EFFECT OF HYDROGEN CHARGING ON THE CRACK-PROPAGATION AND CRACK BRANCHING IN DELAYED FAILURE OF NI-CR-MO STEEL

Delayed failure tests were carried out on Ni-Cr-Mo steel quenched and tempered at 673K in a NaCl aqueous solution in hydrogen charging condition, and the crack branching as well as crack propagation behaviors were investigated. The crack propagation rate da/dt increases with increase in cathode charging current density, i, and da/dt increases to a maximum and then decreases with increase in the increasing rate of stress intensity factor, K. The stress intensity factor at crack branching, K//I//B, decreases to a minimum and then increases with increase in i. These results can be explained by interaction between dislocations and hydrogen atoms, such as dislocation trapping and dislocation transport of hydrogen on the slip planes near the crack tip.