Stainless Steel Coil Tube — Reverse-Bend Certified for Repeated Flexing Applications
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd. offers a premium-grade stainless steel coil tube (Coiltube) engineered for applications demanding superior fatigue resistance under repeated bending and flexing conditions. Unlike conventional straight seamless tubes, coil tubes are formed into continuous helical or serpentine configurations, subjecting the material to cyclic mechanical stress throughout their service life. To address this unique challenge, Wenzhou Jixing has developed and implemented a rigorous Reverse Alternating Bending Test (正反交替弯曲至规定次数) — a coil-tube-specific qualification procedure that is significantly more stringent than standard single-bend tests found in general tubing specifications.
This product line is designed for demanding industrial environments where tubing must endure thousands of flex cycles without cracking, delamination, or loss of mechanical integrity. The coil tube is available in austenitic and duplex stainless steel grades, cold-worked to achieve the necessary combination of ductility, fatigue strength, and corrosion resistance.
Unique Selling Points
- Industry-Leading Reverse Bending Qualification: Every production lot undergoes alternating forward-and-reverse bending to a specified cycle count, simulating real-world repeated flexing conditions far beyond the scope of standard single-bend tests.
- Coil-Tube-Specific Testing Protocol: Recognizing that coil tubes experience fundamentally different stress profiles than straight tubes, Wenzhou Jixing has developed a dedicated test methodology (Test Item No. 102) that accurately predicts field performance.
- Enhanced Fatigue Life: The reverse-bend certified tubes demonstrate superior resistance to low-cycle fatigue failure, extending service life in dynamic thermal and mechanical environments.
- Full Traceability: Each coil tube batch carries complete test records documenting the number of alternating bend cycles achieved, providing end-users with verifiable performance data.
Material & Properties
Available Grades
- 304 / 304L — General-purpose austenitic grade with excellent corrosion resistance and formability
- 316 / 316L — Enhanced chloride and pitting resistance for aggressive chemical environments
- 321 — Stabilized grade with added titanium for high-temperature service
- 310S — High-temperature austenitic grade for furnace and heat-treatment applications
- Duplex 2205 — High-strength duplex grade for demanding mechanical and corrosion environments
Mechanical Properties (Typical — 304L Grade, Cold-Worked)
- Tensile Strength: ≥ 515 MPa (≥ 75 ksi)
- Yield Strength (0.2% offset): ≥ 205 MPa (≥ 30 ksi)
- Elongation (at break): ≥ 40%
- Reverse Alternating Bending Cycles: ≥ 100 cycles to failure (specification-dependent)
- Hardness: ≤ 250 HV (annealed); ≤ 350 HV (cold-worked)
Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) |
| Material Grades | 304, 304L, 316, 316L, 321, 310S, 2205 Duplex |
| Outer Diameter (OD) | 3.0 – 50.8 mm (0.118" – 2.000") |
| Wall Thickness | 0.25 – 3.0 mm (0.010" – 0.118") |
| Coil Diameter | 500 – 2000 mm (19.7" – 78.7") |
| Surface Finish | No. 1, 2B, BA (Bright Annealed), Pickled & Passivated |
| Manufacturing Method | Seamless / Welded (per client requirement) |
| Standards | ASTM A269, ASTM A312, EN 10216-5, ISO 13908 |
| Reverse Bend Test Cycles | As per specification — typically ≥ 100 alternating cycles |
| Delivery Condition | Annealed, Solution-Annealed, or Cold-Worked (H) |
| Maximum Coil Length | Up to 500 m (1,640 ft) per coil |
Manufacturing Process
Production Workflow
- Raw Material Procurement: Certified stainless steel slabs or billets sourced from accredited mills with full chemical composition traceability.
- Piercing & Extrusion: Hot-pierced billets are extruded to produce seamless tube blanks at precise wall thickness tolerances.
- Pilger Milling / Cold Drawing: Extruded blanks are cold-drawn through a series of dies to achieve final OD and wall dimensions within ±0.05 mm tolerance.
- Heat Treatment (Annealing): Tubes are solution-annealed at 1040–1120 °C (1904–2048 °F) followed by controlled water quenching to restore full ductility and optimize grain structure for subsequent forming.
- Pickling & Passivation: Surface oxide scale is removed via acid pickling, followed by passivation treatment to establish a protective chromium oxide layer.
- Coil Forming: Straight tubes are fed through precision forming stations to produce helical or serpentine coil configurations with controlled bend radii and pitch.
- Stress Relief: Post-forming stress relief annealing eliminates residual stresses introduced during the coiling process, ensuring dimensional stability.
- Quality Inspection & Testing: Comprehensive testing including hydrostatic pressure testing, dimensional verification, and the proprietary reverse alternating bending test.
- Packaging & Dispatch: Coils are wrapped in protective material, secured on steel pallets or wooden crates, and prepared for shipment.
Reverse Alternating Bending Test — Detailed Description
Test Protocol (Test Item No. 102 — Process Performance): The Reverse Alternating Bending Test subjects a sample tube specimen to repeated forward and reverse bending cycles around a fixed mandrel. The specimen is bent forward to a specified angle, then reversed back to the original position, and this cycle is repeated until failure or until the specified cycle count is achieved without cracking.
Test Methodology
- Specimen Preparation: A tube segment is cut from the production coil and prepared per ASTM E199 or equivalent methodology.
- Mandrel Diameter: Selected based on tube OD and wall thickness — typically 3D to 6D (where D = tube outer diameter).
- Bend Angle: 90° forward, then 90° reverse, constituting one complete alternating cycle.
- Acceptance Criteria: No cracks, splits, or surface defects exceeding specified limits after the required number of alternating cycles.
- Stringency: This test is inherently more demanding than single-bend tests because the material experiences alternating tensile and compressive stresses on opposite tube walls, accelerating fatigue crack initiation and propagation.
Why Reverse Bending Matters for Coil Tubes
In real-world applications, coil tubes are installed with bends, subject to thermal expansion and contraction, and may experience mechanical vibration. The reverse alternating bending test directly simulates these cyclic loading conditions, providing a far more accurate prediction of in-service fatigue performance than conventional single-bend tests. This makes Wenzhou Jixing's testing protocol a critical differentiator for applications where tube integrity under repeated flexing is paramount.
Applications
- Heat Exchangers & Boilers: Helical coil tubes in shell-and-coil heat exchangers, waste heat recovery units, and industrial boilers where thermal cycling induces repeated flexing.
- Automotive & HVAC Systems: Engine cooling coils, radiator coils, and air conditioning evaporator/condenser tubes subjected to vibration and thermal cycling.
- Chemical Processing: Reactor cooling coils and heat transfer coils in corrosive chemical environments requiring both corrosion resistance and fatigue durability.
- Food & Pharmaceutical: Sanitary heat transfer coils and pasteurization equipment where repeated flexing occurs during thermal processing cycles.
- Power Generation: Superheater coils, economizer coils, and air preheater tubes in fossil-fuel and nuclear power plants experiencing significant thermal cycling.
- Oil & Gas: Subsea heat exchanger coils and process piping coils in offshore platforms and LNG facilities.
Quality Certifications
Company Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational Health & Safety Management System
- CE Marking — Compliance with EU Pressure Equipment Directive (PED 2014/68/EU)
- EN 10204 Type 3.1 Inspection Certificates available upon request
Product Testing & Compliance
- Chemical Analysis (Spectrometric) — per ASTM A269 / ASTM A312
- Hydrostatic Pressure Testing — per ASTM A269 Section 12
- Dimensional Inspection — OD, wall thickness, and coil geometry verification
- Surface Inspection — Visual and eddy-current examination for defects
- Reverse Alternating Bending Test (Test Item No. 102) — Coil-tube-specific fatigue qualification
- Intergranular Corrosion Testing (ASTM A240) — for stabilized and low-carbon grades
- Pitting Resistance Testing (ASTM G48) — for 316/316L and duplex grades
Technical Support & Customization
Wenzhou Jixing Metal Materials Co., Ltd. offers full customization of coil tube specifications including OD, wall thickness, coil diameter, pitch, bend radius, and grade selection. Engineering support is available to assist clients in selecting the optimal material grade and configuration for their specific application, with particular emphasis on fatigue life requirements. Custom reverse-bend test protocols can be developed for applications with unique cyclic loading conditions, ensuring that every coil tube delivered meets the exact performance demands of the end-use application.