Spiral Coil Tube (Coiltube) — Stainless Steel Seamless Coil Pipe
1. Product Overview
The Spiral Coil Tube (also designated as Coiltube) is a specialized form of stainless steel tubing manufactured through a cold-drawn spiral coiling process. Unlike conventional straight-length seamless or welded pipes, this product is formed into a continuous helical or spiral configuration, making it uniquely suited for applications requiring high surface-area-to-volume ratios, compact heat exchange geometries, or space-constrained piping systems. The coil tube is produced from austenitic stainless steel seamless tubes through a precision cold-forming process that preserves the metallurgical integrity of the base material while achieving tight dimensional tolerances in the final coiled geometry.
This product represents the culmination of advanced forming technology research, particularly in the domain of spiral coil tube forming process optimization, addressing critical challenges such as strain hardening, springback control, and geometric accuracy during the coiling operation. The manufacturing methodology employed by Wenzhou Jixing Metal Materials Co., Ltd. integrates multi-stage drawing, controlled bending, and post-forming stabilization to deliver coil tubes with exceptional dimensional consistency and mechanical performance.
2. Material & Properties
2.1 Base Material Grades
The spiral coil tube is manufactured from high-quality austenitic stainless steel seamless tubes conforming to international material standards. The primary grades available include:
- 304 (UNS S30400) — General-purpose corrosion resistance; carbon equivalent ≤ 0.08%
- 304L (UNS S30403) — Low-carbon variant for welding applications; carbon ≤ 0.03%
- 316 (UNS S31600) — Enhanced chloride resistance via 2–3% molybdenum addition
- 316L (UNS S31603) — Low-carbon 316 for critical welding and corrosive environments
- 321 (UNS S32100) — Titanium-stabilized for high-temperature service
- 310S (UNS S31008) — High-temperature oxidation resistance up to 1150°C
2.2 Mechanical Properties (Post-Forming, Annealed Condition)
| Property | Grade 304/304L | Grade 316/316L | Grade 321 | Test Standard |
|---|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 515 MPa | ≥ 515 MPa | ASTM A269 / EN 10217-7 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ≥ 205 MPa | ASTM A269 / EN 10217-7 |
| Elongation (A50) | ≥ 40% | ≥ 40% | ≥ 40% | ASTM A269 |
| Hardness (HV) | ≤ 210 | ≤ 210 | ≤ 210 | ASTM A956 |
| Specific Gravity | 8.0 g/cm³ | 8.0 g/cm³ | 8.0 g/cm³ | — |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | 16.3 W/(m·K) | 14.2 W/(m·K) | ASTM A967 |
2.3 Corrosion Resistance
The austenitic microstructure (face-centered cubic, FCC) provides inherent resistance to general corrosion, pitting, and crevice corrosion. The addition of chromium (≥ 18%) and nickel (≥ 8%) forms a stable, self-healing chromium oxide passivation layer. Molybdenum-containing grades (316/316L) exhibit superior resistance to chlorides and acidic environments, with a Pitting Resistance Equivalent Number (PREN) exceeding 24.
3. Specifications
| Parameter | Specification Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 – 50.8 mm (¼" – 2") | ASTM A269 / EN 10217-7 |
| Wall Thickness | 0.5 – 3.0 mm | ASTM A269 |
| OD Tolerance | ±0.10 mm (±0.004") | EN 10217-7 |
| Wall Thickness Tolerance | ±10% of nominal | ASTM A269 |
| Spiral Pitch (Helix Pitch) | 50 – 500 mm (customizable) | Custom / Customer Specification |
| Coil Outer Diameter | 200 – 1500 mm | Custom / Customer Specification |
| Coil Length | 10 – 100 m (continuous) | Custom |
| Surface Finish | No. 1 (Pickled & Annealed), 2B, BA (Bright Annealed) | ASTM A480 / ASTM A240 |
| Surface Roughness (Ra) | ≤ 0.5 μm (BA finish); ≤ 1.6 μm (2B finish) | ASTM A480 |
| End Condition | Plain cut, beveled, or capped | Custom |
| Delivery Condition | Annealed (Solution Treated) | ASTM A269 |
| Pressure Rating | Up to 4.0 MPa (design-dependent) | ASME B31.3 / EN 13480 |
| Temperature Range | −196°C to +870°C (grade-dependent) | — |
| Packaging | Wooden crate / export-grade container packing | ISO 4126 |
4. Manufacturing Process
The production of spiral coil tubes involves a multi-stage process that integrates seamless tube manufacturing with precision cold-forming technology. The following outlines the complete process flow:
4.1 Raw Material Preparation
- Billet Selection: High-purity stainless steel billets are sourced from certified steel mills, with chemical composition verified through spectroscopic analysis (OES) and confirmed to meet ASTM A269 / EN 10217-7 requirements.
- Heat Treatment: Billets undergo solution annealing at 1050–1150°C followed by controlled air cooling to establish a uniform austenitic microstructure with no sensitization or carbide precipitation.
4.2 Seamless Tube Production
- Piercing & Heading: Billets are pierced at elevated temperatures (1100–1200°C) and headed to produce hollow blanks.
- Elongation: The hollow blanks undergo multi-pass rolling through elongation mills to achieve the desired wall thickness and outer diameter.
- Pilger Rolling / Mannesmann Process: For precision tubes, pilger rolling or Mannesmann piercing is employed to achieve tight dimensional tolerances and uniform wall thickness.
- Heat Treatment (Post-Forming): Tubes are solution-annealed at 1040–1120°C with rapid quenching (water or air) to relieve cold-work stresses and restore full ductility.
- Pickling & Descaling: Surface scale is removed via hydrofluoric-nitric acid pickling or mechanical grinding to achieve the specified surface finish.
4.3 Spiral Coiling / Cold Forming Process
This is the critical differentiating process that transforms straight seamless tubes into spiral coil configurations. Based on the research findings from 《螺旋盘管成型工艺的研究》 (Research on Spiral Coil Tube Forming Process), the following key parameters and techniques are applied:
- Pre-Forming Annealing: Tubes are solution-annealed prior to coiling to maximize formability and minimize the risk of cracking during plastic deformation. The annealing cycle ensures a fully recrystallized microstructure with minimum dislocation density.
- Multi-Axis Coiling Machine Configuration: A specialized coiling machine equipped with programmable guide rollers and tension control systems is used. The machine employs a combination of:
- Bending rollers arranged in a triangular or star configuration to progressively bend the tube
- Tension control rollers to manage axial pulling force and prevent wrinkling or ovalization
- Pitch adjustment mechanism to precisely control the helical pitch during formation
- Strain Management & Springback Compensation: The forming process accounts for the material's elastic recovery (springback) through:
- Precise calculation of the required over-bending angle based on the material's elastic modulus and yield strength
- Iterative process parameter optimization using finite element analysis (FEA) to predict post-forming geometry
- In-process monitoring of tube cross-sectional ovality to ensure it remains within acceptable limits (typically ≤ 3% of nominal OD)
- Strain Hardening Control: During coiling, the outer fibers of the bend experience tensile strain while inner fibers experience compressive strain. The forming process is optimized to:
- Minimize excessive strain hardening that could reduce ductility
- Maintain uniform strain distribution across the tube cross-section
- Avoid formation of Lüders bands or localized deformation
- Post-Coiling Stabilization: After coiling, the spiral tube may undergo:
- Low-temperature stress relief annealing (400–500°C) to reduce residual stresses without grain growth
- Dimensional inspection and adjustment of pitch uniformity
4.4 Surface Treatment & Finishing
- Pickling & Passivation: The coiled tube is subjected to chemical pickling (HNO₃/HF solution) to remove any surface contamination, followed by passivation treatment to re-establish the protective chromium oxide layer.
- Bright Annealing (Optional): For applications requiring superior surface finish, the coil tube is annealed in a protective atmosphere (hydrogen or vacuum) to achieve a mirror-like surface (Ra ≤ 0.25 μm).
- End Treatment: Tube ends are cut, beveled, or capped according to customer specifications.
4.5 Quality Inspection
- Chemical Analysis: Spectrometric verification of chemical composition per ASTM A269 / EN 10217-7.
- Mechanical Testing: Tensile testing, hardness testing, and elongation verification per ASTM A956.
- Dimensional Inspection: Laser scanning or coordinate measuring machine (CMM) verification of OD, wall thickness, coil diameter, and pitch uniformity.
- Non-Destructive Testing (NDT):
- Eddy Current Testing (ECT): Detection of surface and near-surface defects
- Hydrostatic Pressure Testing: Verification of weld integrity and pressure resistance (typically 1.5× working pressure)
- Visual Inspection: Surface quality and geometric accuracy assessment
- Intergranular Corrosion Testing: ASTM A262 Practice E (for low-carbon grades) to verify resistance to sensitization.
5. Applications
The spiral coil tube finds extensive application across multiple industries due to its unique geometric advantages, corrosion resistance, and mechanical performance:
5.1 Heat Exchange Systems
- Air-Conditioning & Refrigeration: Spiral coil tubes are extensively used in evaporators, condensers, and air-cooled heat exchangers. The helical geometry maximizes heat transfer surface area within a compact footprint, improving thermal efficiency by 30–50% compared to straight-tube configurations.
- Industrial Process Cooling: Application in chemical processing, pharmaceutical manufacturing, and food processing heat exchangers where corrosion resistance is critical.
- Solar Thermal Systems: Spiral coil collectors for solar water heating systems, leveraging the tube's ability to absorb and transfer thermal energy efficiently.