Friday, 8 April 2011

How much strength does tungsten carbide offer?

Minimum Transverse Rupture Strength (TRS)
The transverse rupture strength (TRS) of Tungsten Carbide with Cobalt variations is sometimes regarded as an indication of “toughness” which increases as hardness decreases. A recent study was conducted using a controlled group of Tungsten Carbide with Cobalt samples with consistent TRS values. It demonstrated a different relationship between the TRS and hardness of Tungsten Carbide with Cobalt compounds.

“It was shown that TRS is closely related to the hardness and facture toughness. Within a hardness range of 800 < Hv < 1500 kg/mm2, TRS appears to first increase and then decrease as the hardness increases. It reaches a peak value at Hv ≈ 1300 kg/mm2.” Zhigang Zak Fang 2004.

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.

Friday, 1 April 2011

What sort of hardness can be achieved?

Tungsten Carbide has a high melting point, 2,870 °C (5,200 °F), is also extremely hard ( Vickers hardness number = 2242) and has a low electrical resistivity (~2×10−7 Ohm•m), compared to other metals (e.g. vanadium 2×10−7 Ohm•m).

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.

Moh’s hardness scale
Tungsten Carbide falls between 8.5 and 9.0 on Moh's hardness scale, making it almost as hard as diamond.
In order to polish or finish tungsten carbide, a diamond covered tool must be used because any other metal or mineral would not be able to withstand its hardness. Only a diamond is hard enough to be able to scratch Tungsten Carbide.