Aging Mechanism of Sintered WC-Co Hard Metals

Abstract

Properties of two phase WC-Co alloys are not identical but vary remarkably. This is due to the fact that the tungsten content of the binder phase varies extremely from 2∼3% min to 9∼10% max in the two phase range with a very small variation of the carbon content in the alloys. Moreover, properties of the two phase alloys, when their carbon contents are lower, vary abrubtly by low temperature annealing. For example, the transverse-rupture strength, magnetic saturation, electrical resistivity, and lattice parameter of the binder phase decrease, while the hardness of the binder phase increases. Such aging phenomena in cemented carbides are caused by the precipitation of tungsten atoms from the binder phase.
The results mentioned above were fully described in the previous papers^(1)∼(5), following which the present study was carried out on the aging mechanism of cemented carbides, i. e., the mechanism of precipitation of tungsten atoms when low carbon alloys were annealed. The specimens used were two phase WC-Co alloys vacuum-sintered at 1400°C for 1 hr and subsequently vacuum-annealed up to 30 hr at temperature below 950°C. The results are summarized as follows:
(1) Tungsten atoms supersaturated in the binder phase precipitate in the form of CO₃W according to the reaction γ (f.c.c.)→γ (f.c.c)+Co₃W. (2) The precipitate of Co₃W is formed on (001) cubic planes of the face-centered binder phase. (3) The hardness increase of the binder phase is considered due to a precipitation hardening by Co₃W. (4) The decrease in magnetic saturation is also attributed to the precipitation of Co₃W presumed to be nonmagnetic. (5) The solid solubility of tungsten in the cobalt phase is determined. The strength of low carbon alloys seems to be considerably improved by solution-treatment. But, for the purpose of obtaining high strength alloys, the preparation of high carbon alloys are far more desirable than the solution-treatment of low carbon alloys.