Hidden Roles of Time and Temperature in Cyclic Behavior of Stainless Nuclear Piping

Unexpected findings on time and temperature dependent behavior have been recorded during our research on fatigue performance of niobium stabilized stainless steel. Cyclic straining at 325°C and low strain rates resulted in higher stress responses than in higher rate tests. This effect is particular strong in PWR water environment. Subsurface bulk effect in environment is in contrast to the assumption on similar responses in air and environment, which is the foundation of the ‘companion specimen’ method where the strain in environment is measured from a parallel specimen similarly tested in air. Our data shows that environmental effects caused by PWR water cannot be isolated as a separate issue. Environment, temperature and strain rate are factors, which interactively affect the cyclic response and fatigue performance of stainless steel in relevant temperatures and loading conditions. The current ASME Code Section III design curve is based on different translation of the laboratory data than that made by Langer et al. The resulting effect is not as radical as caused by replacement of the original test data to a new highly scattered data base. But also the procedural changes have detectable effects and open a door for a provocative question: “is the design curve still compatible with the code?”