Stainless Steel Coil Tube for Through-Earth Conductor Sheathing
Product Overview
The Stainless Steel Coil Tube for Through-Earth Conductor Sheathing (盘管 / Coil Tube) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product serves as a protective sheathing and conduit for through-earth conductors (贯通地线) in rail transit and railway grounding systems. The coil tube is delivered in continuous coil form, enabling seamless installation over long distances without the need for field welding or mechanical splicing, thereby ensuring electrical continuity and mechanical integrity throughout the entire grounding network.
Through-earth conductor systems are critical safety infrastructure in modern railway and metro transit systems, providing a reliable grounding path for stray current collection, cathodic protection, and lightning strike dissipation. The coil tube sheath protects the embedded conductor from mechanical damage, chemical corrosion, and environmental degradation while maintaining structural compatibility with the surrounding infrastructure.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its superior corrosion resistance, mechanical ductility, and long-term dimensional stability in demanding underground and outdoor environments.
Material Composition
- Base Grade: AISI 304 (UNS S30400) / AISI 304L (UNS S30403) / AISI 316L (UNS S31603)
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–10.5%
- Molybdenum (Mo): 2.0–3.0% (for 316L grade)
- Carbon (C): ≤ 0.08% (standard) / ≤ 0.03% (L-grade, low carbon)
- Manganese (Mn): ≤ 2.0%
- Silicon (Si): ≤ 1.0%
- Phosphorus (P): ≤ 0.045%
- Sulfur (S): ≤ 0.030%
Mechanical Properties
- Tensile Strength (Rm): ≥ 520 MPa (AISI 304) / ≥ 485 MPa (AISI 316L)
- Yield Strength (Rp0.2): ≥ 205 MPa (AISI 304) / ≥ 170 MPa (AISI 316L)
- Elongation (A50): ≥ 40%
- Hardness: ≤ 217 HB (Brinell)
- Impact Energy (23°C): ≥ 70 J (Charpy V-notch)
- Impact Energy (−40°C): ≥ 47 J (Charpy V-notch, 316L)
Corrosion Resistance
- Pitting Resistance (PREN): 18–20 (304L) / 24–26 (316L)
- Intergranular Corrosion: Compliant with ASTM A240 / GB/T 4334
- Crevice Corrosion Resistance: Enhanced by low-carbon and molybdenum additions
- Environmental Compatibility: Suitable for soil burial, concrete encasement, and direct water contact
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Type | Coil Tube / Coil Pipe (盘管) | — |
| Outer Diameter (OD) | Φ15 mm – Φ108 mm (customizable) | GB/T 14976, EN 10216-5 |
| Wall Thickness (WT) | 0.5 mm – 6.0 mm (customizable) | GB/T 14976, EN 10216-5 |
| OD Tolerance | ±0.15 mm (small diameter) / ±0.20 mm (large diameter) | ISO 4200 |
| WT Tolerance | ±10% (thin wall) / ±12.5% (standard) | GB/T 14976 |
| Coil Length | 100 m – 600 m per coil (continuous) | — |
| Coil Diameter | 500 mm – 1200 mm (packing diameter) | — |
| Surface Finish | Mirror Polish (Ra ≤ 0.2 μm) / Matte / Hairline | GB/T 1803 |
| End Condition | Plain / Beveled / Flanged / Threaded | — |
| Manufacturing Method | Seamless (Seamless) / Welded (Welded / ERW) | EN 10216-5, GB/T 14976 |
| Heat Treatment | Solution Annealed (1050°C – 1150°C), Water Quenched | ASTM A270 |
| Maximum Operating Temperature | −196°C to +870°C (304) / −196°C to +870°C (316L) | — |
| Minimum Bend Radius | 3D (cold bending without special tooling) | — |
| Electrical Resistivity | 0.72 × 10−6 Ω·m (at 20°C) | — |
| Thermal Conductivity | 16.3 W/(m·K) (at 100°C) | — |
| Packaging | Wooden pallet with moisture-proof wrapping, steel banding | ISO 4180 |
Manufacturing Process
The production of stainless steel coil tubes for through-earth conductor sheathing follows a rigorously controlled multi-stage manufacturing process to ensure dimensional accuracy, material homogeneity, and corrosion resistance.
Process Flow
- Raw Material Selection: Cold-rolled stainless steel strip (CRSS) or hot-rolled coil (HRC) conforming to ASTM A240 / GB/T 4237 specifications is procured with full mill test certificates (MTC).
- Surface Preparation: The strip undergoes degreasing, pickling, and passivation to remove oxide scale and organic contaminants, ensuring a clean, uniform surface for subsequent processing.
- Coiling & Payoff: The prepared strip is loaded onto a precision pay-off reel with controlled tension to prevent surface marring and dimensional distortion.
- Tube Forming:
- Seamless Method: The strip is drawn through a die set using the cross-roll or spiral-wound seamless process, followed by hot or cold rolling to achieve final dimensions.
- Welded Method (ERW): The strip is roll-formed into a tube shape and longitudinally welded using Electromagnetic Resistance Welding (ERW), followed by upset welding to homogenize the weld zone microstructure.
- Solution Annealing: The formed tube is solution-annealed at 1050°C – 1150°C and rapidly quenched (water or air) to dissolve carbides, restore full austenitic structure, and achieve maximum ductility and corrosion resistance.
- Pickling & Passivation: Post-annealing surface treatment with nitric acid / hydrofluoric acid mixture to remove heat scale, followed by passivation to form a protective chromium oxide layer.
- Dimensional Inspection: Automated optical and ultrasonic measurement systems verify OD, wall thickness, straightness, and roundness against specified tolerances.
- Non-Destructive Testing (NDT): 100% electromagnetic (Eddy Current) inspection for seam integrity; ultrasonic testing for wall thickness and internal defects; hydrostatic or pneumatic pressure testing for leak integrity.
- Coiling: The inspected tube is wound into coils of the specified length and diameter, with protective end caps and inter-layer wrapping to prevent handling damage.
- Final Packaging: Coils are wrapped in moisture-proof polyethylene film, secured with steel strapping, and loaded onto export-grade wooden pallets or crates.
Applications
The stainless steel coil tube for through-earth conductor sheathing is deployed in a wide range of critical infrastructure applications where long-term reliability, corrosion resistance, and electrical continuity are paramount.
Primary Applications
- Rail Transit Grounding Systems: Sheathing for through-earth conductors (贯通地线) in metro subway, high-speed railway, and conventional rail systems. The coil tube protects the embedded conductor from soil corrosion, mechanical stress, and electrolytic attack while maintaining the integrity of the grounding network.
- Cathodic Protection Systems: Used as protective conduit for anode cables and reference electrodes in pipeline cathodic protection networks.
- Lightning Protection Grounding: Serves as sheathed grounding conductors for railway stations, substations, and signal equipment.
- Stray Current Collection: In DC traction systems, the coil tube sheath protects collection conductors that mitigate stray current flow through ballast and surrounding infrastructure.
- Underground Utility Protection: General-purpose protective conduit for electrical and instrumentation cables in chemically aggressive underground environments.
Industry Sectors
- Railway and Metro Transit Engineering
- Power Transmission and Distribution
- Petrochemical and Oil & Gas Pipelines
- Telecommunications Infrastructure
- Marine and Offshore Platforms
- Municipal Water and Wastewater Systems
Failure Analysis Considerations
Based on the technical study of "Failure Analysis of Sheathed Coil Tubes for Through-Earth Conductors" (《贯通地线用护套盘管失效分析》), the following failure modes and preventive measures are documented:
Common Failure Modes
- Corrosion-Induced Wall Thinning: Localized pitting or intergranular corrosion due to chloride contamination or inadequate passivation. Prevention: Specify 316L grade for chloride-rich environments; verify passivation quality per ASTM A967.
- Mechanical Damage During Installation: Crushing, kinking, or indentation from improper handling or backfill compaction. Prevention: Maintain minimum bend radius (≥3D); use protective bedding and controlled compaction methods.
- Weld Seam Defects: Lack of fusion, porosity, or cracking in ERW weld zones. Prevention: 100% eddy current inspection; upset welding post-ERW to homogenize microstructure.
- Electrical Discontinuity: Loss of grounding continuity due to tube separation, insulation contamination, or corrosion at splice points. Prevention: Use continuous coil lengths to minimize splices; specify conductive coating or bare tube for direct electrical contact.
- Stress Corrosion Cracking (SCC): Chloride-induced SCC in sensitized austenitic stainless steel. Prevention: Use low-carbon L-grade (304L/316L); avoid sensitizing temperature range (450°C – 850°C) exposure.
Recommended Inspection Protocols
Periodic in-service inspection should include visual examination of exposed sections, ultrasonic wall thickness measurement at critical points, electrical resistance measurement of the grounding path, and corrosion potential testing. Inspection intervals should be determined based on soil resistivity, stray current density, and environmental aggressivity, typically at 3-year intervals for high-aggressivity environments and 5-year intervals for standard conditions.
Quality Certifications & Standards
All products manufactured by Wenzhou Jixing Metal Materials Co., Ltd. comply with the following international and national standards and hold the following certifications:
Applicable Standards
- GB/T 14976: Steel tubes for fluid conveyance — Dimensions and tolerances (Chinese National Standard)
- GB/T 4237: Cold-rolled stainless steel sheets and strips
- EN 10216-5: Non-alloy and alloy steel tubes for mechanical and technological purposes — Part 5: Technical delivery conditions for seamless austenitic stainless steel tubes
- ASTM A270: Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipes and Tubes
- ASTM A240: Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels
- ISO 4200: Steel tubes — Dimensions and tolerances
- ISO 4180: Steel tubes — Packaging <