a magnet. Cemented Tungsten Carbide is very heavy, with a density of .54 pounds per cubic inch or 15 grams per cubic centimeter.
Cemented Tungsten Carbide is neither a metal nor is it a ceramic. It is a cermetal although this isn't a widely used term. This is a combination of a large percentage of Tungsten Carbide ceramic particles bonded together by a small percentage of a metallic binder, to form a solid mass.
When a mixture of powdered tungsten metal and carbon powder are raised to a very high temperature by an arc of electricity, while the whole mass is held in a carbon crucible, there is a complete chemical combination of the two elements to form a big lump of tungsten carbide ( WC ). This lump is crushed several times and then milled to form a fine powder. The powder is screened down to uniform particle sizes.
The Tungsten carbide powder and powdered cobalt metal are mechanically mixed to form a uniform combination. To this combination is added a polymer or wax that acts as a mechanical binder in the pre-sintered stage. A carefully measured volume of this combined powder is compressed by a double acting press to form the tungsten carbide ball blank, or pre-form. The binder is removed by placing the preformed parts in a vacuum chamber and raising the temperature. The binder out gasses and is trapped down stream as a liquid.
Next, the combined tungsten carbide and cobalt pre-forms are sintered or fused into a solid mass by heating them to a high temperature in a vacuum or hydrogen atmosphere furnace.
Some tungsten carbide ball sizes are Hot Isocratic Pressed or hipped. The newest procedure is to Sinter-HIP the parts in a single operation.
The metallurgically complete ball blanks are then rough ground with course diamond to remove all imperfections from the surface. Using micro size diamond dust, they are then finished lapped and polished to final size and microinch roundness with a sub-microinch surface finish.
Making spectrographic chemical analysis of Tungsten Carbide can be difficult and uncertain, so the accepted procedure is to use a magnetic permascopic evaluation. If anyone tells you that they can analyze tungsten carbide to an exact figure, ask them for the error budget on their process. If they know the uncertainty, it will be in percentage points.
The same exactness of chemistry and physical properties in tungsten carbide that are experienced with alloy metals cannot be expected, as T.C. is made from a physical mixture of a pure metal powder and a powdered ceramic. There will naturally be more variability in performance.
The stiffness or Young's modulus of elasticity given for Tungsten Carbide has been measured in tension, but balls are almost always used in compression. In our experience, the Young's modulus of elasticity of cemented tungsten carbide is actually 10% higher in compression than it is in tension.
There has been a tremendous consolidation within the hard metals industry. The surviving entities have become very large companies that are far less willing to produce any special combinations of material orders. This has dramatically reduced the availability of special grades of cemented tungsten carbide. Many of the exotic grades are simply not available today. Delivery on available grades is in the eight to sixteen week time frame.
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