Influence of cyclic strain rate on environmentally assisted cracking behavior of pressure vessel steel in high-temperature water

Influence of strain rate on low cycle fatigue (LCF) resistance of ASTM A533B pressure vessel steel was investigated in simulated BWR water. Special attention was paid to the environmentally assisted cracking (EAC) behavior. It was found that LCF resistance of the steel degraded with a decrease in cyclic strain rate. An anomalous response for peak tensile stress difference to the strain rate was observed due to dynamic strain aging (DSA). At a higher strain rate, fatigue cracks grew in a tortuous manner and were inclined to the loading axis. Typical hydrogen-induced cracking patterns were observed on the fracture surfaces. With a decrease in the strain rate, however, fatigue cracks tended to become entirely straight and normal to the loading axis. Hydrogen-induced cracking features also diminished on the fracture surfaces. The above cracking behavior may be attributed to a strain-rate-induced change in dominant EAC mechanism from hydrogen-induced cracking to film-rupture/slip-dissolution controlled cracking. Comparison between the experimental data and ASME design fatigue curves indicated that the present steel possessed a certain safety margin in simulated BWR water.