Stainless Steel Coil Tube (盘管/Coiltube) for Cracking Furnace Radiant Section
Product No.: 708 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (盘管/Coiltube) is a critical component engineered specifically for high-temperature service environments, most notably within the radiant section of steam cracking furnaces (裂解炉辐射段). These coil tubes are manufactured as continuous, helically wound or U-bend formed tubular assemblies that serve as the primary heat transfer elements in petrochemical and refining processes. The coil tube design enables efficient thermal energy absorption from furnace combustion gases while simultaneously heating process feedstocks to cracking temperatures exceeding 800°C.
Unlike straight pipe segments, coil tubes are characterized by their continuous curvature, which provides inherent advantages in thermal expansion accommodation, structural compactness, and resistance to thermal fatigue. Wenzhou Jixing Metal Materials Co., Ltd. specializes in the analysis, manufacturing, and optimization of radiant coil tubes, with particular expertise in bending performance analysis (弯曲分析) and structural improvement measures (改进措施) to enhance service life and operational reliability.
2. Material & Properties
2.1 Base Materials
Radiant coil tubes are typically fabricated from high-temperature austenitic and austenitic-ferritic stainless steels and nickel-based alloys selected for exceptional resistance to oxidation, carburization, and thermal fatigue at elevated operating temperatures.
- AISI 310 / 310S (UNS S31008 / S31009): High-chromium, high-nickel austenitic stainless steel offering excellent oxidation resistance up to 1150°C. Widely used for medium-temperature radiant coil applications.
- HP-Nb (UNS N08825): Nickel-iron-chromium alloy with niobium stabilization, providing superior resistance to sulfur scaling and carburization. Preferred for high-severity cracking furnace environments.
- HP40 (UNS N08810): Nickel-based superalloy with enhanced creep strength and carburization resistance, suitable for the hottest zones of radiant coils.
- 347H (UNS S34709): Niobium-stabilized austenitic stainless steel with high creep strength, used in lower-temperature radiant sections and transfer lines.
- Incoloy 800H (UNS N08810): General-purpose high-temperature alloy for radiant coil tubes in ethylene cracking furnaces.
2.2 Mechanical Properties (Typical Values at Elevated Temperatures)
| Property | 310S | HP-Nb | 347H | Incoloy 800H |
|---|---|---|---|---|
| Tensile Strength (RT, MPa) | ≥ 520 | ≥ 485 | ≥ 550 | ≥ 450 |
| Yield Strength (RT, MPa) | ≥ 205 | ≥ 205 | ≥ 275 | ≥ 205 |
| Elongation (RT, %) | ≥ 30 | ≥ 40 | ≥ 30 | ≥ 40 |
| Maximum Service Temperature (°C) | 1150 | 1200 | 1090 | 1200 |
| Thermal Expansion (100–800°C, μm/m·K) | 17.5 | 14.7 | 17.0 | 14.2 |
| Creep Strength (800°C / 100,000h, MPa) | ~45 | ~60 | ~65 | ~55 |
2.3 Key Performance Characteristics
- Oxidation Resistance: Cr₂O₃ protective oxide scale formation at temperatures above 500°C, providing a diffusion barrier against further oxidation.
- Carburization Resistance: Critical for hydrocarbon service environments; niobium and titanium stabilization prevent chromium carbide precipitation at grain boundaries.
- Thermal Fatigue Resistance: Essential for coil tubes subjected to cyclic heating and cooling during furnace start-up, shutdown, and turndown operations.
- Creep Resistance: Maintains dimensional stability under sustained high-temperature stress, preventing excessive elongation or collapse of the coil geometry.
- Hot Corrosion Resistance: Resistance to sulfur, chloride, and ash deposition attack in furnace atmospheres.
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 25.4 mm – 114.3 mm (1" – 4½") | ASTM A213 / A336 |
| Wall Thickness (WT) | 1.5 mm – 6.0 mm (typical); up to 10 mm for high-pressure service | ASTM A213 |
| Coil Bend Radius | 1.5D – 3.0D (minimum 150 mm); optimized per furnace design | ASME B31.3 |
| Coil Length (per pass) | 6.0 m – 25.0 m (configurable per furnace layout) | Customer-specific |
| Number of Passes | 2 – 6 passes (typical for tubular furnaces) | Customer-specific |
| Dimensional Tolerance (OD) | ±0.5% (standard); ±0.3% (precision grade) | ASTM A213 |
| Dimensional Tolerance (WT) | ±10% (standard); ±7% (precision grade) | ASTM A213 |
| Out-of-Roundness | ≤ 1% of OD | ASTM A213 |
| End Condition | Plain ends, beveled ends (37°±2°), or threaded | ASME B31.3 |
3.2 Chemical Composition (Typical, wt%)
| Element | 310S | HP-Nb | 347H | Incoloy 800H |
|---|---|---|---|---|
| C | ≤ 0.08 | 0.05 – 0.10 | ≤ 0.08 | ≤ 0.10 |
| Cr | 24.0 – 26.0 | 20.0 – 23.0 | 17.0 – 19.0 | 19.0 – 23.0 |
| Ni | 19.0 – 22.0 | 66.0 – 71.0 | 9.0 – 13.0 | 30.0 – 35.0 |
| Nb + Ta | — | 1.0 – 3.0 | 10 × C – 1.0 | 0.6 – 1.2 |
| Fe | Balance | Balance | Balance | Balance |
| S | ≤ 0.03 | ≤ 0.01 | ≤ 0.03 | ≤ 0.01 |
3.3 Heat Treatment
| Condition | Temperature | Duration | Cooling |
|---|---|---|---|
| Solution Annealing | 1040 – 1150°C | 1 h per 25 mm thickness | Water quench or air cool |
| Stress Relief (post-forming) | 870 – 950°C | 1 – 2 h | Furnace cool |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Selection of certified stainless steel seamless tube blanks or hot-rolled coil stock meeting ASTM A213/A336 or equivalent specifications. Mill test certificates (MTC) are verified for chemical composition, grain size, and mechanical properties.
- Tube Straightening & Cutting: Seamless tubes are straightened to within 0.3% of length tolerance and cut to required lengths using precision plasma or saw cutting methods.
- End Preparation: Tube ends are beveled, chamfered, or threaded according to the furnace design drawing specifications. Surface roughness at weld preparation areas is maintained at Ra ≤ 3.2 μm.
- Coil Forming / Bending: The tube is formed into the required coil geometry using a CNC-controlled rotary draw bending machine. This process is critical and subject to rigorous analysis:
- Bending Analysis (弯曲分析): Finite Element Analysis (FEA) is performed to predict stress distribution, strain accumulation, and potential thinning at the coil bends. Key parameters include bend radius (R/D ratio), number of bending cycles, and material work-hardening behavior.
- Springback Compensation: The bending machine is programmed with springback compensation values derived from material-specific springback models to ensure the final coil geometry meets dimensional tolerances.
- Minimum Bend Radius Optimization: The minimum bend radius is determined by balancing compact furnace design requirements against the risk of wall thinning, ovalization, and residual stress concentration.
- Welding (if multi-pass coil): For multi-pass coil configurations, tube passes are joined using Gas Tungsten Arc Welding (GTAW) with filler metals matched to the base material (e.g., ER310 for 310S, ERNiCrNb for HP-Nb). Weld procedures are qualified per ASME Section IX. Post-weld heat treatment (PWHT) is applied to relieve welding residual stresses.
- Post-Forming Stress Relief: The formed coil tube assembly undergoes stress relief annealing at 870–950°C to eliminate residual stresses introduced during bending and welding, thereby improving resistance to stress corrosion cracking and thermal fatigue.
- Solution Annealing: Where applicable, a full solution annealing cycle is performed to restore full austenitic microstructure and maximum high-temperature properties.
- Surface Cleaning & Inspection: Coils are cleaned to remove scale, oil, and debris. Internal bores are inspected using borescope or ultrasonic methods for defects.
- Final Non-Destructive Testing (NDT): The completed coil tube assembly undergoes comprehensive NDT including:
- Visual inspection (VT) of all welds and formed areas
- Penetrant testing (PT) of welds per ASME V, Article 6
- Ultrasonic testing (UT) for wall thickness verification at bend locations
- Hydrostatic pressure testing per ASTM A213
- Dimensional verification of coil geometry using laser scanning or 3D coordinate measurement
4.2 Bending Analysis & Improvement Measures (Key Technical Focus)
The core technical expertise of Wenzhou Jixing Metal Materials Co., Ltd. in coil tube manufacturing is reflected in the systematic approach to cracking furnace radiant coil tube bending analysis (裂解炉辐射盘管弯曲分析) and the implementation of improvement measures (改进措施). This includes:
- Strain Mapping: Detailed mapping of major and minor strain components at the inner and outer surfaces of each bend to identify critical thinning zones.