Effect of cyclic quenching on surface martensitic variant selection and mechanical properties of Ni–Cr–Mo microalloyed steel

This study systematically investigates the effects of cyclic heat treatment processes on the surface microstructure and mechanical properties of the experimental steel. The research results indicate that with the increase in the number of cyclic quenching cycles, the microstructure, size, and type of precipitates in the surface of the experimental steel remain essentially consistent. Therefore, this study reveals the role of cyclic quenching in controlling variant selection for toughening mechanisms from the unique perspective of variant reconstruction. Results indicate that as the increase in the number of cycles, grain refinement enhances the selectivity of martensite variants. This in turn reduces the number of CP and Bain groups within the prior austenite grains (PAGs), resulting in an increased length of high angle grain boundaries (HAGBs). Thereby effectively inhibiting the propagation of cracks along the variant interfaces, preventing direct penetration through austenite grains, and inducing crack deflection. In addition, the high density of slip systems and Schmid factor promote plastic deformation in the variant regions of the experimental steel, effectively inhibiting crack propagation. Remarkably, the DQT experimental steel demonstrated superior low-temperature toughness at -40 °C in the surface layer, attaining an impact energy of 197 J, which corresponds to a 4.23-fold increase over that of the SQT experimental steel surface layer. It also exhibited excellent tensile strength (936.5 MPa) and yield strength (858.5 MPa). This study provides new insights for future optimization of surface toughness in thick plates through microstructure control.