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

Slow displacement-rate tensile tests were performed on D6ac. tempered speci mens to investigate the influence of gaseous hydrogen pressure on the notch ed tensile strength (NTS) and the associated fracture characteristics. The susceptibility to hydrogen embrittlement of specimens at a given pressure w as determined by the NTS loss, which tended to decrease as the tempering te mperature increased. The NTS loss for T-300 (300 degreesC tempered) and T-4 50 (450 degreesC tempered) specimens increased with rising hydrogen pressur e up to 5 x 10(5) Pa. and then remained nearly the same up to the pressure of 2 x 10(6) Pa, while no deteriorated NTS in hydrogen has been found for T -600 (600 degreesC tempered) specimens. The extent of intergranular fractur e and/or the region of flat fracture on tensile fractured surfaces were con sistent 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 e ffect of notch tip blunting, the excellent performance of T-600 specimens i n hydrogen could be attributed partly to the presence of less continuous ca rbides at prior austenite grain boundaries. In contrast, T-300 specimens wi th grain boundary carbides in a more continuous manner were highly suscepti ble to hydrogen embrittlement. (C) 2001 Elsevier Science BY, All rights res erved.