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.

Hydrogen embrittlement (HE) behavior of 31CrMoNiNbV steel subjected to austempering below the M_s temperature was investigated. The material exhibited a martensitic/bainitic dual-phase microstructure, with uniformly dispersed V- and Nb-rich MC carbides as well as Cr-rich M₃C carbides precipitated in the matrix. It achieved an ultimate tensile strength of 1688 MPa and an elongation to fracture of 10.7%. Slow strain rate tensile tests revealed that the HE susceptibility index reached 13.5%. Thermal desorption spectroscopy and hydrogen permeation tests showed that the martensite/bainite phase boundaries and the dispersed MC/M₃C carbides acted as effective hydrogen-trapping sites, lowering the hydrogen diffusivity and alleviating local hydrogen accumulation in stress-concentrated regions. Microstructural analysis further indicated that the dual-phase structure may simultaneously mitigate the hydrogen-enhanced localized plasticity and hydrogen-enhanced decohesion mechanisms, through combined stress relaxation at martensite/bainite interfaces and hydrogen immobilization by carbide traps.