Investigation on microstructural and mechanical integrity of GTAW dissimilar welded joint of IN 718/ASS 304L using Ni-based filler IN 625 using EBSD and DHD techniques

The current study investigates the relationship between the microstructural and structural characteristics of dissimilar gas tungsten arc welding (GTAW) between Inconel 718 (IN 718) and austenitic stainless steel 304L (ASS 304L) using Inconel 625 (IN 625) as a filler material. The microstructural characterization of the dissimilar weld includes the application of an optical microscope (OM), field emission scanning electron microscopy (FE-SEM) including energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The optical and FESEM analysis of the base metals (BM) indicates the austenitic nature of the IN 718 BM matrix, which incorporates strengthening precipitates γ' and γ'' distributed throughout the Nickel matrix. In contrast, the ASS 304L BM exhibits an austenitic microstructure along with twins. Moreover, IN 625 weld metal exhibits austenitic microstructure in the form of cellular, columnar, and equiaxed dendrites. Elemental segregation demonstrates the existence of Nb, and Mo-rich carbides, including the laves phases at the interdendritic regions of the IN 625 weld zone and IN 718 Heat-Affected Zone (HAZ). Ferrite stringers were noticed in the HAZ of ASS 304L. EBSD analysis shows the improved texture of the weld zone exhibiting fine equiaxed recrystallized microstructures. EBSD/Kernel average misorientation (KAM) micrographs reveal the existence of high strain at the weld metal grain boundaries. Vickers microhardness assessment of the dissimilar weld demonstrates the high hardness of 257.5 HV for the IN 625 weld zone because of the occurred NbC, laves phases, and Mo-rich carbides. Room temperature tensile characterization shows the failure of tensile specimens from ASS 304L BM reflecting the high strength of the IN 625 weld metal (662 MPa). Tensile tests at high temperatures were performed at 550º C, 600º C, and 650º C. High-temperature tensile result exhibits a higher tensile strength of IN 625 weld zone than the ASS 304L BM, as the tensile specimens fails from 304L BM. The specimen exposed at 550º C exhibits the highest tensile strength of 388 MPa. The Charpy impact toughness test was conducted, with notches at the HAZ of ASS 304L, HAZ of IN 718, and the weld center being evaluated. The toughness value of the weld metal decreases (99 J) due to occurrence of the secondary precipitates. To analyze residual stress, the deep hole drilling (DHD) technique was used, which revealed that the peak residual stress (230 MPa) is induced at 6 mm from the weld surface and falls into the tensile stress category.