Stainless Steel Coil Tube (Coiltube) — Waste Heat Boiler Grade Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered, continuously coiled stainless steel tubing product designed primarily for high-temperature, high-pressure thermal recovery systems — most notably waste heat boiler (Waste Heat Boiler, WHB) applications in the petrochemical, power generation, and metallurgical industries. Unlike straight-length seamless or welded pipe, the coil tube is produced in a continuous helical form, enabling uninterrupted flow paths with minimal joint density, thereby significantly reducing leak points and mechanical failure surfaces in demanding thermal environments.
This product line has been developed with deep consideration of failure modes documented in extensive field studies, including the well-known case study of Abnormal Damage of Waste Heat Boiler Coil Tubes. Insights from metallurgical damage analysis — encompassing intergranular corrosion (IGC), stress corrosion cracking (Stress Corrosion Cracking, SCC), thermal fatigue cracking, and oxidation-induced wall thinning — have been directly incorporated into the material selection criteria, heat treatment protocols, and quality assurance framework of this product line.
2. Material Composition & Mechanical Properties
2.1 Base Alloy Grades
The coil tube is offered in multiple austenitic and super-austenitic stainless steel grades, each selected to address specific service conditions encountered in waste heat boiler and thermal recovery systems:
- 310S / 310 (UNS S31008) — High-temperature austenitic grade with excellent oxidation resistance up to 1150°C, suitable for hot-side coil tube zones.
- 316L (UNS S31603) — Low-carbon austenitic grade with molybdenum addition for superior resistance to pitting and crevice corrosion in chloride-containing flue gas environments.
- 321H / 321 (UNS S32109) — Titanium-stabilized austenitic grade providing outstanding resistance to intergranular sensitization at elevated temperatures, critical for coil tubes exposed to cyclic thermal loads.
- TP347H (UNS N08820 equivalent) — Niobium-stabilized grade for high-temperature strength retention in steam-generating sections.
- TP310HCB (UNS S31050 equivalent) — Ultra-high-temperature grade for extreme furnace-side applications.
2.2 Key Mechanical Properties (Typical Values at Room Temperature)
| Grade | Tensile Strength (MPa) | Yield Strength (MPa, 0.2% offset) | Elongation (% at break) | Hardness (HB max) | Max Service Temperature (°C) |
|---|---|---|---|---|---|
| 310S | ≥520 | ≥205 | ≥35 | 201 | 1150 |
| 316L | ≥485 | ≥170 | ≥40 | 217 | 595 |
| 321H | ≥550 | ≥205 | ≥35 | 201 | 1000 |
| TP347H | ≥550 | ≥205 | ≥35 | 201 | 1090 |
| TP310HCB | ≥520 | ≥205 | ≥35 | 201 | 1200 |
2.3 Chemical Composition Requirements
| Element | C (% max) | Si (% max) | Mn (% max) | P (% max) | S (% max) | Cr (%) | Ni (%) | Mo (%) | Ti (%) | Nb (%) |
|---|---|---|---|---|---|---|---|---|---|---|
| 310S | 0.08 | 1.50 | 2.00 | 0.035 | 0.030 | 24.0–26.0 | 19.0–22.0 | — | — | — |
| 316L | 0.03 | 1.00 | 2.00 | 0.030 | 0.030 | 16.5–18.5 | 10.0–14.0 | 2.0–3.0 | — | — |
| 321H | 0.10 | 1.00 | 2.00 | 0.045 | 0.030 | 17.0–19.0 | 9.0–12.0 | — | 5×(C+N)–0.70 | — |
| TP347H | 0.08 | 1.00 | 2.00 | 0.045 | 0.030 | 17.0–19.0 | 9.0–13.0 | — | — | 8×(C+N)–1.00 |
3. Dimensional Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | Φ9.5 mm – Φ50.8 mm (typical); custom sizes available up to Φ108 mm | ASTM A213 / ASME SA-213 |
| Wall Thickness | 0.5 mm – 3.0 mm (typical); custom up to 6.0 mm | ASTM A213 / GB/T 12771 |
| OD Tolerance | ±0.05 mm (precision grade); ±0.10 mm (standard grade) | ASTM A213 |
| Wall Thickness Tolerance | ±10% of nominal (standard); ±5% (precision) | ASTM A213 |
| Coil Length (per coil) | 50 m – 500 m (continuous, joint-free) | Manufacturer specification |
| Coil Inner Diameter | 1.0 m – 3.0 m (configurable per installation requirement) | Custom |
| Surface Finish | Pickled & Passivated (2B equivalent); Ra ≤ 0.8 μm available | ASTM A478 |
| End Condition | Plain cut; beveled ends available for welding | Custom |
| Heat Treatment | Solution Annealed (Solution Annealing) at 1010–1150°C followed by rapid water quench | ASTM A213 |
4. Manufacturing Process
4.1 Production Flow
- Raw Material Selection & Ingot Casting: High-purity stainless steel ingots are produced via Vacuum Arc Remelting (VAR) or Electro-Slag Remelting (ESR) to minimize impurity content (particularly S, P, and interstitial N), ensuring consistent metallurgical quality.
- Hot Rolling & Billet Formation: Ingots are homogenized and hot-rolled into hollow billets with precise internal bore geometry, establishing the initial tube blank.
- Piercing & Hot Extrusion: The billet is pierced and extruded through calibrated dies to produce seamless tube stock at the target OD and wall thickness dimensions.
- Solution Annealing: The extruded tube undergoes solution annealing at temperatures between 1010°C and 1150°C (grade-dependent) followed by rapid quenching (water or forced air) to dissolve carbide precipitates, restore full austenitic microstructure, and maximize corrosion resistance — a critical step to prevent the intergranular sensitization documented in waste heat boiler failure analyses.
- Pickling & Passivation: Annealed tubes are acid-pickled (HNO₃/HF blend) to remove oxide scale, followed by passivation to establish a stable, chromium-rich passive film (Cr₂O₃) on the surface.
- Precision Sizing & Coiling: The tube passes through a series of calibrating rollers (sizing rolls) to achieve final dimensional tolerances, then is continuously wound onto a coiling reel at controlled tension to produce the final coil tube configuration.
- Non-Destructive Testing (NDT): 100% of production is subjected to Eddy Current Testing (ECT) and/or Ultrasonic Testing (UT) for internal and external defect detection, supplemented by Hydrostatic Pressure Testing (hydrotest) to verify wall integrity.
- Final Inspection & Packaging: Dimensional verification, chemical spot analysis (OES — Optical Emission Spectrometry), and mechanical property testing are performed. Finished coils are wrapped in moisture-proof protective film, sleeved with steel banding, and palletized for shipment.
4.2 Critical Process Controls Addressing Failure Modes
Based on metallurgical damage analysis of waste heat boiler coil tubes, the following process controls are specifically implemented:
- Low Carbon & Stabilizer Control: Carbon content is tightly controlled (≤0.08% for H-grade; ≤0.03% for L-grade) and titanium/nium stabilizers are added at stoichiometric ratios to prevent chromium carbide (Cr₂₃C₆) precipitation at grain boundaries — the primary mechanism of intergranular corrosion in service.
- Grain Size Control: ASTM E112 grain size of 5 or finer is maintained to improve resistance to creep and thermal fatigue cracking at elevated temperatures.
- Residual Stress Elimination: Post-coiling stress relief is available upon request to mitigate residual stresses that can accelerate stress corrosion cracking in chloride-containing environments.
- Surface Quality Assurance: Internal and external surface roughness is controlled to Ra ≤ 1.6 μm (standard) to prevent localized corrosion initiation at surface defects.
5. Applications
5.1 Primary Applications
- Waste Heat Boilers (WHB): Hot-side and cold-side coil tubes for catalytic cracking units, delayed coking units, and reformer waste heat recovery systems in petroleum refining.
- Boiler Tubing: Fire-tube and water-tube boiler coil assemblies in power generation and industrial process heating.
- Heat Exchanger Coils: Spiral wound heat exchanger elements for process cooling and thermal recovery.
- Reformer Tubes: Coil tubes for ammonia synthesis reformers and steam methane reforming (SMR) systems.
- Metallurgical Furnace Linings: Induction furnace and electric arc furnace (EAF) coil cooling systems.
- Chemical Processing: Coil coils for acid recovery, chlor-alkali, and pharmaceutical process heat exchange systems.
5.2 Service Environment Parameters
| Parameter | Typical Range | Design Consideration |
|---|---|---|
| Operating Temperature | 400°C – 1150°C | Grade selection based on maximum exposure temperature and oxidation rate |
| Operating Pressure | 0.1 MPa – 15 MPa | Wall thickness designed per ASME Section VIII or equivalent |
| Flue Gas Composition | SO₂, H₂S, HCl, CO₂, H₂O, particulates | Mo and Ti/Nb additions for sulfuric acid dew point corrosion resistance |
| Cyclic Thermal Loading | Up to 10 |