Sintering plus HIP of ultrafine WC-Co hardmetals

This work analyses the changes induced on the microstructure and mechanical properties of ultrafine (0.2 and 0.4 mu m) hardmetal grade WC-7wt.% Co by HIP after vacuum sintering. A large HIP pressure (155 MPa) is applied to the ultrafine hardmetal system at different temperatures (1000, 1200 and 1400 degrees C. The well-known correlation between porosity reduction and fracture strength improvement is confirmed. Residual porosity left after liquid phase sintering is removed more effectively at HIP temperatures above the eutectic point of the alloy (1400 degrees C. In the absence of grain growth inhibitors, hardness decreases continuously, and WC-Co ultrafine microstructure coarsens as HIP temperature rises. However, for specimens containing VC and Cr3C2 additions, hardness increases as HIP temperature rises from 1200 to 1400 degrees C. It is proposed that this anomalous trend (confirmed by grain size observations) is related to the activation of coalescence mechanisms during solid state HIPing, which are inhibited by the presence of a liquid phase.