Microstructural evolution and low temperature impact toughness of a Fe–13%Cr–4%Ni–Mo martensitic stainless steel

The microstructural evolution of a low carbon Fe–13%Cr–4%Ni–Mo (wt.%) martensitic stainless steel has been studied using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP). The as-quenched sample has a typical lath martensite structure, but carbon atoms were found to form ultra-fine clusters, or carbon-enriched regions, in the martensite. After a single-stage tempering at 680 °C, the steel mainly consists of the martensite and a certain amount of fine M23C6 carbides, which act as heterogeneous nucleation sites of the reversed austenite formed in the two-stage tempered (680 °C × 4 h + 600 °C × 4 h) samples. 3DAP investigations have revealed that the Ni, Cr and Mn atoms are enriched in the reversed austenite as contrasted with the martensite. Phosphorus atoms were uniformly distributed only in martensite, carbon was not detected in either martensite or austenite, and there was no segregation at the martensite–austenite interfaces. Those partitioning behaviors of the alloying elements were thought to be a factor contributing to the stability of the reversed austenite. High impact toughness at low temperature was obtained due to the existence of the reversed austenite.