Stainless Steel Coil Tube (Coiltube) — Technical Product Specification
Product Catalog No.: 597 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered continuous-length tubular product supplied in coil form, designed for seamless integration into HVAC (Heating, Ventilation, and Air Conditioning) systems, concealed fan coil unit (FCU) assemblies, and industrial piping applications. Manufactured from high-grade austenitic stainless steel, this coil tube offers superior corrosion resistance, mechanical integrity, and long-term reliability under demanding operational conditions.
The development and continuous improvement of this product line has been directly informed by post-incident failure analysis — specifically, a documented concealed fan coil unit leakage event at a commercial building complex in the Hangzhou industrial park. This real-world case study revealed critical failure modes associated with material degradation, improper coil formation tolerances, and inadequate joint integrity, all of which have been systematically addressed in the current product design and manufacturing protocol.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international standards, with the following typical chemical composition:
- Grade 304 (UNS S30400): Minimum 18% Chromium (Cr), 8% Nickel (Ni), Carbon (C) ≤ 0.08%
- Grade 316 (UNS S31600): Minimum 16% Cr, 10% Ni, 2% Molybdenum (Mo), C ≤ 0.08% — recommended for aggressive or marine environments
- Grade 316L (UNS S31603): Low-carbon variant of 316 with C ≤ 0.03%, offering enhanced resistance to intergranular corrosion (IGC) during welding
2.2 Mechanical Properties
| Property | Grade 304 | Grade 316 | Grade 316L |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ≥ 485 |
| Yield Strength (MPa) | ≥ 205 | ≥ 205 | ≥ 170 |
| Elongation (%) | ≥ 40 | ≥ 35 | ≥ 40 |
| Density (kg/m³) | 7930 | 8000 | 8000 |
| Thermal Conductivity (W/m·K, 20°C) | 16.3 | 16.3 | 16.3 |
| Operating Temperature Range (°C) | −196 to +870 | −196 to +870 | −196 to +870 |
2.3 Corrosion Resistance
The austenitic microstructure provides exceptional resistance to atmospheric, aqueous, and chemical corrosion. Grade 316 and 316L variants exhibit superior performance against pitting corrosion (PC) and crevice corrosion (CC), with a Pitting Resistance Equivalent Number (PREN) of ≥ 24 and ≥ 26 respectively. The addition of Molybdenum significantly enhances chloride ion resistance, making these grades ideal for concealed applications where leakage-induced moisture exposure is a known risk.
3. Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | 3.0 mm – 50.8 mm (customizable) |
| Wall Thickness (WT) | 0.25 mm – 3.0 mm (customizable) |
| Coil Inner Diameter (ID) | 150 mm – 600 mm |
| Coil Length | Up to 1,000 m per coil (continuous) |
| Diameter Tolerance | ± 0.1 mm (± 2% for OD > 10 mm) |
| Wall Thickness Tolerance | ± 10% of nominal |
| Surface Finish | No. 1 Annealed / 2B / BA (Bright Annealed) / EP (Electro-polished) |
| Surface Roughness (Ra) | ≤ 0.8 μm (BA finish) / ≤ 0.4 μm (EP finish) |
| Forming Quality | Zero visible surface defects; no cracking or thinning > 5% at coil bends |
| Coil Packing | Wooden pallet with VCI (Vapor Corrosion Inhibitor) film wrapping |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Incoming stainless steel strip is verified against mill test certificates (MTC), with spectroscopic analysis confirming chemical composition within specified limits.
- Slitting and Leveling: Wide strip is slit to target coil tube blank dimensions using precision slitting lines, followed by tension leveling to eliminate coil set and ensure flatness (≤ 0.5 mm/m).
- Forming (Coiling): The strip is cold-formed into coil tube geometry using progressive die sets. Forming force is controlled to prevent excessive strain hardening and wall thinning — a critical parameter identified as a root cause in the Hangzhou FCU leakage incident, where over-thinned coil walls contributed to structural failure under thermal cycling.
- Welding (if applicable): Longitudinal seams are welded using Automatic Argon Arc Welding (TIG/IGAW) or Laser Welding (LW) processes, with shielding gas coverage ensuring full penetration and minimal heat-affected zone (HAZ) distortion.
- Post-Weld Heat Treatment: Solution annealing at 1050–1100°C in protective atmosphere (H₂/N₂ or vacuum) followed by controlled cooling to restore full austenitic properties and eliminate residual stresses.
- Surface Treatment: Pickling and passivation to remove oxide scales and establish a stable chromium oxide (Cr₂O₃) passive film. Electro-polishing available for ultra-smooth finishes.
- Final Coiling and Inspection: Tubes are coiled to specified inner diameters, with each coil subjected to dimensional verification, visual inspection, and non-destructive testing (NDT).
4.2 Key Quality Control Measures (Informed by Failure Analysis)
- Wall Thickness Monitoring at Coil Bends: Ultrasonic thickness gauging at every 10 m interval to detect any thinning exceeding 5% of nominal — directly addressing the failure mode observed in the Hangzhou incident.
- Hydrostatic Pressure Testing: Each production batch undergoes hydrostatic testing at 1.5× the maximum design pressure, held for 30 seconds, to verify leak-free integrity.
- Weld Seam Inspection: 100% visual inspection plus 10% sampling via Dye Penetrant Testing (DPT) or Radiographic Testing (RT) for weld seam integrity.
5. Applications
- HVAC Fan Coil Units (FCU): Primary application for concealed and exposed FCU coil assemblies in commercial buildings, where long-term corrosion resistance and dimensional stability are paramount. The Hangzhou case study specifically validated the necessity of stainless steel coil tubes over carbon steel alternatives in concealed applications where corrosion inspection is impractical.
- Refrigeration and Chilled Water Systems: Heat exchanger coils, evaporator tubes, and condenser assemblies.
- Industrial Piping: Continuous coil supply enables efficient on-site pipe fabrication, reducing weld joints and potential leak points.
- Food and Pharmaceutical Processing: Sanitary-grade coil tubes for fluid handling where hygienic surface finish (EP, Ra ≤ 0.4 μm) is required.
- Automotive and Aerospace: Lightweight coil tubing for fuel lines, hydraulic circuits, and HVAC components.
- Architectural and Decorative: Coil-formed structural and aesthetic elements in building facades and interior installations.
6. Quality Certifications & Compliance
- ISO 9001:2015 — Quality Management System certification for consistent manufacturing processes.
- ISO 14001:2015 — Environmental Management System, ensuring sustainable production practices.
- EN 10204 / EN 10204-3.1 — Inspection and test documentation conforming to European standards.
- ASTM A213 / ASTM A269 — Material compliance with American Society for Testing and Materials standards for stainless steel seamless and welded tubing.
- GB/T 14976 — Chinese national standard for cold-rolled stainless steel seamless tubes.
- GB/T 12771 — Chinese national standard for cold-rolled stainless steel welded tubes.
- Mill Test Certificate (MTC): Provided with each shipment, documenting chemical composition, mechanical properties, and traceability.
7. Unique Selling Points
Failure-Informed Engineering: This product line incorporates direct lessons learned from a documented real-world concealed FCU leakage incident in Hangzhou, China. Root cause analysis of that event — which identified wall thinning at coil bends, inadequate weld integrity, and material corrosion as contributing factors — has been systematically integrated into the manufacturing and quality control protocols described above. This results in a product with demonstrably enhanced reliability for concealed and hard-to-inspect applications.
- Continuous Coil Supply: Eliminates field welding and joint assembly, reducing the total number of potential failure points by up to 90% compared to cut-length alternatives.
- Grade Flexibility: Available in 304, 316, and 316L grades to match the corrosivity of the specific operating environment.
- Customizable Dimensions: OD, wall thickness, and coil diameter tailored to project-specific requirements.
- Surface Finish Options: From standard annealed finish to electro-polished sanitary grade, covering a broad spectrum of application needs.
- Full Traceability: Each coil is batch-traceable from raw material mill certificate through final inspection, ensuring complete quality accountability.
8. Conclusion
The Stainless Steel Coil Tube (Catalog No. 597) from Wenzhou Jixing Metal Materials Co., Ltd. represents a robust, failure-informed solution for coil tube applications where long-term integrity, corrosion resistance, and dimensional precision are critical. By systematically addressing the failure modes identified in the Hangzhou concealed FCU leakage incident, this product delivers a measurable improvement in reliability for commercial HVAC systems, industrial piping, and other demanding applications. Customers are advised to specify Grade 316 or 316L for environments with elevated chloride exposure or where the tubes will be concealed and inaccessible for routine inspection.