Heat Input Effect on Dissimilar Super Duplex Stainless Steel (UNS S32750) and Nitronic Steel (N 50) Gas Tungsten Arc Weld: Mechanism, Microstructure, and Mechanical Properties

This research focuses on the effect of heat input of super duplex stainless steel (sDSS 2507) and nitronic steel (N50) dissimilar welded joints. Two heat input combinations selected as lower heat input (LHI) (0.62 kJ/mm) and higher heat input (HHI) (1.01 kJ/mm) were preferred during the operation of the gas tungsten arc welding process (GTAW) process using ER2594 filler. The macrosegregation like an island, peninsulas are observed in both weldments` HAZ and weld zone. The presence of secondary austenites (γ2) precipitation is observed in HAZ and root region of weldments. As heat input increases, the ferrite content of the weld zone in HHI decreases. Nevertheless, HHI had an advantageous effect on austenite nucleation in the HAZ. To ascertain the mechanical characteristics of the dissimilar metal welds (DMWs), tests such as the Vickers micro-hardness test, Charpy impact test, and tensile test were conducted. The mechanical evaluation reveals that the average hardness of LHI shows more hardness of 280 ± 4 Hv0.5 than HHI weldment of 270 ± 3 Hv0.5. Hence, weld zone, HAZ, and interface hardness do not indicate any significant change with the increment in heat input, which can be credited to the phase balance of ferrite/austenite. The tensile strength of LHI and HHI weldment is 895 ± 4 MPa and 900 ± 5 MPa, respectively, but LHI weldment displays better ductility. The Charpy impact test reveals that the impact toughness value decreases with increased heat input (i.e., 165 ± 5 J and 145 ± 3 J for LHI and HHI weldment, respectively) because of the secondary phase formation and increased grain size. However, the weld metal`s Charpy impact toughness was more than the ASME standard value (> 41 J) and the EN 1599:1997 standard value (> 47 J). The microstructural observation and mechanical characterization suggest that lower heat input could be beneficial for weld’s structural integrity.