Hydrogen Embrittlement Resistance of an Austempered Martensite/Bainite Dual-Phase Steel.

Hydrogen embrittlement (HE) behavior of 31CrMoNiNbV steel subjected to austempering below the Ms 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.