Hydrogen embrittlement of a V + Nb-microalloyed high-strength bolt steel subjected to different austenitizing temperatures

•Effect of austenitizing temperature (Taust) on HE of a novel bolt steel was studied.•Coupling effect of grain size and nanoscale MC precipitates on HE was evaluated.•HE resistance decreased with increasing Taust at both tempering temperatures.•The mechanism for the variations of the HE resistance with Taus was explored. The effect of austenitizing temperature (Taust) on the hydrogen embrittlement (HE) of a V + Nb-microalloyed high-strength bolt steel was investigated by slow strain rate tensile (SSRT) testing using pre-hydrogen-charged notched round bar tensile specimen. The steel was austenitized at three different temperatures of 850 °C, 945 °C and 1000 °C followed by tempering at 500 °C and 600 °C. The results show the HE resistance represented by notch tensile strength (NTS) exhibited a decreasing trend with increasing Taust. The NTS for the 600 °C tempered sample was ∼16 % larger than that for the 500 °C tempered one at each Taust. The diffusible hydrogen content CHr exhibited a slight increase with raising Taust for the 500 °C tempered samples while it showed a sharp increase with raising Taust for the 600 °C tempered samples. The increased prior austenite grain (PAG) size were mainly accorded for the deteriorated HE resistance with raising Taust when tempered at 500 °C, and the absorption of a large amount of hydrogen because of the precipitation of profuse nanoscale V-rich MC besides the increased PAG size were mainly accorded for the decreased HE resistance with raising Taust when tempered at 600 °C. It is concluded that the selection of suitable Taust should consider the HE resistance besides obtaining the desired strength level.