Crack Propagation Behavior of SNCM439 Steels in High-pressure Hydrogen Gas

To investigate the effect of high-pressure hydrogen gas on the fracture of high-strength low-alloy steels, rising load tests were conducted on JIS SNCM439 steel in high-pressure (20 MPa) hydrogen gas at room temperature (20-25 degrees C), and its hydrogen-induced crack initiation behavior was investigated. The load-crack opening displacement curve obtained for rising load tests in hydrogen began to deviate from that obtained in air at very low loads, indicating that the stress intensity factor at crack initiation was significantly smaller in hydrogen. Scanning electron microscopy observations of the fractured surfaces of the specimens unloaded during the middle of the rising load test confirmed that hydrogen-induced cracks had already occurred at a load lower than the deviation point. The stretch zone that appeared in the rising load test in air was not observed for the test in hydrogen, and the hydrogen-induced cracks were found to directly initiate from the tip of the fatigue pre-crack. The hydrogen-induced cracks were initiated at almost the same stress intensity factor value as that at which the stretch zone was observed in air, indicating that plastic slip and the resulting hydrogen ingress from the new surface were the causes of the hydrogen embrittlement. In addition, it was shown that the stress intensity factor for crack initiation in hydrogen increased and the effect of hydrogen decreased with the increase in loading rate, inferring that dislocation migration and hydrogen penetration into the steel are key factors for hydrogen embrittlement.