On the formation process of (W.Ti)C powder

The mechanism of formation of (W⋅Ti)C powder, one of the most important raw materials for cemented carbides, has been studied by microscopy, X ray and XMA analyses. Various combinations of starting powders, i.e., W, WC, TiO2, TiC and carbon black, were used. Coarse powders were used for optical microscopic observations.The results obtained were as follows.(1) For all combinations of starting powders, the dissolution of W and Ti began after W had almost changed to WC. When Ti02 was used as a starting powder, the formation of solid solution was assumed to begin at the temperature where TiC was formed from TiO2.(2) With an increase in the heating temperature, (W⋅Ti)C was formed at the surface of WC particles, and the carbide particles showed shell structure of (W⋅Ti)C as observed in the cross sections.(3) The mechanism of formation of (W⋅Ti)C in the solid state reaction was considered as follows; Ti (TiC) diffuses on the surface of the WC particle very rapidly and reacts with WC on the particle surface.The reason for such rapid diffusion of Ti (TiC) on the carbide surface was discussed.(4) When the carbide particle with WC in the core was heated with cobalt above the eutectic temperature, WC which remained as a result of insufficient diffusion in the solid solution reaction, dissolved into the liquid eutectic and W diffused into TiC from the surface. As a result, (W⋅Ti)C phase formed in the shape of ring around TiC particle.