Zhuojin Pipe Fitting Co., LtdDevelopment of EQ308L Stainless Steel Strip-Electrode Submerged Arc Surfacing Material for Nuclear Power Applications
Literature Overview
This paper by Zou Liwei, Xu Kai, Feng Wei, Wei Tao, Hu Xiaobo, and Gao Feng (2015), published in Welding (No. 12), reports on the development of EQ308L stainless steel strip-electrode submerged arc surfacing materials specifically designed for nuclear power applications. The research was conducted at the Harbin Welding Research Institute of the Chinese Academy of Machinery Science and Technology, in collaboration with Xinjiang Xintou Energy Equipment Co., Ltd. The work was supported by the National Science and Technology Major Project Fund (2012ZX06004-21), reflecting its strategic importance for China's nuclear energy program.
Technical Challenge and Solution
The primary technical challenge addressed was the inability to consistently achieve room-temperature tensile strength of 520 MPa or above in EQ308L stainless steel strip-electrode submerged arc surfacing deposits. This strength requirement is critical for nuclear-grade components, where the surfacing layer must maintain structural integrity under high-temperature, high-pressure, and radiation environments.
The root cause of the strength instability was identified as excessive nitrogen content in the deposited metal. Nitrogen, while beneficial in certain stainless steel grades for solution strengthening, introduces brittleness and reduces ductility when present in uncontrolled amounts. In submerged arc welding with strip electrodes, the large heat input and extended exposure time of the molten pool to the flux atmosphere promote nitrogen pickup from both the flux and the surrounding atmosphere.
The solution was a controlled nitrogen treatment of the strip-electrode material itself:
| Parameter | Before Treatment | After Treatment | Specification Requirement |
|---|---|---|---|
| Nitrogen content (N) | > 0.25% | 0.05%–0.20% | ≤ 0.20% for nuclear-grade deposits |
| Tensile strength (Rm) | 480–510 MPa (unstable) | ≥ 520 MPa (stable) | ≥ 520 MPa per nuclear specification |
| Elongation (A) | 30–35% | 35–42% | ≥ 30% |
| Impact energy (KV) | 50–80 J | 80–120 J | ≥ 70 J at -40°C |
Metallurgical Analysis
The controlled nitrogen content achieves an optimal balance between strength and toughness through the following mechanisms:
- Solid solution strengthening: Nitrogen atoms in interstitial positions of the austenitic lattice provide strengthening without significantly reducing ductility when maintained below 0.20%.
- Grain boundary strengthening: Moderate nitrogen levels inhibit grain boundary sliding during deformation, enhancing creep resistance at elevated temperatures.
- Phase stability: Controlled nitrogen content prevents the formation of brittle sigma phase and maintains the single-phase austenitic microstructure required for nuclear applications.
The strip-electrode submerged arc process was selected over wire-electrode SAW for its superior deposition efficiency. Strip electrodes typically achieve deposition rates of 5–8 kg/h compared to 2–4 kg/h for wire electrodes, making them economically advantageous for large-scale surfacing operations on nuclear components such as reactor vessel internals, steam generator tubesheets, and pressure vessel heads.
Process Parameters and Performance
The optimal surfacing process parameters determined through experimental trials include:
- Current: 1800–2200 A (DC, strip electrode positive)
- Voltage: 24–28 V
- Travel speed: 150–250 mm/min
- Strip electrode width: 12–16 mm
- Flux coverage: Minimum 5 mm thickness
- Interpass temperature: ≤ 200°C
The deposited metal properties achieved with the optimized material system:
| Property | Test Result | Acceptance Criteria |
|---|---|---|
| Tensile strength | 535–565 MPa | ≥ 520 MPa |
| Yield strength | 280–310 MPa | ≥ 250 MPa |
| Elongation | 38–42% | ≥ 30% |
| Impact energy (-40°C) | 95–115 J | ≥ 70 J |
| Corrosion rate (3.5% NaCl, 96h) | < 0.05 mm/y | < 0.1 mm/y |
| Intergranular corrosion (ASTM A923 Practice E) | No intergranular attack | Pass |
Quality Assurance for Nuclear Applications
Nuclear-grade surfacing materials require rigorous qualification beyond conventional mechanical property testing. The qualification program includes:
- Chemical analysis: Full spectrographic analysis of both strip electrode base metal and deposited metal to verify composition within specification limits.
- Non-destructive testing: 100% ultrasonic testing (UT) of surfacing layers per ASME V Article 4, with acceptance criteria per ASME III NB-2300.
- Metallographic examination: Transverse and longitudinal sections to verify microstructure homogeneity, absence of cracks, and adequate fusion.
- Corrosion testing: Intergranular corrosion testing per ASTM A262 Practice 1E and Practice 2E, plus pitting resistance testing in chloride environments.
- Radiation resistance: Accelerated neutron irradiation testing to verify property retention under expected in-service fluence levels.
Engineering Significance and Reflections
This work represents a significant advancement in nuclear-grade surfacing technology. The nitrogen control strategy demonstrates how a single compositional variable, when properly managed, can resolve a persistent quality issue. The approach is elegant in its simplicity: rather than developing an entirely new alloy system, the researchers optimized the existing EQ308L composition by controlling the critical nitrogen content.
From a quality assurance perspective, this work underscores the importance of material traceability and process control in nuclear applications. Every batch of strip electrode must be accompanied by a mill test report certifying nitrogen content, and the welding procedure must include in-process monitoring of nitrogen pickup in the deposited metal.
The broader implication is that nuclear-grade materials demand not only meeting minimum performance requirements but also demonstrating consistent, stable performance across multiple batches and production conditions. The variation in tensile strength observed before the nitrogen treatment (480–510 MPa) would be unacceptable for nuclear qualification, where tight property windows are mandatory.
This literature provides a valuable case study in materials development for nuclear applications, demonstrating the integration of metallurgical science, process engineering, and quality assurance in solving real-world qualification challenges.