Stainless Steel Coil Tube (Coiltube) — Technical Specification

1. Product Overview

The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 352), is a precision-engineered continuous coil form of stainless steel tubing designed for applications requiring high flexibility, corrosion resistance, and structural integrity. Unlike traditional straight-length stainless steel pipes, coil tubes are produced in continuous coiled configurations, enabling uninterrupted lengths suitable for large-scale deployment in heat exchange systems, fluid transport networks, hydraulic circuits, and industrial process engineering.

This product category encompasses both austenitic and duplex stainless steel grades, offering a versatile balance of mechanical properties, chemical resistance, and formability. The coil tube format eliminates the need for field welding at intermediate joints, thereby reducing potential failure points and improving overall system reliability. The manufacturing and testing protocols associated with large-scale coil tube installations have been extensively studied, particularly regarding their interaction with surrounding structural materials such as reinforced concrete, as documented in research on the workability changes of concrete before and after large coil tube testing.

2. Material & Properties

2.1 Common Grades

2.2 Mechanical Properties (Typical — Grade 316L Annealed Condition)

Property Specification Standard Reference
Tensile Strength (Rm) ≥ 485 MPa (70 ksi) ASTM A269 / EN 10217-7
Yield Strength (Rp0.2) ≥ 170 MPa (25 ksi) ASTM A269 / EN 10217-7
Elongation (A50mm) ≥ 40% ASTM A269
Hardness (HBW) ≤ 210 HBW ASTM A269
Density 7.98 g/cm³ —
Thermal Conductivity (20°C) 14.9 W/(m·K) —
Specific Heat (20°C) 500 J/(kg·K) —
Thermal Expansion (20–100°C) 17.5 × 10⁻⁶ /K —

2.3 Corrosion Resistance

2.4 Surface Finish Options

3. Specifications

Parameter Range / Value
Outer Diameter (OD) 6.0 mm – 114.3 mm (¼" – 4½")
Wall Thickness 0.5 mm – 4.0 mm (0.020" – 0.157")
Coil Length (Continuous) 10 m – 500 m (typical), up to 1,000 m on special request
Coil Inner Diameter (ID) 300 mm – 600 mm (12" – 24")
Minimum Bend Radius 3× OD (cold bending), 2× OD (hot forming)
Acceptable Standards ASTM A269, ASTM A213, EN 10217-7, GB/T 12771, JIS G3459
Tolerance — OD ±0.05 mm (precision); ±0.15 mm (standard)
Tolerance — Wall Thickness ±10% (standard); ±5% (precision)
Strut Length (for heat exchangers) Custom, typical 200 mm – 6,000 mm
Delivery Condition Annealed (A), Solution Heat Treated (S), Cold Worked (CW)

4. Manufacturing Process

4.1 Process Flow

  1. Raw Material Preparation: Hot-rolled stainless steel coil stock is inspected for chemical composition (via Optical Emission Spectrometry — OES) and mechanical properties. Material certification conforms to EN 10204 Type 3.1.
  2. Cold Drawing / Piercing: The coil stock is drawn through calibrated dies to achieve the target outer diameter and wall thickness. Multi-pass drawing with intermediate annealing ensures dimensional accuracy and uniform microstructure.
  3. Intermediate Annealing: When significant cold work exceeds the strain-hardening threshold, the tube is annealed at 1,050–1,150°C (austenitic grades) in a protective atmosphere (N₂ or H₂) to restore ductility and dissolve carbide precipitates.
  4. Precision Sizing & Cutting: The tube passes through sizing rolls for final dimensional control, followed by precision cutting to the specified coil length.
  5. Surface Treatment: Pickling and passivation using nitric-fluoric acid solutions removes surface oxides and establishes a chromium oxide passive film (Cr₂O₃) for enhanced corrosion resistance.
  6. Coiling & Packaging: The continuous tube is wound onto a precision coiling reel with controlled tension to prevent deformation. Coils are protected with anti-corrosion packaging (VCI paper, shrink wrap, or wooden crates).
  7. Non-Destructive Testing (NDT): 100% eddy current testing (ECT) and/or ultrasonic testing (UT) is performed to detect internal and external defects. Hydrostatic pressure testing is conducted at 1.5× the maximum design pressure.

4.2 Microstructural Considerations

The annealing process is critical to the final microstructural integrity of the coil tube. For austenitic grades, the target microstructure is a fully recrystallized equiaxed austenite grain structure with grain size typically between ASTM 3 and ASTM 8. Over-annealing can result in excessive grain growth, reducing mechanical strength, while under-annealing leaves residual cold-work strains that compromise ductility and fatigue resistance. The carbon content in "L" (low-carbon) grades is restricted to ≤ 0.03% to prevent intergranular carbide precipitation (chromium carbide — Cr₂₃C₆) at sensitization temperatures (450–850°C), which would otherwise degrade weldability and corrosion resistance.

4.3 Formability and Coil Configuration

The coil tube format demands exceptional material formability. During the coiling process, the tube undergoes repeated bending and straightening, which imposes cyclic plastic deformation. The ratio of coil inner diameter to tube outer diameter (D/d ratio) must be carefully controlled to prevent ovalization, residual stress accumulation, or cracking at the inner fiber of the bend. Industry practice maintains a D/d ratio of ≥ 30 for standard applications and ≥ 50 for thin-walled precision tubes.

5. Applications

5.1 Heat Exchanger & Coil Heat Transfer Systems

Stainless steel coil tubes are extensively used in shell-and-coil heat exchangers, air-cooled condensers, and spiral heat exchangers. The continuous coil geometry maximizes heat transfer surface area per unit volume. Research on large-scale coil tube installations has demonstrated their effectiveness in concrete-encased heat transfer systems, where thermal expansion compatibility between the coil tube and surrounding concrete matrix is a critical design parameter. Studies have shown that proper coil tube installation maintains concrete workability and structural integrity, with minimal impact on fresh concrete slump and setting time when appropriate installation protocols are followed.

5.2 Hydraulic & Pneumatic Systems

Continuous-length coil tubing is ideal for hydraulic circuitry in mobile equipment, aircraft landing gear systems, and industrial automation. The absence of intermediate joints eliminates potential leak paths and reduces maintenance requirements.

5.3 Food & Beverage Processing

Electropolished 316L coil tubing meets FDA and 3-A sanitary standards for dairy processing, brewery piping, and pharmaceutical fluid transfer systems. The smooth internal surface (Ra ≤ 0.4 μm) prevents microbial adhesion and facilitates Clean-In-Place (CIP) procedures.

5.4 Marine & Offshore Engineering

Duplex and super-austenitic coil tubes (2205, 2507, 904L) are deployed in seawater cooling systems, ballast water piping, and offshore platform fluid transport where chloride-induced corrosion and stress corrosion cracking are primary concerns.

5.5 Petrochemical & Process Industry

Coil tubes in Alloy 20 and Hastelloy C-276 compositions serve in aggressive chemical environments, including sulfuric acid processing, phosphoric acid production, and flue gas desulfurization (FGD) systems.

5.6 Concrete-Encased Heating & Cooling Systems

As highlighted in the referenced research literature, stainless steel coil tubes are embedded within concrete slabs and structural elements for radiant heating and cooling applications. The thermal compatibility and long-term durability of the coil tube within the alkaline concrete environment (pH 12–13) require careful material selection and corrosion protection strategies, including cathodic protection or the use of passivated surfaces.

6. Quality Certifications & Compliance

6.1 Applicable International Standards

Standard Scope
ASTM A269 Welded Austenitic Stainless Steel Tubing for General, Chemical, and Heat-Transfer Service
ASTM A213 Welded Austenitic Chromium-Chromium-Nickel Alloy Boilers, Heat-Exchanger Tubes, and Condenser Tubes
EN 10217-7 Non-Heat-Resistant Seamless Tubes of Stainless Steels — Technical Delivery Conditions
GB/T 12771 Chinese National Standard for Stainless Steel Seamless Tubes for Fluid Transport
JIS G3459 Japanese Industrial Standard for Stainless Steel Tubes
ISO 11733 Hygienic Stainless Steel Tubes for Food, Dairy, and Pharmaceutical Industry
ASME B31.3 Process Piping Code — Material Selection and Design Requirements

6.2 Factory Certifications

6.3 Testing & Inspection Protocol

  1. Chemical Analysis: OES spectroscopy on each heat lot, with supplementary wet chemistry verification for carbon, sulfur, and nitrogen
  2. Mechanical Testing: Tensile testing (