Activated TIG welding of IN617-P91 dissimilar joints: flux optimization and performance assessment for A-USC applications

This study examines the effects of various flux mixtures on the mechanical and metallurgical integrity of Activated Tungsten Inert Gas (A-TIG) welded P91/Inconel 617 joints. The combination of these materials is particularly advantageous for high temperature components in advanced ultra-super critical (AUSC) power plant applications, where the objective is to achieve a reliable welded joint while reducing overall cost. A Cr₂O₅-dominant flux layer was applied on the Inconel 617 (IN617), whereas a V₂O₅-dominant flux layer was applied on the P91 base metal. A-TIG welding trials were conducted on an 8 mm thick plate, and complete penetration was achieved using this optimized flux configuration. The resulting weld joints were free from imperfections, as confirmed by visual inspection, liquid penetrant testing, and macroscopic examination. Optical microscopy (OM) and field emission scanning electron microscopy (FESEM) revealed an austenitic microstructure at different locations within the weld metal, which was further confirmed using the Schaeffler diagram. The Cr and Mo-rich M₂₃C₆, Mo-rich Mo₆C, and Ti-rich Ti(C/N) phases were observed in the weld metal, as confirmed by energy-dispersive spectroscopy (EDS). Vickers microhardness measurements revealed a hardness of 192 ± 8 HV in the weld metal, while hardness values of 397 ± 3 HV, 385 ± 7 HV, and 232 ± 2 HV were recorded in the CGHAZ, FGHAZ, and ICHAZ of P91 steel, respectively. The highest hardness was observed in the CGHAZ compared to the other regions of the P91 HAZs. The hardness of the IN617 HAZ was measured as 262 ± 3 HV, which is slightly higher than that of the IN617 base metal (257 ± 2 HV). At room temperature, the weld exhibited an ultimate tensile strength (UTS) of 566 ± 16 MPa and an elongation of 33%. High temperature tensile tests were conducted at 500 °C, 550 °C, and 600 °C. The results indicate a progressive decrease in UTS with increasing test temperature, with measured values of 455 MPa, 431 MPa, and 311 MPa, respectively. The weld metal exhibited a Charpy impact toughness of 123 ± 5 J, which lies between those of IN617 (147 ± 6 J) and P91 (112 ± 6 J). The measured Charpy impact toughness of the weld metal exceeded the minimum limits prescribed by ASME and EN ISO standards. Based on the comprehensive metallurgical and mechanical evaluations, it was concluded that the weld joint produced using the flux-assisted A-TIG process is suitable for high-temperature applications, particularly in AUSC power plants.