Stress Corrosion Cracking, Fatigue and Combined Crack Growth Rates of Alloy 600 in a PWR environment

Abstract

By subjecting Alloy 600 to air fatigue (AF), environmentally assisted fatigue (EAF), and stress corrosion cracking (SCC) conditions, different aspects of the crack growth prediction laws and the superposition approach recommended in ASME XI have been tested.

For the heats of Alloy 600 used in this programme the air fatigue behaviour of as-received material is well predicted by ASME fatigue laws. However, upon the introduction of light water reactor (LWR) water conditions, the environmental fatigue laws alone under- predicted the measured crack growth rates for material in a susceptible condition. The prediction is improved significantly if the 30s rise time cap, clearly stipulated in ASME XI, is removed and the full rise-time considered. This aligns better with the original derivation of the fatigue crack growth law from NUREG CR/4667, which is identical to ASME without the rise time cap. The reasons for its inclusion in ASME are unclear due to a lack of references. Both environmental fatigue and SCC predictions were less accurate with increased temperature, despite the ASME laws considering temperature in both the fatigue and SCC predictions, and the introduction of cold work – both known to increase SCC susceptibility.

In general, the superposition approach, whereby the overall crack growth rate is considered as the sum of environmental fatigue and SCC rates, does appear to bound the data for LWR conditions providing the material is not in a condition that significantly enhances SCC (e.g. moderate or heavy cold worked), whereby rates may be under predicted.