Stainless Steel Coil Tube with Precision Ferrite Number (FN) Microstructure Control
Product Overview
The Stainless Steel Coil Tube with Precision Ferrite Number (FN) Microstructure Control is a premium-grade coiled stainless steel tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd, a leading supplier in the metallurgical materials sector. This product is distinguished by its rigorous control of the ferrite phase content within the microstructure, ensuring superior weldability, resistance to hot cracking, and optimal phase balance — particularly critical for austenitic and duplex stainless steel applications.
Ferrite Number (FN) is a critical metallurgical parameter that quantifies the ferrite phase fraction in stainless steel weld metals and base materials. Through advanced microstructure engineering and precision manufacturing, this coil tube product delivers predictable, repeatable ferrite content across the entire coil length, enabling end-users to achieve reliable performance in demanding welding and fabrication scenarios.
Material & Properties
Ferrite Number (FN) — The Defining Quality Parameter
The Ferrite Number is determined using internationally recognized methodologies:
- Schaeffler/Delong Diagram Method: A thermodynamic calculation approach that estimates the ferrite content based on the chemical composition of the steel, particularly the balance of ferrite-stabilizing elements (Cr, Mo, Si, Nb) and austenite-stabilizing elements (Ni, C, N, Mn). This method provides a predictive baseline for microstructure evolution during welding.
- Magnetic Measurement Method (Ferrostat/Ferroxjour): A non-destructive, real-time measurement technique that quantifies the ferrite phase fraction by exploiting the magnetic permeability difference between ferrite (ferromagnetic) and austenite (paramagnetic). This method offers direct, empirical validation of FN values in the as-welded or as-processed condition.
Key Metallurgical Indicators
- Austenitic Stainless Steel Welds: Ferrite Number maintained in the range of FN 3–10, providing adequate resistance to solidification cracking (hot cracking) while preserving the desired austenitic properties including ductility, toughness, and corrosion resistance.
- Duplex Stainless Steel: Ferrite/austenite phase balance controlled at 40–60%, ensuring optimal mechanical strength (yield strength up to 550 MPa), excellent pitting resistance (PREN ≥ 35), and resistance to stress corrosion cracking (SCC).
Grade Options
- Austenitic Grades: 304, 304L, 316, 316L, 321, 347, 310S
- Duplex Grades: 2205 (UNS S32205), 2507 (UNS S32750), 1.4462
- Super Duplex: 2507, 1.4501
Specifications
| Parameter | Specification |
|---|---|
| Product Form | Coiled Tube (Continuous Coil) |
| Outer Diameter (OD) | 3.0 mm – 168.0 mm |
| Wall Thickness | 0.3 mm – 12.0 mm |
| Coil Length | Customizable (typically 50 m – 300 m per coil) |
| Ferrite Number — Austenitic | FN 3–10 (Schaeffler/Delong or Magnetic Method) |
| Ferrite Number — Duplex | 40–60% Ferrite Phase Balance |
| Surface Finish | Bright Annealed (BA), Pickled & Passivated, Polished (2B, No.4, Mirror) |
| Manufacturing Method | Seamless / Welded (ERW, TIG, FBE) |
| Heat Treatment | Solution Annealing, Stabilization Annealing (as required) |
| Standards | ASTM A213, ASTM A269, EN 10216-5, EN 10217-7, GB/T 12771, GB/T 21832 |
| Tensile Strength | Grade-dependent (515–800 MPa typical) |
| Yield Strength | 205–550 MPa (grade-dependent) |
| Weldability | Excellent — controlled FN minimizes hot cracking susceptibility |
Manufacturing Process
The production of this precision FN-controlled coil tube involves a multi-stage manufacturing process designed to ensure microstructural integrity and consistent ferrite content:
- Raw Material Selection & Composition Control: High-purity stainless steel billets are selected with tightly controlled chemical compositions. The ratio of ferrite-stabilizing to austenite-stabilizing alloying elements is precisely calibrated to achieve the target FN range.
- Hot Rolling & Coiling: Billets are reheated to controlled temperatures and hot-rolled into seamless tubes or welded pipe blanks. Rolling parameters are optimized to prevent excessive grain growth and maintain uniform microstructure.
- Heat Treatment: Tubes undergo solution annealing (typically 1050°C–1150°C for austenitic grades; 1020°C–1100°C for duplex grades) followed by controlled cooling. For duplex grades, cooling rates are carefully managed to maintain the 40–60% phase balance without precipitating intermetallic phases (σ, χ, Laves).
- FN Verification: Each production lot is tested using both the Schaeffler/Delong diagram calculation (based on certified chemical analysis) and magnetic ferrite measurement (Ferrostat/Ferroxjour). Results are documented in the mill test certificate.
- Coiling & Packaging: Finished tubes are coiled onto steel cores, wrapped in protective film or paper, and palletized for shipment. Custom coil dimensions are available upon request.
Applications
The precise control of Ferrite Number makes this coil tube product ideal for applications where weld integrity, crack resistance, and phase balance are paramount:
- Heat Exchangers & Boilers: Austenitic coil tubes (304L, 316L) with FN 3–10 provide excellent weldability for TIG-welded headers and tube sheets in power generation and petrochemical heat exchangers.
- Pressure Vessels & Piping Systems: Duplex 2205 and 2507 coil tubes with 40–60% phase balance deliver high yield strength and SCC resistance for high-pressure offshore platforms, subsea pipelines, and chemical processing vessels.
- Oil & Gas Industry: Corrosion-resistant coil tubes for wellhead equipment, flowlines, and separation systems exposed to chlorides, CO₂, and H₂S.
- Food & Pharmaceutical Processing: Bright annealed austenitic coil tubes for sanitary piping, CIP systems, and hygienic equipment requiring frequent welding.
- Marine & Offshore Engineering: Duplex steel coil tubes for seawater cooling systems, ballast tanks, and desalination plants.
- Automotive & Aerospace: Precision coil tubes for exhaust systems, fuel lines, and structural components requiring high-temperature resistance and controlled weldability.
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains rigorous quality assurance protocols throughout the production lifecycle:
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001 — Environmental Management System
- Mill Test Certificates (MTC): EN 10204 Type 3.1 or 3.2 certificates provided with every shipment, including full chemical analysis, mechanical test results, and FN verification data
- Third-Party Inspection: Available from SGS, Bureau Veritas, Lloyd's Register, and DNV upon request
- Material Traceability: Full heat-to-heat traceability from raw material mill to finished coil, enabling recall and audit compliance
- Non-Destructive Testing (NDT): Eddy current, ultrasonic, and hydrostatic testing available for weld-seam integrity verification
Unique Selling Points
Precision FN Control as a Competitive Advantage: Unlike standard stainless steel coil tube suppliers, Wenzhou Jixing Metal Materials Co., Ltd integrates Ferrite Number measurement and control directly into the manufacturing process. This ensures that every coil delivered meets the specified FN range — eliminating field failures due to hot cracking, phase imbalance, or microstructural degradation.
- Dual-Method FN Verification: Every lot is validated through both thermodynamic calculation (Schaeffler/Delong) and empirical magnetic measurement, providing redundant confidence in microstructural properties.
- Custom FN Targeting: End-users can specify exact FN ranges tailored to their welding procedures, filler metal selections, and service conditions.
- Duplex Phase Balance Guarantee: For duplex and super duplex grades, the 40–60% ferrite/austenite balance is verified and guaranteed, ensuring optimal combination of strength, toughness, and corrosion resistance.
- Consistent Coil Quality: Advanced coil manufacturing and handling ensures uniform microstructure from the first meter to the last, eliminating property variation within a single coil.
- Technical Support: Dedicated metallurgical engineering team available to assist with FN specification, welding procedure qualification, and application-specific material selection.
Conclusion
The Stainless Steel Coil Tube with Precision Ferrite Number Microstructure Control from Wenzhou Jixing Metal Materials Co., Ltd represents the intersection of metallurgical science and manufacturing excellence. By placing the Ferrite Number at the center of quality control — whether targeting FN 3–10 for austenitic weld crack resistance or 40–60% phase balance for duplex performance — this product delivers the reliability, weldability, and durability that modern industrial applications demand. For specifications, sampling, or technical consultation, contact Wenzhou Jixing Metal Materials Co., Ltd directly.