Structural Integrity of Welded Pipe Joints Made of Creep–Resistant Steel P91 with Multiple Defects: Experimental Characterization and Engineering Critical Assessment

This study provides a comprehensive analysis of the mechanical properties and structural integrity of butt-welded P91 creep-resistant steel pipe joints utilized in high-pressure piping systems. Two pipe specimens with an outer diameter of Ø48.3 mm and a wall thickness of 10.15 mm were welded using the GTAW process in the PC position under controlled heat input, followed by post-weld heat treatment: one without and one with an axial misalignment of e = 1 mm (10% of wall thickness). Both welded pipes underwent an identical thermal cycle. Experimental results, including tensile testing, Charpy V-notch impact toughness at 20 °C, HV10 hardness testing, and macro/microstructural analysis, confirmed the over-matching effect of the W CrMo9-1 filler metal and satisfactory toughness in both the weld metal and heat-affected zone. The experimental data served as input parameters for Engineering Critical Assessment (ECA) using Failure Assessment Diagrams (FAD) according to the BS 7910 standard. The combined effect of multiple defects in welded joints—axial misalignment and a 0.3 mm root micro-crack (representing the lack of root penetration)—was evaluated under designed internal pressures of 200 and 300 bar. The results of the ECA approach indicate that the presence of multiple welding defects does not compromise the structural integrity of P91 welded pipes, provided that PWHT is properly executed. This underscores the critical role of the ECA/FAD methodology in evaluating the operational safety of high-pressure piping systems, as well as the need for thorough weldment characterization prior to service.