Microstructural and mechanical insights into friction stir welded P91 steel pipes for power plant applications

This research investigates friction stir welding (FSW) of P91 steel pipes widely used in high temperature power plant applications, where conventional welding often leads to defect and degradation. FSW was carried out under force control by varying spindle speed and welding speed to optimize process parameters. Microstructural examination revealed three distinct zones such as stir zone, thermo-mechanically affected zone, and base metal. At the optimized parameter combination (550 rpm, 100 mm/min, plunge depth 6 mm), high-temperature tensile test at 600 °C revealed an ultimate tensile strength (UTS) of 684 MPa, which is approximately 60% higher than the base metal UTS (427 MPa). The significant improvement in strength is attributed to dynamic recrystallization, grain refinement, and strain hardening. High cycle fatigue test at room temperature confirmed fatigue life exceeds 10 6 cycles. Field emission scanning electron microscopy revealed fine equiaxed grains, prior austenite grain boundaries and martensitic lath structure in the stir zone. The presence of carbide and carbonitride precipitations contributed to mechanical strength and microstructural stability. Energy-dispersive X-ray spectroscopy confirmed no tool material contamination. The findings show FSW is an effective technique for joining high quality and defect-free welds that can serve as a promising alternative for P91 steel pipes in power generation application.