FRACTURE PROPERTIES OF A CR-MO FERRITIC STEEL IN HIGH-PRESSURE GASEOUS HYDROGEN

Recently, the measurement of threshold stress intensity factors for various low alloy ferritic steels was performed in high-pressure hydrogen gas, and it was revealed that the threshold for subcritical crack extension under rising displacement was lower than that for crack arrest under constant displacement. We previously examined the threshold for subcritical crack extension of ASME SA-372 Grade J ferritic steels using an unloading elastic compliance method in gaseous hydrogen at pressure up to 115 MPa, and reported that the cracking thresholds under the unloading elastic compliance method are consistent with the cracking threshold under the continuously rising displacement method. In the current study, the threshold stress intensity factor of JIS SCM 435 steel with yield strength of 700 MPa was measured using the unloading compliance method in high-pressure hydrogen gas. The roles of environmental (gas pressure) and testing (displacement rate) parameters were evaluated. The hydrogen gas pressure ranged from 10 to 115 MPa, and the displacement rate was varied between 2x10(-3) and 2x10(-5) mm/s. The measured values of the threshold for subcritical crack extension, K-JIC,K-H, decreased with either increasing hydrogen gas pressure or decreasing displacement rate. When the hydrogen pressure was 115 MPa, however, the cracking threshold showed a modest dependence on displacement rate, varying from 63 to 43 MPa m(1/2). These results demonstrate that the measurement conditions, such as the hydrogen pressure and the displacement rate, affect values of the subcritical cracking threshold under rising displacement.