EFFECT OF SURFACE CONDITION ON THE FATIGUE LIFE OF AUSTENITIC STAINLESS STEELS IN HIGH TEMPERATURE WATER ENVIRONMENTS

摘要

High temperature water environments typical of LWR operation are known to significantly reduce the fatigue life of Type 300 series austenitic stainless steels in laboratory studies relative to air environments. This environmental impact on fatigue life has led to the issue of US-NRC Regulatory Guide 1.207 and supporting document NUREG/CR-6909 which predicts significant environmental reduction in fatigue life (characterised by an environmental factor F_(en)) for a range of actual and design basis transients. However, application of this factor to design curves (obtained from laboratory air data by the application of factors for size, surface finish, environment etc) may be unduly conservative. For example there are reasons to expect that a given surface condition will not have the same impact in air compared to water environments. The aim of the current work was to determine the impact of different surface conditions, typical of operating plant, on the fatigue life in both air and high temperature (300°C) water environments. This work indicates that, in a simulated PWR high temperature water environment, the impact of either a roughly ground/abraded surface finish (simulating flapper wheel dressing) or a simulated surface scratch on fatigue life relative to a polished surface finish is approximately half of the effect observed in ambient temperature air. This suggests that the application of a F_(en) value calculated in accordance with NUREG/CR-6909 to the design curves may indeed unduly over-estimate the impact of surface finish on fatigue life.