Effect of hydrogen on tensile behavior of low alloy steel in the regime of dynamic strain ageing

Abstract

Low alloy steels typically used for reactor pressure vessel (RPV) in light water reactors may undergo different degradations and ageing mechanisms during service like fatigue, strain-induced corrosion cracking and corrosion fatigue or irradiation embrittlement, the latter being recognized as life limiting factor. There is growing concern that hydrogen, absorbed from the high temperature water environment and corrosion reactions, may potentially reduce toughness of RPV steels in synergy (or competition) with other embrittlement mechanisms like irradiation embrittlement, thermal ageing or dynamic strain aging (DSA). Strain rate, temperature and occurrence of DSA in these steels may affect the severity of the effect of hydrogen on toughness. The present investigation was envisaged to characterize the effect of hydrogen on tensile and fracture behavior of low alloy RPV steels at different strain rates and temperatures with a special emphasis on the synergy between DSA and hydrogen embrittlement. For this reason, tensile tests were carried out with as-received and hydrogen pre-charged specimens between 25 and 400 °C and at strain rates between 10-1 and 10-6 s-1. The fracture mode was evaluated by detailed post-test fractography in the scanning electron microscope. DSA in these steels was established by the occurrence of serrations, negative strain-rate sensitivity and a maximum/minimum in strength/ductility at intermediate temperatures and strain rates. The DSA peak and range were found to be shifted to lower temperatures with decreasing strain rates and vice-versa. The hydrogen pre-charging resulted in marginal softening and strain-rate dependent reduction in ductility at 250/288 °C. The hydrogen embrittlement and reduction in ductility were more pronounced and the strain rate range for hydrogen embrittlement significantly extended in the RPV steel with higher DSA susceptibility demonstrating some synergy between DSA and hydrogen effects, probably due to the localization of plastic deformation. In presence of hydrogen, shear dominated ductile fracture (microvoid coalescence) with varying amounts of quasi-cleavage regions and secondary cracking along the prior austenite grain boundaries were observed. A detailed investigation on these aspects and a tentative mechanistic explanation is presented in this paper.