Influence of gaseous hydrogen on the notched tensile strength of D6ac steel

Slow displacement-rate tensile tests were performed on D6ac tempered specimens to investigate the influence of gaseous hydrogen pressure on the notched tensile strength (NTS) and the associated fracture characteristics. The susceptibility to hydrogen embrittlement of specimens at a given pressure was determined by the NTS loss, which tended to decrease as the tempering temperature increased. The NTS loss for T-300 (300°C tempered) and T-450 (450°C tempered) specimens increased with rising hydrogen pressure up to 5×105 Pa and then remained nearly the same up to the pressure of 2×106 Pa, while no deteriorated NTS in hydrogen has been found for T-600 (600°C tempered) specimens. The extent of intergranular fracture and/or the region of flat fracture on tensile fractured surfaces were consistent with the correlation of hydrogen pressure and NTS loss. The smaller the intergranular and/or flat fracture regions, the greater the resistance to hydrogen embrittlement of the specimen would be expected. Besides the effect of notch tip blunting, the excellent performance of T-600 specimens in hydrogen could be attributed partly to the presence of less continuous carbides at prior austenite grain boundaries. In contrast, T-300 specimens with grain boundary carbides in a more continuous manner were highly susceptible to hydrogen embrittlement.