Double-sided GTAW of nuclear grade steel: Mechanical and microstructure perspectives

Abstract

This article investigates the microstructural evolution and mechanical integrity of austenitic stainless steel SS321, a titanium stabilized nuclear grade mainly preferred for severe corrosive environments. The double-sided gas tungsten arc welding (DS-GTAW) technique was utilized to fabricate the butt joint having a plate thickness of 6 mm. A heat input of 1.4058 kJ/mm was used to obtain the maximum depth of penetration (DoP) of 3.3 mm with welding speed 120 mm/min and current 220A. Optical microscopy reveals the microstructure of weldment and base metal (SS321). The DS-GTA weldments were subjected to tensile, bend, impact and microhardness tests and the results are evaluated. The fusion zone consists of columnar, equiaxed dendrites, and intermetallic compounds of Titanium carbide (TiC). From EBSD examination the higher fraction of Low Angle Grain Boundaries is corroborated to the increase in tensile strength and reduction in impact toughness of the weldment. The ferrite measurement reveals the increase in ferrite content in the weldment (6.1 FN) and this attributed to the presence of retained δ-ferrite in comparison to SS321 (1.2 FN). X-Ray Diffraction (XRD) pattern reveals austenite and ferrite peaks are present in the SS321 and weldment. Ductile mode of fracture (using SEM-Scanning electron microscope) was observed in the uni-axial tensile and impact test of weldment specimens.