Dissimilar welding of SA335 P91 and SA218 P22 pipes fabricated through pulsed-GTAW: Metallographic, structural integrity, and molten salt hot corrosion behaviour

The dissimilar tube joints of SA335 P91 alloy steel and the SA218 P22 low-carbon steel were fabricated using the Pulsed Current Gas Tungsten Arc Welding (PC-GTAW) method, employing ER-307 as the SS (stainless steel) filler, utilized in piping systems of thermal exchangers. The tensile studies showed a failure on the SA218 P22 side, with a lower ultimate tensile strength (UTS) of 520 ± 1 MPa and yield strength (YS) of 338 ± 2 MPa, while the notch tensile studies showed a higher strength of 588 ± 4 MPa and yield of 412 ± 3 MPa, confirming an adequate weld strength compared to the base metal. The FESEM-EDS (Field Emission Scanning Electron Microscope) analysis confirmed the presence of Mo-rich phases along with M 23 C 6 carbide precipitates, which resulted in increased hardness of the weld zone. The Charpy V-notch toughness of the joints was found to be higher at 71 ± 2 J, ensuring the minimum requirement for the joints. The corrosion test testing under K 2 SO 4 with 60% NaCl promoted accelerated Type-1 hot corrosion providing the parabolic rate constants of 0.6314, 1.003, and 1.248 (10 ×10 −6 ) g 2 cm −4 s −1 for the weld zone, P91, and P22, respectively, with XRD and SEM/EDS confirming that a protective NiFe 2 O 4 /NiCr 2 O 4 spinel oxide layer on the ER-307 weldment limits corrosive-species ingress.