Stainless Steel Sheathed Coil Tube for Through-Ground Wire (TGW) Systems
1. Product Overview
The Stainless Steel Sheathed Coil Tube is a precision-engineered coiled tubular product designed specifically for through-ground wire (TGW) applications in high-speed railway, metro, and heavy infrastructure projects. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product is supplied in continuous coil form (Coil Tube), enabling efficient spool-based installation and minimizing joint interfaces in the ground continuity network.
The coil tube serves as a protective sheath and mechanical conduit for embedded grounding conductors, providing long-term corrosion resistance, mechanical integrity, and electrical continuity in harsh subterranean environments. The coil configuration allows for rapid deployment along trackside corridors and foundation structures, significantly reducing on-site welding and assembly labor.
2. Material and Properties
2.1 Base Material Grades
- Austenitic Stainless Steel — AISI 304 (UNS S30408): Chromium (Cr) content of 18–20%, Nickel (Ni) content of 8–10.5%. Excellent general corrosion resistance and formability. Suitable for moderate corrosivity soil environments.
- Austenitic Stainless Steel — AISI 316 (UNS S31608): Chromium (Cr) 16–18%, Nickel (Ni) 10–14%, Molybdenum (Mo) 2–3%. Superior resistance to pitting and crevice corrosion; recommended for coastal or high-chloride environments.
- Super Austenitic Stainless Steel — AISI 316L (UNS S31603): Low-carbon variant (C ≤ 0.03%) of 316 grade. Enhanced weldability and resistance to intergranular corrosion (IGC) during welding operations.
2.2 Mechanical Properties (Typical Values — Solution Annealed Condition)
| Property |
304 / 304L |
316 / 316L |
Test Standard |
| Tensile Strength (Rm) |
≥ 515 MPa |
≥ 515 MPa |
ASTM A270 / GB/T 4237 |
| Yield Strength (Rp0.2) |
≥ 205 MPa |
≥ 205 MPa |
ASTM A270 / GB/T 4237 |
| Elongation (A50mm) |
≥ 40% |
≥ 40% |
ASTM A270 |
| Hardness (HBW) |
≤ 217 |
≤ 217 |
ASTM A270 |
| Impact Energy (20°C, Charpy V) |
≥ 47 J |
≥ 47 J |
ASTM A270 |
| Impact Energy (−40°C, Charpy V) |
≥ 27 J |
≥ 27 J |
ASTM A270 |
2.3 Chemical Composition (Maximum, unless noted)
| Element |
304 / 304L |
316 / 316L |
| Carbon (C) |
≤ 0.08% (304) / ≤ 0.03% (304L) |
≤ 0.08% (316) / ≤ 0.03% (316L) |
| Chromium (Cr) |
18.0 – 20.0% |
16.0 – 18.0% |
| Nickel (Ni) |
8.0 – 10.5% |
10.0 – 14.0% |
| Molybdenum (Mo) |
— |
2.0 – 3.0% |
| Silicon (Si) |
≤ 1.0% |
≤ 1.0% |
| Manganese (Mn) |
≤ 2.0% |
≤ 2.0% |
| Phosphorus (P) |
≤ 0.045% |
≤ 0.045% |
| Sulfur (S) |
≤ 0.030% |
≤ 0.030% |
3. Dimensions and Specifications
| Parameter |
Specification |
Notes |
| Outer Diameter (OD) |
φ20.0 – φ63.0 mm |
Custom sizes available upon request |
| Wall Thickness |
1.0 – 3.0 mm |
Tolerance: ±0.1 mm |
| Coil Inner Diameter (ID) |
φ300 – φ600 mm |
Standard: φ500 mm |
| Coil Length |
50 – 200 m per coil |
Custom lengths available |
| Surface Finish |
No. 4 Brushed / 2B Cold Rolled / Mirror Polished (Ra ≤ 0.4 μm) |
Per customer specification |
| End Treatment |
Beveled / Square Cut / Formed Fitting |
Welding preparation included |
| Tolerance — OD |
±0.05 mm (precision grade) |
Per ASTM A270 |
| Tolerance — Wall Thickness |
±10% of nominal |
Per ASTM A270 |
| Coil Weight (Typical) |
200 – 800 kg per coil |
Depends on OD, wall thickness, and coil length |
| Minimum Bend Radius |
≥ 3 × OD |
Without wall thinning or cracking |
| Operating Temperature Range |
−196°C to +870°C |
Austenitic stainless steel range |
| Electrical Resistance (20°C) |
0.74 – 0.80 Ω·mm²/m |
Non-magnetic grade |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Certified stainless steel strip (coil form) sourced from approved mills (e.g., POSCO, Outokumpu, Taiyuan Stainless Steel). Each batch is accompanied by a mill test certificate (MTC) per EN 10204 Type 3.1.
- Slitting and Leveling: The master coil is slit to the required width using precision slitting lines. The strip is then cold-rolled and leveled to achieve flatness within 0.5 mm/m.
- Coil Forming (Roll Forming): The leveled strip passes through a multi-station roll-forming die set, progressively shaped into a semi-circular profile. The forming stations are precision-machined to ensure uniform wall thickness and dimensional accuracy.
- Welding: The formed strip is welded along the longitudinal seam using either:
- Automatic Argon Arc Welding (TIG): Shielded with high-purity argon (≥ 99.99%) on both inside and outside surfaces. Post-weld passivation to restore corrosion resistance at the weld zone.
- Flash Butt Welding (FBW): For larger diameter tubes, flash butt welding is employed to join the seam with controlled heat input and full penetration.
- Post-Weld Heat Treatment: Solution annealing at 1050–1100°C followed by rapid water quenching to dissolve carbides and restore full austenitic microstructure, ensuring maximum corrosion resistance and ductility.
- Precision Sizing: The welded tube passes through a cold-sizing die to achieve exact outer diameter and wall thickness tolerances.
- Coiling: The finished tube is wound into a coil using a hydraulic coiling head with controlled tension to prevent deformation, kinking, or ovality.
- Surface Finishing: Depending on specification, the tube undergoes pickling, passivation, brushing, or mirror polishing.
- Final Inspection and Packaging: Each coil is inspected, tagged, and wrapped in moisture-proof protective film with wooden or steel coil cradles for shipment.
4.2 Key Process Controls
- Weld seam quality verified by Eddy Current Testing (ECT) and Visual Inspection (VT) per ASTM E309 / GB/T 150.
- Weld joint strength validated by Flattening Test and Hydrostatic Pressure Test (1.5× working pressure, 30-second hold).
- Coil geometry verified by Laser Dimensional Scanning to ensure ovality ≤ 1.0% of OD.
5. Applications
5.1 Primary Applications
- High-Speed Railway Through-Ground Wire (TGW): The coil tube serves as the protective sheath for copper or stainless steel grounding conductors embedded along trackside, providing long-term corrosion protection and mechanical shielding in ballasted and slab track foundations.
- Metro and Urban Rail Transit: Ground continuity systems for traction return current collection and electromagnetic compatibility (EMC) shielding.
- Tunnel Grounding Networks: Continuous grounding along tunnel walls and track beds for lightning protection and fault current dissipation.
- Substation and Switching Yard Grounding: Subterranean grounding grids requiring high durability and minimal maintenance.
- Industrial Plant Grounding: Corrosive chemical plant environments where carbon steel grounding systems would fail within years.
5.2 Failure Analysis Insights (Literature Study Reference)
Based on the study "Failure Analysis of Sheathed Coil Tubes for Through-Ground Wire Applications" (《贯通地线用护套盘管失效分析》), the following critical failure modes have been identified and addressed in the product design:
- Weld Seam Cracking: Caused by inadequate post-weld heat treatment or excessive residual stress during forming. Mitigated by mandatory solution annealing and residual stress measurement (X-ray diffraction method).
- Intergranular Corrosion (IGC) at Weld Zone: Due to chromium carbide precipitation at grain boundaries. Mitigated by using low-carbon (L-grade) materials and strict thermal cycle control during welding.
- Coil Deformation During Installation: Caused by insufficient coil support or improper uncoiling technique. Addressed by specifying minimum bend radius and providing installation guidance.
- Galvanic Corrosion at Dissimilar Metal Joints: When coil tube contacts carbon steel or aluminum components. Mitigated by providing isolation gaskets and recommending compatible material pairings.
6. Quality Certifications and Standards Compliance
6.1 Applicable Standards
| Standard |
Description |
| ASTM A270 / A270M |
Standard Specification for Seamless and Wrought Stainless Steel Tubular Products (ASME) |
| ASTM A269 / A269M |
Standard Specification for Seamless, Welded, and Heavy-Walled Stainless Steel Pipes and Tubes |
| EN 10217-7 |
Non-ferrous metallic materials — Seamless tubes of stainless steels — Technical delivery conditions — Part 7: Tubes of austenitic stainless steels for general applications |
| GB/T 12771 |
Stainless Steel Seamless Tubes for Fluid Transport |
| GB/T 4237 |
Stainless Steel Plates, Sheets and Strips |
| GB/T 150 |
Pressure Vessel — Welding Procedure Qualification |
| ISO 14288 |
Stainless Steel — Seamless Tubes for General Applications |
| TB/T 3345 |
Railway Industry Standard — Through-Ground Wire Technical Requirements (China) |
6.2 Certifications
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- EN 10204 Type 3.1 — Mill Test Certificate for every production batch
- Railway Industry Supplier Qualification — Qualified supplier for China State Railway Group (CR) and related entities
- CE Marking — Conformity with European New Approach Directives (where applicable)
- NADCAP / AS9100 — Available upon request for aerospace-grade applications
7. Unique Selling Points
- Continuous Coil Supply: Eliminates field joints, reducing potential failure points in the grounding continuity network by up to 90% compared to straight-length pipe segments.
- Failure-Informed Design: Product specifications directly incorporate lessons learned from documented field failure analysis studies, ensuring that known failure modes (weld cracking, IGC, galvanic corrosion) are systematically mitigated.
- Dual Protection: The stainless steel sheath provides both mechanical protection and electrical continuity, serving as both a conduit and an integral component of the grounding path.
- Full Traceability: Every coil is individually traceable to raw