FURTHER EVIDENCE OF MARGIN FOR ENVIRONMENTAL EFFECTS, TERMED F_(EN-THRESHOLD), IN THE ASME SECTION Ⅲ DESIGN FATIGUE CURVE FOR AUSTENITIC STAINLESS STEELS THROUGH THE INTERACTION BETWEEN THE PWR ENVIRONMENT AND SURFACE FINISH

摘要

Fatigue rules within the ASME Boiler and Pressure Vessel Code have undergone significant change over the past decade, especially with the inclusion of Code Case N-792-I as an acceptable method to describe the effects of BWR and PWR environments on the fatigue life of components. The incorporation of the environmental effects into the fatigue calculations is performed using an environmental factor, F_(en), which attempts to describe the difference in fatigue life of polished specimens between air and high-temperature water environments. The F_(en) depends on parameters such as the temperature, dissolved oxygen and strain rate. The deleterious effects on fatigue life of a wide range of other factors are not accounted for by the standard constant amplitude testing, performed on small polished specimens that was used to develop the mean air curve. These factors are accounted for in the design curve, which is defined by adjusting the mean air fatigue curve with transference factors. Evidence, obtained in recent years, now suggests that the assumed deleterious effect of surface finish on fatigue life may be excessive. Therefore, the combination of the air design fatigue curve and F_(en) results in overly-conservative estimates for fatigue life in high temperature water environments. Approaches have been developed to quantify the scale of this excess-conservatism and define this as a margin within the design fatigue curve that can be used to offset F_(en). In the ASME framework this is termed F_(en-Threshold). This paper presents further evidence in support of the approach by extending the database of test results to 316-type steels and extending the range of environmental and loading conditions. The additional stainless steel data demonstrates that the size of the margin is insensitive to strain amplitude over an extended range of strain amplitudes. Furthermore, the analysis indicates that in conditions where the environmental effect is low (less than the value of F_(en-Threshold)) the margin remains great enough to offset the entire F_(en), returning such sites to an assessment against the air design curve. This paper also presents an extension of the research program to nickel based alloys through testing of Alloy 600. The intent is to investigate whether there is evidence of similar excessive conservatism that can be translated into an equivalent margin for environmental effects in nickel-based alloys.