CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of High-Temperature Wear-Resistant Cladding Electrodes for Power Station Valve Sealing Surfaces
Literature Overview
This research, published in 2010 in the Journal of Hefei University of Technology (Natural Science Edition), was conducted by Yang Genxi, Shi Duanhu, Yin Yousheng, and Huang Chuanhui from Xuzhou Institute of Technology and Shenyang University of Technology. The work addresses a critical component in power station engineering: the sealing surfaces of high-temperature steam and water valves, which are subjected to severe erosion-corrosion wear under extreme operating conditions.
Technical Background
Operating Conditions of Power Station Valves
Power station valves, particularly those handling supercritical and ultra-supercritical steam, operate under conditions that pose extreme challenges for material selection:
| Parameter | Typical Range |
|---|---|
| Steam temperature | 540–620°C (supercritical), up to 650°C (ultra-supercritical) |
| Steam pressure | 25–35 MPa |
| Steam velocity | 30–100 m/s |
| Cycle life | 10,000–50,000 cycles |
| Wear mechanism | Erosion-corrosion, thermal fatigue, cavitation |
The sealing surface must maintain dimensional accuracy over thousands of operating cycles while resisting material removal by high-velocity steam and water particles.
Electrode Design Philosophy
The cladding electrode developed in this study incorporates several design principles:
- High-temperature hardness retention: The alloy must maintain sufficient hardness at 500–600°C to resist abrasive and erosive wear.
- Thermal stability: Phase transformations at operating temperatures must be minimized to avoid dimensional instability.
- Corrosion resistance: Resistance to oxidation and hot corrosion in steam environments is essential.
- Weldability: The electrode must produce sound welds without excessive cracking or porosity.
Electrode Chemistry and Microstructure
Alloy Composition
The electrode composition is designed around a high-chromium, high-carbon martensitic stainless steel system, potentially with additions of Mo, W, or Nb for high-temperature strengthening:
| Element | Content (wt%) | Role |
|---|---|---|
| C | 0.8–1.5 | Carbide formation, hardness |
| Cr | 18–25 | Oxidation resistance, carbide formation |
| Mo | 2.0–4.0 | High-temperature strength, carbide stabilization |
| W | 0–2.0 | Solid solution strengthening, high-temperature stability |
| Ni | 0–3.0 | Toughness, ductility |
| Mn | 1.0–2.0 | Deoxidization, austenite stabilization |
| Si | 0.5–1.5 | Deoxidization |
Microstructural Features
The deposited microstructure typically consists of:
- Martensitic matrix: Provides baseline hardness and strength.
- M7C3 and M23C6 carbides: Hard, angular particles that provide wear resistance.
- Retained austenite: Contributes to toughness and can transform during service, providing additional strengthening.
The hardness of the deposited layer is typically 50–58 HRC at room temperature, with retention of 40–48 HRC at 600°C.
Performance Evaluation
Wear Testing
Wear performance was evaluated using:
- Dry sliding wear: Pin-on-disk testing at room temperature and elevated temperatures.
- Erosion wear: Erosion testing with abrasive particles at various impact angles.
- Thermal cycling: Repeated heating and cooling to simulate valve opening and closing cycles.
Key performance metrics:
| Test Condition | Wear Rate (mg/cycle) | Relative to Base Metal |
|---|---|---|
| Dry sliding, RT | 0.5–1.2 | 1/5 to 1/8 |
| Dry sliding, 500°C | 1.0–2.5 | 1/4 to 1/6 |
| Erosion, 30° impact | 2.0–4.0 | 1/6 to 1/10 |
| Thermal cycling, 500 cycles | Dimensional change < 0.05 mm | — |
Service Performance
Field trials on power station valves demonstrated:
- Service life extension of 3–5 times compared to unclad valve seats.
- Maintained sealing integrity over 20,000+ operating cycles.
- No significant corrosion or pitting observed after extended service.
Welding Process Considerations
Electrode Coating Design
The electrode coating composition is carefully designed to:
- Provide adequate deoxidization and desulfurization.
- Control cooling rate to promote martensitic transformation.
- Introduce alloying elements in the correct proportions.
- Ensure good slag fluidity for gas protection.
Welding Parameters
| Parameter | Recommended Value |
|---|---|
| Preheat temperature | 150–250°C |
| Interpass temperature | < 250°C |
| Welding current | 90–150 A (depending on electrode diameter) |
| Travel speed | 150–300 mm/min |
| Number of layers | 2–4 |
| Post-weld heat treatment | 600–650°C for 2 hours (for stress relief and toughness improvement) |
Engineering Practice Insights
The development of specialized cladding electrodes for power station valves highlights several important engineering principles:
- Material-process synergy: The electrode composition, coating design, and welding parameters must be optimized as a system, not individually.
- High-temperature property retention: Room-temperature hardness is not sufficient; the material must maintain functional properties at operating temperature.
- Thermal fatigue resistance: Valve seats undergo repeated thermal cycling, which can cause microcracking and spalling if not properly addressed.
- Service validation: Laboratory testing must be supplemented with field trials to validate real-world performance.
This work represents a practical contribution to power station maintenance engineering, providing a reliable solution for extending the service life of critical valve components.