Zhuojin Pipe Fitting Co., LtdDevelopment of EQ308L Strip Electrode Submerged Arc Overlay Welding Material for Nuclear Power Applications
Literature Overview
The research article by Zou Liwei, Xu Kai, Feng Wei, Wei Tao, Hu Xiaobo, and Gao Feng, published in Welding (2015, Issue 12), reports on the development of EQ308L stainless steel strip electrode submerged arc overlay welding materials for nuclear power applications. Funded by the National Science and Technology Major Project and other national programs, this work was conducted jointly by the Harbin Welding Research Institute of the Chinese Academy of Machinery Science and Technology and Xinjiang Xintou Energy Equipment Co., Ltd. The study addresses a critical technical challenge: achieving stable tensile strength of at least 520 MPa in the deposited metal of EQ308L strip electrode submerged arc overlay welding materials through nitrogen control.
Core Technical Findings
The EQ308L designation refers to a low-carbon austenitic stainless steel welding consumable, corresponding to AWS A5.4/E308L and ISO 3583/1 G 308 L. In nuclear power applications, the mechanical properties of overlay welds are subject to stringent requirements, particularly minimum tensile strength. The key technical achievement of this study was the identification of nitrogen content as the critical variable controlling tensile strength stability.
| Parameter | Specification | Effect on Properties |
|---|---|---|
| Nitrogen content in deposited metal | 0.05%–0.20% | Controls tensile strength stability; optimal range for ≥520 MPa |
| Tensile strength | ≥520 MPa | Nuclear power requirement |
| Yield strength | Typically ≥205 MPa | Adequate for nuclear applications |
| Elongation | Typically ≥30% | Ensures ductility and crack resistance |
| Corrosion resistance | Excellent in nuclear environments | Meets nuclear grade requirements |
The nitrogen control treatment involved precise adjustment of the welding flux composition and welding process parameters to achieve the target nitrogen range in the deposited metal. Nitrogen acts as an interstitial solid solution strengthening element in austenitic stainless steel, increasing tensile strength without significantly reducing ductility. However, excessive nitrogen can lead to increased susceptibility to sensitization and intergranular corrosion, while insufficient nitrogen results in inadequate strength.
Process Analysis and Engineering Interpretation
Strip electrode submerged arc welding (SESAM) is a highly productive process that uses a flat strip electrode (typically 0.5–2.0 mm thick, 15–50 mm wide) instead of a wire electrode. The process offers several advantages for overlay welding applications: high deposition rates (10–25 kg/h), excellent weld geometry control, low dilution rates, and consistent mechanical properties. For nuclear power applications, these advantages translate into improved quality assurance and reduced production costs.
The nitrogen control challenge in SESAM is more complex than in conventional submerged arc welding (SAW) because the large electrode width and high current density create different arc conditions and metal transfer characteristics. The arc temperature, which governs nitrogen pickup from the atmosphere and flux, varies across the width of the strip electrode. Additionally, the cooling rate in the deposited metal is influenced by the electrode geometry, current density, and travel speed, all of which affect nitrogen solubility and precipitation behavior.
The metallurgical performance of the developed EQ308L SESAM material was evaluated through comprehensive testing: mechanical properties (tensile strength, yield strength, elongation, hardness), corrosion resistance (intergranular corrosion per ASTM A262 Practice E, pitting corrosion, stress corrosion cracking), and microstructural examination (grain size, phase composition, inclusion morphology). The results confirmed that the material met all nuclear power requirements, with excellent metallurgical properties and corrosion resistance.
Integration with Engineering Practice
In the context of nuclear power pipe fitting manufacturing, overlay welding is used for several purposes: corrosion protection of carbon steel pipes and fittings, restoration of worn surfaces on valves and pumps, and enhancement of surface properties for high-temperature service. The development of reliable strip electrode submerged arc overlay welding materials is particularly important for large-diameter nuclear piping components, where the productivity and quality consistency of SESAM provide significant advantages over manual or wire electrode processes.
The following practices are relevant for nuclear power applications:
- Weld procedure qualification (WPQ) must be performed in accordance with applicable codes (ASME III, RCC-M, or national nuclear codes), including full mechanical and corrosion testing of deposited metal.
- Nitrogen content control requires precise flux composition management and process parameter optimization. In-line monitoring of nitrogen content in deposited metal is recommended for production control.
- Non-destructive testing (NDT) requirements for nuclear overlay welds are stringent, typically including 100% ultrasonic testing (UT) and radiographic testing (RT) per applicable inspection codes.
- Material traceability and quality documentation are critical for nuclear applications. Each batch of strip electrode and flux must be accompanied by a certificate of analysis including chemical composition, mechanical properties, and nitrogen content.
Key Questions and Reflections
The study's success in solving the tensile strength stability problem through nitrogen control raises an important question about the long-term stability of nitrogen in the deposited metal during service. At elevated temperatures, nitrogen can diffuse out of solution, potentially reducing strength over time. For nuclear power applications where components operate at temperatures up to 300–350°C for decades, the long-term stability of nitrogen-strengthened deposits is a critical consideration. Further research on nitrogen diffusion kinetics in austenitic stainless steel at service temperatures would be valuable.
Another reflection concerns the scalability of the developed material to different electrode widths and thicknesses. The study optimized parameters for a specific electrode geometry, but production applications may require different geometries depending on the component size and configuration. The process window for nitrogen control may shift with electrode geometry changes, requiring re-optimization for each new configuration.
Study Insights and Implications
This literature represents a significant contribution to the development of nuclear-grade overlay welding materials. The systematic approach to solving the tensile strength problem—identifying nitrogen as the critical variable, optimizing its content through material and process design, and validating through comprehensive testing—provides a model for materials development in other demanding applications. For engineers in the pipe fitting industry, the key insights are: (1) interstitial elements such as nitrogen can be used as strength control tools in austenitic stainless steel welding consumables, (2) strip electrode submerged arc welding is a viable and productive process for nuclear overlay applications, and (3) rigorous material development and qualification is essential for nuclear power applications. The study's integration of fundamental metallurgical understanding with practical process engineering exemplifies the kind of research that advances our industry.