CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of EQ308L Stainless Steel Strip Submerged Arc Weld Overlay Material for Nuclear Power Applications
Literature Overview
This research by Zou Liwei, Xu Kai, Feng Wei, Wei Tao, Hu Xiaobo, and Gao Feng (2015), conducted at the Harbin Welding Research Institute of the China Academy of Machinery Science and Technology in collaboration with Xinjiang Xintou Energy Equipment Co., Ltd., reports on the development of a specialized stainless steel strip submerged arc welding (strip SAW) consumable designated EQ308L for nuclear power applications. The work was supported by multiple national funding programs including the National Science and Technology Major Project (2012ZX06004-21), the National Energy Application Technology Research and Engineering Demonstration Project (NY20111201-1), and the Heilongjiang Provincial Scientific Research Institution Innovation Capability Enhancement Program (YC2015D009).
Technical Background and Requirements
Nuclear power reactors require clad components that provide corrosion resistance to high-temperature water, steam, and nuclear-grade chemical environments while maintaining structural integrity under reactor operating conditions. The 304L/304 stainless steel grade is widely used for nuclear-grade cladding due to its excellent resistance to stress corrosion cracking (SCC) in high-temperature water environments, low carbon content to prevent sensitization, and proven performance record in nuclear applications worldwide.
The strip submerged arc welding (strip SAW) process, also known as electroslag welding (ESW) overlay when using continuous strip electrode, is preferred for nuclear-grade cladding due to:
- High deposition rate (20-40 kg/h) enabling efficient cladding of large components
- Low dilution rate (5-15%) preserving the corrosion resistance of the overlay
- Low hydrogen content reducing the risk of delayed cracking
- Consistent quality with minimal operator influence
- Suitability for thick overlay layers required in nuclear applications
| Nuclear Application Requirement | Specification |
|---|---|
| Overlay grade | 304L (EQ308L equivalent) |
| Carbon content | ≤ 0.03% |
| Dilution rate | ≤ 10% |
| Intergranular corrosion resistance | Pass 1000h ASTM A923 Practice A |
| Weld metal composition | Meets ASTM A268/A269M requirements |
| Surface quality | Smooth, no undercut, no porosity |
| NDE acceptance | RT or UT per ASME V, no indications above 0.5 mm |
Material Development Approach
The EQ308L strip consumable was developed through systematic optimization of the following parameters:
Strip Composition Design
The strip composition was designed to ensure that even with maximum allowable dilution, the weld metal would maintain the required corrosion resistance:
- Carbon: 0.02-0.03% (ultra-low carbon to prevent sensitization)
- Chromium: 18-21% (ensures passive film stability)
- Nickel: 8-11% (stabilizes austenitic structure)
- Molybdenum: 0.5-2.0% (enhances pitting resistance)
- Sulfur: ≤ 0.005% (reduces hot cracking tendency)
- Phosphorus: ≤ 0.03% (reduces hot cracking tendency)
Flux Development
A specialized low-silica, low-fluoride flux was developed to:
- Maintain low hydrogen pickup (< 5 ml/100g)
- Provide stable arc and smooth weld bead profile
- Control dilution rate through slag composition
- Minimize slag inclusion in the weld metal
- Facilitate easy slag removal for visual inspection
Process Parameter Optimization
| Parameter | Optimized Value | Effect |
|---|---|---|
| Strip width | 30-50 mm | Controls bead width and penetration |
| Strip thickness | 1.5-2.0 mm | Controls deposition rate and dilution |
| Current | 3000-5000 A | Controls melt pool volume |
| Voltage | 32-38 V | Controls arc length and penetration |
| Travel speed | 150-300 mm/min | Controls heat input and bead shape |
| Preheat temperature | 100-150°C | Reduces residual stress |
| Interpass temperature | ≤ 200°C | Prevents sensitization and grain growth |
Quality Verification
The developed consumable was verified through comprehensive testing:
- Chemical analysis: All elements within specified ranges, carbon consistently below 0.03%
- Mechanical properties: Tensile strength 550-650 MPa, elongation 35-45%, hardness 180-220 HV
- Intergranular corrosion: Passed 1000-hour ASTM A923 Practice A test without intergranular attack
- Side bend test: 100% qualified, no cracking on either surface
- Radiographic testing: No porosity or lack of fusion detected
- Dilution rate measurement: 6-9% confirmed by spectroscopic analysis
Study Insights and Reflections
This research represents a significant contribution to China's nuclear power industry by developing a domestically produced nuclear-grade strip SAW consumable, reducing dependence on imported materials. The multi-faceted development approach — simultaneously optimizing strip composition, flux chemistry, and process parameters — demonstrates the systems engineering mindset required for nuclear-grade consumable development. The emphasis on dilution control is particularly noteworthy, as it directly impacts the corrosion resistance of the final cladding layer. For engineers involved in nuclear pressure vessel fabrication, this work provides a validated consumable option that meets the stringent requirements of ASME VIII Division 3 and RCC-M codes. The long-term reliability of nuclear-grade cladding depends on consumable quality consistency, and this research establishes the technical foundation for achieving that consistency.