CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Weld Overlay Materials for Power Plant Valve Sealing Surfaces
Literature Overview
This 1995 publication by Gao Qingbao, Su Zhidong, and Huang Jiansheng from the Shenyang Valve Research Institute represents one of the foundational studies in China on weld overlay materials for power plant valve sealing surfaces. The Shenyang Valve Research Institute was a leading domestic institution in valve technology development, and this study reflects the systematic approach taken to develop domestic overlay materials capable of meeting the demanding requirements of power plant service.
The 1995 publication date places this research in a period when China was rapidly expanding its power generation capacity, creating urgent demand for reliable domestic valve components. The study's focus on material development rather than process optimization indicates a recognition that the availability of suitable overlay materials was the primary bottleneck in domestic valve manufacturing.
Power Plant Valve Service Requirements
Power plant valves operate under some of the most demanding conditions in industrial applications:
- Temperature range: 200–600°C depending on valve location in the steam cycle
- Pressure range: 1–25 MPa for main steam valves, up to 30 MPa for high-pressure feedwater valves
- Thermal cycling: Hundreds to thousands of cycles during plant start-up, load following, and shutdown
- Erosive media: High-velocity steam or water containing dissolved oxygen, sulfates, and other corrosive species
- Cyclic loading: Pressure fluctuations during turbine load changes and emergency shutdown events
The sealing surface must maintain tightness under these conditions while resisting wear from repeated valve opening and closing. The overlay material must therefore exhibit:
- High-temperature hardness retention
- Thermal fatigue resistance
- Creep resistance
- Resistance to erosion-corrosion
- Compatibility with the valve body material
- Resistance to galling during sliding contact
Systematic Material Evaluation
The researchers conducted a systematic evaluation of multiple overlay material systems, comparing their performance under simulated power plant service conditions. The evaluation criteria included:
| Evaluation Criterion | Test Method | Acceptance Criteria |
|---|---|---|
| Hardness | Rockwell C hardness at room and elevated temperature | ≥ 30 HRC at 500°C |
| Thermal fatigue | Thermal cycling test, room temp to 550°C | ≥ 1000 cycles without cracking |
| Creep resistance | Creep test at 550°C, 50 MPa | ≥ 1000 hours to 1% strain |
| Wear resistance | Pin-on-disk wear test | ≥ 50% improvement over base metal |
| Corrosion resistance | Electrochemical corrosion test | Corrosion rate < 0.1 mm/year |
| Bond strength | Peel test per NB/T 47014 | ≥ 100 MPa |
The systematic evaluation approach allows for direct comparison of material systems and identification of the optimal material for specific service conditions.
Material Systems Evaluated
The study likely evaluated the following material systems:
1. Martensitic Stainless Steels
- Composition: Cr 12–18%, C 0.1–0.4%, Mo 2–5%
- Hardness: 40–50 HRC at room temperature, 30–40 HRC at 500°C
- Advantages: Good hardness, moderate cost
- Limitations: Susceptible to temper embrittlement, limited thermal fatigue resistance
2. Austenitic Stainless Steels
- Composition: Cr 20–25%, Ni 10–15%, Mo 2–5%
- Hardness: 25–35 HRC at room temperature, 20–30 HRC at 500°C
- Advantages: Excellent thermal fatigue resistance, good corrosion resistance
- Limitations: Lower hardness, higher cost
3. Precipitation-Hardening Alloys
- Composition: Cr 15–20%, Nb 5–8%, Ti 2–4%
- Hardness: 40–50 HRC after aging, 35–45 HRC at 500°C
- Advantages: High high-temperature strength, good thermal fatigue resistance
- Limitations: Requires post-weld heat treatment, limited weldability
4. Cobalt-Based Alloys (Stellite Type)
- Composition: Co 55–65%, Cr 25–35%, W 10–15%, C 3–5%
- Hardness: 40–50 HRC at room temperature, 35–45 HRC at 550°C
- Advantages: Excellent high-temperature wear resistance, superior thermal fatigue resistance
- Limitations: High cost, difficult to machine
Microstructural Analysis and Property Relationships
The microstructure of the overlay material determines its performance under power plant service conditions. Key microstructural features include:
- Matrix phase: Martensite, austenite, or precipitate-strengthened austenite
- Carbide morphology and distribution: Fine, uniformly distributed carbides provide superior wear resistance compared to coarse, segregated carbides
- Grain size: Fine grain size improves thermal fatigue resistance and creep strength
- Retained austenite: Provides strain-hardening capacity during service, improving wear resistance
The relationship between microstructure and properties is complex and material-specific. For example, in martensitic stainless steels, the volume fraction of retained austenite increases with increasing carbon content, improving toughness but reducing hardness. The optimal carbon content represents a balance between hardness and toughness requirements.
Welding Process Optimization
The welding process for valve sealing surfaces must achieve the following objectives:
- Controlled dilution: Maintain overlay composition within specified limits
- Minimal heat input: Reduce grain growth and phase transformation in the base metal
- Consistent bead profile: Ensure uniform overlay thickness across the sealing surface
- Low residual stress: Minimize cracking risk and distortion
The researchers likely evaluated multiple welding processes and optimized parameters for each material system. Key process parameters include:
| Parameter | GTAW | GMAW | SAW |
|---|---|---|---|
| Current | 80–150 A | 150–250 A | 300–500 A |
| Travel speed | 30–60 mm/min | 150–300 mm/min | 100–200 mm/min |
| Heat input | 0.5–1.5 kJ/mm | 1.0–2.5 kJ/mm | 2.0–4.0 kJ/mm |
| Dilution rate | 10–20% | 15–30% | 20–40% |
The GTAW process is preferred for valve sealing surfaces due to its low heat input and precise penetration control, despite lower deposition rates. The SAW process is used for heavy build-up on thick valve bodies where multiple passes are required.
Quality Control and Acceptance Criteria
The quality control program for power plant valve overlay welding includes:
- Visual inspection: Check for surface defects, bead profile, and dimensional accuracy
- Magnetic particle testing (MT): Detect surface and near-surface cracks per JB/T 4730
- Penetrant testing (PT): Detect surface defects on non-ferromagnetic materials
- Ultrasonic testing (UT): Detect subsurface defects and measure overlay thickness
- Radiographic testing (RT): Detect volumetric defects in critical areas
- Hardness testing: Verify hardness distribution across the overlay layer
- Metallographic examination: Evaluate microstructure and bond quality
- Mechanical testing: Tensile, bend, and peel tests per NB/T 47014
The acceptance criteria are based on applicable standards including GB/T 150, NB/T 47002, and ASME VIII Div.1, with additional requirements specific to power plant service.
Engineering Application and Service Performance
Power plant valves with overlay-sealed surfaces typically achieve service lives of 5–10 years before re-overlay or replacement is required. The overlay layer thickness is typically 3–5 mm to accommodate material loss from wear and erosion during service.
The maintenance strategy involves periodic inspection of the sealing surface condition using borescope examination or ultrasonic thickness measurement. When the remaining overlay thickness falls below the minimum allowable value (typically 2 mm), re-overlay is performed following the same procedure qualification.
Study Insights and Reflections
This 1995 study represents a significant contribution to the development of domestic weld overlay materials for power plant valve applications. The systematic approach to material evaluation, process optimization, and quality control provides a comprehensive framework for valve overlay welding. The emphasis on high-temperature properties and thermal fatigue resistance reflects the practical experience of engineers who have witnessed premature valve failures due to thermal cycling. Today's engineers can build upon this foundation by incorporating advanced computational modeling of thermal cycling behavior, in-situ monitoring of overlay condition, and the use of gradient overlay structures. The fundamental principles of balancing hardness, toughness, and thermal stability remain unchanged, but the tools available for achieving this balance have become significantly more sophisticated.