A study on WC-Ni cemented carbides: Constitution, alloy compositions and properties, including corrosion behaviour

The manuscript presents a systematic study on three groups of Ni-based cemented carbides (WC-20 wt% Ni, WC-20 wt% Nil5Cr and WC-20 wt% Ni11Cr6Mo) based on thermodynamic calculations and experimental studies. Samples were prepared with different gross carbon contents (carbon activities) with the aim to provide a comprehensive overview on: constitution and phase formation, solubility of elements in the binder phase, binder and composite hardness, as well as their corrosion behaviour. A WC-20 wt% Co8Cr was used as standard binder system for comparison. The results show that the amount of alloying elements that can be dissolved in the binder phase is significantly higher in the Ni binder as compared to the Co binder (e.g., the solubility of W in WC-Ni is about double of that observed in conventional WC-Co). The degree of alloying is primarily affected by the carbon content. The lower the gross carbon content within the processing window, the higher is the degree of binder alloying. In WC-Ni, up to 12 at% W is observed in solution in low carbon alloys, as compared to 4 at% W in high carbon grades. Additions of Cr are limited to 9-11 at% Cr, depending on the gross carbon content. When exceeding this value, Cr3C2 is formed in high carbon Ni alloys. The solubility of Mo in WC-NiCrMo cemented carbides is limited to about 2 at%, at higher additions a carbide is formed, most likely (W,Mo)C. The high hardness values observed in NiCr and NiCrMo alloys when compared to plain Ni alloys can be attributed to the effect of Cr and Mo as WC grain growth inhibitors, and not to a significant enhancement of the intrinsic binder hardness. In contrast, our standard Co grade demonstrated a significantly higher binder and composite hardness than all of the nickel variants studied. Finally, high-alloyed materials (low carbon NiCr- and NiCrMo alloys) did present a superior corrosion resistance compared to the CoCr grade even in the most severe corrosion environments. Higher degree of alloying (i.e., higher amounts of Cr, W, and Mo present in solid solution in the binder) is indicative of superior corrosion behaviour due to the nature of the protective layers formed under corrosive environments.