Hydrogen embrittlement of a V plus Nb-microalloyed medium-carbon bolt steel subjected to different tempering temperatures

The effect of different tempering temperatures on the hydrogen embrittlement (HE) resistance of a V + Nbmicroalloyed medium-carbon bolt steel was studied by slow strain rate tensile testing and hydrogen thermal analysis. The HE resistance represented by notch tensile strength 6bN of hydrogen-charged specimens increases with increasing tempering temperature Ttem, and the 6bN of the in situ hydrogen-charged sample is considerably lower than that of the pre-hydrogen-charged sample. Both the diffusible hydrogen content Cdiff and hydrogentrapping capability of the precipitates show a significant increase-decrease change trend with Ttemp. The ratio of Cdiff primarily trapped by nanoscale V-rich MC precipitates increases linearly with increasing Ttemp. The control of V-rich MC through suitable increasing Ttemp is vital for obtaining the best HE resistance although at the cost of decreased strength. It is thus suggested that a compromise between strength and HE resistance should be considered for the practical application of this type of steel.