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

• Effect of austenitizing temperature ( T aust ) 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 T aust at both tempering temperatures. • The mechanism for the variations of the HE resistance with T aus was explored. The effect of austenitizing temperature ( T aust ) 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 T aust . The NTS for the 600 °C tempered sample was ∼16 % larger than that for the 500 °C tempered one at each T aust . The diffusible hydrogen content C Hr exhibited a slight increase with raising T aust for the 500 °C tempered samples while it showed a sharp increase with raising T aust for the 600 °C tempered samples. The increased prior austenite grain (PAG) size were mainly accorded for the deteriorated HE resistance with raising T aust 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 T aust when tempered at 600 °C. It is concluded that the selection of suitable T aust should consider the HE resistance besides obtaining the desired strength level.