Stainless Steel Sheath Coil Tube for Continuous Ground Wire Systems (Coiltube)
1. Product Overview
The Stainless Steel Sheath Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered flexible conduit designed specifically for the protection and routing of continuous ground wires (CGW — Continuous Ground Wire) in railway tunnel infrastructure. This product falls under the category of protective sheath coiltubes used in high-speed rail and metro transit systems, where reliable grounding continuity is critical for both electrical safety and electromagnetic compatibility (EMC).
The coiltube serves as a durable, corrosion-resistant outer jacket that encases the continuous ground wire within tunnel walls and trackside infrastructure, shielding it from mechanical damage, chemical attack, and environmental degradation over the service life of the railway system. The product is supplied in coil form to facilitate installation in curved tunnel geometries and complex routing scenarios without the need for additional bending or forming operations.
2. Material & Properties
The sheath coiltube is fabricated from austenitic stainless steel, with the primary grade being 304 (UNS S30400) or 316 (UNS S31600), selected based on the corrosive environment of the installation site. The material composition and mechanical properties are as follows:
2.1 Chemical Composition (Typical, wt%)
| Element | Grade 304 (UNS S30400) | Grade 316 (UNS S31600) |
|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.08 |
| Si (Silicon) | ≤ 1.00 | ≤ 1.00 |
| Mn (Manganese) | ≤ 2.00 | ≤ 2.00 |
| P (Phosphorus) | ≤ 0.045 | ≤ 0.045 |
| S (Sulfur) | ≤ 0.030 | ≤ 0.030 |
| Cr (Chromium) | 18.00 – 20.00 | 16.00 – 18.00 |
| Ni (Nickel) | 8.00 – 10.50 | 10.00 – 14.00 |
| Mo (Molybdenum) | — | 2.00 – 3.00 |
2.2 Mechanical Properties
- Tensile Strength (Rm): ≥ 520 MPa (Grade 304); ≥ 515 MPa (Grade 316)
- Yield Strength (Rp0.2): ≥ 205 MPa
- Elongation (A50mm): ≥ 40%
- Hardness: ≤ 217 HBW
- Corrosion Resistance: Excellent resistance to intergranular corrosion, pitting corrosion, and stress corrosion cracking (SCC — Stress Corrosion Cracking) in chloride-containing environments
- Temperature Range: −196 °C to +870 °C (continuous service)
2.3 Surface Finish
- Outer surface: Polished or matte finish with Ra ≤ 0.8 μm
- Inner surface: Smooth, burr-free, suitable for wire insertion without abrasion
- Optional: Passivated surface treatment (citric acid or nitric acid passivation) to enhance passive film integrity
3. Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Sheath Coil Tube (Coiltube) for Continuous Ground Wire |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd. |
| Product Code / Serial No. | 336 |
| Material Grade | 304 / 304L / 316 / 316L (stainless steel) |
| Outer Diameter (OD) | 12.7 mm – 50.8 mm (customizable) |
| Wall Thickness | 0.5 mm – 2.0 mm (± 10% tolerance) |
| Inner Diameter (ID) | Determined by OD and wall thickness; tolerance ± 0.1 mm |
| Coil Length per Roll | 30 m – 100 m (adjustable per project requirement) |
| Minimum Bending Radius | ≥ 3 × OD (cold bending without cracking) |
| Surface Treatment | Passivated; optional epoxy or PVC inner lining available |
| Standards Compliance | ASTM A213 / ASTM A269 / GB/T 12771 / EN 10216-5 / TB/T 3079 |
| Delivery Condition | Annealed and pickled (AP); solution annealed (SA) |
| Inspection Method | 100% eddy current testing; hydrostatic pressure testing; visual inspection |
| Packaging | Coiled on wooden or steel reels; moisture-proof wrapping; export-grade palletization |
4. Manufacturing Process
The production of the stainless steel sheath coiltube follows a rigorous, multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Hot Rolling (HR — Hot Rolling): Continuous casting billets are reheated to 1200–1300 °C and hot-rolled into semi-finished tube blanks (ERW — Electric Resistance Welded, or seamless depending on grade).
- Piercing & Enlarging: For seamless variants, solid billets are pierced into hollow blooms and progressively enlarged to target dimensions.
- Cold Drawing (CD — Cold Drawing): The semi-finished tubes are cold-drawn through calibrated dies to achieve precise final dimensions and enhanced mechanical properties. This process also improves surface finish and dimensional tolerance.
- Solution Annealing: Tubes are heated to 1050–1150 °C in a controlled-atmosphere furnace (protective gas or vacuum) and rapidly quenched to dissolve carbide precipitates and restore full austenitic structure.
- Pickling & Passivation: Post-annealing oxide scale is removed via acid pickling (HNO₃/HF mixture or citric acid). A protective passive chromium oxide layer is then formed through chemical passivation.
- Coiling: The finished tube is wound into coils of specified length and diameter on precision winding machines with controlled tension to avoid kinking or ovality.
- Non-Destructive Testing (NDT): 100% inline eddy current (EC) inspection detects wall thickness variations, cracks, and surface defects. Hydrostatic pressure testing verifies weld integrity (for ERW tubes) at 1.5× maximum working pressure.
- Dimensional Verification: Sampling inspection of OD, wall thickness, straightness, and coil geometry per ASTM A751.
5. Applications
The stainless steel sheath coiltube for continuous ground wire systems finds primary application in the following domains:
- High-Speed Railway Tunnels: Protection of the continuous ground wire (CGW) that runs the full length of tunnel infrastructure, ensuring equipotential bonding between track structures and preventing stray current corrosion.
- Metro / Underground Transit Systems: Grounding continuity in underground stations and tunnel sections where moisture, chloride ions, and vibration are prevalent.
- Electromagnetic Compatibility (EMC) Shielding: The stainless steel sheath provides a conductive enclosure that helps suppress electromagnetic interference along signal and power cables routed alongside the ground wire.
- Corrosive Environments: Coastal railway corridors, tunnel sections with high groundwater chloride content, and chemically aggressive subsoil conditions where carbon steel alternatives would fail prematurely.
- Long-Term Infrastructure: Designed for a service life of 30–50 years without replacement, reducing lifecycle maintenance costs in railway assets.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and product certification portfolio:
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- ISO/TS 22163:2017 — Railway-specific Quality Management System (formerly EN 15085)
- EN 10204 — Type 3.1 Material Test Certificates (MTC) provided with every shipment
- CE Marking — Conformity with applicable European Directives for railway components
- CRCC Certification — China Railway Certification Center approval for railway material supply
- Third-Party Inspection: Available upon request through SGS, TÜV, or DNV for critical project deliveries
Failure Analysis Insight: Post-service failure analysis of sheath coiltubes used in continuous ground wire applications has identified common degradation mechanisms including chloride-induced pitting corrosion, stress corrosion cracking at weld seams, and mechanical fatigue from repeated thermal cycling. Wenzhou Jixing's products address these failure modes through: (1) strict control of sulfur and carbon content to minimize sensitization; (2) solution-annealed delivery condition to eliminate residual stresses; (3) 100% NDT coverage to reject sub-surface defects prior to shipment; and (4) optional inner protective coatings to provide redundant corrosion protection in the most aggressive tunnel environments.
7. Unique Selling Points
- Railway-Specific Engineering: Products are engineered specifically for continuous ground wire applications in railway tunnels, with dimensional tolerances and mechanical properties tailored to TB/T 3079 and international railway standards.
- Proven Failure-Resistant Design: Informed by extensive failure analysis studies, the material grade, wall thickness, and surface treatment are optimized to resist the primary degradation mechanisms observed in service.
- Full Traceability: Each coil is traceable to its heat number, mill certificate, and inspection records, ensuring complete supply chain accountability.
- Customization Capability: Flexible sizing (OD, wall thickness, coil length) and surface treatment options to meet project-specific design requirements.
- Export-Ready: Packaging and documentation conform to international export standards, with experience in supplying railway infrastructure projects across Asia, Europe, and Africa.