Enhanced microstructural and mechanical performance of P91–304L dissimilar welded joints using gas tungsten arc welding with a Ni-1 intermediate buffer layer and ERNiCrMo-3 filler metal

• Ni-1 intermediate buffer layer effectively restricted carbon migration, , which led to resistance against the formation of CDZ and CEZ • Ni-1 buffer with PWHT enhanced P91 HAZ impact toughness by 14%. • GTAW with Ni-1/IN625 provided enhanced P91–304 L weld performance. • The Ni-1 buffer resulted in homogeneous delta ferrite distribution. • Highest hardness was in CGHAZ and lowest in ICHAZ across all joints. The pursuit of efficiency in nuclear power plant components operating at high steam temperatures and pressures has driven the use of advanced alloys like P91 steel and 304 L stainless steel, yet the structural integrity of their dissimilar welds remains challenged by carbon migration and premature failure; prompting this study to investigate the effectiveness of a Ni-1 (∼92% Ni) intermediate buffer layer combined with ERNiCrMo-3 (IN625) filler metal via multi-pass Gas Tungsten Arc Welding to enhance their metallurgical and mechanical performance. The study compared three IN625 buttering applications: as-welded, post-weld heat-treated, and post-weld heat-treated with an intermediate Ni-1 buffer layer. Microstructural analysis, employing optical microscopy and scanning electron microscopy, revealed peninsula features and an unmixed zone at the interface. The fusion zone features included Mo, Nb, and Ti-rich precipitates. The P91 heat-affected zone demonstrated a soft delta ferrite zone. Mechanical testing showed tensile strengths of 674 MPa (as-welded), 653 MPa (post-weld heat-treated), and 633 MPa (post-weld heat-treated with buffering). Notably, the Ni-1 buffer with post-weld heat treatment resulted in a 14% improvement in P91 heat-affected zone impact toughness compared to the as-welded joint. Microhardness profiling revealed the highest hardness in the coarse-grain heat-affected zone and the lowest in the intercritical heat-affected zone. The study concludes that the Ni-1 buffer layer successfully suppresses carbon migration, preventing the formation of detrimental carbon-depleted and enriched zones. By mitigating metallurgical degradation and improving toughness, the Ni-1/IN625 dual-filler approach provides a robust solution for the structural reliability of P91-304 L SS joints in demanding nuclear applications.