Sequential Carbide Precipitation During Tempering and Its Influence on Strength-Toughness Balance in 31CrMoNiNbV Secondary Hardening Martensitic Steel.

The temperature-dependent carbide precipitation behavior and mechanical properties in 31CrMoNiNbV secondary hardening martensitic steel were investigated over a tempering temperature range of 200-650 °C. Sequential carbide precipitation behavior was characterized using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy, which was further correlated with tensile properties, hardness and low-temperature toughness. At the high tempering temperature of 580 °C, the investigated material achieved a well-balanced combination of tensile strength of 1633 MPa and total elongation of 15.1%, wherein tempering-induced M2C precipitation contributed to strength retention, while dislocation recovery and the formation of high-angle grain boundaries led to concurrent improvement in ductility and toughness. A predictive framework linking tempering temperature to carbides evolution and the resultant mechanical properties has been established for the investigated material, guiding the microstructural and heat treatment parameter design of ultra-high strength secondary hardening martensitic steels.