An investigation into the effects of HIP after sintering of WC-ZrC-Co-Cr3C2 cemented carbides.

Abstract

The sintering behaviour of cemented carbides based on WC-ZrC-Co-Cr3C2 powder mixtures have been analyzed by dilatometric and calorimetric methods for different cobalt contents and WC/ZrC ratios. As expected, powder oxide reduction in these compositions is mainly of carbothermic nature. However, depending on the milling conditions, some highly stable Zr-rich oxides are retained in the binder phase after sintering. Hot isostatic pressing (HIP) cycles have been successfully applied for closing residual porosity after vacuum sintering. For a fixed amount of binder phase and a WC/ZrC ratio, the hardness of these materials depends on the amount of residual porosity and WC grain growth control. The best combination of hardness and toughness is found for alloys with 8 wt%Co and WC/ZrC wt. ratios of 6.46. HIP treatments induce the formation of a compact and well adhered layer mainly comprised of Zr oxides and WC grains. The cobalt binder phase migrates from this layer towards the sample bulk likely due to the loss of wettability on these Zr rich oxides. Hot hardness is higher for the alloy with higher WC/ZrC ratio suggesting that this property depends on both the volume fraction of (ZrxW1-x)C and WC phases and their degree of contiguity.