Evaluation of microstructural and mechanical qualities in optimised TIG-welded SDSS 2507 joints

Super duplex stainless steel (SDSS) is gaining attraction owing to its excellent mechanical strength and superior corrosion resistance. In this study, tungsten inert gas (TIG) was implemented for welding the SDSS thin sheet. The Taguchi method and analysis of variance (ANOVA) were carried out by selecting L25 orthogonal arrays. The optimum TIG parameters were a welding current of 75 A, an arc potential of 15 V, a welding rate of 120 mm/min, and an argon gas consumption rate of 12 L/min. An ANOVA study found that welding current (46.95%) was the largest contributor in producing the excellent welded joint. The microstructural research indicated increased grain size in the heat-affected zone (HAZ) and fusion zone (FZ), represented by distinct grain boundary layers, intragranulars, and Widmanstätten austenite. This was due to heat input and rapid cooling inclusion as well as re-crystallisation of the ferrite matrix. The elemental mapping analysis showed that chromium must be present to generate a shielding oxide layer, which decreased from 25.50% in the parent material to 23.40% in the TIG welded joint. The tensile test found that TIG welds had an ultimate tensile strength (UTS) of 789 MPa. This value was equivalent to the base metal UTS value of 800 MPa. The micro-hardness test of the TIG welded joint confirmed that the HAZ (350 HV) and FZ (325 HV) were higher than that of the base metal (305 HV). The hardness value near the FZ boundary experienced a significant increase due to the development of hard microscopic components and element migration during the TIG process.