Test method for threshold of hydrogen-induced crack growth <i>K</i>_I,H of SCM435 in 115 MPa hydrogen gas

Abstract

In order to investigate the suitable test methods for determining threshold of hydrogen-induced crack growth, K I,H , J Ic , none-unloading J Ic , constant-load and constant-displacement tests were conducted in 115 MPa hydrogen gas at room temperature, using SCM435 low alloy steels whose tensile strengths were between 824 and 1127 MPa. Constant-load and constant-displacement tests have been standardized by ASME KD-10 for determining K I,H in hydrogen gas. J Ic test has been standardized by ASTM E1820 for determining fracture toughness J Ic or K Ic in air. Recently, J Ic test was done in hydrogen gas to obtain K I,H . In this study, there were many cases when J Ic tests were not conducted in hydrogen gas, because CT specimens broke before the first unloading. Then, none-unloading J Ic test, where unloading processes were eliminated in J Ic test, was employed in hydrogen gas. All four tests were done under small scale yielding in 115 MPa hydrogen gas. K I,H obtained by four tests was almost the same. KD-10 demands that constant-load and constant-displacement tests of ferritic steels such as SCM435 are done for 1000 hours. Contrary to this, none-unloading test at displacement rate of 2×10 -3 mm/s was finished within one hour. It was concluded from these results that none-unloading J Ic test was the best test for determining K I,H in 115 MPa hydrogen gas. On the other hand, J Ic test was the only test for determining K Ic in air, because K Ic was obtained under large scale yielding.