Effect of initial microstructure on microstructure evolution and mechanical properties of 0.12C martensitic steels during quenching and tempering

The microstructure evolution and mechanical properties of a Fe–0.12C–0.2Si–1.6Mn–0.3Cr–0.0025B (wt.%) steel with different initial microstructures, i.e., hot rolled (HR) and cold rolled–annealed (CRA), were studied through optical microscopy, scanning electron microscopy, electron channeling contrast imaging, microhardness and room temperature uniaxial tensile tests. After water quenching from 930 °C to room temperature, a fully martensitic microstructure was obtained in both as-quenched HR and CRA specimens, which shows a microhardness of 480 ± 5 HV, and no significant difference in microstructure and microhardness was observed. Tensile test results show that the product of tensile strength and total elongation (UTS × TE) of the as-quenched HR specimen, i.e., 24.1 GPa%, is higher than that of the as-quenched CRA specimen, i.e., 18.9 GPa%. While, after being tempered at 300 °C, the martensitic microstructures and mechanical properties of the two as-quenched specimens change significantly due to the synergy role of the matrix phase softening and the precipitation strengthening. Concerning the maximum UTS × TE, it is 18.9 GPa% obtained in the as-quenched CRA one, while that is 24.4 GPa% obtained in the HR specimen after tempered at 300 °C for 5 min.