Stainless Steel Coil Tubes for Ethylene Cracking Furnace Dilution Steam Systems
1. Product Overview
The stainless steel coil tube (Coiltube) product line manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a specialized high-temperature tubing solution engineered for critical service in Ethylene Cracking Furnace (ECF) dilution steam generation systems. These coil tubes serve as the core heat exchanger elements within the dilution steam superheater coils, where they absorb radiant heat from the cracking furnace furnace wall to superheat process steam to temperatures exceeding 500–700°C, ensuring adequate steam dilution for hydrocarbon cracking reactions and preventing coke formation on coil surfaces.
Dilution steam coil tubes represent one of the most demanding applications in the petrochemical industry. The tubes are subjected to extreme thermal cycling, high-temperature oxidation, sulfur corrosion from feedstock impurities, and significant thermal stress gradients. Failure analysis studies—such as the case documented in "Failure Analysis of Dilution Steam Heating Coil Tubes in Ethylene Cracking Furnaces"—have revealed that tube rupture can result from a combination of high-temperature creep, oxidation-induced wall thinning, thermal fatigue cracking, and sulfidation attack. Understanding these degradation mechanisms is essential for selecting appropriate alloy grades, specifying manufacturing quality, and establishing maintenance intervals.
2. Material & Properties
2.1 Recommended Alloy Grades
Based on the operating environment of ethylene cracking furnace dilution steam systems, the following nickel-based and austenitic stainless steel alloys are recommended for coil tube applications:
- Alloy 800H (UNS N08810) — Nickel-Iron-Chromium alloy with high creep strength and excellent resistance to sulfur attack; the industry benchmark for dilution steam coil applications.
- Alloy 800HT (UNS N08811) — A modified variant of Alloy 800H with enhanced creep-rupture strength at elevated temperatures, offering extended service life in demanding furnace environments.
- Alloy 617 (UNS N06617) — High-temperature nickel-chromium-iron-molybdenum-tungsten alloy providing outstanding oxidation resistance and strength retention at temperatures up to 1200°C.
- Alloy 600 (UNS N06600) — Classic nickel-chromium-iron alloy with excellent resistance to oxidizing and reducing environments; suitable for moderate-temperature dilution steam service.
- Alloy 601 (UNS N06601) — Nickel-chromium-iron alloy with improved high-temperature strength over Alloy 600; suitable where moderate sulfur resistance is required.
- Alloy 625 (UNS N06625) — Nickel-chromium-molybdenum alloy offering excellent corrosion resistance and high strength; utilized in applications requiring combined resistance to sulfur and chloride environments.
- 310S / 310 (UNS S31008) — Austenitic stainless steel with high chromium and nickel content; cost-effective option for lower-temperature dilution steam applications.
2.2 Key Mechanical and Physical Properties
| Property | Alloy 800H | Alloy 800HT | Alloy 617 | Alloy 600 | 310S |
|---|---|---|---|---|---|
| Composition (Ni, wt%) | 30–35 | 30–35 | ~62 | 65–73 | 19–22 |
| Composition (Cr, wt%) | 19–23 | 19–23 | ~21 | 15–17 | 24–26 |
| Maximum Service Temperature (°C) | 1100 | 1100 | 1200 | 1095 | 1150 |
| Creep Rupture Strength (100 MPa @ 800°C, 10⁴ h) | Yes (meets criterion) | Yes (enhanced) | Yes | Limited | Limited |
| Oxidation Resistance | Excellent | Excellent | Outstanding | Excellent | Good |
| Sulfur Attack Resistance | Excellent | Excellent | Very Good | Good | Poor |
| Thermal Conductivity (W/m·K @ 100°C) | ~13.5 | ~13.5 | ~11.7 | ~14.6 | ~22.7 |
| Linear Thermal Expansion (µm/m·K, 20–800°C) | ~16.5 | ~16.5 | ~13.5 | ~16.5 | ~18.5 |
| Yield Strength (MPa @ 20°C) | ≥205 | ≥205 | ≥205 | ≥205 | ≥205 |
| Density (g/cm³) | 8.1 | 8.1 | 8.7 | 8.4 | 8.0 |
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 32 mm, 38 mm, 48 mm, 60 mm, 76 mm (custom available) | ASTM B167 / ASTM B407 |
| Tube Wall Thickness | 3.0 – 6.0 mm (typical range for dilution steam coils) | ASTM B167 |
| OD Tolerance | ±0.15 mm (±0.4% of nominal) | ASTM B167 |
| Wall Thickness Tolerance | ±10% of nominal | ASTM B167 |
| Coil Inner Diameter (ID) | 1500 – 3000 mm (depending on furnace design) | Project-specific |
| Coil Length | Up to 60 m per coil (continuous coil) | Project-specific |
| Surface Finish | Pickle and passivate; surface roughness Ra ≤ 1.6 µm | ASTM A380 |
| End Condition | Plain end, or with forged/rolled fittings as specified | ASME B31.3 |
| Deliverable Form | Continuous coil tube on coil former | — |
3.2 Heat Treatment Specifications
| Alloy Grade | Solution Treatment Temperature | Duration | Cooling Method |
|---|---|---|---|
| Alloy 800H / 800HT | 1065 – 1120°C | 1 hour per 25 mm thickness | Air cool or controlled furnace cool |
| Alloy 617 | 1065 – 1120°C | 1 hour per 25 mm thickness | Air cool |
| Alloy 600 | 1040 – 1120°C | 1 hour per 25 mm thickness | Air cool or water quench |
| 310S | 1040 – 1120°C | 1 hour per 25 mm thickness | Air cool or water quench |
3.3 Applicable Standards and Specifications
- ASTM B167 — Standard Specification for Nickel-Chromium-Iron Alloy (Alloy 600) Seamless Pipe, Tube, and Fittings
- ASTM B407 — Standard Specification for Nickel-Chromium-Iron Alloy (Alloy 617) Seamless Pipe, Tube, and Fittings
- ASTM B575 — Standard Specification for Nickel-Chromium-Iron Alloy (Alloy 800) Seamless Pipe, Tube, and Fittings
- ASTM B580 — Standard Specification for Nickel-Chromium-Iron Alloy (Alloy 800H) Seamless Pipe, Tube, and Fittings
- ASTM A380 — Standard Practice for Chemical Passivation of Stainless Steel Parts
- ASME B31.3 — Process Piping Code
- ASME BPV Code Section I — Power Boilers (where applicable)
- EN 10217-7 — Seamless heat-resistant steel tubes
- GB/T 12771 — Chinese national standard for stainless steel seamless tubes (where applicable)
4. Manufacturing Process
4.1 Production Workflow
- Raw Material Procurement: High-purity nickel, chromium, and iron ingots or master alloys are sourced from certified suppliers. Incoming materials undergo 100% chemical composition verification via OES (Optical Emission Spectroscopy) and ICP-AES (Inductively Coupled Plasma - Atomic Emission Spectroscopy) to ensure conformance to the specified alloy grade.
- Ingot Casting: Alloy compositions are melted in vacuum induction furnaces or argon oxygen decarburization (AOD) vessels under controlled atmospheres to minimize oxygen and nitrogen pickup. The molten metal is cast into ingots of appropriate size for subsequent hot working operations.
- Hot Rolling / Hot Extrusion: Cast ingots are reheated to forging temperatures and processed via hot rolling or hot extrusion to produce seamless tube blanks (rings or bars). This step establishes the initial tubular geometry with controlled microstructure.
- Piercing and Enlarging: The tube blanks are pierced and progressively enlarged through multiple passes to achieve the target outer diameter and wall thickness. Multi-stand mill configurations ensure uniform dimensional accuracy along the tube length.
- Coiling: The straight seamless tube is wound into a continuous coil of the specified inner diameter using hydraulic coiling equipment. Coil tension is carefully controlled to prevent deformation or ovality of the tube cross-section.
- Solution Heat Treatment: The coiled tube undergoes solution heat treatment at the specified temperature range to dissolve carbides and intermetallic phases, achieving a single-phase austenitic microstructure with maximum corrosion resistance and ductility.
- Pickling and Passivation: The tube surface is cleaned of oxide scale and mill scale through acid pickling (typically using a mixed acid solution of hydrofluoric and nitric acid), followed by passivation to form a protective chromium oxide film on the surface.
- Quality Inspection: Each coil undergoes comprehensive non-destructive testing (NDT) including eddy current testing (ECT) for surface and near-surface defect detection, hydrostatic testing for wall integrity verification, and dimensional measurement at multiple intervals along the coil length.
- Packaging and Delivery: The finished coil is wrapped in protective material, secured on a coil former or pallet, and prepared for shipment with full documentation including mill test certificates, traceability data, and inspection reports.
4.2 Critical Process Controls
Given the severe operating conditions of ethylene cracking furnace dilution steam systems, the following process controls are essential to ensure product reliability:
- Microstructural Control: Grain size must be controlled per ASTM E112 to prevent intergranular corrosion and ensure adequate creep resistance. Grain size number ≥ 3 is typically required.
- Sulfur and Phosphorus Limitation: Sulfur content must be maintained below 0.015% and phosphorus below 0.025% to prevent hot shortness during hot working and intergranular degradation at elevated temperatures.
- Carbon Equivalent Control: Total carbon content is restricted to prevent carbide precipitation at grain boundaries, which can lead to intergranular corrosion and reduced creep strength.
- Coil Flatness and Ovality: Ovality of the tube cross-section must not exceed 1.5% of the nominal outer diameter to ensure proper installation and stress distribution in the furnace coil configuration.
5. Applications
5.1 Primary Application: Ethylene Cracking Furnace Dilution Steam Coils
The primary application of these stainless steel coil tubes is in ethylene cracking furnace dil