Thermomechanical and isothermal fatigue behavior of 347 and 316L austenitic stainless tube and pipe steels

Many components of nuclear power plant piping systems are made of austenitic stainless steels. These structures undergo degradation by thermomechanical loading caused by thermal transients and stratifications. In scientific literature, most of the studies deal with this problem under isothermal fatigue conditions, which is different from the typical service conditions. In addition, less attention has been paid to thermo-mechanical fatigue (TMF). This work aims to understand and compare the cyclic deformation behavior during TMF and isothermal fatigue (IF) testing in air for two of the most commonly used grades of austenitic stainless tube and pipe (TP) steel, the non-stabilized TP 316L and the niobium-stabilized TP 347, under light water reactor relevant temperature conditions. Three types of tests, i.e. in-phase, out-of-phase TMF in the 100–340 °C temperature range, and IF tests at the maximum temperature Tmax of TMF were performed. All the tests were carried out under total strain control, for two different mechanical strain amplitudes (Δεmech/2 = 0.3% and 0.5%). Results revealed that irrespective of the tested strain amplitude and material, in-phase TMF showed a higher lifetime over out-of-phase TMF and IF at Tmax. A crossover in fatigue lifetime was observed in TP 316L between out-of-phase TMF and IF test at Tmax with decreasing strain amplitude. In TP 347, out-of-phase TMF and IF test at Tmax showed similar fatigue lifetimes for both strain amplitudes. Finally, three lifetime prediction models were selected and compared to evaluate the possibility of estimating TMF lifetime by using IF data. Satisfactory predictions were obtained from all the prediction approaches.