Residual stress evolution enhanced martensite phase transition and texture development in cryogenic-tempered WC-Co ultra-coarse grained cemented carbide

The aim of this study is to clarify the effect of residual stress on the microstructure, martensite phase transition of Co binder phase, texture development of WC hard phase and mechanical properties enhancement in cryogenic-tempered WC-6wt.%Co ultra-coarse grained cemented carbides. Sin²(ψ)-2θ method was used to analyze the residual stress, and EBSD was used to reveal the phase transition of Co phase and texture of WC phase. The results showed that the density, grain size and carbon balance of cemented carbide revealed little difference after cryogenic-tempering treatment. The mean compressive residual stress of WC phase increased by 60.84% after cryogenic treatment, while tempering treatment resulted in a significant relaxation of the residual stress by 101.97% and even transformed the compressive residual stress to tensile stress. EBSD analysis showed that the mean ratio of hcp-to-fcc Co increased from 1.75% to 4.22% and 11.61%. It is difficult to distinguish the effect of residual stress increase and the temperature decrease on the martensite transition during the cooling process. However, the occurrence of martensite phase transition during tempering is not temperature-dependent but highly related to the residual stress evolution. After cryogenic-tempering treatment, {0001}<11 2‾ 0> preferred orientation of WC phase was concentrated, which was related to the basal plane slip and grain deflection of WC grains during the evolution of residual stress. Due to the martensite phase transition strengthening of Co and the preferred orientation of WC, the mean Vickers hardness increased from 1068.4 HV30 to 1149.4 HV30. The mean transverse rupture strength firstly decreased from 2747.5 MPa to 2515.3 MPa, and then increased to 2705.0 MPa, which was related to the increase and relief of residual stress during the post treatment. Furthermore, mean fracture toughness showed a slight increase from 22.5 MPa m^1/2 to 23.4 MPa m^1/2. The simultaneous increase of fracture toughness with hardness can be attributed to the residual stress evolution and martensite phase transition.