Deformation and damage assessment in type 316 LN stainless steel weld joint under isothermal and thermomechanical cyclic loading

Present investigation is aimed at understanding the deformation behavior and the development of damage in a type 316 LN austenitic stainless steel (SS) weld joint (WJ) under isothermal low cycle fatigue (IF) and thermomechanical fatigue (TMF). In-phase (IP) and out-of-phase (OP) TMF tests were carried out by maintaining a phasing relation of 0° and 180° respectively, between the mechanical strain and temperature cycles. Dynamic strain ageing (DSA) was found to exert a strong influence on the cyclic stress response (CSR) of the joint under both IF and TMF cycling. The CSR was observed to be higher under TMF compared to IF cycling due to the activation of additional hardening mechanisms in the former tests. The difference in fatigue life under TMF and IF is rationalized based on the development of deformation and damage through optical and scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) studies. A clear demarcation of the strain distribution in the base metal (BM) and weld metal (WM) region of the joint is achieved through detailed EBSD analysis of the tested specimens. Cyclic plastic deformation and the associated development of damage take place independently in the BM and the WM parts of the joint, competing to cause the failure. The build-up of damage under IF and TMF cycling is dictated by a combination of DSA and δ-ferrite transformation, depending on the applied strain amplitude and the strain-temperature phasing employed. The observed life variations have been rationalized in terms of the substructural evolution and fracture behavior under different testing conditions. •The CSR and fatigue life depend on the type of fatigue cycle (IF and IP/OP TMF).•Thermal cycling and imposed strain influenced the dynamic strain ageing behavior.•Failure location across the joint changes with the applied strain and type of cycle.•Localized deformation in the joint depends on the δ-ferrite transformation and DSA.