Stellite Alloy Overlay Welding of Supercritical Main Steam Inlet Nozzle Study Reflection

Literature Overview

This paper presents a comprehensive study on the overlay welding of Stellite alloy onto the supercritical main steam inlet nozzle of a power boiler, addressing a critical engineering challenge in the power generation industry. The supercritical main steam system operates at temperatures exceeding 600°C and pressures above 25 MPa, creating an extremely harsh environment where thermal fatigue, creep, and erosion-corrosion degradation are prevalent failure modes.

The research focuses on the selection of Stellite alloy composition, welding process parameters, heat input control, and post-weld inspection procedures to ensure the integrity and long-term serviceability of the overlay. The study bridges the gap between laboratory-scale welding trials and full-scale component fabrication, providing practical guidance for the repair and fabrication of critical steam system components.

Core Technical Points

The supercritical main steam inlet nozzle is typically fabricated from martensitic stainless steel such as P91 or P92, which provides adequate creep strength and thermal fatigue resistance at operating temperatures. However, the nozzle is susceptible to erosion at the steam entry region where high-velocity steam impinges on the internal surface, and to thermal fatigue cracking at stress-concentrated locations. The application of a Stellite alloy overlay provides a protective barrier that significantly extends the service life of the component.

Material Selection and Compatibility

The selection of Stellite alloy composition is critical for achieving the desired performance in this application. The study evaluates Stellite 6, Stellite 6B, and Stellite 21, with the following considerations:

Stellite 6B was selected for the nozzle application due to its superior hot corrosion resistance, which is important considering the presence of vanadium and other trace elements in the fuel oil that can deposit on the steam-side surfaces and accelerate corrosion at elevated temperatures.

Welding Process Selection

The study evaluates submerged arc welding (SAW) and gas tungsten arc welding (GTAW) as the primary processes for applying the Stellite overlay. The final process selection was a multi-pass approach combining GTAW for the root pass and SAW for the fill and cap passes:

1. Root pass: GTAW with Stellite 6B wire electrode, current 80-100 A, travel speed 80-100 mm/min. This ensures complete fusion at the interface and establishes a sound metallurgical bond.
2. Fill passes: SAW with Stellite 6B wire and flux, current 280-340 A, travel speed 180-220 mm/min, wire feed rate 5.0-6.0 m/min. The flux composition is critical for achieving the desired microstructure and preventing hot cracking.
3. Cap pass: SAW with reduced current (240-280 A) to minimize dilution and ensure adequate coverage.

Heat Input Control

Heat input control is paramount in this application due to the susceptibility of the P91 substrate to temper embrittlement and the need to maintain the overlay microstructure. The study establishes the following heat input limits:

• Maximum heat input per pass: 2.5 kJ/mm
• Interpass temperature: 150-250°C (strictly controlled to prevent excessive grain growth)
• Total heat input: minimized through the use of multiple thin passes rather than fewer thick passes

The thermal cycling effect during multi-pass welding must be considered in the design of the welding sequence. A symmetric welding pattern is recommended to minimize residual stress and angular distortion in the nozzle component.

Defect Analysis and Quality Control

Common Defects in Stellite Overlay Welding

Inspection Requirements

The quality control program for the Stellite overlay welding of the supercritical main steam inlet nozzle includes:

• Visual inspection: 100% of the weld surface, checking for undercut, overlap, excessive reinforcement, and surface defects.
• Penetrant testing (PT): 100% of the overlay surface to detect surface-breaking cracks and pores.
• Ultrasonic testing (UT): 100% of the overlay thickness to detect lack of fusion and internal defects. The test must be performed from both the overlay side and the substrate side.
• Radiographic testing (RT): 10% of the weld length, or 100% if required by the project specification.
• Hardness testing: 3-5 locations across the overlay thickness and along the weld length to verify the hardness profile and dilution level.
• Bond strength testing: Performed on coupon specimens to verify adequate adhesion between the overlay and the substrate.

Engineering Practice and Service Performance

The Stellite overlay on the supercritical main steam inlet nozzle has demonstrated excellent performance in service, with field data indicating a 3-5 fold improvement in erosion resistance compared to the uncoated P91 base material. The overlay thickness of 3-5 mm provides adequate protection for the expected service life of 10-15 years before re-overlay or replacement is required.

Several practical lessons have been learned from the field application of Stellite overlay welding on supercritical steam components:

1. The overlay must be applied with adequate thickness at the steam entry region where erosion is most severe, while the thickness can be reduced in regions with lower erosion rates to minimize cost and weight.
2. The transition from the overlay to the bare base metal should be gradual to avoid stress concentration at the edge of the overlay, which can initiate thermal fatigue cracking.
3. Regular inspection of the overlay thickness is essential during scheduled maintenance to monitor erosion rates and plan for re-overlay or component replacement.
4. The overlay surface should be polished to a smooth finish after welding to minimize flow resistance and reduce the risk of flow-induced vibration.

Study Insights and Conclusions

This research provides valuable practical guidance for the overlay welding of Stellite alloys on supercritical steam system components. The systematic approach to process parameter optimization, combined with rigorous quality control procedures, establishes a reliable methodology that can be replicated for similar applications. The emphasis on heat input control and dilution management is particularly important, as these factors directly influence the microstructure, mechanical properties, and long-term performance of the overlay.

The study also highlights the importance of understanding the interaction between the base material metallurgy and the overlay process. The P91 substrate, with its precipitation-hardened microstructure, is sensitive to thermal cycling, and the welding process must be designed to minimize adverse effects on the substrate properties. The thermal cycling effect of multi-pass welding can actually be beneficial in some cases, as it provides a tempering effect that relieves residual stresses in the heat-affected zone, but this must be carefully controlled to avoid over-tempering and loss of strength.

Future work should focus on the development of advanced overlay materials with improved thermal fatigue resistance, such as functionally graded coatings that transition from the Stellite alloy at the surface to a thermal barrier coating material near the interface. Additionally, the application of laser cladding or cold spray technology for applying thinner, more precisely controlled overlay layers represents an area of active research that may offer advantages over conventional arc welding processes for this application.