Microstructure anomaly due to the heat from the laser welding process and its effect on the mechanical behavior of the dissimilar welded joint between P92 steel and Inconel 625 for AUSC boiler applications

High-power lasers have demonstrated exceptional effectiveness in welding plates of 10 mm thickness or more, providing substantial benefits over traditional multi-pass fusion welding methods. In this research, a dissimilar welded joint of ferritic/martensitic P92 steel and Inconel 625 superalloy, with a plate thickness of 10 mm, was successfully fabricated using the laser beam welding process. This study focuses on an in-depth microstructural analysis of the weldments utilizing optical microscopy and scanning electron microscopy and presents the microstructure. To evaluate the joint’s performance, room-temperature and high-temperature tensile properties, Charpy impact toughness, and hardness of the dissimilar weldments have been analyzed. The findings establish a correlation between the evolved microstructure and the resulting mechanical properties of the weld. The characterization of the weld metal revealed a columnar and cellular austenitic microstructure, which varied from top to bottom and across the interfaces. The EDS line mapping and point analysis indicated heterogeneity in chemical composition at both the micro and macro levels. The weight percentages of Ni, Cr, Fe, and Mo varied across the weld metal from one side of the interface to the other, attributed to the mixing of two materials with distinct microstructures and chemical compositions. Macrosegregation manifested as beaches, peninsulas, islands, and partially melted zones was detected on both sides of the interface. Elemental diffusion was also evident across the interfaces. The SEM/EDS analysis confirmed the presence of two types of phases: spherical-shaped precipitates, identified as NbC, M₂₃C₆, and Mo₆C, and an interconnected chain-like phase along inter-dendritic areas, enriched with Nb and Mo, which could be identified as a Laves phase. Tensile tests showed that the welded joint experienced failure in the P92 base metal at both ambient and high temperatures. The UTS at room temperature was 675 ± 4 MPa for the as-welded joint and 671 ± 2 MPa for the post-weld heat-treated (PWHT) joint. At elevated temperatures, the UTS of the as-welded joint was 403 MPa at 600 °C and 351 MPa at 650 °C. The PWHT have observed a less effect on tensile properties however it showed a significant impact on hardness of the P92 HAZ and Charpy toughness of the weld metal. The hardness of the weld metal in the as-welded joint was 196 ± 12 HV, which increased by 36 HV after PWHT, likely due to the evolution of new carbide phases. Additionally, the hardness varied across the weld metal, from the Inconel 625 interface to the P92 interface, with higher values on the P92 side. For the weld metal, the Charpy impact toughness was found to be 54 ± 5 J in the as-welded condition and 34 ± 3 J after PWHT which was mainly affected by the carbides and other intermetallic phases present in the weld metal.