Thermo-Mechanical and Isothermal Low-Cycle Fatigue Behavior of Type 316L Stainless Steel in High-Temperature Water and Air

Two unique facilities for isothermal low-cycle fatigue (LCF) and in-phase (IP) or out-of-phase (OP) thermo-mechanical fatigue (TMF) testing of tubular specimens under boiling water reactor (BWR) and pressurized water reactor (PWR) coolant conditions were set up and successfully tested. The systems allow both strain-and stress-controlled fatigue experiments with very small strain amplitudes or complex stress/strain profiles with superimposed rather rapid temperature changes (100 degrees C to 340 degrees C) under flowing conditions. The present article introduces the new test facility and briefly discusses the first results of these experiments. In contrast to air, a strong effect of temperature on LCF and TMF lives was observed in deoxygenated and hydrogenated high-temperature water environment, where both lives decrease with increasing temperature above 100 degrees C. The TMF life is between that of the isothermal LCF tests at minimum and maximum temperature. The IP TMF life is shorter than that of OP TMF and close to the LCF life at maximum temperature. The OP TMF life is close to that of LCF at minimum temperature. In a hydrogenated environment, the LCF and TMF lives seem to be slightly shorter than predicted by the NUREG/ CR-6909 approach of the United States Nuclear Regulatory Commission (NRC) Regulatory Guide 1.207. Significant environmental effects were observed at 100 degrees C, which is significantly below the temperature threshold of 150 degrees C.