Thermal cycling effects on the creep-fatigue interaction in type 316LN austenitic stainless steel weld joint

Thermomechanical fatigue (TMF) tests under in-phase temperature-mechanical strain combination were performed on a type 316 LN austenitic stainless steel weld joint using temperature ranges of 573-823 K, 623-873 K and 673-923 K employing a mechanical strain amplitude (Delta epsilon(mech)/2) of +/- 0.4%. Isothermal low cycle fatigue (designated as IF) tests were also carried out at the maximum temperatures (T-max) of TMF cycling. The interaction of creep deformation with IF and TMF cycling was assessed by incorporating hold periods of 1 min and 5 min at the maximum tensile strain. TMF cycling resulted in considerably lower lives compared to IF, with the life reduction being more significant under hold conditions. Further, a clear difference in the cyclic stress response (CSR) was observed between creep-TMF and creep-IF cycling, depending on the hold duration and temperature range. The contribution of creep deformation to the overall damage was found to be higher under TMF compared to IF cycling at the T-max. The observed variations in the cyclic lives under IF and TMF cycling with and without hold times were explained on the basis of the dominant fracture mechanisms identified through detailed microstructural characterization of the crack initiation and propagation modes. The extent of intergranular damage was seen to increase with an increase in the temperature range and the hold period. The role of oxidation in the development of damage was observed to be different under creep-TMF and creep-IF cycling. Numerous grain boundary microvoids and their inter-linkages caused enhanced crack propagation under creep-TMF, compared to creep-IF cycling. A comparison of the deformation behaviour, cyclic life and damage developed in the material under IF and TMF suggested that the performance of the weld joint under TMF cycling cannot be satisfactorily represented through IF tests at the T-max.