Stainless Steel Coil Tube for Primary Reforming Furnace – Cold-End Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Catalog No.: 569 | Product Category: Technical Reference & Engineering Literature
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Cold-End Coil Tube") is a critical process component employed in the Primary Reforming Furnace of steam methane reforming (SMR) units within petrochemical and natural gas processing facilities. The cold-end coil tube serves as the upstream section of the reforming tube bundle, responsible for preheating the feedstock mixture (hydrocarbon feed gas and superheated steam) prior to its introduction into the hot-end reforming zone where catalytic reactions occur.
During the start-up phase of a primary reforming furnace, the cold-end coil tube is subjected to rapid and non-uniform temperature gradients that can lead to thermal over-temperature conditions (exceeding the maximum allowable operating temperature). This phenomenon poses significant risks including accelerated oxidation, creep deformation, thermal fatigue cracking, and ultimately catastrophic tube failure. Wenzhou Jixing Metal Materials Co., Ltd. provides both the coil tube product and associated technical literature addressing the root causes and mitigation strategies for cold-end over-temperature events during furnace commissioning and start-up operations.
Key Engineering Challenge: The cold-end coil tube operates in a thermally transitional zone where the temperature rises from ambient (approximately 25–80 °C) to the reforming inlet temperature (typically 600–800 °C). During start-up, improper sequencing of fuel firing, steam flow rates, and feed gas introduction can cause localized hot spots exceeding material limits by 100–200 °C, necessitating robust material selection and precise process control.
2. Material & Properties
The cold-end coil tube is manufactured from high-performance austenitic and austenitic-ferritic stainless steel alloys engineered to withstand the demanding thermal, chemical, and mechanical environments of primary reforming furnace service.
2.1 Primary Alloy Grades
- SAE 310 / AISI 310 / UNS S31008 – High-temperature austenitic stainless steel with elevated chromium (19–22%) and nickel (24–26%) content, offering superior oxidation resistance up to 1150 °C.
- SAE 309 / AISI 309 / UNS S30908 – Austenitic stainless steel with enhanced thermal shock resistance due to higher nickel content (19–22%), suitable for cyclic thermal loading conditions.
- SAE 347 / AISI 347 / UNS S34700 – Niobium-stabilized austenitic stainless steel resistant to intergranular sensitization during welding and heat treatment.
- SAE 304 / AISI 304 / UNS S30408 – General-purpose austenitic stainless steel used in lower-temperature sections of the cold-end coil.
- Custom Alloy Compositions – Tailored grades incorporating titanium (Ti), niobium (Nb), or hafnium (Hf) for specialized service requirements.
2.2 Mechanical Properties (Representative Values at Room Temperature)
| Property | SAE 310 (UNS S31008) | SAE 309 (UNS S30908) | SAE 347 (UNS S34700) |
|---|---|---|---|
| Tensile Strength (UTS), MPa | ≥ 515 | ≥ 515 | ≥ 515 |
| Yield Strength (0.2% offset), MPa | ≥ 205 | ≥ 205 | ≥ 205 |
| Elongation, % | ≥ 40 | ≥ 40 | ≥ 40 |
| Density, g/cm³ | 8.0 | 8.0 | 8.0 |
| Thermal Conductivity at 100 °C, W/(m·K) | 16.3 | 16.3 | 16.3 |
| Thermal Expansion (20–1000 °C), ×10⁻⁶/°C | 18.0 | 18.0 | 17.5 |
| Maximum Continuous Service Temperature, °C | 1150 | 1100 | 980 |
| Creep Strength at 1000 °C (10⁴ h), MPa | ~140 | ~120 | ~85 |
2.3 Chemical Composition (Wt.%)
| Element | SAE 310 | SAE 309 | SAE 347 |
|---|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.08 | ≤ 0.08 |
| Si (Silicon) | ≤ 1.50 | ≤ 1.50 | ≤ 1.00 |
| Mn (Manganese) | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| Cr (Chromium) | 19.00–22.00 | 19.00–22.00 | 17.00–19.00 |
| Ni (Nickel) | 24.00–26.00 | 19.00–22.00 | 9.00–13.00 |
| Nb (Niobium) | — | — | 10×C–1.00 (min) |
| Mo (Molybdenum) | — | — | — |
| N (Nitrogen) | — | — | — |
2.4 High-Temperature Performance Characteristics
- Oxidation Resistance: The high chromium and nickel content forms a stable, self-healing chromium oxide (Cr₂O₃) scale layer on the tube surface, providing excellent resistance to scaling in oxidizing and mildly reducing atmospheres at temperatures up to 1150 °C.
- Thermal Shock Resistance: The austenitic microstructure with its face-centered cubic (FCC) lattice offers superior ductility at elevated temperatures, accommodating thermal cycling without brittle fracture.
- Creep Resistance: Solid solution strengthening from chromium and nickel, combined with grain boundary stabilization, provides adequate resistance to time-dependent deformation under sustained high-temperature loading.
- Resistance to Decarburization: Low carbon content (≤ 0.08 wt%) minimizes the risk of surface carbon depletion, which can lead to loss of strength and increased oxidation susceptibility at the tube inner surface.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Designation | Cold-End Coil Tube for Primary Reforming Furnace | — |
| Tube Material | SAE 310 / 309 / 347 Stainless Steel | ASTM A213 / ASME SA-213 |
| Outer Diameter (OD) | 25.4 mm – 101.6 mm (1" – 4") | ASTM A213 |
| Wall Thickness | 1.5 mm – 6.35 mm (0.059" – 0.250") | ASTM A213 |
| Coil Diameter (Mean) | 300 mm – 1500 mm | Customer Specification |
| Coil Length | 15 m – 80 m (per coil) | Customer Specification |
| Tolerance on OD | ±0.5% (per ASTM A213) | ASTM A213 |
| Tolerance on Wall Thickness | ±10% (per ASTM A213) | ASTM A213 |
| Surface Finish | Mill finish / Pickled and passivated (Ra ≤ 1.6 μm) | ASTM A478 |
| Heat Treatment | Solution annealing at 1050–1150 °C, followed by water quench | ASTM A213 |
| Maximum Operating Temperature | ≤ 1100 °C (SAE 310) / ≤ 1050 °C (SAE 309) | Material datasheet |
| Operating Pressure | Up to 3.5 MPa (gauge) | Design specification |
| Fluid Medium | Hydrocarbon feed gas + superheated steam mixture | Process design |
| Welding Method | TIG (GTAW) / Orbital welding with matching filler wire | ASME Section IX |
| Coil Packaging | Wooden crate with moisture barrier; spool supports to prevent deformation | ISO 1496 |
4. Manufacturing Process
The production of cold-end coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and conformance to applicable international standards.
4.1 Raw Material Procurement and Inspection
- Material Certification: Each heat of stainless steel coil stock is accompanied by a mill test certificate (MTC) in accordance with EN 10204 Type 3.1, verifying chemical composition and mechanical properties.
- Incoming Inspection: Visual examination for surface defects, dimensional verification (OD, wall thickness), and ultrasonic testing (UT) for internal soundness.
- Spectroscopic Analysis: Portable optical emission spectrometry (OES) is employed to confirm chemical composition within specified limits.
4.2 Coil Winding and Forming
- Coil Winding: The stainless steel tube is wound onto a mandrel of the specified mean coil diameter using a precision coil-winding machine equipped with hydraulic forming rolls to achieve the desired curvature without inducing plastic deformation or work hardening.
- Springback Compensation: The winding machine is calibrated to account for elastic springback, ensuring the final coil geometry meets dimensional tolerances after removal from the mandrel.
- Support Spooling: Intermediate support spools are installed at calculated intervals (typically every 1.5–2.0 m along the coil length) to prevent sagging, ovalization, and residual stress accumulation.
4.3 Welding and Fabrication
- End Preparation: Tube ends are beveled to ASME B31.3 specifications with a 37.5° included angle and 1.6 mm root face for butt weld joints.
- TIG Welding (GTAW): Gas tungsten arc welding is performed using argon shielding gas (99.99% purity) with a back-purging arrangement to prevent internal oxidation. Welding parameters are optimized based on wall thickness and material grade.
- Filler Metal: Matching composition filler wire (e.g., ER310 for SAE 310 tube, ER309 for SAE 309 tube) is used in accordance with AWS A5.9 / A5.10 specifications.
- Weld Joint Inspection: 100% visual inspection (VT) and radiographic testing (RT) or ultrasonic testing (UT) of weld joints per ASME Section V, Article 2 and Article 4 respectively.
4.4 Post-Weld Heat Treatment and Surface Treatment
- Post-Weld Annealing (PWHT): Welded assemblies are solution annealed at 1050–1150 °C followed by rapid water quench to restore full austenitic microstructure, relieve welding residual stresses, and homogenize the heat-affected zone (HAZ).
- Pickling and Passivation: The tube surface is pickled using a mixed acid solution (nitric acid + hydrofluoric acid) to remove mill scale, weld discoloration, and oxide layers. Subsequent passivation with a 20–30% nitric acid solution forms a uniform chromium oxide passive film to enhance corrosion resistance.
- Cleaning: Final cleaning with deionized water and low-lint wiping to remove all chemical residues and particulate contamination.
4.5 Final Inspection and Testing
- Dimensional Verification: OD, wall thickness, coil diameter, and total length are measured at multiple points along the coil length using calibrated instruments.
- Hydrostatic Testing: Each coil tube is pressure tested at 1.5× the maximum design pressure (typically 5.25 MPa) with hold time of minimum 10 minutes, in accordance with ASTM A213.
- Leak Testing: Helium leak detection (sensitivity ≤