Comparative assessment of Ni-based electrodes on the integrity of Inconel 617-AISI 304 H dissimilar welds for advanced ultra-supercritical boilers

This study examines the influence of Ni-based electrode composition on the microstructure and mechanical behavior of dissimilar weld joints (DWJs) between IN617 and AISI 304 H stainless steel, key candidate materials for advanced ultra-supercritical (AUSC) boiler systems. The DWJs were fabricated using shielded metal arc welding (SMAW) with three Ni-based electrodes: ENiCrFe-3 (Inconel 182), ENiCrCoMo-1 (Inconel 617), and ENiCrMo-3 (Inconel 112). All welded joints were confirmed defect-free through liquid penetrant testing, X-ray radiography, and macroscopic examination, showing no cracks, porosity, or discontinuities. Optical and scanning electron microscopy (SEM) analyses revealed an austenitic dendritic microstructure in all electrode welds, marked by elemental segregation and distinct morphological variations across the weld cross-section. The weld fusion interfaces displayed macrosegregation with distinct features of the island, peninsula, and unmixed zones, accompanied by elemental diffusion between the dissimilar base metals and Ni-based weld metal. Tensile testing at ambient temperature revealed that all Ni-based electrode welds achieved room and high-temperature ultimate tensile strength (UTS) comparable to AISI 304 H base metal, with failure consistently in the AISI 304 H base metal, underscoring their suitability for AUSC boiler service. At room temperature, Ni-based electrode welds exhibited UTS values between 651 ± 2 MPa (ENiCrCoMo-1) and 662 ± 7 MPa (ENiCrFe-3), closely matching the AISI 304 H base metal (666 ± 7 MPa) but remaining well below that of IN617 (826 ± 1 MPa). At 650 °C and 700 °C, the weld joints exhibited UTS values of 305–318 MPa and 241–255 MPa, respectively, aligning closely with AISI 304 H (322 MPa at 650 °C and 261 MPa at 700 °C) but remaining significantly lower than IN617 base metal (669 MPa at 650 °C and 583 MPa at 700 °C). Charpy testing confirmed ductile fracture in all specimens, with Charpy toughness values of 82–91 J for the Ni-based welds, significantly lower than AISI 304 H (194 ± 5 J) and IN617 (142 ± 4 J) base metals but still well above EN ISO (47 J) and ASME (41 J) standards. The lower Charpy toughness in the welds was attributed to segregation of alloying elements (Mo, Nb) and the formation of secondary phases such as Laves, which promoted crack initiation. Vickers hardness testing revealed the highest weld metal hardness with ENiCrCoMo-1 electrode (233 ± 4 HV, root) and the lowest with ENiCrFe-3 electrode (192 ± 4 HV, top). Among all electrodes, ENiCrCoMo-1 offered the most favorable combination of microstructural stability, strength, toughness, and hardness, making it the most suitable electrode for dissimilar SMAW joints of IN617 and AISI 304 H in AUSC boiler applications. • Defect free IN617/AISI304H dissimilar welds were produced using SMAW with three Ni-based electrodes. • All welds showed austenitic dendritic structures; ENiCrCoMo-1 minimized diffusion and PMZ width at the IN617 interface. • ENiCrCoMo-1 welds achieved superior hardness, balanced strength, and toughness due to fine Mo ₆ C, M ₂₃ C ₆ , and Ti(C,N) precipitates. • ENiCrCoMo-1 was identified as the most reliable electrode for high-temperature AUSC boiler applications.