Stainless Steel Coil Tube (Spiral Coil Pipe) — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (also referred to as Spiral Coil Pipe or Coiltube) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is formed by the continuous winding of seamless or welded stainless steel tube into a helical or spiral configuration, producing a compact, spring-like structure with controlled geometry and mechanical properties. Coil tubes are integral components in hydraulic systems, heat exchangers, refrigeration circuits, and high-pressure fluid transfer applications where space efficiency and fatigue resistance are critical design parameters.
The manufacturing of spiral coil tubes on hydraulic presses involves the precise bending of straight-length stainless steel tube under controlled radial and axial loads to achieve a consistent helical pitch, bend radius, and coil diameter. The process demands rigorous control over material springback, work hardening, and geometric tolerances to ensure the final product meets stringent performance requirements.
2. Material & Properties
2.1 Commonly Used Stainless Steel Grades
- AISI 304 (UNS S30400) — Austenitic stainless steel with excellent corrosion resistance and formability; widely used for general-purpose coil tube applications.
- AISI 316L (UNS S31603) — Low-carbon austenitic grade with superior resistance to pitting and crevice corrosion due to molybdenum addition; preferred for chemical and marine environments.
- AISI 321 (UNS S32100) — Titanium-stabilized austenitic grade offering enhanced resistance to intergranular corrosion at elevated temperatures.
- AISI 310S (UNS S31008) — High-temperature austenitic grade suitable for coil tubes operating above 1000 °C.
2.2 Mechanical Properties (Typical, Annealed Condition)
| Property | AISI 304 | AISI 316L | AISI 321 |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ≥ 515 |
| Yield Strength (MPa) | ≥ 205 | ≥ 205 | ≥ 205 |
| Elongation (% min.) | ≥ 40 | ≥ 40 | ≥ 40 |
| Density (g/cm³) | 8.00 | 8.03 | 8.00 |
| Thermal Conductivity at 100 °C (W/m·K) | 16.3 | 14.6 | 16.3 |
| Maximum Service Temperature (°C) | 870 | 870 | 925 |
| Minimum Service Temperature (°C) | −196 | −196 | −196 |
2.3 Corrosion Resistance Characteristics
Stainless steel coil tubes exhibit outstanding resistance to atmospheric corrosion, acidic and alkaline environments, and chlorinated media (grade-dependent). The passive chromium oxide film (Cr₂O₃) self-repairs under oxidizing conditions, providing long-term structural integrity without the need for additional protective coatings. This eliminates the risk of galvanic corrosion commonly encountered with carbon steel alternatives.
3. Specifications
| Parameter | Range / Value | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 50.8 mm (⅛" – 2") | ASTM A213 / A269 |
| Wall Thickness | 0.3 mm – 3.0 mm | ASTM A213 / A269 |
| Coil Diameter (Center-to-Center) | 50 mm – 500 mm | Customer-Specific |
| Helical Pitch | 10 mm – 80 mm | Customer-Specific |
| Bend Radius (Minimum) | ≥ 4× OD | ASME B31.3 |
| Coil Length (Unwound Equivalent) | 1 m – 30 m | — |
| Tolerance on OD | ± 0.1 mm (≤ 10 mm OD); ± 0.2 mm (> 10 mm OD) | ASTM A213 |
| Tolerance on Wall Thickness | ± 10% | ASTM A269 |
| Surface Finish | Mirror Polish (Ra ≤ 0.4 μm) / Brushed / Pickled & Passivated | ASTM A270 |
| End Condition | Plain End / Beveled / Flanged / Threaded | Customer-Specific |
| Operating Pressure (Design) | Up to 420 bar (6,000 psi) | ASME B31.3 / PED |
| Operating Temperature Range | −196 °C to +870 °C | Material-Dependent |
4. Manufacturing Process
4.1 Raw Material Procurement and Inspection
High-purity stainless steel ingots are sourced from certified mills and subjected to spectrographic analysis (OES/PES) to verify chemical composition. Hot-rolled coils are inspected for surface defects, dimensional accuracy, and metallurgical homogeneity prior to downstream processing.
4.2 Seamless Tube Production
Seamless stainless steel tubes are produced via the APM (Assel-Pilger-Mandel) or PLT (Piercing-Laminating-Trirolling) method, followed by cold drawing and cold rolling to achieve precise dimensional tolerances and superior grain structure. The resulting tube stock is supplied in the annealed (solution-treated) condition to maximize ductility for subsequent coiling operations.
4.3 Spiral Coiling on Hydraulic Press
The core forming operation involves the bending of straight-length stainless steel tube into a helical coil configuration using a specialized hydraulic press with programmable tooling. Key process parameters include:
- Die Geometry: Precision-machined mandrel and counter-die sets matched to the target coil diameter and tube OD to prevent ovality and wall thinning.
- Hydraulic Force Control: Closed-loop pressure regulation ensures consistent bending force, minimizing residual stress and dimensional variation between coils.
- Bend Speed: Controlled at 0.5–2.0 rev/min to limit strain rate sensitivity and avoid adiabatic heating of the material.
- Springback Compensation: Pre-calculated over-bend angles (typically 2°–5°) compensate for elastic recovery inherent in austenitic stainless steels.
- Intermediate Annealing: For multi-turn coils or high-work-hardening grades, in-process solution annealing at 1050–1100 °C in inert atmosphere may be applied to restore ductility and prevent cracking.
4.4 Post-Forming Heat Treatment
Following coiling, the product undergoes solution annealing (1040–1120 °C, water quench) to relieve bending-induced residual stresses, restore full solution-treated properties, and enhance corrosion resistance. Stabilization treatment (850–880 °C) is applied for grades 321 and 347 to prevent carbide precipitation at welding zones.
4.5 Surface Treatment and Passivation
Coil tubes are pickled and passivated using citric acid or nitric-hydrofluoric acid solutions to remove scale, contaminants, and free iron particles. Final surface finishing (electropolishing, mechanical polishing) is performed to achieve the specified roughness profile.
4.6 Quality Inspection and Testing
- Dimensional Inspection: Laser scanning and CMM (Coordinate Measuring Machine) verification of coil diameter, pitch, and straightness.
- Hydrostatic Testing: Each coil tube is pressure-tested at 1.5× design pressure per ASTM A213 Section 7.
- Corrosion Testing: Salt spray testing (ASTM B117) and intergranular corrosion testing (ASTM A262 Practice E) as applicable.
- Non-Destructive Testing: Eddy current testing (ASTM E309) for surface and near-surface defect detection; X-ray radiography for weld integrity verification on welded tube variants.
- Metallurgical Examination: Transverse sectioning and optical microscopy to verify grain structure, absence of deformation bands, and proper annealing response.
5. Applications
5.1 Hydraulic Systems
Coil tubes serve as compact, high-pressure fluid conduits in hydraulic power units, mobile machinery (excavators, cranes), aerospace actuation systems, and industrial press equipment. The helical geometry provides inherent shock absorption and fatigue resistance under cyclic loading.
5.2 Heat Exchange Systems
In spiral heat exchangers, coil tubes maximize surface-area-to-volume ratio, enabling efficient heat transfer between process fluids. Applications include district heating, chemical process cooling, and cryogenic fluid handling.
5.3 Refrigeration and HVAC
Stainless steel coil tubes are employed in evaporators and condensers for ammonia-based and fluorocarbon refrigeration systems where corrosion resistance and long service life are paramount.
5.4 Food and Pharmaceutical Processing
Compliant with FDA 21 CFR and EHEDG standards, coil tubes in grades 316L are used for CIP (Clean-In-Place) compatible fluid transfer, pasteurization circuits, and aseptic processing equipment.
5.5 Oil and Gas Industry
Used in subsea production systems, wellhead assemblies, and offshore platforms where resistance to sour gas (H₂S/CO₂) environments and extreme temperature differentials is required.
5.6 Automotive and Aerospace
Compact coil tubes function as fuel lines, hydraulic lines, and pneumatic conduits in vehicles and aircraft where weight reduction and vibration resistance are design imperatives.
6. Quality Certifications and Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing excellence and traceability. |
| ISO 3834-2 | Quality requirements for fusion welding of metallic materials. |
| EN 10204 Type 3.1 | Inspection certificate verifying conformity of material and product to specified standards. |
| ASME BPV Code Section I | Boiler and Pressure Vessel Code compliance for pressure-containing applications. |
| PED 2014/68/EU | European Pressure Equipment Directive conformity for Category II–IV equipment. |
| ASTM A213 | Standard specification for seamless austenitic stainless steel tubing for high-temperature service. |
| ASTM A269 | Standard specification for seamless and welded austenitic stainless steel tubing. |
| EN 10216-5 | European standard for non-alloy and alloy steel tubes for pressure purposes (austenitic grades). |
| NORSOK M-650 | Oil and gas industry standard for materials and fabrication of piping systems. |
| NSF/ANSI 61 | Health effects standard for drinking water system components (food-grade applications). |
7. Unique Selling Points
Hydraulic Press Forming Expertise: Wenzhou Jixing Metal Materials Co., Ltd leverages advanced hydraulic press technology to achieve superior geometric precision (pitch tolerance ≤ ±0.5 mm), minimal wall thinning (≤ 3%), and consistent mechanical properties across the entire coil length — distinguishing their products from competitor offerings produced by simpler mandrel-bending methods.
- Full Traceability: Every coil tube is traceable from raw material mill certificate through final inspection, ensuring complete supply chain transparency.
- Custom Engineering Support: The manufacturer offers finite element analysis (FEA) simulation of coil behavior under operational loads, enabling optimized design for specific customer applications.
- Wide Grade Availability: Stock availability of multiple stainless steel grades (304, 316L, 321, 310S, duplex 2205) allows rapid response to diverse corrosion and temperature requirements.
- Low-Minimum-Order Flexibility: Capability to produce short-run custom coils with minimum order quantities as low as 50 pieces, supporting prototyping and specialized industrial needs.
- Extended Service Life: Solution-annealed austenitic stainless steel coil tubes deliver a design life exceeding 25 years in aggressive environments, significantly outperforming carbon steel and aluminum alternatives.
8. Storage, Handling, and Packaging
Coil tubes are individually wrapped in acid-free kraft paper and sealed in VCI (Vapor Corrosion Inhibitor) bags to prevent atmospheric corrosion during transit. Bulk shipments are palletized and shrink-wrapped with corner protectors. Storage should be in a dry, well-ventilated warehouse with relative humidity maintained below 65%. Avoid contact with carbon steel surfaces to prevent free iron contamination and subsequent rust staining. Coil tubes should be stored on wooden or polymer pallets, never directly on concrete floors.
9. Conclusion
The stainless steel coil tube product offered by Wenzhou Jixing Metal Materials Co., Ltd represents a synergistic combination of premium austenitic stainless steel metallurgy, precision hydraulic press forming technology, and rigorous quality assurance protocols. This product line addresses the demanding requirements of modern industrial applications where compact geometry, exceptional corrosion resistance, high-pressure capability, and long-term reliability are non-negotiable performance criteria. The manufacturer's commitment to standards compliance, process innovation, and customer-centric engineering support positions this product as a preferred solution for critical infrastructure and high-value industrial systems worldwide.