Stainless Steel Coil Tube — Mandrel Bending 180° Process Performance Test Specification
1. Product Overview
The stainless steel coil tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a cold-rolled, seamless or welded tubular product supplied in coil form, designed for secondary forming operations including bending, rolling, and welding. This technical specification focuses on the Process Performance (工艺性能) verification of the coil tube, specifically the Bending Test (弯曲试验) conducted around a mandrel to 180°, which serves as the foundational validation of the material's tube-bending processability (弯管加工适应性基础验证).
The mandrel bending test at 180° is a critical quality gate that evaluates the ductility, strain-hardening behavior, and microstructural integrity of the stainless steel coil tube under severe plastic deformation conditions. This test simulates the most demanding forming scenarios encountered in downstream manufacturing, ensuring that the coil tube can withstand complex geometric transformations without material failure.
2. Material & Properties
Stainless steel coil tubes used for mandrel bending applications are typically fabricated from austenitic or austenitic-ferritic grades, selected for their exceptional formability, corrosion resistance, and weldability. The following material characteristics are essential for successful 180° mandrel bending:
- Grade Options: Commonly available in grades such as SUS304 (UNS S30400), SUS316 (UNS S31600), SUS321 (UNS S32100), and SUS430 (UNS S43000), with additional grades available upon request.
- Microstructure: A fully annealed austenitic structure provides maximum ductility and minimizes the risk of strain-induced martensitic transformation during bending.
- Ductility Requirements: Elongation (A) values of ≥40% for austenitic grades ensure sufficient plastic strain capacity to accommodate the 180° bend without fracture.
- Strain Hardening Exponent (n-value): Higher n-values indicate superior formability and uniform strain distribution during bending operations.
- Surface Quality: Bright annealed (BA) or 2B finish surfaces reduce friction during mandrel bending and prevent surface damage initiation sites.
3. Specifications — Bending Test Parameters
| Parameter | Specification | Standard Reference |
|---|---|---|
| Test Designation | Mandrel Bending 180° (绕心轴弯曲180°) | ASTM A269 / ASTM A554 / GB/T 12771 |
| Bending Angle | 180° | — |
| Mandrel Diameter (D) | Varies by tube OD and wall thickness; typically D ≥ 3×OD for thin-wall, D ≥ 5×OD for thick-wall | ASTM A269 Table 12 |
| Acceptance Criteria — Outer Surface | No cracks (外侧无裂纹) | Visual inspection under 2× magnification |
| Acceptance Criteria — Inner Surface | No obvious wrinkles (内侧无明显皱褶) | Visual inspection under 2× magnification |
| Test Temperature | Ambient temperature (20°C ± 5°C), unless otherwise specified | ASTM A269 §9 |
| Number of Test Specimens | Minimum 3 specimens per heat/lot | Internal QC protocol |
| Applicable Tube OD Range | 6 mm – 150 mm | — |
| Applicable Wall Thickness Range | 0.5 mm – 6.0 mm | — |
4. Manufacturing Process
The production of stainless steel coil tube suitable for 180° mandrel bending involves a tightly controlled multi-stage manufacturing sequence:
- Hot Rolling: Billet or slab is hot-rolled into strip or round bar stock with precise dimensional tolerances.
- Pickling and Descaling: The hot-rolled product undergoes acid pickling to remove scale and restore the base metal surface for subsequent cold working.
- Cold Forming (Piercing/Seam Welding/ERW): The strip or bar is cold-formed into tube via seamless piercing (for seamless products) or electric resistance welding (ERW) / submerged arc welding (SAW) processes.
- Cold Drawing / Cold Reduction: Further dimensional precision and surface quality improvement through cold drawing or cold rolling passes.
- Solution Annealing (Solution Heat Treatment): Tubes are annealed at 1050°C – 1150°C followed by rapid quenching to achieve a fully austenitic, strain-free microstructure. This step is critical for maximizing ductility and ensuring successful mandrel bending performance.
- Pickling and Passivation: Post-annealing pickling removes any scale, followed by passivation to establish a stable chromium oxide protective layer.
- Coiling: The finished tube is wound onto a coil with controlled tension to prevent deformation and surface damage during storage and transportation.
5. Applications
Stainless steel coil tubes that pass the 180° mandrel bending test are suitable for a wide range of applications requiring complex forming operations:
- Automotive Industry: Exhaust system components, fuel lines, turbocharger piping, and structural brackets requiring tight-radius bends.
- Appliance Manufacturing: Refrigerator coils, dishwasher racks, washing machine components, and heating element housings.
- Medical Devices: Orthopedic implant fabrication, surgical instrument handles, and endoscope tubing.
- Chemical Processing: Heat exchanger tubes, reactor internals, and process piping requiring corrosion-resistant bent geometries.
- Architecture and Decoration: Handrails, balustrades, window frames, and ornamental features with complex profiles.
- Aerospace Components: Fuel lines, hydraulic tubing, and structural elements requiring high formability and integrity.
- Furniture Manufacturing: High-end furniture frames, shelving systems, and decorative elements.
6. Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems ensuring compliance with international standards:
- ISO 9001:2015 — Quality Management System certification for consistent product quality and process control.
- ASTM A269 — Standard Specification for Welded, Austenitic Chromium and Chromium-Nickel Stainless Steel Tubes for Boiler, Heat Exchanger, and Similar Heat Absorbing Applications (bending test methodology).
- ASTM A554 — Standard Specification for Cold-Wrought Austenitic Stainless Steel Tube for Chemical and General Applications.
- EN 10216-5 — Technical delivery conditions for seamless and welded stainless steel tubes for pressure purposes.
- GB/T 12771 — Chinese national standard for stainless steel seamless tubes for general purposes.
- JIS G3445 — Japanese Industrial Standard for austenitic stainless steel tubes.
- Mill Test Certificates (MTC): EN 10204 Type 3.1 certificates provided with each shipment, including bending test results.
7. Unique Selling Points
- Verified 180° Bendability: Every production lot undergoes mandatory mandrel bending verification to 180°, providing customers with documented proof of exceptional formability and eliminating downstream forming failures.
- Dual-Surface Quality Assurance: Comprehensive inspection of both the outer surface (crack-free) and inner surface (wrinkle-free) ensures complete integrity under maximum strain conditions.
- Foundation for Complex Forming: The 180° mandrel bend test serves as the baseline verification for tube-bending processability (弯管加工适应性基础验证), guaranteeing that the coil tube can withstand subsequent manufacturing operations including rolling, welding, and multi-axis forming.
- Full Traceability: Each coil is traceable to its heat number, annealing batch, and test results, enabling complete supply chain accountability.
- Wide Grade Availability: Multiple stainless steel grades available to match specific corrosion, mechanical, and forming requirements.
- Coil Supply Advantage: Continuous coil format maximizes material utilization and minimizes waste during automated tube-bending production lines.
Note: The mandrel bending test at 180° represents the most stringent process performance verification for stainless steel coil tubes. Passing this test confirms that the material possesses adequate ductility, strain-hardening resistance, and microstructural stability to survive severe plastic deformation. This specification ensures that customers receive coil tube products with guaranteed forming capability, reducing production rejects and ensuring manufacturing continuity in downstream applications.