Factors influencing the rate of growth of fatigue cracks in RPV steels exposed to a simulated PWR primary water environment

Abstract

Fatigue crack propagation has been studied in A533B plate exposed to simulated PWR primary coolant environment, as part of a U.K. collaborative programme. Compact tension specimens 25 mm or 50 mm thick were fatigued in a non-refreshed autoclave over the temperature range 130–290°C and frequency range 0.00167–0.1 Hz. A wide range of ΔK values were studied using stepped loading techniques on steels with nominal sulphur contents in the range 0.006–0.019%. MnS inclusion distributions were characterized by sulphur printing. It was found that a threshold stress intensity amplitude ( ΔK ThEAC), needed to be exceeded to trigger sustained environmentally assisted fatigue crack growth rates in PWR primary coolant environments. Below ΔK ThEAC crack growth was found to be transgranular with ductile striations and secondary cracking giving rates per cycle similar to those observed in air. Above ΔK ThEAC the cracking remained transgranular, but the fracture morphology changed to facets with some branching and crack growth rates became independent of ΔK. Factors found to influence ΔK ThEAc and subsequent plateau crack growth rates were fatigue cycle frequency, water chemistry and flow rate, temperature and the size and distribution of MnS inclusions in the steel. The complex interplay of these variables in relation to plant operation and the use of the ASME XI (1980) crack growth assessment procedures are discussed with reference to crack velocity parameters.