Implications of cryogenic treatment on microstructure, phase formation, mechanical and tribological properties of tungsten carbide cutting bits with varying cobalt content for mining applications

Abstract

This study aims to provide a comprehensive analysis on the effect of cryogenic treatment on two different grades of WC rock cutting bits (low cobalt grade: WC-9 wt% Co and high cobalt grade: WC-25 wt% Co). The cutting bits are exposed to cryogenic treatment at different holding time such as 12, 24 and 36 h. The treated and untreated bits are characterized for low temperature DSC, phase analysis, microstructural study, elemental composition, hardness, fracture toughness and scratch test. Phase shift was observed for both the cases due to the martensitic phase transformation from α-Co (fcc) to ε Co (hcp) during cryogenic treatment. This transition was confirmed with low temperature DSC analysis between −90 °C and − 60 °C. New η carbide particles (Co6W6C) was formed for WC-25%Co and no such carbides was found for WC-9%Co. CT 36 of WC-25%Co exhibited a higher level of precipitation of η carbides. The results show that CT has enhanced hardness for both low (14.08%) and high cobalt (23.34%) WC cutting bits compared to untreated bits whereas density is not altered evidently. CT 24 and CT 36 possessed higher hardness and lower fracture toughness for WC-9%Co and WC-25%Co respectively. Higher CoF and formation of tribo layer was observed for treated bits whereas untreated bit experiences surface fracture and cracks as a result of scratch test. As a finding, mining sectors can utilize cutting bits that have undergone sustainable cryogenic treatment for rock cutting applications.