Mechanistic understanding of environmentally assisted fatigue crack growth of austenitic stainless steels in PWR environments

Abstract

This paper describes an investigation into the mechanisms influencing environmentally assisted enhancement of fatigue crack growth of 304L stainless steel in PWR primary water. Pre-cracked specimens were tested under loading conditions containing hold periods, either using a trapezoidal waveform, or periods of saw-tooth loading interspersed with long hold periods. Several post-test characterization methods were used to provide insight into the mechanisms influencing crack growth behavior. A correlation was observed between steel sulfur content and reduced environmental enhancement, which was more pronounced under trapezoidal loading than for saw-tooth loading following extended hold periods. Post-test examination linked enhanced crack growth with highly faceted fracture surfaces, whilst lower levels of enhancement showed a less faceted and more heavily oxidized appearance. The observations suggest that, whilst enhanced corrosion due to MnS dissolution from the steel is the cause of retarded crack growth rates, different retardation mechanisms appear to contribute at high and low stress ratios. This programme was sponsored by The Electric Power Research Institute (EPRI) and a full report describing the full programme of work is available as EPRI report#3002007973.