Stainless Steel Coil Tube (Coiltube) — Medical-Grade Flexible Tubing System
1. Product Overview
The Stainless Steel Coil Tube (also referred to as Coiltube or Coil Pipe) is a precision-engineered flexible tubing product designed for demanding medical, industrial, and sanitary applications. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product series integrates advanced coil-forming technology with high-purity stainless steel materials to deliver superior flexibility, corrosion resistance, and structural integrity. The coil tube is supplied in continuous coil (spool) configuration, enabling efficient downstream processing, secondary forming, and automated assembly in high-volume production environments.
Distinct from rigid straight-length tubing, the coil tube retains its helical geometry during storage and transport, reducing handling losses and eliminating the need for intermediate straightening operations. The product has been extensively studied and validated for its application in medical plastic pipe systems, particularly in conjunction with buckle-locking (snap-fit) joining mechanisms, where dimensional accuracy and surface finish play critical roles in connection reliability.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from high-grade austenitic stainless steel conforming to international standards. The primary alloy grades utilized include:
- 304 (UNS S30400) — The most widely used general-purpose austenitic stainless steel, offering excellent corrosion resistance, formability, and weldability.
- 316 (UNS S31600) — Enhanced molybdenum content provides superior resistance to pitting and crevice corrosion, making it ideal for medical and pharmaceutical applications.
- 316L (UNS S31603) — Low-carbon variant of 316, further reducing intergranular corrosion susceptibility during welding operations.
- 321 (UNS S32100) — Titanium-stabilized grade for applications requiring resistance to sensitization at elevated temperatures.
2.2 Mechanical Properties
| Property | 304 Grade | 316L Grade | Test Standard |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 480 | ASTM A270 / EN 10217 |
| Yield Strength (MPa) | ≥ 205 | ≥ 170 | ASTM A270 / EN 10217 |
| Elongation (%) | ≥ 40 | ≥ 40 | ASTM A270 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | ASTM E10 |
| Density (g/cm³) | 8.00 | 8.03 | — |
| Thermal Conductivity (W/m·K, at 100°C) | 16.3 | 16.3 | ASTM E1461 |
2.3 Chemical Composition (Typical, wt%)
| Element | C | Si | Mn | P | S | Cr | Ni | Mo | N |
|---|---|---|---|---|---|---|---|---|---|
| 304 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 18.00–20.00 | 8.00–10.50 | — | — |
| 316L | ≤ 0.030 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 16.00–18.00 | 10.00–14.00 | 2.00–3.00 | ≤ 0.10 |
2.4 Surface Characteristics
- Surface Finish: Available in pickled (matte), bright annealed, polished (2B, BA), and electropolished finishes.
- Surface Roughness (Ra): As low as 0.4 μm for electropolished grade; ≤ 1.6 μm for standard pickled finish.
- Chromium Oxide Passivation Layer: Self-forming protective film providing inherent corrosion resistance in most environments.
3. Specifications
| Parameter | Specification Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 108.0 mm | ASTM A270 / EN 10217-7 |
| Wall Thickness | 0.3 mm – 6.0 mm | ASTM A270 / EN 10217-7 |
| Coil Diameter | φ 200 mm – φ 1,200 mm | Manufacturer Specification |
| Coil Length per Spool | 100 m – 500 m (typical) | — |
| Coil Weight per Spool | Up to 500 kg | — |
| OD Tolerance | ± 0.05 mm (fine tolerance); ± 0.10 mm (standard) | ASTM A270 |
| Wall Thickness Tolerance | ± 10% (standard); ± 5% (fine tolerance) | ASTM A270 |
| Roundness | ≤ 0.5% of OD | EN 10217 |
| Straightness (post-uncoiling) | ≤ 1.5 mm/m | EN 10217 |
| Operating Temperature Range | −196 °C to +870 °C (304); −196 °C to +870 °C (316L) | ASTM A270 |
| Pressure Rating (Schedule-dependent) | Up to 16.0 MPa (PN160) | EN 10217-7 |
| Welding Compatibility | TIG (GTAW), MIG (GMAW), Laser Welding | ISO 6947 |
| Maximum Bending Radius | ≥ 3D (D = outer diameter) for cold bending | EN 10217 |
| Hydrostatic Test Pressure | 1.5 × design pressure, minimum 2.0 MPa | ASTM A270 |
4. Manufacturing Process
The production of stainless steel coil tubes follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional precision, metallurgical integrity, and surface quality. The process flow is outlined below:
4.1 Process Flow Diagram
- Raw Material Preparation: Selection of certified stainless steel strip (cold-rolled or hot-rolled and pickled) from approved mill sources. Each batch undergoes incoming inspection for chemical composition (OES/Spectrometry) and mechanical properties.
- Strip Annealing (if required): Solution annealing in a controlled atmosphere (vacuum or high-purity nitrogen) at 1050–1150 °C followed by rapid cooling to restore full austenitic microstructure and eliminate cold-work hardening.
- Coil Forming (Spiral Winding / Rolling): The flat strip is fed through a precision coil-forming machine equipped with synchronized roll stations. The forming rollers progressively bend the strip into a helical coil shape. Critical process parameters include roll gap settings, feed speed, and tension control to prevent ovality, wrinkles, or springback.
- Seam Welding: The longitudinal seam is welded using Automatic Submerged Arc Welding (SAW) or High-Frequency Induction Welding (HFIR). The weld zone undergoes in-line optical inspection and ultrasonic testing (UT) for defect detection.
- Weld Zone Annealing: The welded coil passes through a localized heat treatment zone (induction or flame) to homogenize the heat-affected zone (HAZ), restoring corrosion resistance and mechanical properties in the weld region.
- Full-Length Annealing and Pickling: The completed coil tube is subjected to full-length solution annealing (1050–1100 °C) in a protective atmosphere, followed by acid pickling (nitric-hydrofluoric acid bath or electrolytic pickling) to remove scale and restore the passive chromium oxide film.
- Surface Finishing: Optional processes include mechanical polishing, electropolishing (for medical-grade surfaces), or passivation treatment to achieve the required surface roughness and corrosion resistance.
- Quality Inspection and Testing: Each coil undergoes comprehensive non-destructive testing (NDT) including eddy current testing (ECT), ultrasonic testing (UT), hydrostatic pressure testing, and dimensional verification (OD, wall thickness, roundness, straightness).
- Coil Winding and Packaging: The inspected tube is wound onto a protective coil core with inter-layer paper or plastic film wrapping. The outer surface is protected with stretch film and marked with product identification labels including heat number, grade, dimensions, and batch traceability codes.
4.2 Key Process Parameters
| Process Stage | Critical Parameter | Target Value |
|---|---|---|
| Solution Annealing | Temperature | 1050 – 1150 °C |
| Solution Annealing | Atmosphere Dew Point | ≤ −60 °C (vacuum) or ≤ −40 °C (N₂) |
| Seam Welding (HFIR) | Welding Frequency | 200 – 500 kHz |
| Seam Welding (HFIR) | Welding Current | 80 – 250 kA (diameter-dependent) |
| Pickling | Bath Temperature | 50 – 80 °C |
| Pickling | Acid Concentration | HNO₃: 10–15%; HF: 0.5–1.5% (wt) |
| Electropolishing | Current Density | 150 – 300 A/dm² |
| Electropolishing | Electrolyte Temperature | 40 – 60 °C |
5. Applications
The stainless steel coil tube from Wenzhou Jixing Metal Materials Co., Ltd. is engineered for a broad spectrum of high-performance applications. The following are the primary application domains:
5.1 Medical Device Manufacturing
- Endoscope Sheaths and Channels: The flexibility and dimensional precision of coil tubes make them ideal for endoscope instrument channels, where repeated flexing and minimal hysteresis are required.
- Surgical Instrument Shafts: Used in laparoscopic and arthroscopic instruments requiring biocompatibility and resistance to autoclave sterilization cycles (up to 134 °C, 121 kPa steam).
- Medical Tubing Assemblies: When combined with buckle-locking (snap-fit) joining mechanisms, coil tubes provide reliable fluid transport in infusion systems, dialysis circuits, and respiratory devices. The coil geometry facilitates coiled-length tubing assemblies that maintain kink resistance while fitting within compact device housings.
- Implantable Device Components: 316L-grade coil tubes are used in lead wires for cardiac pacemakers and neurostimulators, where long-term biostability and fatigue resistance are paramount.