The welding of dissimilar alloys is technically challenging due to mismatches in melting range, solidification behaviour, and thermal expansion, which predispose joints to segregation, cracking, and formation of deleterious phases. In advanced industrial sectors such as chemical processing, power generation, and oil & gas, the joining of nickel-based superalloys with nitrogen-strengthened stainless steels offers an attractive balance of corrosion resistance, mechanical performance, and cost efficiency. Therefore, this study examines the gas tungsten arc welding of Hastelloy C-22, a Ni-Cr-Mo superalloy, with Nitronic 50, an austenitic stainless steel, comparing autogenous welding with ERNiCrMo-4 filler-assisted welding. Microstructural assessment by optical microscopy and scanning electron microscopy, complemented by energy-dispersive spectroscopy, revealed distinct solidification modes. Autogenous welds exhibited a predominantly fine columnar dendritic fusion zone, whereas filler-assisted welds contained a mixture of columnar and equiaxed dendrites with coarser grain morphology. Both joint types showed pronounced elemental redistribution at the interfaces, with evidence of grain boundary migration. Filler-assisted welds further promoted the precipitation of secondary Ni-Mo-rich Laves and μ phases within interdendritic regions, absent in autogenous welds, signifying complex segregation phenomena associated with filler addition. Mechanically, ERNiCrMo-4 welds achieved superior ultimate tensile strength (817 MPa) and impact toughness (37.17 J) compared to autogenous welds (709 MPa and 15.21 J, respectively). Despite promoting secondary phases, the filler metal minimized macro segregation and enhanced weld bead uniformity. Corrosion tests revealed a lower degree of sensitization for filler-assisted welds (12.66%) compared to autogenous welds (14.78%), indicating improved resistance to intergranular attack. These findings clarify the interplay between welding method, filler chemistry, and microstructural evolution in Hastelloy-stainless steel dissimilar joints.
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