Residual Stress and Mechanical Properties of P91 Steel Pipe Welded Joints

Literature Overview

This study investigates the residual stress distribution and mechanical properties of P91 (ASTM A335 P91, 9Cr-1Mo-V-Nb) steel pipe welded joints, which are critical components in ultra-supercritical power plant steam piping systems. P91 steel is a martensitic ferritic heat-resistant alloy that requires careful welding procedures due to its high hardenability, susceptibility to hydrogen-induced cracking, and tendency to develop high residual stresses during welding. The residual stress state directly influences the long-term creep rupture life, stress corrosion cracking resistance, and fatigue performance of welded joints, making its characterization essential for reliable component design and lifecycle management.

Welding Process and Heat Treatment

Welding Procedure Specification

The study employed the following welding procedure for a 325 mm OD × 22 mm wall thickness P91 pipe butt joint:

PWHT Effectiveness

Post-weld heat treatment at 745 °C for 4 hours is critical for P91 welded joints to achieve:

• Tempering of the as-welded martensitic structure to a tempered martensite with fine carbide precipitation
• Reduction of residual stresses by 70–85% through stress relaxation at elevated temperature
• Homogenization of the microstructure across the weld, HAZ, and base metal
• Recovery of ductility and toughness in the HAZ, which is particularly susceptible to over-tempering or under-tempering

Residual Stress Analysis

Measurement Methodology

Residual stresses were measured using the incremental hole-drilling method (strain gauge method) in accordance with ASTM E837. Measurements were taken at multiple locations across the weld cross-section:

• Weld center: Along the weld axis at the fusion line
• HAZ: 2 mm, 5 mm, and 10 mm from the fusion line
• Base metal: 20 mm, 50 mm, and 100 mm from the fusion line

Residual Stress Distribution

The results show that even after PWHT, significant residual stresses remain, particularly in the HAZ region where longitudinal stresses reach 210 MPa. This is attributed to:

1. Incomplete stress relaxation during PWHT due to the short hold time and limited diffusion distances
2. Differential thermal expansion between the weld, HAZ, and base metal during cooling from PWHT temperature
3. Constraint from the pipe geometry that prevents full stress relief through plastic deformation

Effect of PWHT Parameters on Residual Stress

The study compared residual stresses for different PWHT conditions:

The diminishing returns beyond 760 °C × 6 h, combined with the risk of over-tempering and grain growth at higher temperatures, confirm that 745 °C × 4 h represents an optimal balance between stress relief and microstructural integrity for P91 welded joints.

Mechanical Property Characterization

Hardness Distribution

Microhardness measurements (Vickers, 1 kg load) across the weld cross-section reveal:

The HAZ exhibits the highest hardness after PWHT (295 HV), which is 23% higher than the base metal. This is attributed to the partial tempering of the HAZ microstructure, where the prior austenite grain boundaries and carbide-free zones create localized regions of higher strength. The hardness profile shows a characteristic "W" shape with peaks in the HAZ and weld metal, indicating potential concerns for stress concentration and crack initiation.

Tensile and Creep Properties

The HAZ exhibits the highest tensile strength but the lowest ductility and creep rupture life, confirming it as the critical location for long-term failure. The reduced creep life in the HAZ is attributed to:

• Coarsening of carbide precipitates during PWHT, reducing precipitation strengthening
• Formation of carbide-free zones along prior austenite grain boundaries, which promote intergranular creep cavitation
• Higher residual stresses that add to the applied stress, accelerating creep damage

Defect Analysis and Quality Control

Common Defects in P91 Welded Joints

FMEA-Based Risk Assessment

A Failure Mode and Effects Analysis (FMEA) was conducted for the welding process:

The highest Risk Priority Numbers (RPN) are for creep cavitation and residual stress exceedance, both of which are long-term degradation mechanisms that are difficult to detect during initial inspection. This underscores the importance of process control during welding and PWHT rather than relying solely on post-fabrication inspection.

Study Insights and Reflections

This research provides comprehensive data on the residual stress state and mechanical properties of P91 welded joints that is directly applicable to engineering design and quality assurance. The finding that the HAZ is the critical location for both short-term (fatigue) and long-term (creep) failure is consistent with other studies on 9Cr steels and reinforces the need for special attention to HAZ quality during welding.

A particularly important insight is the residual stress profile after PWHT. The persistence of 185–210 MPa longitudinal stresses in the HAZ, despite proper PWHT, challenges the common assumption that PWHT eliminates residual stresses. Engineers must recognize that PWHT reduces but does not eliminate residual stresses, and that the stress state in the HAZ can significantly influence long-term performance. This has implications for stress-based life assessment methods, which must account for the actual residual stress distribution rather than assuming a fully stress-relieved condition.

The practical recommendations from this study—optimizing PWHT parameters, controlling hydrogen levels, and implementing comprehensive inspection protocols—should be incorporated into welding procedure specifications for P91 components. Furthermore, the FMEA analysis provides a structured framework for identifying and mitigating the most critical failure modes, which can be adapted to other welding applications involving advanced heat-resistant alloys. As the power industry continues to push toward higher operating temperatures and pressures, the understanding and control of residual stresses in welded joints will become increasingly important for ensuring the long-term reliability of critical components.