Stainless Steel Coil Tube (Coiltube) — Wenzhou Jixing Metal Materials Co., Ltd.
Product Catalog No.: 332
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a continuous-length tubular product formed by cold-drawing or cold-rolling processes, wound into a helical coil configuration for efficient storage, transport, and downstream processing. This product occupies a critical niche in the metallurgical and materials engineering domain, serving as a versatile semi-finished component across heating systems, fluid conveyance, structural assemblies, and precision engineering applications.
1. Product Overview
The Coil Tube represents a specialized form of stainless steel tubing characterized by its continuous, uncut helical winding configuration. Unlike discrete-length pipe or tube products, the Coiltube is supplied in continuous coils, enabling downstream users to cut-to-length without generating butt-weld joints, thereby minimizing material waste and preserving the metallurgical integrity of the tube wall. This configuration is particularly advantageous in applications requiring long, seamless tube runs such as floor radiant heating systems, fan coil units, and hydronic heating circuits.
In the context of building services engineering, the stainless steel coil tube finds extensive deployment in floor radiant heating systems (Floor Radiant Heating, FRH) and fan coil units (Fan Coil Units, FCU) in regions classified as "summer hot and winter cold" (夏热冬冷地区), where thermal comfort demands necessitate reliable, corrosion-resistant, and long-service-life heating media. The comparative analysis of floor radiant heating versus fan coil heating in such climatic zones has demonstrated that stainless steel coil tubes offer superior thermal transmittance, resistance to galvanic corrosion, and hygienic fluid transport properties compared to conventional carbon steel alternatives.
The Coiltube product line from Wenzhou Jixing Metal Materials Co., Ltd. encompasses a range of diameters, wall thicknesses, and stainless steel grades tailored to specific application requirements, from domestic heating circuits to industrial process piping.
2. Material and Properties
2.1 Base Material Grades
The Coiltube is fabricated from austenitic stainless steels conforming to international standards, with the following primary grades available:
- SAE 304 / AISI 304 (UNS S30400): The most widely specified grade, offering excellent general corrosion resistance, formability, and weldability. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni).
- SAE 316 / AISI 316 (UNS S31600): Enhanced corrosion resistance due to the addition of 2–3% Molybdenum (Mo), making it suitable for aggressive chemical environments and chloride-containing fluids.
- SAE 321 / AISI 321 (UNS S32100): Stabilized with Titanium (Ti), providing superior resistance to intergranular corrosion at elevated temperatures.
- SAE 304L / AISI 304L (UNS S30403): Low-carbon variant with carbon content ≤ 0.03%, offering improved weldability and resistance to sensitization.
2.2 Mechanical Properties
| Property | SAE 304 (Annealed) | SAE 316 (Annealed) | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 515 MPa | ASTM A213 / GB/T 12771 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ASTM A213 / GB/T 12771 |
| Elongation (A) | ≥ 40% | ≥ 40% | ASTM A213 / GB/T 12771 |
| Hardness (HV) | ≤ 217 HV | ≤ 217 HV | ASTM A213 |
| Density (ρ) | 7.93 g/cm³ | 8.00 g/cm³ | — |
2.3 Physical and Thermal Properties
- Thermal Conductivity (k): Approximately 16.3 W/(m·K) for SAE 304 at 20°C, providing efficient heat transfer in hydronic heating applications.
- Specific Heat Capacity (cp): Approximately 500 J/(kg·K), contributing to thermal inertia in heating circuits.
- Linear Thermal Expansion Coefficient (α): Approximately 17.3 × 10⁻⁶ /K (20–600°C), requiring consideration in long-run coil installations.
- Maximum Service Temperature: Up to 870°C for SAE 304 and 925°C for SAE 316 in non-oxidizing atmospheres.
2.4 Corrosion Resistance
The austenitic microstructure of the stainless steel Coiltube provides inherent resistance to oxidation and general corrosion. The passive chromium oxide film (Cr₂O₃) that forms spontaneously on the surface acts as a protective barrier against atmospheric and aqueous corrosion. In chloride-rich environments, the Mo-bearing SAE 316 grade demonstrates superior pitting corrosion resistance, with a higher Corrosion Resistance Index (CRI) as defined by ASTM G48.
3. Technical Specifications
| Parameter | Specification Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 159.0 mm | GB/T 12771 / ASTM A213 |
| Wall Thickness | 0.5 mm – 6.0 mm | GB/T 12771 / ASTM A213 |
| OD Tolerance | ±0.05 mm to ±0.5 mm (dependent on OD) | GB/T 12771 |
| Wall Thickness Tolerance | ±10% of nominal | GB/T 12771 |
| Surface Finish | No. 1, 2B, BA (Bright Annealed), Polished | ASTM A480 / ASTM A666 |
| End Condition | Plain, Beveled, or Threaded | — |
| Coil Length | Up to 30 m (continuous, uncut) | — |
| Coil Weight | Up to 2,000 kg per coil (typical) | — |
| Pressure Rating | Up to 6.4 MPa (PN64), dependent on OD and wall thickness | EN 10217 / GB/T 12771 |
| Working Temperature Range | −196°C to +870°C (SAE 304); −196°C to +925°C (SAE 316) | — |
| Available Grades | SAE 304, 304L, 316, 316L, 321 | AISI / UNS Designation |
4. Manufacturing Process
The production of the stainless steel Coiltube follows a rigorous multi-stage metallurgical processing route, ensuring dimensional accuracy, mechanical integrity, and surface quality.
4.1 Process Flow
- Raw Material Preparation: Cold-rolled stainless steel strip (Cold-Rolled Strip, CRS) is sourced from certified steel mills, conforming to the specified chemical composition and mechanical properties. Incoming material undergoes spectroscopic verification (Optical Emission Spectrometry, OES) to confirm grade compliance.
- Forming (Coil Forming): The strip is fed through a series of progressively reducing rolls (Roll Forming Dies) in a high-precision forming machine, gradually bending the flat strip into a circular cross-section. The forming process is controlled to maintain uniform wall thickness and minimize residual stress.
- Welding: The longitudinal seam is closed using High-Frequency Resistance Welding (HFRW) or Transverse Magnetic Field (TMF) welding, achieving full-penetration welds with minimal heat-affected zone (HAZ). Weld integrity is verified through 100% inline ultrasonic testing (UT) and visual inspection.
- Heat Treatment (Solution Annealing): The welded tube undergoes solution annealing at 1050–1150°C followed by controlled cooling (water quench or air cooling), restoring the austenitic microstructure, eliminating welding-induced residual stresses, and re-establishing the passive oxide film. For stabilized grades (e.g., SAE 321), a subsequent stabilization anneal at 870–900°C may be performed.
- Pickling and Passivation: The annealed tube is subjected to acid pickling (typically a mixture of nitric acid HNO₃ and hydrofluoric acid HF) to remove the oxide scale formed during annealing, followed by passivation to restore the chromium oxide passive layer.
- Dimensional Calibration: The tube passes through a cold-calibration (Cold-Piercing) or cold-rolling station to achieve final dimensional tolerances on OD and wall thickness.
- Surface Finishing: Depending on the required surface quality, the tube may undergo electropolishing, mechanical polishing, or passivation to achieve the specified surface finish (e.g., Ra ≤ 0.4 μm for sanitary applications).
- Coiling: The continuous tube is wound onto a coil reel at controlled tension, ensuring uniform coil geometry and preventing mechanical damage. Coils are wrapped in protective film and packaged in wooden or steel crates.
- Final Inspection: Each coil undergoes 100% visual inspection, dimensional verification, hydrostatic pressure testing, and sample-based mechanical testing (tensile, hardness, impact) prior to release.
4.2 Key Process Controls
- Weld Seam Integrity: 100% inline ultrasonic testing ensures no porosity, lack of fusion, or cracks in the longitudinal weld.
- Dimensional Consistency: In-process laser measurement systems monitor OD and wall thickness in real-time, with automatic feedback control of roll positions.
- Microstructural Verification: Metallographic examination (per ASTM E3) confirms full austenitic structure with no delta-ferrite or martensite formation.
- Chemical Analysis: Each heat of raw material is verified by OES and/or wet chemical analysis against ASTM A213 chemical composition requirements.
5. Applications
5.1 Building Services and Heating Systems
The Coiltube finds primary application in floor radiant heating systems and fan coil units in "summer hot, winter cold" climatic zones. In floor radiant heating, the continuous coil configuration allows the tube to be laid in serpentine or spiral patterns within the floor slab, providing uniform heat distribution with minimal pressure drop. The stainless steel material ensures long-term resistance to corrosion from heating fluids and compatibility with concrete encasement.
In fan coil units, the Coiltube serves as the heat exchange medium, transferring thermal energy between the circulating fluid and the air stream. The high thermal conductivity and corrosion resistance of stainless steel contribute to sustained heat transfer efficiency over extended service life.
5.2 Additional Application Domains
- Fluid Conveyance Systems: Food and beverage processing, pharmaceutical piping, and chemical process lines where hygienic and corrosion-resistant requirements are critical.
- Automotive and Aerospace: Fuel lines, brake lines, and exhaust system components requiring high-temperature resistance and fatigue durability.
- Medical Equipment: Sterilizable tubing for medical gas delivery and dialysis systems, requiring ultra-clean surface finishes.
- Structural and Mechanical: Spring coils, expansion loops, and flexible connections in industrial piping systems.
- Heat Exchangers: Tubular heat exchanger bundles where continuous coil construction eliminates butt-weld joints, reducing failure points.
- Hydronic Cooling Systems: Chilled water distribution in commercial HVAC systems.
6. Quality Certifications and Standards Compliance
The Coiltube product is manufactured and tested in accordance with the following international and national standards:
| Standard | Scope | Relevant Clause |
|---|---|---|
| ASTM A213 / A213M | Seamless and welded austenitic stainless steel tubes for heat exchangers and high-temperature service | Chemical, mechanical, hydrostatic test |
| GB/T 12771 | Stainless steel welded tubes for general purposes (Chinese National Standard) | Dimensional tolerances, surface quality, mechanical properties |
| EN 10217-7 | Stainless steel tubes for pressure purposes — |