Stainless Steel Coil Tube (Coiltube) — Tubular Furnace Convective Section Grade
Product Code: No. 327 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The stainless steel Coiltube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered seamless tubular product specifically designed for the convective section of tubular heating furnaces (管式加热炉对流段). These coil tubes are supplied in continuous coil form, enabling efficient spool fabrication, reduced welding requirements, and enhanced thermal efficiency in industrial process heating systems.
Based on extensive technical research and field application studies — particularly the study titled "Discussion on Improving the Utilization Rate of Tubular Materials in Convective Coil Tubes of Tubular Furnaces" — this product line addresses the critical industry challenge of maximizing material utilization while maintaining structural integrity under cyclic thermal loading conditions. The coil tube design allows for optimal heat transfer surface configuration within compact furnace geometries, achieving superior thermal performance compared to conventional straight-tube arrangements.
The product is engineered to meet the rigorous demands of continuous operation in high-temperature environments, where convective heat exchange occurs between process gases and the working fluid within the tube. Key design considerations include resistance to thermal fatigue, oxidation at elevated temperatures, and dimensional stability during repeated heating and cooling cycles.
2. Material & Properties
2.1 Material Grades
Wenzhou Jixing offers multiple stainless steel grades tailored to specific furnace operating conditions:
- 304 (UNS S30400): General-purpose austenitic stainless steel with excellent corrosion resistance; suitable for moderate-temperature convective sections up to 870°C (1600°F).
- 321 (UNS S32100): Titanium-stabilized austenitic grade offering superior resistance to intergranular corrosion and thermal fatigue; recommended for temperatures up to 925°C (1700°F).
- 310S (UNS S31008): High-temperature austenitic grade with exceptional oxidation resistance; suitable for extreme service conditions up to 1150°C (2100°F).
- 347H (UNS S34708): High-carbon, niobium-stabilized grade providing enhanced creep strength at elevated temperatures.
- Custom Alloy Grades: Available upon request for specialized applications requiring tailored mechanical properties.
2.2 Mechanical Properties (Representative Values for Grade 321)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) | ASTM A213 / A269 |
| Yield Strength (0.2% offset) | ≥ 205 MPa (29,700 psi) | ASTM A213 / A269 |
| Elongation (in 50 mm / 2 in.) | ≥ 35% | ASTM A213 / A269 |
| Thermal Conductivity (at 100°C) | ~ 16.3 W/(m·K) | ASTM E153 |
| Thermal Conductivity (at 600°C) | ~ 26.7 W/(m·K) | ASTM E153 |
| Coefficient of Thermal Expansion (20–800°C) | 17.5 × 10⁻⁶ /°C | ASTM E228 |
| Maximum Continuous Service Temperature | 925°C (1700°F) | Manufacturer Data |
2.3 Chemical Composition (Grade 321 — Typical)
| Element | C | Mn | P | S | Si | Cr | Ni | Ti | N |
|---|---|---|---|---|---|---|---|---|---|
| Max/Min % | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 18.0–20.0 | 9.0–12.0 | 5×C–0.70 | 0.10 |
2.4 Key Material Advantages
- Thermal Fatigue Resistance: Titanium stabilization (Grade 321/347H) prevents chromium carbide precipitation at grain boundaries, maintaining ductility during cyclic thermal loading — a critical factor in convective coil tube service.
- Oxidation Resistance: Chromium-rich oxide scale formation provides long-term protection against scaling at temperatures up to 925°C, reducing maintenance intervals and extending service life.
- Cold Workability: Austenitic microstructure permits coil forming without intermediate annealing, enabling tight bending radii essential for compact convective section designs.
- Creep Strength: Optimized grain structure and alloy composition provide resistance to time-dependent deformation under sustained high-temperature loading.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Product Form | Continuous Coil (盘管) | — |
| Outer Diameter (OD) | 12.7 mm (1/2") – 101.6 mm (4") | ASTM A213 / A269 |
| Wall Thickness | 0.89 mm (0.035") – 6.35 mm (0.250") | ASTM A213 / A269 |
| OD Tolerance | ±0.05 mm (±0.002") | ASTM A213 |
| Wall Thickness Tolerance | ±10% (standard); ±5% (precision) | ASTM A213 |
| Minimum Bend Radius | 3× OD (cold bent); 2× OD (hot bent) | Manufacturer Specification |
| Coil Length | 15 m – 200 m (customizable) | — |
| Surface Finish | Pickled & Passivated (P&P); Mirror Polish (Ra ≤ 0.4 μm) | ASTM A479 |
| End Condition | Plain; Beveled (per ASME B31.3); Flanged | ASME B31.3 |
| Heat Treatment | Solution Annealed (1040–1120°C, water/air quench) | ASTM A213 |
| Delivery Condition | Coiled; Spooled; Bundled | — |
| Applicable Standards | ASTM A213, A269, ASME SA-213, EN 10216-5, GB/T 12771 | Multi-standard compliance |
3.1 Dimensional Configuration Options
| OD (mm) | Wall Thickness (mm) | Coil Length (m) | Typical Application |
|---|---|---|---|
| 19.05 | 1.65 | 30–80 | Convective section, low-pressure |
| 25.4 | 2.11 | 25–100 | Convective section, medium pressure |
| 33.4 | 3.05 | 20–60 | High-pressure convective service |
| 50.8 | 4.57 | 15–50 | Large-bore convective coil |
| 76.2 | 5.56 | 15–40 | Heavy-duty furnace application |
4. Manufacturing Process
The production of Wenzhou Jixing stainless steel coil tubes follows a rigorous multi-stage manufacturing process designed to ensure dimensional accuracy, metallurgical quality, and consistent mechanical properties:
- Raw Material Selection & Ingot Production: High-purity stainless steel ingots are produced via Electric Arc Furnace (EAF) melting with Argon Oxygen Decarburization (AOD) or Vacuum Induction Melting (VIM) processes. Chemical composition is verified by Optical Emission Spectroscopy (OES) and Lateral Mass Spectrometry (LMS) to ensure compliance with specified grade requirements.
- Hot Rolling & Billet Preparation: Ingots are hot rolled into round billets of the required diameter. Billets undergo ultrasonic testing (UT) to detect internal defects such as inclusions, voids, and cracks.
- Seamless Tube Piercing & Elongation: Billets are heated to 1150–1200°C and pierced using a Mannesmann-type piercing mill to produce seamless tube blanks. The elongation process achieves the desired length-to-diameter ratio for subsequent reduction.
- Multi-stand Reduction & Sizing: The tube blanks pass through a series of reduction stands (plug mill or rotary piercing) to achieve the target outer diameter and wall thickness. Precision sizing is achieved through a final sizing mill with tight tolerance control.
- Solution Annealing: Tubes are annealed at 1040–1120°C in a protective atmosphere (endothermic gas or vacuum) to dissolve carbides, restore full solution-hardened microstructure, and eliminate residual stresses from cold working. Rapid quenching (water or controlled air) follows to maintain the austenitic single-phase structure.
- Pickling & Passivation: Annealed tubes undergo pickling in mixed acid (HNO₃ + HF) to remove oxide scale, followed by passivation with dilute nitric acid to establish a protective chromium oxide film. This ensures corrosion resistance and a clean surface finish.
- Coil Forming & Spooling: The finished tubes are fed into a precision coil forming system that winds the tube into the desired coil configuration. Coil diameter, pitch, and overlap are controlled to ensure uniform geometry suitable for furnace installation.
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Non-Destructive Testing (NDT): Every production lot undergoes comprehensive NDT including:
- Electromagnetic (Eddy Current) Testing — 100% inspection for surface and near-surface defects
- Hydrostatic Testing — at 1.5× design pressure per ASTM A213
- Ultrasonic Testing — for wall thickness verification and internal defect detection
- Visual Inspection — surface quality and dimensional verification
- Final Inspection & Packaging: Tubes are inspected for dimensional accuracy, surface finish, and marking compliance. Protective end caps are installed, and coils are wrapped in moisture-proof packaging with desiccant for export.
5. Applications
Wenzhou Jixing stainless steel coil tubes are engineered for demanding industrial applications where thermal efficiency, material utilization, and long service life are paramount:
5.1 Primary Applications
- Tubular Heating Furnace Convective Sections (管式加热炉对流段): Primary application — coil tubes serve as heat exchange elements where flue gas convective heat transfer preheats process fluids. The coil configuration maximizes heat transfer area within constrained furnace volumes.
- Petrochemical Process Heating: Used in reforming furnaces, cracker furnaces, and steam superheaters where cyclic thermal loading demands superior fatigue resistance.
- Power Generation: Applied in superheater and reheater sections of waste heat recovery boilers (WHRB) and organic Rankine cycle (ORC) systems.
- Chemical Processing: Employed in acid concentrators, chlorinator furnaces, and specialty chemical reactors requiring corrosion-resistant heat transfer surfaces.
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