Stainless Steel Coil Tube (Coiltube) for High-Temperature Heat Extraction Service

1. Product Overview

The Stainless Steel Coil Tube (Coiltube), designated as Product No. 1585 by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered tubular product specifically designed for high-temperature heat extraction applications in severe industrial environments. These coil tubes are manufactured in continuous coiled form, eliminating the need for field welding during installation and thereby significantly reducing the probability of weld-induced failure in critical service conditions.

The development and refinement of this product line have been heavily informed by metallurgical failure analyses conducted on heat extraction coil tubes (HECT) deployed in catalytic cracking unit (CCU) regenerators, such as the landmark investigation at the Luoyang Refinery. These studies revealed that primary failure mechanisms in such environments include oxidation-induced wall thinning, carburization (carbon penetration), thermal fatigue cracking, and stress corrosion cracking (SCC). The present product is engineered with material selection, dimensional tolerances, and manufacturing protocols explicitly targeted at mitigating these well-documented failure modes.

2. Material & Properties

Coil tubes for regenerator heat extraction duty are typically fabricated from austenitic stainless steels and high-temperature nickel-based alloys. The following material grades are available:

• Type 310 (UNS S31008 / 06Cr25Ni20): Contains approximately 25% Chromium (Cr) and 20% Nickel (Ni), offering excellent resistance to oxidation and carburization at temperatures up to 1150°C. Suitable for moderate thermal cycling regimes.
• Type 310S (UNS S31008): A low-carbon variant of Type 310 with carbon content ≤ 0.08%, providing superior resistance to intergranular corrosion (IGC) during welding and high-temperature exposure.
• 310HT (UNS N08810): A heat-treated grade with enhanced creep strength and thermal stability, recommended for extended service above 1000°C.
• Incoloy 800 (UNS N08800) / Incoloy 800H (UNS N08810): Nickel-iron-chromium alloys with 30% Ni and 19% Cr, exhibiting exceptional resistance to thermal fatigue, sulfidation, and carburization. Incoloy 800H is the low-carbon version preferred for high-temperature applications.
• Haynes 230 (UNS N06023): A Ni-Cr-Ti-Al superalloy for extreme-temperature service exceeding 1200°C, with outstanding resistance to oxidation and hot corrosion.
• 347H (UNS S34709): A low-carbon, niobium-stabilized austenitic stainless steel with enhanced creep resistance, suitable for moderate-temperature heat extraction coils.

Key Metallurgical Properties:

• Microstructure: Fully austenitic (Face-Centered Cubic, FCC) grain structure with controlled grain size (ASTM E112 Grain Size No. 6–10) to balance thermal fatigue resistance and mechanical strength.
• Carburization Resistance: Achieved through high Cr content and low carbon activity, limiting carbon diffusion depth to < 50 μm after 10,000 hours at 1050°C in endothermic gas atmosphere.
• Thermal Conductivity: 25–35 W/(m·K) at 1000°C, providing efficient heat transfer from the hot gas side to the coolant (typically steam or water) in the tube lumen.
• Thermal Expansion Coefficient: 16–18 × 10⁻⁶ /°C (20–1000°C), requiring proper expansion accommodation in the coil support structure.
• Creep Strength: Minimum 50 MPa at 1000°C for 10,000 hours for Type 310S; > 80 MPa for Incoloy 800H at the same conditions.

3. Specifications

Parameter	Specification	Standard / Reference
Product Designation	Stainless Steel Coil Tube (Coiltube), Product No. 1585	Wenzhou Jixing Metal Materials Co., Ltd.
Outer Diameter (OD)	Φ25 mm, Φ32 mm, Φ38 mm, Φ45 mm, Φ51 mm (custom sizes available)	ASTM A213 / ASME SA-213
Wall Thickness	1.5 mm – 6.0 mm (typical range; 3.0–4.0 mm for regenerator duty)	ASTM A213
OD Tolerance	±0.15 mm	ASTM A213
Wall Thickness Tolerance	±10% of nominal	ASTM A213
Coil Inner Diameter (ID)	300 mm – 1500 mm (configurable per installation geometry)	Custom / Project-Specific
Coil Length per Spool	Up to 2000 m per continuous coil (weld-free)	—
Surface Finish	Inside: Ra ≤ 1.6 μm (polished); Outside: Ra ≤ 3.2 μm (bright annealed)	ASTM A696 / EN 10217-7
Heat Treatment	Solution Annealed (1050–1150°C) + Water Quenched; optional Stress Relief at 870°C	ASTM A213
Maximum Service Temperature	1150°C (Type 310S); 1200°C (Incoloy 800H); 1300°C (Haynes 230)	Material-Specific
Design Pressure	Up to 2.5 MPa (steam side)	ASME BPV Code Section I
Applicable Standards	ASTM A213, ASME SA-213, EN 10217-7, GB/T 14976, JIS G3459	—
Non-Destructive Testing (NDT)	100% Eddy Current (ET) + Hydrostatic Test (2.5× design pressure)	ASTM A751

4. Manufacturing Process

The manufacturing of stainless steel coil tubes for heat extraction service follows a rigorous, multi-stage process chain designed to ensure metallurgical homogeneity, dimensional accuracy, and freedom from defects that could precipitate premature failure in high-temperature environments.

1. Raw Material Selection and Heat Traceability:

   Only certified ingots or continuous-cast slabs from qualified steelmakers are procured. Each heat is accompanied by a mill test report (MTR) documenting chemical composition (including trace elements such as S ≤ 0.015%, P ≤ 0.030%, N ≤ 0.10%), and heat treatment history. Full heat traceability is maintained throughout the manufacturing process.

2. Hot Rolling / Seamless Tube Forming:

   The tube is produced via the seamless tube manufacturing route (Piercing → Hot Rolling → Elongation) or, for smaller diameters, via the cold drawing route (Hot Rolled Billet → Cold Heading → Piercing → Cold Drawing). Seamless construction eliminates longitudinal weld seams that are vulnerable to intergranular attack and creep rupture.

3. Solution Annealing:

   The tube is solution annealed at 1050–1150°C in a protective atmosphere (endothermic gas or vacuum) followed by rapid water quenching. This dissolves any precipitated carbides (particularly chromium carbides such as Cr₂₃C₆ and Cr₇C₃) and restores full austenitic homogeneity, ensuring optimal corrosion and oxidation resistance.

4. Coiling:

   The annealed tube is coiled onto a mandrel of the specified inner diameter using a controlled-speed coiling machine. The coiling process is optimized to minimize residual stresses and prevent work hardening that could compromise fatigue life. Tension and bending radius parameters are carefully calibrated to prevent ovality exceeding 2% of OD.

5. Internal and External Surface Treatment:

   The internal bore is mechanically polished or chemically passivated to achieve Ra ≤ 1.6 μm, minimizing fouling and deposit buildup during service. The external surface receives a bright annealing pass to remove scale and restore a clean, oxidation-resistant oxide layer.

6. Quality Inspection and NDT:

   100% of the coil length undergoes eddy current testing (ET) per ASTM E709 to detect wall thinning, cracks, and inclusion-related defects. A hydrostatic pressure test at 2.5× design pressure is performed to verify integrity. Spectroscopic verification (OES) is conducted on each coil to confirm chemical composition compliance.

7. Packaging and Dispatch:

   Each coil is wrapped in VCI (Vapor Corrosion Inhibitor) paper and shrink-wrapped in moisture-barrier film. Coils are secured on wooden or steel skids with steel banding and labeled with heat number, material grade, dimensions, and certification documentation.

5. Applications

The primary application domain for these stainless steel coil tubes is in the heat extraction systems of catalytic cracking unit (CCU) regenerators in petroleum refineries. In a CCU regenerator, the hot flue gas (typically 700–750°C, with local hot spots exceeding 1000°C due to coke combustion) flows over the exterior of the coil tubes, while water or steam flows through the tube lumen, generating high-pressure steam for power recovery.

Key application areas include:

• CCU Regenerator Heat Extraction Coils (HECT): The principal application. Coil tubes are arranged in a bundle configuration within the regenerator vessel, extracting heat from the hot flue gas produced by catalytic coke combustion. Failure of these coils results in catastrophic loss of containment, hydrocarbon release, and unplanned unit shutdown.
• Fluid Catalytic Cracking (FCC) Unit Heat Recovery: Similar duty to CCU regenerator service, including riser reactor heat extraction and stripper systems.
• High-Temperature Furnace Heating Elements: In metallurgical and ceramic processing furnaces where temperatures exceed 1000°C and oxidizing atmospheres prevail.
• Boiler and Waste Heat Recovery Systems: Where continuous coil geometry is advantageous for compact heat exchange surfaces.
• Chemical Process Heat Exchangers: In severe-service chemical reactors and heat exchangers requiring resistance to sulfidation, halide corrosion, and thermal cycling.
• Power Generation (Supercritical/Ultra-Supercritical Boilers): For superheater and reheater sections operating at steam temperatures above 550°C.

6. Quality Certifications and Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and ensures full compliance with international standards:

• ISO 9001:2015 — Quality Management System Certification, demonstrating consistent capability to provide products meeting customer and regulatory requirements.
• ISO 14001:2015 — Environmental Management System Certification, ensuring environmentally responsible manufacturing practices.
• ASTM A213 / ASME SA-213 Compliance — Full conformance to American standards for seamless austenitic stainless steel tubing, including chemical composition, mechanical properties, and heat treatment requirements.
• EN 10217-7 — European standard for austenitic stainless steel tubes for heat exchangers and heat-transfer equipment.
• NB (National Board) Registration — For products intended for pressure vessel service in the United States.
• Mill Test Reports (MTR) — Each coil is accompanied by a comprehensive MTR including chemical analysis (OES), mechanical test results (tensile, hardness), heat treatment records, NDT results, and dimensional verification.
• Third-Party Inspection (TPI) — Available upon request; inspection by SGS, Bureau Veritas, or TUV is accommodated during manufacturing and prior to shipment.

7. Unique Selling Points

The engineering philosophy behind this coil tube product is directly informed by post-mortem metallurgical investigations of failed heat extraction coils in industrial service, most notably the comprehensive failure analysis conducted at the Luoyang Refinery catalytic cracking unit. The following differentiators distinguish this product:

• Failure-Informed Material Selection:

  Material grades are selected not merely on nominal temperature ratings but on demonstrated resistance to the specific failure mechanisms identified in field investigations: carburization depth control, oxidation scale spallation resistance, and thermal fatigue crack initiation life. For example, the transition from Type 310 to Incoloy 800H is recommended where carburization depth exceeds 100 μm after 5,000 hours of service.

• Weld-Free Continuous Coil Geometry:

  By delivering the tube in continuous coiled form (up to 2000 m per spool), the product eliminates field-weld joints that are the predominant initiation sites for intergranular corrosion and creep rupture in regenerator heat extraction service. This represents a fundamental reliability improvement over straight-length tube assemblies.

• 100% Eddy Current Inspection:

  Full-length eddy current testing per ASTM E709 ensures detection of sub-surface inclusions, wall thinning, and micro-cracks that could serve as failure nucleation sites under high-temperature cyclic loading. This is critical given that inclusions from the steelmaking process have been identified as root causes of premature coil tube failure in multiple field