Strength–toughness improvement of 13Cr4NiMo martensitic stainless steel with thermal cyclic heat treatment

To improve the strength–toughness of 13Cr4NiMo martensitic stainless steel (13-4MSS), a thermal cyclic heat treatment (TCHT) combined with the advantage of tempering was proposed. The microstructures were characterized by scanning electron microscopy, X-ray diffraction and electron backscattered diffraction, and the mechanical behaviors in terms of tensile properties and impact toughness were analyzed in correlation with microstructural evolution. It was found that grains and the martensitic matrix were refined by TCHT through the cyclic quenching transformation and austenite recrystallization, which was conducive to more nucleation quantity of reversed austenite during tempering. Two-spherical-cap nucleation model was used to explain the effect of refined grains of TCHT on the nucleation of reversed austenite. Grain refinement by TCHT improved the brittle fracture stress to reduce the ductile–brittle transition temperature and thus improved the cryogenic impact toughness of 13-4MSS. Reversed austenite distributed at the martensitic lath boundary enhances the crack arrest performance and increases the brittle fracture stress. It is concluded that reasonable TCHT plus tempering process significantly improves the strength–toughness of 13-4MSS, reflecting the comprehensive effect of grain refinement and reversed austenite.