WC-Co by Direct Reduction Carburisation Reactions

Abstract

Thermogravimetry, XRD and SEM have been used to study the direct reduction/carburisation of non-stoichiometric CoW0₄ with hydrogen/methane mixtures over the temperature range 900°C to 1100°C. The evolution of phases during reduction/carburisation has been related to differences in starting materials, temperature and gas phase composition. The thermochemistry of the reduction/carburisation process is detailed and actual results of processing are related to thermodynamic predictions. It is shown that the first stage of reaction is reduction of the tungstate to give the intermetallic compounds Co₃W and Co₇W₆ as well as tungsten metal. During this stage no carburisation occurs. The subsequent carburisation stage of the process yields a sequence of phases passing through carbides of the general formulae M₁₂C and M₆C before giving a mixture of WC and Co. In W-rich mixtures a transitory existence of W₂C is shown to exist.