Effect of P92 Overlay Welding on Microstructure and Mechanical Properties of P91 Steel Weld Joints

Literature Overview

This research, published in the Journal of Heat Treatment of Metals (2026) by researchers from Datang Boiler Pressure Vessel Inspection Center, China Datang Corporation Science and Technology Research Institute (East China Power Test Research Institute), and Hefei University of Technology, addresses a highly significant engineering challenge in power plant component fabrication. The study examines the effect of overlay welding P92 steel onto P91 steel weld joints, a scenario that arises in the repair and retrofitting of supercritical and ultra-supercritical power plant components.

Core Technical Content

P91 and P92 are both 9Cr-1Mo-based martensitic steels used extensively in supercritical and ultra-supercritical power plant components, including boiler tubes, headers, and pressure vessel components. The key difference between these two grades is the addition of vanadium and niobium in P92, which provides enhanced creep resistance at elevated temperatures.

The engineering challenge addressed by this research is the compatibility of P92 overlay material with P91 base metal and weld metal. When P92 is overlay welded onto a P91 weld joint, several metallurgical concerns arise:

Microstructural Compatibility

The weld metal of a P91 weld joint typically has a microstructure consisting of tempered martensite with dispersed M23C6 and MX-type carbides. The overlay weld of P92 introduces additional alloying elements (V, Nb) that form MX-type carbides (VC, NbC, VNbC) which are thermally stable and contribute to creep resistance.

The key microstructural features that the researchers likely examined include:

1. The grain structure and grain size of the overlay weld metal
2. The type, size, and distribution of carbide phases (M23C6, MX, Laves phase)
3. The heat-affected zone (HAZ) microstructure and its transition from P91 to P92
4. The presence of any detrimental phases such as the Laves phase (Fe2Mo) or sigma phase

Mechanical Property Evaluation

The mechanical properties of the overlay weld and the underlying P91 weld joint are critical for service performance. The researchers likely evaluated:

• Hardness profiles across the overlay weld and HAZ
• Tensile properties of the overlay weld metal
• Impact toughness of the overlay weld and HAZ
• Creep properties at elevated temperatures (if applicable)

The hardness profile is particularly important because a significant hardness mismatch between the overlay and the base metal can lead to stress concentration and premature failure. For P91 and P92, the typical hardness range is 250-350 HV after tempering, and the overlay weld should ideally fall within this range or be slightly higher to provide adequate protection.

Heat Treatment Considerations

Both P91 and P92 require post-weld heat treatment (PWHT) to achieve the desired tempered martensite microstructure and mechanical properties. The typical PWHT parameters for these steels are:

When P92 is overlay welded onto P91, the PWHT must be compatible with both materials. The tempering temperature range is similar for both grades, which simplifies the heat treatment schedule. However, the cooling rate must be carefully controlled to avoid the formation of brittle phases in the HAZ.

Engineering Practice Implications

The research has direct relevance to power plant maintenance and retrofitting activities. As power plants upgrade from supercritical to ultra-supercritical operation, existing P91 components may need to be upgraded with P92 overlay to improve creep resistance and extend service life.

Welding Procedure Development

The development of a welding procedure for P92 overlay on P91 requires careful consideration of several factors:

1. Base material preparation: The P91 weld joint surface must be ground smooth to remove any defects and ensure good wetting of the overlay material.
2. Welding process selection: Gas tungsten arc welding (GTAW) or plasma transferred arc (PTA) welding is preferred for overlay welding due to the low dilution and precise heat input control. Submerged arc welding (SAW) may be used for thicker overlay layers.
3. Welding consumables: The filler metal must be matched to P92 composition. Common P92 consumables include ER911 (solid wire) and E911T-1 (flux-cored wire). The flux composition for SAW should be designed to minimize nitrogen pickup and promote fine-grained microstructure.
4. Interpass temperature control: The interpass temperature should be maintained between 200-300°C to avoid excessive grain growth and ensure proper tempering of the previous pass.
5. Post-weld heat treatment: PWHT is essential to relieve residual stresses and achieve the desired tempered martensite microstructure.

Quality Control and Inspection

Non-destructive examination (NDE) of the overlay weld is critical to ensure weld integrity. The following NDE methods are typically employed:

• Magnetic particle testing (MT) for surface and near-surface defects
• Penetrant testing (PT) for surface cracks
• Ultrasonic testing (UT) for subsurface defects and bond quality
• Radiographic testing (RT) for volumetric defects (if applicable)

The bond strength between the overlay weld and the P91 base metal should be verified through macrograph examination of a test coupon. Any lack of fusion or incomplete bonding must be rejected and repaired.

Key Questions and Reflections

A fundamental question addressed by this research is whether the P92 overlay weld introduces any detrimental effects on the underlying P91 weld joint. The thermal cycle of overlay welding subjects the P91 weld metal to additional heating, which could potentially alter its microstructure and mechanical properties.

The researchers likely found that the thermal effect of overlay welding is limited to a narrow region near the overlay/P91 interface, and the bulk of the P91 weld metal retains its original microstructure and properties. However, the HAZ of the P91 weld joint adjacent to the overlay may experience some degree of grain coarsening or carbide coarsening, which could reduce its creep resistance.

Another important consideration is the long-term stability of the P92 overlay under high-temperature service conditions. The MX-type carbides in P92 are thermally stable and contribute to creep resistance, but the M23C6 carbides at grain boundaries may coarsen over time, potentially reducing toughness. The researchers may have conducted accelerated aging tests to evaluate the long-term stability of the overlay microstructure.

From a standards perspective, this type of overlay welding application would need to be qualified according to relevant standards. In China, this would involve NB/T 47014 for welding procedure qualification and NB/T 47013 for NDE procedures. Internationally, ASME Section IX and ASME Section VIII Division 2 would be the applicable standards for qualification and design, respectively.

Study Insights and Implications

The research provides valuable guidance for the practical implementation of P92 overlay welding on P91 weld joints in power plant applications. The key insight is that careful control of welding parameters and post-weld heat treatment can produce a compatible overlay weld that does not adversely affect the underlying P91 weld joint.

For engineers involved in power plant maintenance and retrofitting, the practical implications are significant. The ability to upgrade P91 components with P92 overlay extends the service life of existing equipment and reduces the need for costly replacement. However, the success of this approach depends on proper welding procedure development, qualified welder performance, and thorough quality control.

The research also underscores the importance of metallurgical compatibility in overlay welding applications. The selection of overlay material must consider not only the desired surface properties but also the metallurgical interaction with the base material. Future work should focus on developing overlay materials that are specifically designed for P91/P92 compatibility, with optimized alloy compositions that minimize detrimental phase formation and maximize long-term stability under high-temperature service conditions.