Stainless Steel Coil Tube for PET Process Columns
Product Overview
The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled tubing product designed specifically for high-temperature, high-pressure chemical process applications, with particular emphasis on Polyethylene Terephthalate (PET) polymerization reactors and process columns. These coil tubes serve as critical heat-exchange and fluid-conveyance components within the PET production line, where they endure extreme thermal cycling, corrosive chemical media (including Monoethylene Glycol (MEG), Terephthalic Acid (TPA), and Antimony Trioxide (Sb₂O₅) catalyst residues), and sustained mechanical stress.
Unlike conventional straight seamless tubes, coil tubes are formed into continuous helical configurations during the manufacturing process, providing superior heat transfer efficiency, compact footprint, and enhanced resistance to thermal fatigue cracking. The product is engineered to address the well-documented challenge of crack initiation and propagation in PET process column coil tubes, incorporating metallurgical and process controls that significantly extend service life.
Material & Properties
The coil tubes are fabricated from austenitic and super-austenitic stainless steel grades selected for their exceptional resistance to pitting corrosion, intergranular corrosion, and stress corrosion cracking (SCC) under PET process conditions.
Available Material Grades
- SAE 316L (UNS S31603) — Low-carbon austenitic grade with molybdenum addition; standard choice for general PET service. Carbon content ≤ 0.03% to minimize sensitization risk during welding.
- SAE 321 (UNS S32100) — Titanium-stabilized austenitic grade; recommended where elevated temperatures (up to 870°C) are encountered, providing superior resistance to intergranular corrosion.
- SAE 316Ti (UNS S31635) — Titanium-stabilized variant of 316L; combines molybdenum's pitting resistance with titanium's sensitization resistance.
- SAE 347H (UNS S34708) — High-temperature, niobium-stabilized grade; suited for high-temperature heat exchanger applications within the PET process.
- SAE 904L (UNS N08904) — Super-austenitic grade with high Ni/Mo content; reserved for highly aggressive or chloride-rich environments requiring maximum corrosion resistance.
- SAE 2205 (UNS S32205) — Duplex stainless steel; offers approximately twice the yield strength of austenitic grades, suitable for high-pressure coil tube applications.
Mechanical Properties (Typical for SAE 316L)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (抗拉强度) | ≥ 485 MPa | ASTM A213 / GB/T 12771 |
| Yield Strength (屈服强度) | ≥ 170 MPa | ASTM A213 / GB/T 12771 |
| Elongation (延伸率) | ≥ 40% | ASTM A213 / GB/T 12771 |
| Hardness (硬度) | ≤ 235 HV (Vickers) | ASTM A262 |
| Impact Toughness (冲击韧性) | ≥ 47 J @ −40°C | ASTM A370 |
| Corrosion Resistance (耐腐蚀性) | Pitting Resistance Equivalent Number (PREN) ≥ 24 | ASTM G48 / ASTM A262 |
Chemical Composition (SAE 316L — Typical)
| Element | Maximum Content (%) |
|---|---|
| Carbon (C) | 0.03 |
| Manganese (Mn) | 2.00 |
| Phosphorus (P) | 0.045 |
| Sulfur (S) | 0.030 |
| Chromium (Cr) | 18.00 – 20.00 |
| Nickel (Ni) | 12.00 – 15.00 |
| Molybdenum (Mo) | 2.00 – 3.00 |
| Iron (Fe) | Balance |
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 108.0 mm (customizable) | ASTM A213 / GB/T 12771 |
| Wall Thickness (WT) | 0.5 mm – 6.0 mm (customizable) | ASTM A213 / GB/T 12771 |
| Coil Length (per coil) | 10 m – 200 m (configurable per application) | — |
| Coil Inner Diameter (CID) | 50 mm – 500 mm | — |
| OD Tolerance | ±0.05 mm (precision grade); ±0.10 mm (standard grade) | ASTM A213 |
| WT Tolerance | ±10% of nominal | ASTM A213 |
| Surface Finish | Mirror polish (Ra ≤ 0.4 μm), Electropolished (Ra ≤ 0.5 μm), or Commercial passivated (Ra ≤ 0.8 μm) | ASTM A262 / GB/T 19228 |
| End Condition | Plain end, Beveled end, or Threaded end (BSPT/NPT) | — |
| Maximum Operating Temperature | 870°C (316L); 1100°C (347H); 400°C (2205 duplex) | ASME BPV Code |
| Maximum Operating Pressure | Up to 40 MPa (design-dependent) | ASME BPV Code Section VIII |
| Minimum Bend Radius | 3× OD (cold bend); 5× OD (hot form) | — |
| Welding Method | Orbital TIG (GTAW); Pulse TIG; Laser welding | ASME BPV Code Section IX |
| Heat Treatment | Solution annealing at 1050°C – 1150°C, followed by water quench | ASTM A213 |
Manufacturing Process
The manufacturing of stainless steel coil tubes for PET process columns follows a rigorously controlled, multi-stage process designed to eliminate the root causes of cracking observed in conventional coil tube production:
- Raw Material Selection & Inspection — Virgin stainless steel billets or slabs are sourced from certified mills with full Chemical Composition (CC) and Mechanical Test Report (MTR) traceability. Incoming material undergoes spectroscopic analysis (OES) to verify alloy content within specified ranges.
- Hot Rolling & Coiling — The billet is heated to 1150°C – 1250°C and hot-rolled into a seamless tube blank. The coiling process is performed at controlled temperatures to minimize residual stresses and prevent grain boundary segregation.
- Pilgering / Cross-Rolling (Tube Drawing) — The hot-rolled tube blank is cold-worked through a pilger mill or cross-roller mill to achieve precise OD and wall thickness dimensions. This stage is critical for achieving the required dimensional tolerances.
- Solution Annealing (固溶处理) — Tubes are solution-annealed at 1050°C – 1150°C in a controlled-atmosphere furnace (dew point ≤ −60°C) followed by rapid water quenching. This step dissolves any precipitated chromium carbides, restoring full corrosion resistance and preventing sensitization-induced intergranular cracking.
- Coil Forming (盘管成型) — The straight tube is formed into a helical coil configuration using a precision coil former. The forming process is optimized to minimize plastic strain accumulation, which is a primary contributor to crack initiation in PET service. Controlled forming speed and temperature are maintained to prevent work-hardening and micro-crack formation.
- Post-Form Stress Relief (成形后应力消除) — A critical step to address cracking: the formed coil is subjected to a stress relief anneal at 650°C – 850°C (below the sensitization range of 450°C – 850°C, carefully controlled) to relieve residual forming stresses without inducing sensitization.
- Welding & Joining — Orbital TIG welding (GTAW) is employed for end-to-end joints and any required internal welds. Weld parameters are optimized to minimize heat input and avoid the sensitization temperature range. Welding is performed under argon gas shielding (purity ≥ 99.99%) to prevent oxidation.
- Post-Weld Heat Treatment (PWHT) — Welded assemblies undergo a solution anneal at 1100°C ± 20°C followed by water quench to homogenize the weld microstructure and restore full corrosion resistance.
- Surface Finishing & Passivation — Tubes are subjected to electropolishing or acid pickling (HNO₃/HF solution) followed by passivation to remove surface contaminants, scale, and free iron, establishing a stable chromium oxide passive film.
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Non-Destructive Testing (NDT) — Comprehensive inspection including:
- Visual inspection (VT) — 100% coverage
- Eddy Current Testing (ECT) — 100% coverage for internal and external surface defects
- Hydrostatic Pressure Testing — 100% at 1.5× maximum operating pressure, per ASTM A213
- Ultrasonic Testing (UT) — for wall thickness verification and volumetric defect detection
- Magnetic Particle Testing (MT) — where applicable
- Final Inspection & Packaging — Each coil tube undergoes final dimensional verification, chemical spot-checking, and is packaged with anti-corrosion wrapping, wooden crating, and full traceability documentation (Mill Test Report, Heat Number, NDT certificates).
Crack Prevention & Mitigation — Unique Selling Points
Based on extensive field experience and metallurgical analysis of PET process column coil tube failures, Wenzhou Jixing Metal Materials Co., Ltd. incorporates the following engineering solutions that distinguish their product from conventional offerings:
- Ultra-Low Carbon Control: Carbon content is maintained at ≤ 0.025% (below the standard 0.03% limit for 316L) to further reduce the risk of chromium carbide precipitation at grain boundaries, which is the primary mechanism of intergranular cracking in PET service.
- Stabilizing Element Addition: Where applicable, titanium (Ti ≥ 5×C) or niobium (Nb ≥ 10×C) stabilizers are added to bind free carbon, preventing sensitization even under prolonged exposure to the 450°C – 850°C temperature range common in PET process columns.
- Controlled Cold Working: The coil forming process is optimized to limit accumulated plastic strain to below 30%, preventing work-hardening-induced micro-crack initiation. Post-forming stress relief annealing is performed to further reduce residual stresses to below 50 MPa.
- Grain Size Control: Fine grain structures (ASTM grain size ≥ 8) are achieved through controlled thermomechanical processing, improving resistance to stress corrosion cracking and enhancing fatigue life.
- Surface Integrity Optimization: Electropolishing removes surface micro-defects (up to 25 μm of material) that could serve as crack initiation sites. The resulting smooth surface (Ra ≤ 0.4 μm) also reduces fluid friction and improves heat transfer efficiency.
- Full Traceability: Every coil tube is traceable from raw material heat number through every processing stage, enabling root cause analysis in the event