SUS316LN Stainless Steel Pipe Welding for EAST Superconducting Magnet CICC Conductor

Literature Overview

This paper by Wu Jiefeng, Chen Siyue, Weng Peide, and Gao Daming, published in Welding (2005, No. 9), presents the welding technology and quality control methods developed for the CICC (Conductor on the Inside of a Cooling Channel) superconducting conductor used in the EAST (Experimental Advanced Superconducting Tokamak) device at the Institute of Plasma Physics, Chinese Academy of Sciences. The CICC conductor consists of a multi-strand Nb-Ti superconducting cable housed within a SUS316LN austenitic stainless steel jacket, formed by welding 1.5 mm thick seamless stainless steel pipe segments into 600 m long tubes. This paper represents a significant contribution to the welding technology of thin-wall austenitic stainless steel pipes in a demanding cryogenic application.

Core Technical Content

CICC Conductor Structure

The CICC conductor for the EAST TF (Toroidal Field) and PF (Poloidal Field) coils has the following configuration:

The SUS316LN steel is a low-nitrogen variant of SUS316L, specifically designed for cryogenic applications where the nitrogen content affects the mechanical properties and weldability. The "N" designation indicates reduced nitrogen content (typically ≤0.025% compared to 0.030–0.050% in standard 316L), which improves weldability and reduces the risk of weld cracking.

Welding Challenge Analysis

The welding of 1.5 mm thick SUS316LN pipe into 600 m long continuous tubes presents several unique challenges:

1. Thin wall thickness: 1.5 mm wall requires precise heat input control to prevent burn-through and excessive distortion
2. Long continuous weld: 600 m of continuous welding requires high welding speed and consistent quality
3. Cryogenic service: The weld must maintain mechanical integrity at 4.2 K (liquid helium temperature)
4. Superconducting cable insertion: The interior of the pipe must be clean and free of oxide scale to allow cable insertion
5. Subsequent drawing: The welded tube must withstand the cold drawing process to form a square cross-section
6. Corrosion resistance: The SUS316LN material must maintain its corrosion resistance after welding

Welding Process Selection

The paper describes the use of automatic GTAW (Gas Tungsten Arc Welding) for the butt welding of the pipe segments. The selection of GTAW is based on the following considerations:

The automatic GTAW process provides the necessary precision and repeatability for long continuous welds. The use of automatic welding equipment ensures consistent welding parameters throughout the 600 m length, which is critical for maintaining uniform mechanical properties and avoiding weak spots.

Welding Parameters

The typical welding parameters for the SUS316LN pipe welding are as follows:

The low heat input characteristic of GTAW is essential for maintaining the microstructure of the SUS316LN steel. Excessive heat input would cause grain growth in the heat-affected zone, reducing the cryogenic toughness and potentially causing distortion that would affect the subsequent drawing process.

Quality Control Methods

The quality control program for the CICC conductor welding is comprehensive, reflecting the critical nature of the application:

The paper emphasizes the importance of root weld quality, as the interior of the pipe must accommodate the superconducting cable. Any root defects or oxide scale would interfere with cable insertion or create stress concentrations during the subsequent drawing process.

Microstructural Considerations

The welding of SUS316LN steel requires careful control of the heat-affected zone microstructure to maintain cryogenic toughness. Key microstructural considerations include:

1. Grain growth in HAZ: Excessive heat input causes austenite grain growth, reducing cryogenic toughness
2. Sigma phase precipitation: Long exposure to temperatures between 600–900°C can cause sigma phase formation, reducing ductility
3. Sensitization: Chromium carbide precipitation at grain boundaries reduces corrosion resistance
4. Weld metal composition: The weld metal should match or slightly exceed the base metal composition in chromium and nickel content

The low-nitrogen content of SUS316LN reduces the risk of intergranular corrosion compared to standard 316L, but the welding process must still be controlled to prevent sensitization.

Engineering Practice Integration

Comparison with Conventional Pipe Welding

The welding of SUS316LN pipe for CICC conductors differs significantly from conventional stainless steel pipe welding in several respects:

Drawing Process Considerations

After welding, the 600 m long SUS316LN tube must be cold drawn to form a square cross-section. This process imposes additional requirements on the welded joint:

• Uniform mechanical properties: The weld must have similar strength and ductility to the base metal to avoid localized deformation during drawing
• Residual stress management: Welding residual stresses must be minimized to prevent cracking during drawing
• Dimensional accuracy: The welded tube must have uniform diameter and wall thickness to ensure consistent drawing results

The welding parameters must be optimized not only for weld quality but also for the subsequent drawing process. This requires a systems approach that considers the entire manufacturing sequence rather than optimizing the welding process in isolation.

Lessons from EAST Project

The EAST project represents one of the most demanding applications of stainless steel pipe welding in terms of length, quality, and service conditions. Key lessons include:

1. Automation is essential: Manual welding cannot achieve the required consistency over 600 m of continuous welds
2. Back purge is critical: Root oxidation must be prevented to ensure cable insertion capability
3. In-process monitoring: Real-time monitoring of welding parameters is necessary to detect and correct deviations
4. Sequential quality control: Quality checks at multiple stages (before welding, during welding, after welding, after drawing) are essential

Key Questions and Reflections

Long-term Reliability at Cryogenic Temperature

The paper describes the welding process and quality control but does not extensively discuss the long-term behavior of the welded joints at cryogenic temperature. Engineers should consider:

• Cyclic fatigue: The superconducting magnet undergoes repeated magnetization and demagnetization cycles, imposing cyclic stress on the jacket
• Hydrogen embrittlement: Potential hydrogen pickup during service could affect weld integrity
• Radiation effects: The proximity to plasma may expose the jacket to neutron and ion radiation, affecting material properties

Scalability to Future Projects

The welding technology developed for EAST has implications for future fusion projects such as ITER and DEMO. The key question is whether the technology can be scaled to the larger dimensions and longer lengths required for these projects. The ITER CICC conductors will have larger diameters and longer lengths, requiring further development of welding automation and quality control systems.

Standardization and Documentation

The welding procedures developed for EAST are project-specific and may not be directly applicable to other applications. Engineers should document the welding procedures, qualification results, and quality control methods in a manner that allows adaptation to similar applications. The experience gained in this project should be shared through technical publications and industry standards.

Study Insights and Implications

This paper represents a significant contribution to the welding technology of thin-wall austenitic stainless steel pipes for extreme applications. The successful welding of 600 m of SUS316LN pipe into continuous tubes demonstrates the feasibility of automatic GTAW for long, thin-wall pipe welding with high quality requirements.

The key technical insight is that the welding process must be designed as part of a complete manufacturing sequence that includes subsequent cold drawing. The welding parameters, quality control methods, and residual stress management are all optimized not just for weld quality but for the entire manufacturing process. This systems approach is essential for complex manufacturing operations where multiple processes interact.

The paper also highlights the importance of material selection for specific applications. The use of SUS316LN rather than standard SUS316L provides improved weldability and cryogenic performance, demonstrating that even small compositional differences can have significant impact on manufacturing capability and service performance.

For engineers involved in cryogenic or fusion applications, this paper provides a valuable reference for the design and execution of thin-wall stainless steel pipe welding operations. The emphasis on automation, in-process monitoring, and sequential quality control represents a mature approach to welding in demanding applications. The experience documented here should inform the development of welding technology for future fusion projects and other extreme applications requiring long, continuous, high-quality welds in thin-wall stainless steel pipes.