Investigating the Effect of Temperature and Rise Time on the Fatigue Crack Growth Rates of Austenitic Stainless Steel Operating in Hydrogenated Water

Fatigue Crack Growth Rates (FCGRs) of austenitic stainless steels tested in a simulated Pressurized Water Reactor (PWR) primary coolant environment can be significantly enhanced when compared to those tested in air. At temperatures above 150 °C the relationship between temperature and FCGR enhancement follows an Arrhenius-type relationship. Historically, only very limited data exist at lower temperatures, and these data have led to different interpretations of actual low temperature behavior. In the present work, the effect of temperature has been investigated. FCGR tests were conducted using a variety of temperatures, with a particular focus on temperatures below 100 °C. The results from these tests revealed greater FCGRs than would be expected from extrapolation of the high temperature Arrhenius behavior from higher temperatures, suggesting a more complex interaction between material, deformation and environment. In order to understand the effect of temperature on FCGRs, specimens from these tests have been investigated using a range of complementary electron microscopy techniques. Fracture surfaces of specimens exhibited significant differences in morphology between stages conducted above and below 150 °C. The analysis of cross-sectioned samples tested at low temperatures also highlighted the presence of strain-induced martensite, which was confirmed by crystallographic analysis using EBSD and low voltage EDX semiquantitative analysis. The presence of α’-martensite is considered to be consistent with the ‘upturn’ in behavior observed at temperatures below 150 °C; the role of the environment and micro-deformation at the crack tip in relation to the enhanced FCGR is proposed and discussed.