Fatigue of Stainless Steel in Simulated Operational Conditions: Effects of PWR Water, Temperature and Holds

Abstract

Our experimental research on fatigue performance of stainless steels and transferability of laboratory data to plant operational conditions focuses in niobium stabilized stainless steel (1.4550, X6CrNiNb1810mod) taken from a pipe manufactured as primary piping for a German NPP Good fatigue performance both in air and in PWR water was reported in previous PVP papers. The NUREG/CR-6909 report proposes Fen factors based on stroke controlled experiments in hot water for non-stabilized steels. Since PVP2013-97500 we have new data in 200°C PWR water to compare with predictions by NUREG/CR-6909. Our strain controlled tests in 325°C and 200°C PWR water give longer lives resp. smaller Fen factors. For the slowest tested strain rate 4·10−6 in 325°C water the prediction according to NUREG/CR-6909 goes just below the current ASME design curve, but our results remains well above. Including also the relevant design temperature effect, our result Fen = 4 is well below the predicted Fen = 14,5. The gap is smaller for higher strain rates and low Fen values. Simplified simulations of fatigue transients combined with normal operation indicated that relevant loading patterns as hold-time effects may result to notably longer lives than in standard laboratory tests. A concern was raised on transferability of data to thermal transients separated with months of normal operation. Cyclic strain (transients) followed by hot holds (normal operation) lead to time and temperature dependent hardening with reduction in cyclic plastic strain and fatigue usage, i.e. extension of life. This paper reports new data, challenges met and our progress towards developing realistic design factors for effects both reducing and extending fatigue endurance in nuclear power plant operational conditions.