Influence of heat input on the microstructures, mechanical properties, and corrosion behaviour of dissimilar Nimonic 75 superalloy and Nitronic 50 austenitic stainless steel GTA welds

The dissimilar weldments of Nimonic 75 superalloy and Nitronic 50 austenitic stainless steel (ASS) are of great importance for high-temperature structural applications where cost-effectiveness and performance are critical. In this study, dissimilar weldments were prepared using the Gas Tungsten Arc Welding (GTAW) process with varying heat inputs. The microstructural, mechanical, and corrosion behaviour were analysed using optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) to study the weld metal microstructure and grain boundary characteristics. The EBSD analysis revealed epitaxial growth at the weld interface, with equiaxed dendritic structures in low- and medium-heat input specimens, and columnar grains with random orientation in the high heat input weld metal. The grain boundary analysis showed a higher fraction of low-angle grain boundaries (LAGBs) with increasing heat input. The SEM-EDS analysis confirmed segregation of Ti- and Cr- rich precipitates in the interdendritic regions of the high heat input joint. The mechanical tests demonstrated superior hardness and tensile strength for the low- and medium-heat input specimens compared to the high-heat input specimen. The fracture of the dissimilar weldments was found to be at the center of the weld, depicting ductile failure. Furthermore, corrosion studies indicated that the medium heat input weldment exhibited the best resistance to corrosion in chloride environments. Overall, the findings highlighted the critical role of welding heat input in tailoring the microstructure, mechanical performance, and corrosion resistance of dissimilar GTA-welded joints.