The formation of tungsten carbide in cobalt-base wear-resistant coating alloys

Abstract

Tungsten carbide-cobalt alloys cannot be produced by melting because of a peritectic reaction in the W-C system; they are produced by sintering. Tungsten carbide-cobalt powders (mixed, agglomerated, sintered) can be plasma sprayed or deposited by other techniques but they cannot be fused afterwards without decomposition of the tungsten monocarbide that provides the best mechanical properties for many applications. Wear-resistant cobalt alloys were developed 60 years ago and are based on cobalt-chromium-tungsten-carbon. During studies of the CoCrWC system with various carbon concentrations and at various temperatures we identified MC, M 2C, M 7C 3, M 23C 6, M 6C, M 12C and M 28C carbides. The solidifying M 6C carbide is unstable over a certain concentration range of chromium and decomposes to form tungsten carbide (WC). On heat treatment the tungsten-containing M 6C forms WC in a cobalt-chromium matrix if the chromium content is less than 5 wt.%. It is therefore possible to produce a sprayed and fused or welded layer of WC-cobalt alloy. The rate of WC formation depends on temperature and time. Time-temperature-decomposition diagrams have been established for four alloys. The structures of the heat-treated alloys are similar to those of sintered tungsten carbide-cobalt alloys.