Influence of mean stress and frequency on the fatigue crack growth behaviour of A508 cl III steel in a low temperature pressurised water reactor environment

Abstract

The present paper attempts to generate fatigue crack growth data for RPV steels in a pressurised water reactor (PWR) environment at lower temperatures than those normally encountered at the PWR water outlet temperatures of around 290°C. Significant effects of frequency on the fatigue crack growth behaviour were observed in ambient PWR water at low and high R- ratio . The enhanced fatigue crack growth rates at low frequency were coincident with the appearance of significant amounts of isolated intergranular failure facets. At a PWR water temperature of 120°C, marked effects of frequency and R- ratio were recorded. Indeed lowering the frequency and increasing the R- ratio significantly increased fatigue crack growth rates as a result of environmental assisted crack (EAC) growth which was typified by the appearance of significant amounts of flat, fan-shaped EAC growth on the fatigue fracture surfaces. A powerful effect of material on the fatigue crack growth rates were evident in a PWR environment at 120°C inasmuch that the RPV A508 steel exhibited growth rates which, at a ΔK level of 30 MPa√ m , were over an order of magnitude faster than those recorded for an A533B RPV steel. Such differences were explained in terms of non-metallic sulphide composition because while the A533B steel contained many more sulphide inclusions, over 80% of them were pure manganese sulphide inclusions while the A508 steel contained nearly 90% active sulphide inclusions which were manganese sulphides containing significant amounts of iron, chromium, aluminium and phosphorous and traces of sodium, potassium, zinc and nickel. Súch active sulphide inclusions, which were either completely or partially dissolved in PWR water at 120°C, produced surface active chemical species that promote EAC growth, while the predominantly pure manganese sulphides prevalent in the A533B steel were not attacked and remained undissolved.