Hydrogen-assisted cracking of Cr-Mo steel in slow strain rate tensile test with high-pressure gaseous hydrogen

Slow strain rate tensile (SSRT) tests were performed using smooth specimens of quenched and tempered JIS-SCM435 steels with three different tensile strengths (TS), which are ranged from 824 to 1127 MPa. The tests were carried out in 115 MPa hydrogen gas and reference gases (air or 115 MPa nitrogen gas) at three different temperatures; 233 K, room temperature (RT) and 393 K. In the reference gases, the specimens exhibited the so-called cup-and-cone fracture at every temperature. On the other hand, in hydrogen gas, a number of cracks initiated at specimen surface and grew, which led to a marked reduction in ductility at every temperature. The crack growth curves were obtained as a function of true strain by observing the specimen surface of the fractured specimens. The true strain at which the hydrogen-assisted cracking starts was strongly dependent on the microstructure, strength level and test temperature. However, in all the materials tested at RT, the hydrogen-assisted cracking did not occur during the uniform deformation, but occurred in the necking process. Even at 233 K and 393 K, the material with a moderate strength did not exhibit the hydrogen-enhanced cracking before reaching the TS. The result ensured that the Cr-Mo steel with a moderate strength can maintain the TS even in 115 MPa hydrogen from the viewpoint of fracture mechanism.