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Cobalt Content and WC Grain Size determine Wear Resistance and Toughness of a Carbide

In general, carbides with finer microstructures can be manufactured with a decrea-sing grain size of the tungsten carbide. A requirement for this is the prevention of grain growth during the sintering process by adding suitable doping components in the right amount, adjusted to the cobalt content. The latter has been determined based on the required performance specifications for the carbide. Since the specific surface of a carbide depends reciprocally on its grain size, a finely-grained carbide can adsorb more binder than a coarsely-grained WC. If one considers the Iso hard-ness curve of a carbide in a diagram as a function of the carbide grain size and its cobalt content, the curve behaves as a decreasing polynomial function (cf. Figure).

In general, an increase in cobalt content results in increased toughness while hardness and wear resistance are reduced.

This opposite development of the two desirable parameters, hardness and toughness, can be countered by reducing the carbide grain size. The result is an increased hardness on account of the finer basic grain of the carbide which at the same time permits a high binding metal content as the grain structure offers a large specific surface, allowing for a high toughness. Consequently, superfine grain carbide grades offer increased hardness while maintaining toughness.

The Figure illustrates this relationship by displaying the properties of the GC grades GC-K6UF, GC-K40UF, GC-K44UF and GC-K55SF. GC-K40UF exhibits an average grain size of 0.6 µm and consists of 10 weight percent cobalt. The grade GC-K44UF is made of 0.5 µm tungsten carbide powder and contains 12 percent cobalt. While the toughness of both grades is approximately the same, the Vickers hardness of GC-K44UF is 5 percent higher. Our grade GC-K55SF with a grain size of 0.2 µm and a cobalt content of 9 percent offers a 24 percent higher Vickers hardness (HV30 = 1920 kg/mm), while ex-hibiting a reduction in toughness of only 5 percent.