Stainless Steel Process Air Coil Tube for Industrial Conversion Furnaces
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a high-performance tubular product engineered specifically for demanding industrial thermal applications, particularly within First-Stage Conversion Furnaces (一段转化炉) used in ammonia synthesis, methanol production, and hydrogen generation processes. These coil tubes serve as critical heat-transfer elements that circulate process air through the furnace's radiant zone, absorbing thermal energy from the combustion of fuel gas and natural gas mixtures.
Unlike straight-segment tubes, coil tubes are formed into continuous helical or serpentine configurations that maximize surface-area-to-volume ratios, thereby enhancing convective and radiative heat transfer efficiency. The product line is designed to withstand prolonged exposure to extreme thermal cycling, oxidizing atmospheres, and mechanical stresses inherent in reforming furnace environments operating at temperatures ranging from 800°C to 1,200°C under pressures up to 3.5 MPa.
Building upon extensive field experience—including systematic root-cause analyses of coil tube leakage incidents in first-stage conversion furnaces—this product incorporates advanced metallurgical design and manufacturing protocols to mitigate failure modes such as hot corrosion, creep rupture, thermal fatigue cracking, and oxidative thinning.
Material & Properties
The coil tubes are fabricated from premium-grade austenitic and ferritic-martensitic stainless steels selected for their exceptional high-temperature strength, oxidation resistance, and thermal stability. The following material grades are available:
- TP310H (UNS S31008) — High-temperature austenitic grade with chromium content of 24–27% and nickel content of 19–22%, offering superior resistance to thermal fatigue and creep at temperatures exceeding 1,100°C. Suitable for radiant coil applications in the hottest zones of conversion furnaces.
- TP310S (UNS S31008) — Low-carbon variant of TP310H with carbon content ≤0.08%, providing enhanced resistance to intergranular corrosion during welding and high-temperature service.
- TP309H (UNS S30908) — Chromium-nickel austenitic grade (23–25% Cr, 12–15% Ni) offering good resistance to sulfur attack and thermal shock, suitable for intermediate-temperature coil sections.
- TP347H (UNS S34708) — Niobium-stabilized austenitic grade with improved resistance to intergranular corrosion and good mechanical properties at elevated temperatures.
- TP446 (UNS S44600) — Ferritic stainless steel (17–19% Cr, ≤1.0% C) offering excellent resistance to oxidation and nitridation at temperatures up to 1,150°C, with lower thermal expansion coefficient compared to austenitic grades.
Key Mechanical and Physical Properties (at Operating Temperature)
| Property | TP310H | TP309H | TP347H | TP446 |
|---|---|---|---|---|
| Maximum Continuous Service Temperature (°C) | 1,200 | 1,100 | 1,050 | 1,150 |
| Yield Strength at 1,000°C (MPa) | ≥ 35 | ≥ 45 | ≥ 55 | ≥ 40 |
| Tensile Strength at 1,000°C (MPa) | ≥ 120 | ≥ 150 | ≥ 170 | ≥ 130 |
| Thermal Expansion Coefficient (10⁻⁶/K, 20–1000°C) | 18.5 | 18.0 | 17.5 | 11.0 |
| Oxidation Resistance Rating (°C) | Excellent | Very Good | Good | Excellent |
| Coefficient of Thermal Conductivity at 100°C (W/m·K) | 26.2 | 23.0 | 16.3 | 29.0 |
| Creep Life at 1,000°C / 100 MPa (hours) | ≥ 10,000 | ≥ 15,000 | ≥ 20,000 | ≥ 12,000 |
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 25 mm – 102 mm (standard); custom sizes available up to 152 mm | ASTM A312 / ASME SA-312 |
| Wall Thickness | 1.5 mm – 6.0 mm (standard); up to 10.0 mm on request | ASTM A312 / ASME SA-312 |
| OD Tolerance | ±0.5% (±0.02 mm minimum) | ASTM A312 |
| Wall Thickness Tolerance | ±10% (±0.10 mm minimum) | ASTM A312 |
| Coil Bend Radius (Inner) | ≥ 6 × OD (standard); ≥ 4 × OD for special applications | Manufacturer Specification |
| Coil Configuration | Helical, Serpentine, U-Turn, or Custom Geometry | Customer Drawing |
| End Preparation | Plain, Beveled (37.5°), Threaded (NPT/BSP), or Flanged | ASME B1.20.1 / ASME B16.5 |
| Surface Finish | 2B (Cold Rolled), BA (Bright Annealed), Pickled & Passivated | ASTM A478 |
| Maximum Operating Pressure | Up to 3.5 MPa (design dependent) | ASME Section VIII Div. 1 |
| Heat Treatment | Solution Annealed at 1050–1150°C, followed by controlled cooling | ASTM A312 |
| Delivery Condition | Coiled, Straight Length, or Pre-Formed Assemblies | — |
Manufacturing Process
The production of high-performance stainless steel coil tubes involves a meticulously controlled multi-stage manufacturing sequence designed to ensure metallurgical integrity, dimensional accuracy, and resistance to the failure mechanisms commonly observed in conversion furnace service.
- Raw Material Procurement & Inspection — Ingot or slab stock is sourced from certified mills with full chemical composition analysis (spectrographic and wet chemistry). Each heat lot is verified against ASTM/ASME compositional requirements for Cr, Ni, C, Nb, Mn, Si, S, P, and trace elements.
- Hot Rolling & Annealing — Billets are reheated to 1,150–1,250°C and hot-rolled into seamless tube blanks with controlled reduction ratios to refine grain structure. Intermediate annealing is performed to relieve residual stresses and restore ductility.
- Piercing & Cross-Rolling — The heated billet is pierced to form a hollow tube, followed by cross-rolling (Roulo) or plug-rolling to achieve the target OD and wall thickness with uniform cross-sectional geometry.
- Cold Drawing / Cold Expansion — Tubes undergo cold working (drawing through a die and mandrel, or internal expansion) to achieve precise dimensional tolerances, improve surface finish, and enhance mechanical properties through strain hardening.
- Solution Annealing — Tubes are solution-annealed at 1,050–1,150°C in a controlled atmosphere (vacuum or inert gas) to dissolve carbides, homogenize the microstructure, and maximize corrosion and oxidation resistance.
- Pickling & Passivation — Surface oxides and scale are removed via acid pickling (HNO₃/HF blend), followed by passivation to form a protective chromium oxide layer, ensuring a clean, corrosion-resistant surface.
- Coil Forming — Straight tubes are formed into helical or serpentine configurations using hydraulic or mechanical coiling equipment. The forming process is carefully controlled to prevent excessive plastic deformation, work hardening, or micro-cracking at the inner bend radius. Post-forming stress relief annealing may be applied.
- Non-Destructive Testing (NDT) — Comprehensive inspection includes:
- Eddy Current Testing (ECT) — Full-length inspection for wall thickness uniformity, surface and near-surface defects.
- Ultrasonic Testing (UT) — Detection of internal laminations, inclusions, and subsurface cracks.
- Visual Inspection (VT) — Examination of ends, surfaces, and bend regions for geometric conformity and surface defects.
- Leak Testing — Helium mass spectrometry or hydrostatic testing at 1.5× design pressure to verify weld integrity and tube body soundness.
- Final Dimensional Verification — OD, wall thickness, straightness (for straight sections), and coil geometry are verified using laser measurement systems and coordinate measuring machines (CMM).
- Packaging & Dispatch — Tubes are wrapped in protective film or paper, bundled with wooden or steel cradles, and labeled with material certificates, heat numbers, and traceability documentation.
Applications
Primary Applications
- First-Stage Conversion Furnaces (一段转化炉) — Process air coil tubes circulate air through the radiant section of ammonia synthesis converters, absorbing heat from the combustion of natural gas and fuel oil to preheat incoming process gas streams. This is the primary design application of this product line.
- Second-Stage Conversion Furnaces — Steam and gas coil tubes operating at slightly lower temperatures for secondary reforming.
- Steam Reformers — Heat recovery coils for waste heat utilization in steam methane reforming (SMR) processes.
- Hydrogen Production Units — Process coils in partial oxidation (POX) and autothermal reforming (ATR) reactors.
- Cracking Furnaces — Ethylene cracking furnaces in petrochemical complexes, where tubes operate at temperatures up to 1,100°C.
Secondary Applications
- Power generation boilers — Superheater and reheater tubes
- Aluminum smelting — Preheater tubes for Soderberg pot technology
- Industrial kilns — Heat exchanger elements in calcination and calcining processes
- Fertilizer production — Ammonia synthesis loop heat exchangers
- Chemical processing — High-temperature reactor internals and jacketed vessels
Quality Certifications & Standards Compliance
All products manufactured by Wenzhou Jixing Metal Materials Co., Ltd. conform to the following international standards and quality management systems:
| Standard / Certification | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent product quality and continuous improvement |
| ISO 14001:2015 | Environmental Management System certification for sustainable manufacturing practices |
| ISO 45001:2018 | Occupational Health and Safety Management System certification |
| ASTM A312 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for High-Pressure High-Temperature Boilers and Heat Exchanger Tubes |
| ASME SA-312 | American Society of Mechanical Engineers Specification for Seamless and Welded Austenitic Stainless Steel Tubes |
| ASTM A213 | Standard Specification for Seamless Austenitic Stainless Steel and Heat-Resisting Alloy Tubes for Heat-Transfer Equipment |
| EN 10216-5 | European Standard for Tubes for Heat Exchangers and Heat Transfer Equipment — Heat-Resisting Stainless Steel Tubes |
| GB/T 14976 | Chinese National Standard for Seamless Stainless Steel Tubes for Fluid Transport |
| GB/T 13296 | Chinese National Standard for Seamless Stainless Steel Tubes for Heat Exchangers and Heat Transfer Equipment |
| PED 2014/68/EU | European Pressure Equipment Directive compliance for pressure-retaining applications |
| NACE MR0175 | Resistance to Sulfide Stress Cracking in Carbon, Low Alloy, and High Strength Steels (for sour service applications) |
Each shipment is accompanied by a Mill Test Certificate (MTC) conforming to EN 10204 Type 3.1, including full chemical analysis, mechanical test results, heat treatment records, and NDT reports. Third-party inspection by recognized agencies (e.g., SGS, Bureau Veritas, TÜV) is available upon customer request.
Unique Selling Points
Field-Validated Design Philosophy — The product development program is informed by systematic failure analysis of coil tube leakage in first-stage conversion furnaces, incorporating lessons learned from real-world service to address root causes including hot corrosion from sulfur compounds, creep deformation at elevated temperatures, thermal fatigue from cyclic heating/cooling, and mechanical vibration-induced cracking at support points.