Microstructure evolution under various austenitizing and tempering temperatures and its strengthening mechanism in a low-alloy martensitic steel

This study systematically investigates the effects of austenitizing temperature on the microstructure-property relationship of D406A steel subjected to quenching and tempering. The microstructural characteristics are analyzed, and the kernel average misorientation (KAM) parameter is used to quantify local orientation variations. The prior austenite grain (PAG) size is determined via EBSD-based reconstruction, and the sizes of martensite packets, blocks, and laths are quantitatively measured. The results indicate that the sizes of PAGs, martensite packets, and martensite blocks decrease with decreasing austenitizing temperature, whereas the martensite lath size slightly increases. The highest strength-ductility product of 11719.8 MPa·% can be obtained for the QT890 sample, revealing an outstanding strength-ductility synergy. The strengthening mechanism of D406A steel is mainly attributed to solid solution strengthening, dislocation strengthening and grain boundary strengthening.