Stainless Steel Coil Tube — Localized Solution Heat Treatment After Bending Forming
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. (Wenzhou Jixing Metal Materials Co., Ltd.)
Product Code: No. 379 — Coil Tube (Coiltube)
Classification: Literature Study & Technical Reference
1. Product Overview
Stainless steel coil tubes are continuous-length hollow cylindrical products supplied in coiled form, designed for downstream bending, forming, and fabrication processes. Unlike seamless or welded straight pipes, coil tubes are delivered in a wound configuration that facilitates automated forming operations, minimizes material waste, and enables high-throughput production of custom geometries such as elbows, headers, and complex tubing assemblies.
The defining technical challenge associated with stainless steel coil tube fabrication lies in the metallurgical consequences of cold bending. During the bending process, plastic deformation is imposed on the tube wall, particularly on the outer radius of the bend, introducing residual stresses, localized work hardening, and potential distortion of the austenitic microstructure. If left unaddressed, these conditions can compromise corrosion resistance, reduce ductility, and introduce susceptibility to stress corrosion cracking (SCC). Therefore, localized solution heat treatment (Localized Solution Annealing) applied selectively to the bent regions constitutes a critical post-forming operation to restore the original metallurgical properties of the stainless steel matrix.
2. Material & Properties
Coil tubes are typically manufactured from austenitic stainless steel grades selected for their superior corrosion resistance, formability, and weldability. The most commonly specified grades include:
- AISI 304 / 1.4301 (UNS S30400): The baseline austenitic grade with 18% Cr and 8% Ni, offering excellent general corrosion resistance and cold workability.
- AISI 316L / 1.4404 (UNS S31603): Low-carbon variant with 2–3% Mo addition, providing enhanced resistance to pitting and crevice corrosion in chloride-containing environments.
- AISI 321 / 1.4541 (UNS S32100): Titanium-stabilized grade resistant to intergranular corrosion, suitable for high-temperature service up to 870°C.
- AISI 310S / 1.4845 (UNS S31008): High-temperature grade with elevated Cr and Ni content for oxidation resistance in extreme thermal environments.
Key Metallurgical Properties (Post Heat Treatment)
- Microstructure: Fully austenitic (γ-phase) with minimal or no delta-ferrite, confirmed by metallographic examination per ASTM E407.
- Grain Size: Controlled to ASTM E112 grain size range 4–8 to balance ductility and corrosion resistance.
- Pitting Resistance (PREN): ≥ 19 for 304L; ≥ 24 for 316L, calculated per ASTM G48.
- Hardness: ≤ 223 HB (annealed condition); localized bending zones may exhibit 250–350 HB without heat treatment.
- Residual Stress: Reduced to near-zero levels in treated zones, verified by X-ray diffraction (XRD) or hole-drilling method per ASTM E653.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Form | Coil Tube (Coiltube) — Continuous Length | — |
| Outer Diameter (OD) | 6.0 mm – 168.3 mm (1/4" – 6") | ASTM A213 / A269 |
| Wall Thickness | 0.5 mm – 6.0 mm (0.020" – 0.236") | ASTM A213 / A269 |
| Coil Length | Up to 300 m (continuous) | Manufacturer Specification |
| Coil Weight | Up to 3,000 kg per coil | — |
| Surface Finish | No. 1 Annealed / 2B / BA / No. 4 / Mirror (6K–12K) | ASTM A480 |
| End Condition | Plain / Beveled / Threaded / Flanged | — |
| Tolerance (OD) | ±0.05 mm – ±0.5 mm (depending on diameter) | ASTM A463 |
| Tolerance (Wall) | ±10% of nominal | ASTM A463 |
| Heat Treatment — Solution Annealing | 1010°C – 1150°C, followed by rapid air or water quench | ASTM A213 / A269 |
| Localized Post-Bend Heat Treatment | 1050°C ± 20°C, 3–10 min hold, controlled quench | Wenzhou Jixing Internal Standard |
| Maximum Bending Radius | ≥ 1.5D (cold bend) / ≥ 1.0D (hot bend with pre-heat) | Manufacturer Specification |
| Minimum Bend Count (Continuous) | ≥ 500 bends without wall cracking | Internal Testing Protocol |
4. Manufacturing Process
4.1 Coil Tube Production Sequence
- Hot Rolling & Annealing: Stainless steel strip is hot rolled to near-final thickness, followed by full annealing in a continuous annealing line (CAL) under a protective atmosphere (dew point ≤ -40°C) to achieve a fully recrystallized austenitic structure.
- Pickle & Passivate: The annealed strip is pickled to remove scale and surface oxides, followed by passivation to establish a stable chromium oxide passive film.
- Forming / Welding: The flat strip is formed into a tube shape via cold forming (for smaller diameters) or resistance welding (for larger diameters, per ASTM A999).
- Coiling: The formed tube is wound onto a pay-off reel to create the coil configuration, with controlled tension to prevent deformation during storage and transport.
4.2 Bending Forming Process
- Uncoiling & Feeding: The coil tube is fed from the pay-off reel through a series of guide rollers into the bending station.
- Hydraulic or Mechanical Bending: The tube is bent to the desired geometry using hydraulic press brakes, rotary draw bending machines, or mandrel-assisted bending equipment. Bending parameters (bend radius, bend rate, mandrel position) are controlled to minimize wall thinning and ovality.
- Post-Bend Inspection: Visual and dimensional inspection of bent sections for cracks, excessive wall thinning (typically limited to ≤ 15% reduction), and ovality (≤ 5% of OD).
4.3 Localized Solution Heat Treatment (Core Technology)
This is the distinctive and proprietary process that differentiates Wenzhou Jixing's coil tube offerings from conventional bent tube products. The process involves the following critical steps:
- Identification of Affected Zones: The metallurgically altered regions around each bend (typically extending 2–3D from the bend centerline) are identified as the target zones for localized heat treatment.
- Induction Heating or Gas Torch Annealing: A focused energy source — high-frequency induction coil or controlled oxy-fuel torch — is applied to selectively heat the bent zone to the solution annealing temperature range of 1010°C – 1150°C without affecting the adjacent untreated tube sections.
- Dwell Time Control: The temperature is held for 3–10 minutes (depending on wall thickness and alloy grade) to ensure complete dissolution of precipitated carbides (particularly chromium carbides, Cr₂₃C₆) and recovery of the austenitic grain structure.
- Controlled Quenching: Rapid cooling is achieved via forced air cooling, water spray, or controlled convection to suppress carbide re-precipitation and grain coarsening. Quench rate must exceed the critical cooling rate for the specific alloy grade.
- Passivation Restoration: Following heat treatment, the surface is re-passivated (typically via citric acid or nitric acid dip) to re-establish the protective chromium oxide film that was disrupted during the thermal cycle.
4.4 Process Control Parameters
| Control Variable | Target Range | Monitoring Method |
|---|---|---|
| Peak Temperature | 1050°C ± 20°C | Pyrometer / Thermocouple (Type K or Type R) |
| Dwell Time | 3–10 min (wall-thickness dependent) | Timer / Automated PLC Control |
| Quench Rate | ≥ 100°C/min (air); ≥ 500°C/min (water) | Thermal Imaging / IR Camera |
| Maximum Temperature Rise in Untreated Zone | ≤ 200°C above ambient | Infrared Thermography |
| Surface Oxidation (Scale) | ≤ 0.1 mm scale thickness | Visual / Caliper Measurement |
5. Applications
The coil tube product with localized post-bend heat treatment is engineered for demanding applications where both geometric complexity and metallurgical integrity are paramount:
- Chemical & Petrochemical Processing: Heat exchanger tubes, reactor internals, and process piping systems requiring resistance to aggressive chemical media. The restored corrosion resistance after bending is critical for long-term service life in chloride-rich and acidic environments.
- Food & Pharmaceutical Industry: Sanitary piping, CIP (Clean-in-Place) systems, and process vessels where surface integrity and freedom from carbide precipitation (to prevent pitting) are mandatory per FDA and GMP standards.
- Oil & Gas / Offshore Platforms: Subsea umbilicals, wellhead tubing, and platform structural tubing requiring resistance to sulfide stress cracking (SSC) and chloride-induced SCC.
- Aerospace & Defense: Fuel lines, hydraulic tubing, and exhaust components where weight optimization, fatigue resistance, and corrosion immunity are non-negotiable.
- Power Generation: Boiler tubes, superheater elements, and turbine exhaust systems operating at elevated temperatures where thermal stability and oxidation resistance are required.
- Automotive & Transportation: Exhaust systems, turbocharger manifolds, and fuel delivery lines requiring complex bending geometries with maintained material performance.
- Medical Devices: Endoscope tubing, implantable stent precursors, and surgical instrument components where biocompatibility and surface finish are critical.
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management framework ensuring conformance to international standards:
- ISO 9001:2015 — Quality Management System certification covering all manufacturing and heat treatment processes.
- ISO 14001:2015 — Environmental Management System, particularly relevant to emissions control during induction heating and passivation operations.
- EN 10204 3.1 — Manufacturer's Test Report (MTR) documentation for each production batch, including chemical composition, mechanical properties, and heat treatment records.
- ASTM A213 / A269 / A999 — Compliance with American Society for Testing and Materials specifications for stainless steel tubing.
- ASME B31.3 / B31.1 — Conformance for pressure piping applications in process and power industries.
- NACE MR0175 / ISO 15156 — Qualification for sour service environments (H₂S-containing) where applicable.
- EN 10217-7 — European standard for heat-treated steel tubes and pipes.
- Third-Party Inspection (TPI): Available upon request through SGS, BV (Bureau Veritas), or DNV for independent verification of material and process compliance.
7. Unique Selling Points
"Bend Without Compromise — Restore Without Damage"
- Proprietary Localized Heat Treatment Technology: Unlike conventional approaches that require full-length re-annealing (which is impractical for bent coils and introduces distortion in previously formed sections), Wenzhou Jixing's localized induction heating process targets only the metallurgically affected zones, preserving the integrity of unaffected tube sections while fully restoring corrosion resistance and ductility in bent regions.
- Reduced Material Waste: The coil configuration combined with automated bending and localized heat treatment minimizes offcuts and scrap, offering a cost-effective solution for high-volume production of complex tube geometries.
- Enhanced Service Life: By eliminating chromium carbide precipitation and restoring the passive film in bent zones, the product achieves corrosion performance equivalent to the original annealed condition, extending service life by an estimated 30–50% compared to untreated bent tubing.
- Process Flexibility: The technology accommodates a wide range of tube diameters (6 mm – 168 mm