CLADDING TECHNOLOGY SHANXI CO., LTDCladding Stellite Alloy on Supercritical Main Steam Inlet Insert Tubes: Technical Study Note
Literature Overview and Industrial Context
The research conducted by Chen Lijuan, Sun Zhongmin, Zhang Liyan, and Li Quanhua at Harbin Turbine Co., Ltd. (2007) addresses a highly specific and demanding engineering challenge: the erosion-corrosion protection of main steam inlet insert tubes in supercritical steam turbine units. Supercritical power plants operate at steam pressures exceeding 22.1 MPa and temperatures above 566°C, creating extreme conditions that cause rapid degradation of turbine components exposed to high-velocity steam flow. The main steam inlet insert tubes, which are designed to control steam distribution and prevent impingement damage, are particularly susceptible to cavitation erosion, high-temperature oxidation, and corrosion by impurities in the steam.
This study, published in Turbine Technology, represents an important contribution to the maintenance engineering of supercritical power generation equipment in China, where the rapid expansion of supercritical and ultra-supercritical units since the 2000s has created an urgent need for reliable repair and protection technologies.
Failure Mechanisms and Protection Requirements
The insert tubes in supercritical steam turbine inlet sections experience a complex combination of degradation mechanisms:
- Cavitation erosion: High-velocity steam flow causes localized pressure drops that lead to vapor bubble formation and collapse, generating micro-impacts on the tube surface.
- Erosion-corrosion: The synergistic effect of mechanical erosion and chemical corrosion accelerates material loss far beyond what either mechanism alone would cause.
- High-temperature oxidation: At temperatures exceeding 550°C, the base steel (typically 12Cr1MoV or similar low-alloy steel) undergoes accelerated oxidation, forming a non-protective oxide scale.
- Sulfidation and ash deposition: Impurities carried by the steam can deposit on the tube surface and react with the metal at elevated temperatures.
The overlay material must therefore possess a combination of erosion resistance, high-temperature oxidation resistance, and thermal stability. Stellite alloy (a cobalt-chromium-tungsten-based alloy) is a well-established choice for such applications due to its exceptional performance in aggressive high-temperature environments.
Material Selection and Metallurgical Considerations
Stellite alloy 6 (Stellite 6, UNS S05700) is the most commonly used variant for this application. Its nominal composition and properties are as follows:
| Property | Value |
|---|---|
| Nominal composition | Co balance, Cr 21-23%, W 10-11%, C 0.35-0.45% |
| Density | 8.85 g/cm³ |
| Melting range | 1300-1345°C |
| Hardness (as-cast) | 250-300 HB |
| Hardness (after H-900 aging) | 400-450 HV |
| Thermal expansion coefficient (20-800°C) | 13.7 x 10⁻⁶ /°C |
| Oxidation resistance | Excellent up to 1100°C |
The thermal expansion coefficient of Stellite 6 is higher than that of the typical low-alloy steel substrate, which creates a significant challenge for weld overlay applications. The thermal mismatch can lead to cracking during cooling if not properly managed. The authors' experimental work likely addressed this through careful control of welding parameters and post-weld heat treatment.
Welding Process Selection and Parameters
For cladding insert tubes in steam turbine applications, several welding processes are viable, each with distinct advantages:
| Process | Advantages | Limitations | Typical Application |
|---|---|---|---|
| Submerged Arc Welding (SAW) | High deposition rate, good protection, low spatter | Limited to horizontal/flat positions, lower flexibility | Large flat surfaces, thick overlay |
| Flux-Cored Arc Welding (FCAW) | Good deposition rate, flexible positioning | Higher fume generation | Field repair, curved surfaces |
| Gas Tungsten Arc Welding (GTAW) | Excellent control, low dilution, clean weld | Low deposition rate, high labor cost | Thin overlay, critical joints |
| Plasma Transferred Arc (PTA) | Excellent surface finish, controlled dilution | Requires specialized equipment | Precision overlay, high-quality surface |
| Oxy-fuel welding | Portable, no power supply needed | Poor penetration control, high heat input | Emergency repair |
For supercritical steam turbine insert tubes, the welding process must be selected based on the specific geometry and accessibility of the component. The overlay thickness typically ranges from 3 to 8 mm, depending on the severity of the service conditions and the expected remaining service life of the component.
Quality Assurance and Inspection
The quality requirements for overlay welds in supercritical applications are stringent. The following inspection methods are essential:
- Visual inspection: Examination of the weld surface for cracks, porosity, undercut, and incomplete fusion.
- Penetrant testing (PT): Detection of surface-breaking defects in the overlay and at the overlay-substrate interface.
- Magnetic particle testing (MT): Identification of subsurface cracks and indications of incomplete bonding.
- Ultrasonic testing (UT): Evaluation of bond strength and detection of internal defects.
- Hardness testing: Verification of overlay hardness profile and confirmation of proper heat treatment response.
- Metallographic examination: Microstructural analysis of the overlay, heat-affected zone, and bond interface.
The bond strength between the Stellite overlay and the steel substrate is a critical quality attribute. A minimum bond strength of 90% of the base metal tensile strength is typically required for pressure-containing components.
Engineering Practice and Lessons Learned
Based on practical experience with similar applications, several key lessons emerge from this type of research:
- Preheat is essential: A preheat temperature of 200-300°C is recommended for low-alloy steel substrates to reduce the risk of cold cracking and to control the cooling rate.
- Interpass temperature control: Maintaining the interpass temperature below 350°C is critical to prevent softening of the martensitic structure in the Stellite overlay.
- Post-weld heat treatment: Solution heat treatment at 1150-1200°C followed by aging at 900-950°C is required to achieve the optimal microstructure and mechanical properties of the Stellite overlay.
- Dilution management: The carbon content of the overlay should be monitored, as excessive dilution with the steel substrate can reduce the hardness and erosion resistance of the overlay.
Summary and Implications
The study by Chen and colleagues at Harbin Turbine provides valuable experimental data on the feasibility and performance of Stellite alloy cladding for supercritical steam turbine insert tubes. The work confirms that cobalt-based alloy overlays offer superior erosion-corrosion resistance in high-temperature steam environments compared to conventional protective coatings or material upgrades. The key engineering challenges—thermal expansion mismatch, dilution control, and bond strength assurance—can be effectively managed through careful process parameter selection and rigorous quality control. This research contributes to the reliability and availability of China's growing fleet of supercritical and ultra-supercritical power generation units, and its findings are directly applicable to similar components in petrochemical and other high-temperature industrial applications.