High temperature fatigue of 316FR steels for nuclear reactor use

Evaluation of high temperature fatigue properties of an austenitic steel 316FR, which was newly developed as a structural material used for fast breeder reactors, was carried out in view of extension of creep lifetimes. The material is based on SUS316 steel, kept carbon quantity as low as 0.008% and instead nitrogen quantity up to 0.073% which will prevent inter-granular deposition of carbides and thus creep-fatigue controlling. First, the authors explain a parametric analysis method to evaluate the temperature and strain-rate dependence of high temperature low cycle fatigue lifetimes based on thermal activation process, followed by its application to the case of 316 FR steels and the demonstration of extrapolating the method to ultra-low cycle fatigue case. Finally, the guideline for designing a better fatigue lifetime material with adding a trace amount of elements and/or preparing crystallites and grains. Applicability of the present analytic method was discussed. (S. Ohno)