Stainless Steel Coil Tube – Minimum Bending Radius Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Product No. 218) is a precision cold-formed tubular product supplied in coiled form, designed for flexible installation and maintenance applications requiring on-site bending and forming operations. This specification defines the Minimum Bending Radius — the critical geometric parameter governing the feasibility and quality of bending operations on stainless steel coil tubes. Compliance with the minimum bending radius ensures structural integrity, dimensional accuracy, and resistance to mechanical failure during and after the bending process.
This product falls under the classification of Installation & Maintenance (一级分类), specifically within the Bending Processing (弯曲加工) sub-category, with the technical focus on determining the minimum permissible bending radius based on material temper condition.
2. Material & Properties
Stainless steel coil tubes are typically manufactured from austenitic stainless steel grades, most commonly SUS304 (UNS S30400), SUS316 (UNS S31600), and SUS321 (UNS S32100), among others. The mechanical properties governing bendability are primarily determined by the work-hardening temper (冷作硬化状态) of the tube material:
- Soft Annealed (Soft / SA — Solution Annealed): The material has undergone full annealing treatment, resulting in maximum ductility and elongation. This temper offers the greatest formability and the smallest minimum bending radius.
- Semi-Hard (Half-Hard / HH): The material has undergone partial cold work, providing a balance between strength and ductility. The minimum bending radius increases accordingly.
- Hard (Full-Hard / F): The material has undergone extensive cold working, achieving high yield strength but significantly reduced ductility. Bending operations are not recommended in this condition.
Key material properties influencing bendability include:
- Elongation at Break (A50mm): Typically ≥ 35% (SA), ≥ 25% (HH)
- Yield Strength (Rp0.2): 205–310 MPa (SA), 310–520 MPa (HH), ≥ 520 MPa (F)
- Tensile Strength (Rm): 515–750 MPa (SA), 520–750 MPa (HH), ≥ 750 MPa (F)
- Work Hardening Exponent (n-value): Higher n-value indicates superior formability
3. Minimum Bending Radius Specifications
The minimum bending radius (Minimum Bend Radius, MBR) is expressed as a multiple of the outer diameter (OD) of the tube and is directly dependent on the temper condition. The following table summarizes the reference values as specified in the manufacturer's technical manual:
| Temper Condition | Temper Designation | Minimum Bending Radius (Tangent) | Bending Recommendation | Typical Formability Rating |
|---|---|---|---|---|
| Soft Annealed | SA (Solution Annealed) | ≈ 3 × OD | Recommended — Optimal for tight bends | Excellent |
| Semi-Hard | HH (Half-Hard) | ≈ 5 × OD | Acceptable with appropriate tooling | Moderate |
| Hard | F (Full-Hard) | Not Applicable | Not Recommended — Bending prohibited | Poor / None |
3.1 Defect Modes When Minimum Bending Radius Is Exceeded
Failure to observe the minimum bending radius results in characteristic manufacturing defects that compromise the structural integrity and service life of the bent tube:
| Defect Type | Location | Root Cause | Severity |
|---|---|---|---|
| Excessive Outer Wall Thinning (外侧减薄超标) | Outer surface of bend (tension side) | Tensile strain exceeds material's uniform elongation limit; cross-sectional area reduction surpasses acceptable tolerance (typically > 15–20%) | Critical — Risk of rupture, corrosion acceleration, pressure vessel failure |
| Inner Wall Wrinkling (内侧起皱) | Inner surface of bend (compression side) | Compressive buckling of the tube wall due to insufficient mandrel support or excessive bend tightness | Major — Flow restriction, surface defect, fatigue initiation site |
3.2 Acceptance Criteria for Bent Tubes
- Wall Thinning Tolerance: Maximum permissible reduction in wall thickness on the outer bend radius shall not exceed 15% for pressure-containing applications and 20% for non-pressure applications (per ASME B31.3 / EN 13480 reference standards).
- Wrinkling Tolerance: No visible wrinkles or folds on the inner bend radius; surface profile deviation shall not exceed 0.5 mm.
- Dimensional Tolerance: Bend angle accuracy ± 1°; bend radius accuracy ± 5% of specified value.
4. Manufacturing Process
4.1 Coil Tube Production
The stainless steel coil tube is manufactured through the following sequential processes:
- Strip Milling: Stainless steel strip (typically 2B or BA finish) is slitted to the required width.
- Coil Forming & Welding: The strip is roll-formed into a tube shape and longitudinally welded using TIG (Tungsten Inert Gas) welding or laser welding technology, followed by internal and external weld seam grinding.
- Sizing & Cutting: The formed tube is passed through sizing rollers to achieve precise outer diameter and wall thickness tolerances, then cut to coil length.
- Heat Treatment (if applicable): Solution annealing in a protective atmosphere (argon or hydrogen) at 1050–1150 °C followed by water quenching to achieve the desired temper condition.
- Surface Finishing: Pickling and passivation (酸洗钝化) to remove scale and restore corrosion resistance.
- Coiling & Packaging: The finished tube is coiled on a mandrel and packaged for shipment.
4.2 Bending Process Considerations
- Tooling Selection: Use of properly sized bend dies (上模/下模) and mandrel (芯轴) to support the inner wall during bending.
- Bend Ratio Calculation: The minimum bend radius must be calculated as a factor of OD based on temper: R_min = k × OD, where k = 3 (SA) or k = 5 (HH).
- Speed Control: Bending speed should be controlled to minimize strain-rate sensitivity effects and reduce work-hardening during the operation.
- Post-Bend Inspection: Visual inspection, wall thickness measurement (ultrasonic), and dimensional verification are mandatory.
5. Applications
Stainless steel coil tubes with controlled minimum bending radii find extensive application in the following industries:
- Hydraulic & Pneumatic Systems: Fluid conveyance lines requiring tight bends in confined spaces (automotive, aerospace, industrial machinery).
- Heat Exchangers & Refrigeration: Coil tubes for condensers, evaporators, and chiller systems where serpentine bending is required.
- Automotive Industry: Exhaust systems, fuel lines, brake lines, and turbocharger piping.
- Medical Equipment: Tubing for endoscopes, surgical instruments, and medical gas delivery systems.
- Food & Pharmaceutical Processing: Hygienic piping systems requiring in-place bending and welding.
- Construction & Architectural: Handrails, guardrails, structural framing, and decorative elements.
- Energy Sector: Solar thermal collectors, geothermal systems, and nuclear-grade coolant loops.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. ensures that all coil tube products conform to internationally recognized quality standards and certifications:
- ISO 9001:2015 — Quality Management System certification
- ISO 3834 — Quality requirements for welding of metallic materials
- EN 10216-5 — Technical delivery conditions for seamless and welded circular hollow sections
- ASTM A554 / ASTM A513 — Standard specifications for stainless steel welded tubing
- JIS G 3459 — Cold-rolled stainless steel tubes (Japanese Industrial Standard)
- GB/T 14976 — Chinese National Standard for cold-rolled seamless stainless steel tubes
- CE Marking — Conformity with European pressure equipment directives (PED 2014/68/EU)
- Mill Test Certificates (MTC / EN 10204 3.1) — Provided with each shipment
Important Note: The minimum bending radius values specified in this document are derived from the manufacturer's technical manual and represent reference guidelines. Actual bendability may vary depending on specific alloy composition, wall thickness-to-diameter ratio (t/D), tube finish, and environmental conditions (temperature, strain rate). It is strongly recommended to perform trial bends (试弯) on sample material prior to mass production to validate the minimum bend radius under actual production conditions. Always consult the latest edition of the manufacturer's technical data sheet for updated specifications.
7. Unique Selling Points
- Temper-Specific Bendability Data: Precise minimum bending radius values provided for each temper condition, enabling engineers to select the optimal material state for their specific application.
- Defect Prevention: Clear identification of failure modes (outer wall thinning and inner wall wrinkling) with quantified acceptance criteria ensures first-time-right bending operations.
- Full Traceability: Every coil tube batch is traceable to its heat number, with complete mechanical property and chemical composition documentation.
- Technical Support: Dedicated engineering team available for bend simulation, tooling design consultation, and on-site process validation.
- Comprehensive Grade Range: Availability across multiple stainless steel grades and temper conditions to meet diverse performance and regulatory requirements.