Influence of Microstructural Inhomogeneity on the Creep Rupture Behaviour of 316LN SS Weld Joints

Abstract

The fusion zone is usually the weakest link in austenitic steel weld joints and it is usually in this region where damage initiates during creep deformation. The heterogeneity in microstructure of the fusion zone enhances the complexity in understanding the damage evolution during creep exposure. The major source of heterogeneity in the microstructure is the weld thermal cycle, which the weld joint is inevitably subjected to during multi-pass welding. Subsequent thermal cycles are likely to alter the microstructure of the previously deposited fusion zone thereby influencing the mechanical properties of the joints. In this study, the effect of thermal cycle on creep rupture behaviour of 316LN SS weld joint has been studied. Weld joints were fabricated by two welding processes—activated tungsten inert gas (ATIG) and shielded metal arc (SMA) welding. In case of ATIG weld joints, the creep properties of single and dual-pass weld joints were compared and for the SMAW joints, the comparison was made between weld joints made with two different electrode sizes. In ATIG weld joint, the subsequent thermal cycle in the dual pass weld joint improved its creep rupture life. In the case of the SMAW joint, the weld joint made with larger electrode, which was subjected to less number of thermal cycles exhibited better creep properties. The weld thermal cycle induces a ‘thermo-mechanical treatment’ and changes the morphology of delta ferrite in the previously deposited fusion zone which has a synergistic role in dictating the creep rupture life of the 316LN SS weld joints.