Effect of Microalloying of V, Nb and Mo on Hydrogen Embrittlement Susceptibility of 2 GPa-grade Medium-carbon Si–Cr Spring Steel with Tempered Martensite Microstructure

Hydrogen embrittlement (HE) susceptibility was evaluated on JIS–SUP12-based steel (SB), and V-, Nb- and (Nb+Mo)-added steels (SV, SNb and SNbMo, respectively) under uniaxial tension and high stress triaxiality conditions, to elucidate the roles of the microalloying elements in the HE mechanisms of 2 GPa-grade medium-carbon Si–Cr spring steels, which were obtained via low-temperature tempering. The SV steel contained solute V and undissolved V carbides, the SNb steel undissolved Nb carbides and the SNbMo steel solute Mo and undissolved Nb carbides. The microalloying of V, Nb and Mo decreased the apparent hydrogen diffusivity owing to strong hydrogen attraction by solute V and Mo, and reversible hydrogen trapping with V and Nb carbides. Although all the steels attained the 2 GPa tensile strength in hydrogen-uncharged state, hydrogen significantly reduced the tensile strength through premature failure before the onset of yielding under uniaxial tension condition. In the hydrogen-charged specimens, the strength was strongly correlated with the shear fracture surface area. The HE susceptibility was increased in the following order: SNbMo ≈ SNb < SB < SV. This suggests that hydrogen-induced plasticity mitigates the HE susceptibility in the SNb and SNbMo steels, whereas the solute V facilitates the hydrogen-induced plastic inhomogeneity, which leads to premature fracture. Under high stress triaxiality condition in micro-cantilever specimens, hydrogen decreased the intrinsic fracture resistance to one third compared to the uncharged specimens, regardless of the steels. In the microalloyed specimens, hydrogen suppressed intergranular fracture, whereas the dependence of fracture resistance on the microalloying element was minor.