Insights into microstructure and mechanical integrity of P91 steel pipe repaired by temper bead welding using GTAW and SMAW

This work investigates the temper bead welding repair (TBWR) of SA335-P91 steel pipe fabricated via combined GTAW/SMAW processes, focusing on microstructural configuration and mechanical integrity. Pipes in service at a Thai power plant (Electricity Generating Authority of Thailand, EGAT) were joined using gas tungsten arc welding (GTAW) for the root pass and shielded metal arc welding (SMAW) for the hot and cover passes. Metallographic and mechanical testing were performed to characterize the fusion zone (FZ), heat-affected zone (HAZ) and base metal (BM) after repair. The as-received P91 pipe exhibited a hardness of 196–265 HV, while the repair-welded joint showed average hardness values of 210 HV (BZ), 330 HV (HAZ) and 230 HV (WZ). Guided bend testing produced deformation below 3 mm in both the WZ and HAZ, satisfying ASME BPVC Section IX-2015 (QW-153). Average ultimate tensile strength reached 688 MPa, exceeding the ASME SA-335 minimum specified tensile strength for P91 (585 MPa), with fracture consistently located in the base-metal region and a ductile-dimple morphology. Non-destructive testing performed to ASME Section V confirmed the absence of rejectable defects after repair. These results indicate that a properly sequenced temper bead procedure can restore, and in this case improve, the short-term mechanical integrity of SA335-P91 girth welds, supporting its use as a maintenance strategy for in-service power-plant piping.