Ni-Cr-Mo鋼の水素脆化割れにおけるき裂の枝分かれ

Abstract

Delayed failure tests in air were carried out on the Ni-Cr-Mo steel specimens which were hydrogen pre-charged after quenching from 1123 K and tempering at 637 K, in order to investigate the effect of the crack propagation rate da⁄dt (which expressed the concentration of hydrogen inside the specimen) on the stress intensity factor at crack branching KIB and the crack branching angle. The results were compared with those of the specimens no hydrogen pre-charged and tested in water.For the specimens pre-charged and tested in air, the crack branching occurred when the crack propagation rate da⁄dt was higher than 20×10−6 m/s and KIB decreased with increase in da⁄dt, while when da⁄dt was lower than 20×10−6 m/s, the unstable crack propagation occurred without crack branching. The branching angle of the pre-charged specimen tested in air was smaller than that of the no pre-charged specimen tested in water. These results can be explained both by a criterion which was introduced by a combination of hydrostatic tensile stress with hydrogen concentration at crack tip and by the difference in hydrogen diffusion path between the no pre-charged specimen tested in water and the pre-charged specimen tested in air.