Stainless Steel Coil Tube — Continuous Cold-Formed Tubing with Nitrogen-Infusion Process
Product Overview
The Stainless Steel Coil Tube (Coiltube) is a continuously cold-formed tubular product manufactured from high-grade austenitic stainless steel strip. Unlike discrete-length seamless or welded pipes, coil tubes are produced in continuous lengths wound onto coils, enabling downstream users to cut to precise required dimensions with minimal material waste. This product line represents the advanced application of continuous forming technology combined with internal nitrogen infusion (Nitrogen Injection Process), a proprietary technique that enhances mechanical properties, surface quality, and corrosion resistance of the final tube product.
Coil tubes are widely recognized in international markets for their cost-efficiency, dimensional consistency, and adaptability to automated downstream processing lines. They serve as critical components in heat exchangers, condensers, evaporators, refrigeration systems, chemical processing equipment, and precision instrumentation.
Material & Properties
Base Material Grades
- SUS304 / AISI 304 (1.4301) — General-purpose austenitic stainless steel with excellent corrosion resistance and formability.
- SUS316 / AISI 316 (1.4401) — Molybdenum-enhanced grade offering superior resistance to chlorides and pitting corrosion.
- SUS321 / AISI 321 (1.4541) — Titanium-stabilized grade for high-temperature service up to 870°C.
- SUS316L / AISI 316L (1.4435) — Low-carbon variant for welding-intensive applications requiring resistance to intergranular corrosion.
- SUS304L / AISI 304L (1.4307) — Low-carbon grade for deep drawing and welding applications.
Mechanical Properties (Typical, Post-Nitrogen Infusion)
- Tensile Strength (Rm): ≥ 520 MPa (2H hardness state)
- Yield Strength (Rp0.2): ≥ 205 MPa
- Elongation (A50): ≥ 35%
- Hardness: 80–220 HV (depending on tempering state)
- Thermal Conductivity: 16.3 W/(m·K) at 100°C
Chemical Composition (SUS304, wt%)
| Element | Specification |
|---|---|
| C (Carbon) | ≤ 0.08 |
| S (Sulfur) | ≤ 0.030 |
| P (Phosphorus) | ≤ 0.045 |
| Cr (Chromium) | 18.0 – 20.0 |
| Ni (Nickel) | 8.0 – 10.5 |
| Si (Silicon) | ≤ 0.75 |
| Mn (Manganese) | ≤ 2.00 |
| Fe (Iron) | Balance |
Specifications
| Parameter | Specification / Range |
|---|---|
| Outer Diameter (OD) | 2.0 mm – 159.0 mm |
| Wall Thickness (WT) | 0.25 mm – 6.00 mm |
| Coil Length | Up to 5,000 m per coil (continuous) |
| OD Tolerance | ± 0.05 mm – ± 0.20 mm (per EN 10217-7) |
| WT Tolerance | ± 10% of nominal (thin wall); ± 0.1 mm (thick wall) |
| Surface Finish | No.1, 2B, BA (Bright Annealed), HL (Hairline), Mirror (8K) |
| Internal Surface | Ra ≤ 0.4 μm (electropolished option available) |
| Delivery Condition | Annealed & Pickled (AP), Solution Heat Treated (SHT), Cold Worked (CW) |
| Coil Weight | Up to 3,000 kg per coil |
| Standard Compliance | EN 10217-7, ASTM A554, ASTM A213, GB/T 14976, JIS G3459 |
| Nitrogen Infusion Content | 0.02% – 0.10% N (by weight, controlled) |
Manufacturing Process
The production of high-quality stainless steel coil tubes involves a multi-stage continuous manufacturing process, with the nitrogen infusion step serving as a critical differentiator in achieving superior mechanical and metallurgical performance.
Process Flow
- Strip Preparation: Cold-rolled stainless steel strip is inspected for surface defects, dimensional accuracy, and chemical composition conformity. Strip is uncoiled and fed into the forming line at controlled tension.
- Edge Trimming & Inspection: Strip edges are trimmed to remove mill burrs. Online optical inspection systems detect surface imperfections prior to forming.
- Continuous Forming (Roll Forming): The strip passes through a series of precision roller stations that progressively bend it into the desired circular or oval cross-section. Roller profiles are CNC-ground to ensure dimensional accuracy throughout the production run.
- Longitudinal Welding (TIG / Laser): A continuous longitudinal weld is formed along the tube seam using either Tungsten Inert Gas (TIG) welding or high-power fiber laser welding. Shielding gas (argon or argon-helium mix) prevents oxidation of the weld zone.
- Internal Nitrogen Infusion (Nitrogen Injection Process): This is the proprietary differentiating step. The tube is passed through a sealed chamber where pure nitrogen gas (≥ 99.99% purity) is introduced under controlled pressure (typically 0.5 – 3.0 MPa) into the tube interior. The nitrogen diffuses into the austenitic matrix at elevated temperatures (850°C – 1050°C), creating solid-solution strengthening. This process:
- Increases tensile strength by 15% – 30% without reducing ductility.
- Enhances resistance to stress corrosion cracking (SCC).
- Improves fatigue life under cyclic loading conditions.
- Reduces susceptibility to intergranular corrosion by promoting nitrogen-rich grain boundary segregation.
- Post-Weld Heat Treatment (Solution Annealing): The welded and nitrogen-infused tube undergoes solution annealing in a controlled atmosphere furnace (hydrogen or vacuum) to restore full austenitic structure and relieve residual stresses.
- Pickling & Passivation: The tube is immersed in a mixed acid solution (nitric + hydrofluoric acid) to remove the heat-treatment scale and restore chromium-rich passive oxide layer. Passivation treatment ensures long-term corrosion resistance.
- Dimensional Calibration: The tube passes through a calibration ring or mandrel system to correct any ovality or dimensional deviation, ensuring roundness within ± 0.5% of nominal OD.
- Online Quality Inspection: Ultrasonic testing (UT) detects internal defects; eddy current testing (ECT) verifies weld seam integrity; optical systems monitor OD and WT continuously.
- Coiling & Packaging: The finished tube is wound onto a heavy-duty steel coil core with protective inner wrapping (PE film or kraft paper) and outer packaging (wooden crates or stretch-wrap pallets) for shipment.
Key Technical Considerations of the Nitrogen Infusion Process
The nitrogen infusion process requires precise control of three critical parameters: temperature (affects diffusion rate), pressure (drives nitrogen into the matrix), and residence time (ensures uniform distribution). Excessive nitrogen content (> 0.12%) may cause grain boundary precipitation of Cr₂N, which can reduce ductility. The process window is optimized for each grade and wall thickness through computational thermodynamic modeling and empirical validation.
Applications
- Refrigeration & Air Conditioning: Evaporator and condenser coils for commercial and industrial HVAC systems. The continuous coil format enables automated bending and brazing operations.
- Heat Exchangers: Shell-and-tube heat exchangers, plate heat exchanger gaskets, and plate-fin compact exchangers in petrochemical and power generation industries.
- Chemical Processing: Reactor internals, distillation column components, heat transfer surfaces in aggressive chemical environments (SUS316/316L grades).
- Food & Pharmaceutical Equipment: Sanitary-grade tubing for CIP (Clean-in-Place) systems, pasteurizers, and fermentation equipment (BA or electropolished finish).
- Automotive Industry: Fuel lines, brake lines, exhaust system components, and turbocharger manifolds requiring high-temperature resistance.
- Medical Devices: Endoscopy tubing, surgical instrument shafts, and dialysis equipment components requiring biocompatibility and precision dimensions.
- Oil & Gas: Subsea flow lines, wellhead components, and offshore platform instrumentation tubing (corrosion-resistant grades).
- Household Appliances: Water heater heat exchangers, dishwasher spray arms, and coffee machine steam tubes.
Quality Certifications & Compliance
- ISO 9001:2015 — Quality Management System certification ensuring consistent product quality and process control.
- ISO 14001:2015 — Environmental Management System demonstrating sustainable manufacturing practices.
- EN 10204 3.1 — Material Test Certificates (MTC) with full traceability from mill certificate to final product.
- CE Marking — Conformity with European Directive 2014/68/EU (Pressure Equipment Directive) where applicable.
- PED 2014/68/EU — Pressure Equipment Directive compliance for tubes used in pressurized systems.
- NSF/ANSI 61 — Drinking Water System Components certification (for food-grade and potable water applications).
- 3-A Sanitary Standards — Dairy, food, and beverage industry compliance for hygienic equipment tubing.
- ASME BPV Code — Boiler and Pressure Vessel Code compliance for tubes used in pressure-containing equipment.
- GB/T 14976 — Chinese national standard for seamless and welded stainless steel tubes.
- ASTM A554 / A213 — American Society for Testing and Materials standards for stainless steel tubing.
- JIS G3459 — Japanese Industrial Standard for stainless steel tubes.
Unique Selling Points
- Nitrogen-Infused Microstructure: Proprietary nitrogen injection process delivers 15%–30% higher tensile strength compared to conventionally annealed tubes, enabling thinner-wall designs with equivalent pressure rating — reducing weight and material cost.
- Superior Weld Seam Quality: Continuous laser or TIG welding with real-time monitoring ensures weld integrity exceeding parent material strength, eliminating the traditional weak-point concern associated with welded tubing.
- Minimal Material Waste: Continuous coil delivery format eliminates butt-welding losses between discrete tube lengths, offering customers 5%–12% material savings compared to cut-length pipe purchases.
- Multi-Grade Flexibility: Full range from SUS304 to specialty alloys (SUS316L, SUS321, SUS904L, etc.) accommodates diverse corrosion and temperature requirements from a single supply platform.
- Custom Surface Finishes: From mill finish through mirror-polished (8K, Ra ≤ 0.05 μm) internal surfaces, enabling direct integration into sanitary and optical applications without secondary finishing.
- Full Traceability: Every coil is serialized with heat number, production date, chemical analysis, mechanical test results, and dimensional inspection data — ensuring complete supply chain accountability.
- Scalable Production: Continuous production capability supports both small-batch custom orders (minimum 500 m) and large-volume industrial supply (up to 5,000 m per coil), serving OEMs and fabricators alike.
Technical Support & Engineering Services
Wenzhou Jixing Metal Materials Co., Ltd. provides comprehensive technical support including:
- Material selection consulting based on service environment (temperature, pressure, corrosive media).
- Finite Element Analysis (FEA) for tube performance prediction under operating conditions.
- Custom coil configuration design to optimize downstream forming and fabrication processes.
- Welding procedure qualification support for customer fabrication shops.
- Non-destructive testing (NDT) report provision upon request.