Microstructure and mechanical properties of a dissimilar metal welded joint of Inconel 617 and P92 steel with Inconel 82 buttering layer for AUSC boiler application

The application of the novel dissimilar metal welded (DMW) joint, utilizing Inconel 617 and P92 steel, was showcased in the advanced ultra-supercritical (AUSC) boiler. The work has been performed to investigate the effect of Inconel 82 (ERNiCr-3) buttering layer on microstructure and mechanical properties (high-temperature tensile strength, impact strength and microhardness) of gas tungsten arc welded (GTAW) dissimilar joint between Inconel 617 and P92 steel fabricated using the Inconel 617 (ERNiCrCoMo-1) filler. For optical microscopy and scanning electron microscopy (SEM), samples were machined along a transverse direction which comprised the butter layer, weld metal, and heat-affected zone of both sides. The energy-dispersive X-ray spectroscopy (EDS) was used to map the interface of the buttering layer and weld metal and butter layer and P92 steel. The high-temperature tensile testing and Charpy impact testing at room temperature were conducted for the integrity assessment of the welded joint. The examination of microstructure and hardness revealed that the buttering layer of Inconel 82 filler successfully mitigated a significant portion of the brittle martensitic microstructure from the coarse-grained heat-affected zone (CGHAZ), along with hardness peaks on the side of P92 steel. The conventional method of DMW joint fabrication, without the use of a buttering layer, has been demonstrated to be less favourable compared to the new fabrication method, which incorporates a buttering layer. The TiC/NbC carbides were identified in the Inconel 82 buttering layer, whereas M23C6 and Mo6C carbides were found in the Inconel 617 filler weld. Near the interface of the Inconel 82 buttering layer and P92 steel, the formation of peninsula and island structures, as well as Type I and Type II boundaries, were confirmed. Additionally, element diffusion of Ni, Cr, and Fe was observed. The tensile test results indicated an ultimate tensile strength of 620 ± 4 MPa and % elongation of 19 ± 4 % at room temperature, with fracture occurring in the buttering layer near the interface of the buttering layer and P92 steel. At temperatures of 550 °C and 650 °C, the ultimate tensile strength decreased to 448 MPa and 326 MPa, respectively, with fractures occurring in the P92 steel, irrespective of temperature. The hardness of the Inconel 82 buttering layer and Inconel 617 filler weld were 219 ± 10 HV and 248 ± 11 HV, respectively. The Charpy impact toughness of the Inconel 82 buttering layer and Inconel 617 filler weld were 138 ± 6 J and 115 ± 4 J, respectively. The study comprehensively investigates and discussed the correlations between microstructure and mechanical properties.