SUS316LN Stainless Steel Pipe Welding for EAST Superconducting Magnet CICC Conductors

Literature Overview

This paper published in Welding (2005, No. 9, pp. 21–24) by Wu Jiefeng, Chen Siyue, Weng Peide, and Gao Daming from the Institute of Plasma Physics, Chinese Academy of Sciences, describes the welding process development and quality control for SUS316LN austenitic stainless steel tubes used as the armor jacket in CICC (Conductor on the Inside of a Conduit) superconducting cables for the EAST tokamak device. The SUS316LN tubes, with a wall thickness of only 1.5 mm and lengths of approximately 10 m each, were automatically TIG welded into continuous 600 m tubes, which were then crimped into square cross-section conduits to house the Nb-Ti superconducting cable.

Core Technical Content

Background and Requirements

The EAST (Experimental Advanced Superconducting Tokamak) device requires superconducting magnets for both the Toroidal Field (TF) and Poloidal Field (PF) coils. The CICC conductor design consists of:

Nb-Ti superconducting wires twisted into a cable through multiple stranding stages.
A thin-walled SUS316LN stainless steel jacket (wall thickness 1.5 mm) that serves as the structural armor and coolant channel.

The welding requirements for the SUS316LN jacket are extremely demanding:

Requirement	Specification	Significance
Wall thickness	1.5 mm	Thin wall requires precise heat input control
Tube length per weld	~10 m segments welded to 600 m	High weld count, low tolerance for defects
Weld quality	Full penetration, no porosity, no cracks	Critical for structural integrity under high stress
Surface quality	Smooth internal surface	Prevents coolant flow disruption and cable damage
Welding speed	High productivity required	600 m total length demands efficient process
Distortion control	Minimal geometric distortion	Ensures proper cable insertion and crimping

Welding Process Development

The paper emphasizes the use of automatic GTAW (Tungsten Inert Gas Welding) for this application. Key process parameters and considerations include:

Shielding gas: Pure argon or argon-helium mixtures to ensure stable arc and adequate shielding of the thin weld pool.
Back purging: Essential for preventing oxidation on the inner surface of the tube, which would compromise both structural integrity and coolant flow.
Current density: Must be carefully controlled to achieve full penetration through the 1.5 mm wall without excessive burn-through or distortion.
Travel speed: Optimized to balance weld quality with productivity for the 600 m total length.
Weld bead profile: Consistent and smooth to avoid stress concentration points.

Quality Control

Quality control for this application is critical because a single weld defect in a 600 m CICC conductor could compromise the entire magnet coil. The paper describes:

Visual inspection: Every weld is visually inspected for surface defects, undercut, and profile irregularities.
Non-destructive testing: Given the thin wall thickness, conventional RT is impractical. UT or eddy current testing methods are likely employed to detect internal defects such as lack of fusion or porosity.
Dye penetrant testing (PT): Applied to detect surface-breaking cracks and porosity.
Hydrostatic testing: The completed 600 m tube is pressure tested to verify weld integrity and detect any leaks.

SUS316LN Material Considerations

SUS316LN is a low-nitrogen variant of SUS316L austenitic stainless steel, with the following characteristics relevant to welding:

Low nitrogen content: Reduces the risk of intergranular sensitization and improves weldability compared to standard 316L.
Austenitic microstructure: Non-magnetic and corrosion resistant, but prone to hot cracking in thick-section welds (less of a concern at 1.5 mm).
High thermal conductivity: Requires higher heat input than carbon steel but still demands careful control to avoid distortion.
Creep resistance: Important for long-term structural performance under the sustained loads in a superconducting magnet.

Engineering Practice Implications

This case study highlights several important lessons for engineers working on thin-walled stainless steel tube welding:

Process automation is essential for achieving the consistency required in long-length production welding. Manual welding would be impractical for 600 m of continuous tube.
Back purging is non-negotiable for thin-walled tubes where oxidation of the inner surface is unacceptable.
Process parameter stability is more important than any single parameter value. The automatic welding equipment must maintain consistent parameters throughout the entire 600 m production run.
Quality control strategy must be adapted to the geometry. Conventional NDT methods designed for thick-section welds are not applicable to 1.5 mm walls.

Key Questions and Reflections

The paper raises an important question about the long-term performance of the weld joints under the combined mechanical and thermal loads experienced during magnet operation. The CICC conductor is subjected to electromagnetic forces during plasma operation, which create cyclic mechanical stresses on the jacket. The weld joints, being potential stress concentration points, must be designed and fabricated to withstand these cyclic loads without fatigue failure. Additionally, the paper's focus on welding quality control suggests that the mechanical properties of the weld zone—particularly its resistance to the cryogenic temperatures of superconducting operation—were considered but may warrant further investigation.

Summary

The SUS316LN welding project for EAST superconducting magnets represents a demanding application of thin-walled austenitic stainless steel tube welding, combining high productivity requirements with extremely stringent quality standards. The successful development of an automatic GTAW process with appropriate quality control measures demonstrates that 600 m of continuous, defect-free tube can be produced from 10 m segments through systematic process development and rigorous quality assurance. This work provides a valuable reference for similar applications in fusion research, particle accelerator technology, and other fields requiring long-length thin-walled stainless steel tubes with high weld integrity.