A Comparative Study on the Microstructural Evolution and Mechanical Behavior of 316LN Stainless Steel Welds Made Using Hot-Wire Tungsten Inert Gas and Activated Tungsten Inert Gas Process

In this study, the influence of hot wire tungsten inert gas (HW-TIG) welding and Activated-TIG (A-TIG) welding on the microstructure and mechanical properties of type 316LN stainless steel joints was investigated. 11 mm thick 316LN austenitic stainless steel plates were used for the weld joint fabrication. The plates with V-groove and straight butt configuration were joined by the HW-TIG and A-TIG welding techniques, respectively. 316L filler wire was used during HW-TIG welding while A-TIG welding was performed autogenously. The microstructural variation in the different zones of the weld joint is characterized using optical microscopy, scanning electron microscopes (SEM) equipped with electron backscatter diffraction (EBSD) detector, and energy Dispersive x-ray Spectroscopy (EDS). To evaluate mechanical properties, miniature tensile, standard tensile, Charpy impact, tensile creep, Vickers microhardness, and nano-indentation tests were used. The research findings revealed that the variations in mechanical properties were attributed to microstructural variations and pre-existing dislocations caused by work hardening during successive thermal cycles in the HW-TIG compared to A-TIG fusion zones. Additionally, the microstructure of A-TIG weld metal exhibited uniformly distributed delta (delta)-ferrite within the austenite matrix. In particular, the autogenous nature of A-TIG welding substantially reduced the delta-ferrite content in A-TIG welds than HW-TIG welds. The presence of microstructural heterogeneity, including dendrite orientation and delta-ferrite content, predominantly undermined the creep rupture strength of HW-TIG weld joints as compared to A-TIG weld joints. In conclusion, the A-TIG weld joint has a uniform microstructure, adequate strength, comparable ductility, and superior creep rupture strength in contrast to the HW-TIG weld joint. Therefore, A-TIG welding is recommended for welding of 316LN stainless steels for high temperature applications.