Abstract
Considering in design by analysis, four types of tests, slow-strain-rate tensile (SSRT), fatigue life, fatigue crack-growth (FCG), and elasto-plastic fracture toughness (JIC) tests, were conducted with low-alloy steels, JIS-SCM435 and JIS-SNCM439, in 115 MPa hydrogen gas and air at room temperature (RT). In addition to above tests at RT, the SSRT tests were also conducted in 115 MPa hydrogen gas and air at 120 oC and in 106 MPa hydrogen gas and 0.1 MPa nitrogen gas at -45 oC. The low-alloy steels used in this study had tensile strengths (σB) ranging from 824 to 1201 MPa with fine and coarse tempered-martensitic microstructures. In the SSRT and fatigue life tests, the tensile strength and fatigue limit were not degraded in hydrogen gas. The FCG tests revealed that the FCG rate (da/dN) was accelerated in hydrogen gas; however, there existed an upper bound of the FCG acceleration, showing the FCG rate in hydrogen gas was about 30 times larger than that in air, when σB was lower than 900 MPa. The JIC tests demonstrated that the fracture toughness (KIC) in air was 207 MPa·m1/2 at σB = 900 MPa, whereas the hydrogen-induced crack-growth threshold (KI,H) was 57 MPa·m1/2 at σB = 900 MPa. Based on these results, we proposed advanced guidelines on the use and design for SCM435 and SNCM439 on design by analysis in 115 MPa hydrogen gas, which enable to design the storage cylinders used in 70 MPa hydrogen station with lower cost without compromising safety.
