Pulsed current GTAW Inconel 625/AISI 304 L steel dissimilar joint: Microstructure and mechanical properties

The present study aims to investigate the microstructure and mechanical behavior of Pulsed Current Gas Tungsten Arc Welded (PCGTAW) dissimilar joints of Inconel 625 and AISI 304 L steel. The dissimilar metal welded (DMW) joint was obtained using the Ni-based ERNiCrMo-3 filler. The welded joint was subjected to a microstructural study using optical microscopy and Scanning electron microscopy (SEM) with energy dispersive X-Ray spectroscopy (EDS). Mechanical characterization involved measuring microhardness, Charpy impact toughness, and tensile properties at both room temperature and high temperatures. Microstructural characterization revealed that the weld metal exhibits a fine equiaxed dendritic structure, which varies from the top to the root region (from the capping to the root pass). This variation is attributed to the heating effect of successive passes and the difference in cooling rates from the top to the root region. The electron probe micro-analyzer (EPMA) mapping and SEM-EDS analysis revealed the presence of Ti and Nb-rich NbC/TiC phases and Nb and Mo-rich Laves phases in the austenitic matrix of the weld metal. Additionally, the presence of Mo, Cr, and Fe-rich M₂₃C₆ carbides was also confirmed. Optical and SEM observations of the interface revealed the macrosegregation near the interface of AISI 304 L steel and ERNiCrMo-3 filler weld metal, characterized by filler-deficient zones (unmixed zone, peninsula, and island). Line mapping and EPMA analysis confirmed a sharp compositional gradient and elemental diffusion across the interface of weld metal and AISI 304 L steel. For the peninsula and island, EDS line/area elemental; mapping and EPMA mapping showed Fe and Cr as the predominant elements, indicating that the composition in these areas is intermediate between the weld metal and AISI 304 L base metal, closely resembling the composition of the AISI 304 L base metal. The pulsed current resulted in the development of a fine equiaxed structure, with a uniform distribution of small secondary phase particles, which has positively influenced the mechanical properties. The Charpy impact toughness and hardness of the weld metal were 152 ± 4 J and 232 ± 6 HV, respectively. The failure of the tensile-tested specimens from the AISI 304 L base metal rather than the weld metal, at both room and high temperatures, indicates its suitability for the end application. The average tensile strength at room temperature was 668 MPa. At elevated temperatures, the tensile strengths were 370 MPa at 550 °C, 368 MPa at 600 °C, and 311 MPa at 650 °C.