Influence of Creep on Isothermal and Thermomechanical Low Cycle Fatigue of Type 316 LN SS Weld Joint

Thermomechanical fatigue (TMF) behaviour of a type 316LN austenitic stainless steel weld joint under in-phase (IP) and out-of-phase (OP) temperature-mechanical strain combinations were investigated. IP TMF was generally found to be more detrimental compared to OP cycling. The interaction of creep deformation with TMF was studied by incorporating dwell periods of 1 min and 5 min at peak tensile strain under IP TMF. Isothermal low cycle fatigue (IF) and tensile hold creep-IF interaction tests were concurrently conducted at the maximum temperature (T-max) of TMF cycles. Results obtained were compared with the behaviour exhibited by the base metal. The cyclic stress response (CSR) of the base metal and weld joint under creep-TMF and creep-IF was found to depend on the hold period. The microstructural transformation in the weld metal played a significant role in the observed difference in CSR between the base metal and the weld joint. Creep-TMF resulted in a higher stress relaxation and consequently led to lower lives compared to creep-IF cycling. The crack initiation and propagation modes were characterized through detailed optical and scanning electron microscopy whereby, the dependence of fracture morphology on the testing conditions under continuous cycling and hold time conditions were established. The extent of intergranular damage was seen to increase with an increase in the hold period. A combined influence of creep and metallurgical transformations in the weld metal contributed to significant reduction in creep-TMF life with hold period.