Tungsten Carbide

We use Tungsten Carbide everyday in a variety of products, for use in a number of different applications throughout many industries. It is an incredibly versatile and useful material due to its unique properties which are outlined below:

General

When evaluating or finding equivalents of Tungsten Carbide grades the important criteria is to specify two of three factors. Binder content, hardness or grain size. In straight matrix materials any two of these will match the third. A 15% binder with hardness of 88.0 RA would have to be fine grain material whereas with a hardness of 86.0 RA would need a very coarse grade to achieve it. Specifying cobalt binder only can be a dangerous game. Take control of each situation and ensure you know what you are using and why so that consistency of specification can be obtained from whatever source. We can achieve hardness differential on 15% cobalt: purely by varying the grain size, that would need a spread of 11% cobalt over same gain size materials. I.e. We can manufacture 15% material with a hardness of a 6% or 18% grade just by using sub micron or coarse grains !!

The Tungsten Carbide properties chart below shows basic data for each grade manufactured. All specifications are designed and engineered for a purpose and rigid controls are kept throughout production processes to ensure adherence to grade engineering.

Quality control properties such as hardness, density, and minimum transverse rupture strength were determined from tests made on each batch of powder before it is used in the manufacturing process. Other properties such as Young’s modulus of Elasticity , Poisson’s ratio , Coefficient of Thermal Expansion, Thermal Conductivity and Electrical Conductivity are used by engineers for design calculations. Properties such as compressive strength, grain size and abrasion resistance give the designer additional information about the suitability of the grade for the part being designed.

Binder

The binder in most grades of Tungsten Carbide is cobalt. The other binder used is nickel. The binder is added as a percentage by weight varying from 3% to 30%. The amount of binder used is a very important factor in determining the properties of each grade. As a rule of thumb the lower the cobalt content the harder the material will become. However variation in grain size and additives can upset this rule.

Density

Determined by comparison of mass with volume and usually stated in g/cm3.

Grain Size

The majority of grades we machine are made with standard size grains varying between 1 and 3 microns in size. Using larger grains of 2 – 6 microns will greatly increase the strength and toughness of the material because the larger grains interlock better. The trade off is that larger grain materials do not offer as much resistance to wear as finer grain sized materials. Sub micron materials that vary between 0.4 and 1.0 micron grain size are harder than standard grain materials with the same cobalt content. The sub micron grains are much more uniform in size and hence give improved hardness as well as increased carbide strength. However, as specs show the transverse rupture strength is perhaps 20% improved on 15% sub micron compared to 15% fine grain material but this can give a false impression as sub micron carbide is not as resistant to impact and may chip more easily.

Rockwell Hardness

The hardness of Tungsten Carbide grades is determined by using the Rockwell hardness tester. A pointed diamond indenter is forced into the carbide. The depth of the hole is a measure of the hardness. The Rockwell “A” scale is used for tungsten carbide. Rockwell “C” readings are only shown on the data sheet so that tooling people can compare values of carbide against tool steel. The “A” scale is used on tungsten carbide because the lower indenting force of 60 KGs is less likely to damage the diamond than the 150 KGs force used on the “C” scale.

Minimum Transverse Rupture Strength (TRS)

TRS is a measure of the strength of Tungsten Carbide. Tensile strength is not used on tungsten carbide because it is too brittle and accurate readings cannot be obtained. As a rule of thumb the tensile strength of tungsten carbide is approx. half of the transverse rupture strength.

Transverse rupture strength values are determined by the amount of force needed to break standard test pieces under the same test conditions.

Compressive Strength

Compressive strength is measured by compressing a right cylinder test piece between two tungsten carbide blocks held in line by an outer sleeve assembly. The CS of Tungsten Carbide is higher than for virtually all metals and alloys. This high compressive strength makes it possible to compress carbon at one million P.S.I. from man made diamonds.

Impact Strength

This measures the resistance of Tungsten Carbide to shock loading by a drop weight impact test. This is a more reliable indication of toughness than TRS readings.

Young’s Modulus of Elasticity

Young’s Modulus of Elasticity is an indication of the elasticity or bendability of Tungsten Carbide. Bendability increases with the increase in binder.

Poisson’s Ratio

Poisson’s ratio may most easily be described by thinking of a marshmallow held between two flat plates. As the plates are pushed together the marshmallow is compressed and squashes out. All metals, even Tungsten Carbide, squash out at least a very small amount. The ratio varies only slightly with the amount of cobalt binder.

Mean Coefficient of Thermal Expansion

This indicates the amount of expansion which might be expected when heat is applied to the material. The expansion rate increases with temperature increase. The more binder present the higher the expansion rate. Tungsten Carbide is about 1/3 to 1/2 that of tool steel.

Thermal Conductivity

Tungsten Carbide conducts heat much more rapidly than tool steel. Thermal conductivity rates go down as the binder content goes up.

Electrical Conductivity

Electrical conductivity of Tungsten Carbide is determined by comparing it to that of copper. Thereby the copper standard being 100% . Generally the conductivity of Tungsten Carbide increases as the cobalt content increases but the most conductive tungsten carbide grade we manufacture would only make 10.7% of copper.

Electrical Resistivity

Determined by voltage drop for known current over known cross section area and test piece length.

Detailed Grades

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