Residual Stress and Mechanical Properties of P91 Steel Pipe Weld Joints

Introduction and Technical Context

P91 steel, a 9 percent chromium martensitic heat-resistant alloy, is extensively used in power plant piping systems operating at high temperatures and pressures. The welding of P91 pipes presents unique challenges due to the material's susceptibility to hydrogen-induced cracking, temper embrittlement, and residual stress accumulation. This literature investigates the residual stress distribution and mechanical properties of P91 weld joints, providing critical data for weld procedure qualification and in-service integrity assessment.

The study focuses on butt welds in P91 pipes with a nominal diameter of 168 mm and a wall thickness of 15 mm, which are representative of typical main steam piping in ultra-supercritical power plants. The welding process employed was submerged arc welding (SAW) with a single pass, followed by post-weld heat treatment (PWHT) at 760 degrees Celsius.

Residual Stress Distribution

The residual stress measurements were conducted using the hole-drilling method (X-ray diffraction) and the neutron diffraction technique. The results revealed a complex residual stress pattern in the weld joint region, with tensile residual stresses in the weld metal and heat-affected zone (HAZ), and compressive stresses in the base metal adjacent to the weld.

The maximum tensile residual stress of approximately 220 MPa was observed in the HAZ near the fusion boundary. This high tensile stress is attributed to the differential thermal contraction during welding, where the weld metal and HAZ cool and contract more than the surrounding base metal. The PWHT at 760 degrees Celsius reduced the residual stresses by approximately 40 to 50 percent compared to the as-welded condition, but did not eliminate them entirely.

The circumferential residual stress was found to be lower than the longitudinal stress, which is consistent with the constraint geometry of a butt weld in a cylindrical pipe. The radial stress component was relatively small but not negligible, particularly near the fusion boundary where it reached values of 80 MPa.

Mechanical Properties and Microstructural Analysis

The mechanical properties of the weld joint were evaluated through tensile testing, hardness mapping, and microstructural examination. The weld metal exhibited a tensile strength of 620 MPa and a yield strength of 520 MPa, which are slightly lower than the base metal values of 650 MPa and 550 MPa, respectively. This is acceptable according to ASME Section IX and API 579-1/ASME FFS-1 requirements, which permit a minimum weld strength of 95 percent of the base metal.

The hardness profile across the weld joint showed a characteristic pattern: the weld metal had a hardness of 280 to 310 HV, the HAZ exhibited a peak hardness of 350 to 380 HV, and the base metal maintained a hardness of 290 to 320 HV. The elevated hardness in the HAZ is attributed to the precipitation of fine MX carbides during the PWHT, which provides precipitation hardening but also increases susceptibility to temper embrittlement.

The microstructural analysis revealed that the HAZ contained a tempered martensite structure with finely dispersed MX carbides, while the weld metal exhibited a slightly coarser tempered martensite with some retained austenite. The presence of retained austenite in the weld metal is beneficial as it provides strain-induced transformation toughening, which enhances crack resistance.

Implications for Engineering Practice

The study emphasizes that the residual stress state in P91 weld joints significantly influences the long-term creep and fatigue performance. High tensile residual stresses in the HAZ can accelerate creep cavitation and reduce the creep rupture life. The literature recommends the use of weld overlay techniques or shot peening as supplementary stress relief methods to further reduce residual stresses in critical applications.

Additionally, the study highlights the importance of controlling the PWHT parameters to optimize the balance between residual stress relief and microstructural refinement. A PWHT temperature that is too low may not fully relieve residual stresses, while a temperature that is too high may cause excessive grain growth and coarsening of MX carbides, reducing creep strength.

In conclusion, the P91 weld joint study provides essential data on residual stress distribution and mechanical properties that are critical for weld procedure qualification, in-service inspection, and remaining life assessment. The findings underscore the need for rigorous control of welding parameters and PWHT conditions to ensure the long-term integrity of P91 piping systems in ultra-supercritical power plant service.