Influence of PWR Primary Coolant Environment on Corrosion Fatigue Crack Growth of Austenitic Stainless Steel

The rate of growth of flaws in reactor circuit components by fatigue is usually determined using the reference crack growth curves in Section XI of the ASME Boiler and Pressure Vessel Code. These curves describe the rate of crack propagation per cycle (da/dN) as a function of the applied stress intensity factor range (ΔK). No reference curves for water-wetted defects in austenitic stainless steels are currently available. This paper describes the results of testing of Type 304L stainless steel in simulated PWR primary coolant over a range of temperatures and mechanical loading conditions. The data on wrought stainless steel presented in this paper demonstrate that crack growth rates can be significantly enhanced by the PWR primary environment at temperatures between 150°C and 300°C. The degree of enhancement increases significantly with reducing loading frequency and decreases with decreasing water temperature. The environmental influence on fatigue is also smaller at very high R ratio (≥0.85). At long rise times the maximum enhancement of crack growth rate over inert crack growth rates was between 1 and 2 orders of magnitude at 250–300°C. However there is evidence that at very long rise times the environmental effect starts to decrease again. The conditions under which this occurs are influenced by temperature and water flowrate, with turbulent flow conditions appearing to have a limited beneficial effect. Due to the strong time dependence of crack growth rate, the data are best rationalized using a time domain (a˙_e–a˙_i) approach.