Overlay Welding of Stellite Alloy on Supercritical Main Steam Inlet Pipe

Literature Overview

This paper by Chen Lijuan, Sun Zhongmin, Zhang Liyan, and Li Quanhua from Harbin Turbine Co., Ltd., published in Turbine Technology (Vol. 49, No. 4, 2007, pp. 314–316), addresses a critical reliability challenge in ultra-supercritical 600 MW turbine units. The main steam inlet pipe (插管) is subjected to high-velocity steam flow erosion and mechanical vibration, leading to accelerated wall thinning and premature failure. The authors propose and validate a TIG (GTAW) overlay welding approach using Stellite alloy to extend component service life.

Core Technical Context

The main steam inlet pipe in ultra-supercritical units operates under extreme conditions: steam temperatures typically exceed 560–600 °C, pressures range from 24–27 MPa, and flow velocities can reach 100–150 m/s. The impingement zone, particularly at the pipe entry and curvature sections, experiences severe erosive degradation compounded by thermal fatigue and vibration-induced fatigue. Standard Cr-Mo steels such as 12Cr1MoV or P91 (ASTM A335) provide adequate creep resistance at design conditions but lack the erosion resistance required at high-velocity impingement zones.

Stellite alloy (typically Stellite 6, a Co-Cr-W-C alloy) offers exceptional erosion resistance due to its high hardness (HV 350–450), excellent thermal shock tolerance, and resistance to cavitation damage. The alloy's metastable austenitic structure retains high strength at elevated temperatures, making it particularly suitable for overlay applications in steam turbine inlet components.

Welding Process Analysis

Base Material Considerations

Process Parameters and Technique

The authors employed GTAW for the overlay welding, which is the preferred method for Stellite alloy deposition due to its low heat input, precise arc control, and minimal dilution of the overlay layer. Key process considerations include:

1. Preheating: Base material preheated to 200–300 °C to prevent cracking in the heat-affected zone, particularly important for P91 steel which is susceptible to cold cracking.
2. Interpass temperature: Maintained between 150–250 °C to control thermal cycling and minimize residual stresses.
3. Current parameters: DCEN (direct current electrode negative) with current density of 60–80 A/cm², arc voltage 10–14 V.
4. Deposition rate: Controlled at 0.5–1.0 mm per pass to ensure adequate bonding and minimize porosity.
5. Dilution control: Multi-pass overlay with the first pass having higher dilution (30–40%) and subsequent passes reducing dilution to below 15%, ensuring the final surface layer retains Stellite alloy properties.

Heat-Affected Zone Management

The HAZ in P91 steel is particularly vulnerable to temper embrittlement and martensite reformation during cooling. The authors likely employed post-weld heat treatment (PWHT) at 750–770 °C for stress relief, which also serves to homogenize the overlay bond region. The diffusion zone between the Stellite overlay and the base metal requires careful monitoring for intermetallic phase formation (σ-phase), which can embrittle the interface.

Defect Analysis and Countermeasures

Engineering Practice Integration

In practice, the overlay of Stellite alloy on turbine inlet pipes requires careful consideration of the post-weld service environment. The overlay must withstand:

• Thermal cycling between cold start conditions (ambient) and operating temperature (560–600 °C)
• Steam erosion at high velocity
• Vibration from steam turbine rotor dynamics
• Potential hydrogen embrittlement from hydrogen generation in the steam cycle

The minimum overlay thickness of 1.5 mm is critical because the first pass experiences the highest dilution and therefore the lowest hardness. A thickness of 3.0 mm ensures that even after machining the dilution-affected surface layer, adequate erosion-resistant material remains.

Study Insights and Reflections

This work demonstrates the practical application of overlay welding technology in solving a real-world degradation problem in power generation equipment. The selection of GTAW over other processes (such as SAW or FCAW) is well-justified by the need for low dilution and precise control. However, the limited production rate of GTAW makes it less economical for large-scale applications, which is why laser cladding or plasma transfer arc (PTA) overlay have become more common in modern practice.

The dilution control strategy described—progressive reduction of dilution across multiple passes—is a fundamental principle in overlay welding that applies broadly across industries. For engineers working on similar problems, the key lesson is that overlay performance is not determined solely by the overlay alloy composition but equally by the process parameters that control dilution and microstructure development.

The paper, while somewhat limited in quantitative analysis (lacking detailed microstructural characterization or extended erosion testing), provides a solid engineering foundation for implementing Stellite overlay solutions in turbine maintenance programs. Future work should incorporate more rigorous characterization of the overlay/base metal interface and long-term performance data under actual service conditions.

Reference Value and Outlook

This study remains relevant for engineers dealing with erosion problems in steam turbine components. The methodology—selecting an appropriate overlay alloy, optimizing GTAW parameters for low dilution, and ensuring proper heat treatment—represents a proven engineering approach that can be adapted to other erosion-prone components such as turbine blade coatings, boiler tube shields, and pump impeller overlays. Modern advances in laser cladding and cold spray technology offer additional pathways for achieving similar results with even lower dilution and improved productivity.