Stainless Steel Coil Tube — Reverse Bending Test Specification (Alternating Bending to Specified Cycles)
Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered austenitic stainless steel tubing product designed for applications requiring exceptional fatigue resistance under repeated mechanical deformation. Unlike conventional straight-length stainless steel tubes, coil tubes are supplied in continuous helical coil form, enabling seamless integration into bending-intensive manufacturing processes such as automotive exhaust systems, heat exchanger coil winding, and hydraulic hose production.
The defining quality assurance protocol for this product is the Reverse Bending Test (Alternating Bending Test) — a dedicated mechanical performance evaluation that subjects the tube to repeated forward-and-reverse bending cycles up to a specified number of repetitions without cracking, splitting, or surface defect initiation. This test methodology is inherently more stringent than single-direction bending tests, as it simulates the true fatigue loading conditions encountered in coil tube service applications.
Material & Properties
Coil tubes are typically fabricated from austenitic stainless steel grades selected for their superior ductility, workability, and cyclic deformation tolerance. The material must maintain full recrystallized grain structure through multiple bending cycles to prevent fatigue crack propagation at the outer bending radius.
- Primary Grades: SUS304 (1.4301), SUS316 (1.4401), SUS321 (1.4845), SUS316L (1.4404)
- Microstructure: Fully austenitic (FCC — Face-Centered Cubic), with controlled grain size per ASTM E112 standards
- Key Mechanical Properties:
- Tensile Strength (UTS): ≥ 520 MPa (for 304 grade)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation: ≥ 40% (in 50 mm gauge length)
- Recovery Coefficient: ≥ 25%
- Surface Finish: Bright annealed (BA), 2B cold-rolled, or pickled & passivated (P&H) — free from scale, oxide inclusions, and surface discontinuities
- Chemical Composition Control: Carbon content ≤ 0.08% (304) or ≤ 0.03% (304L) to minimize sensitization risk during repeated deformation
Specifications
| Parameter | Specification / Range | Test Standard |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 50.8 mm (1/8" – 2") | — |
| Wall Thickness (WT) | 0.3 mm – 3.0 mm | — |
| Coil Diameter | 300 mm – 1,200 mm (ID) | — |
| Coil Length | Continuous coil, typically 500 m – 5,000 m per coil | — |
| Reverse Bending Test — Number of Cycles | ≥ 30 to ≥ 100 alternating cycles (grade-dependent) | GB/T 247, ASTM E290 |
| Reverse Bending Test — Mandrel Diameter (D) | D/t ≥ 3 (where t = wall thickness) | GB/T 247 |
| Reverse Bending Test — Angle per Cycle | 180° forward / 180° reverse per complete cycle | — |
| Acceptance Criteria | No cracks, splits, or surface defects ≥ 1 mm visible to naked eye on outer or inner surface | GB/T 247, ASTM E290 |
| Test Temperature | Ambient (room temperature, 20 ± 5°C) | — |
| Material Condition | Full solution annealed (1050°C – 1150°C, water quenched) | — |
Manufacturing Process
The production of stainless steel coil tubes adheres to a rigorous multi-stage manufacturing sequence to ensure consistent material properties and dimensional accuracy:
- Slit Coil Preparation: Cold-rolled stainless steel strip (slit coil) is received and inspected for surface quality, thickness uniformity (±5% tolerance), and flatness (within 0.5 mm/m).
- Forming (Roll Forming / Seamless Piercing): The slit coil is fed through a precision roll-forming die set to produce the desired tube cross-section (round, oval, rectangular, or D-shape) at controlled feed rates to minimize strain hardening.
- Welding (for welded coil tubes): TIG (Tungsten Inert Gas) welding is applied at the longitudinal seam with argon shielding (99.99% purity) to prevent oxidation. Weld bead height is controlled to ≤ 0.1 mm above the base metal surface.
- Solution Annealing: The formed tube is heat-treated at 1050°C – 1150°C in a controlled atmosphere (vacuum or protective gas) followed by rapid quenching to achieve full recrystallization and eliminate cold-work residual stresses.
- Coiling: The annealed tube is wound into helical coils on precision mandrels with controlled winding tension to prevent inter-coil contact damage and maintain dimensional stability.
- Surface Treatment: Optional pickling and passivation (HNO₃/HF mixture) to remove any residual oxide and establish a protective chromium oxide passive layer.
- Final Inspection & Packaging: Each coil undergoes dimensional verification, visual surface inspection, and the critical reverse bending test before being packaged in protective wrapping with coil guards.
Reverse Bending Test — Detailed Methodology
The reverse bending test (also referred to as the Alternating Bending Test or Reciprocating Bend Test) is a coil-tube-specific quality assurance protocol that exceeds the severity of standard single-bend evaluations:
Test Procedure
- Specimen Preparation: A tube segment of minimum 150 mm length is cut from the coil at a position representative of the production batch (typically the middle section).
- Fixture Setup: The specimen is mounted in a specialized reciprocating bend fixture with a hardened steel mandrel of specified diameter (D/t ratio per grade requirements).
- Alternating Cycles: The tube is bent forward through 180° around the mandrel, then immediately reversed through 180° in the opposite direction. Each forward-reverse pair constitutes one complete cycle.
- Cycle Count: Bending continues for the specified number of cycles (typically 30–100 cycles depending on grade, OD, and wall thickness).
- Post-Test Examination: After completion, the specimen is inspected under magnification (5× minimum) for:
- Cracks on the outer bending surface (tensile side during each cycle)
- Splits or delamination on the inner bending surface (compressive side)
- Surface roughening or grain boundary cracking indicative of fatigue degradation
- Weld seam integrity (for welded tubes) — no weld cracking or seam separation
Rationale for Enhanced Stringency
The reverse bending test is inherently more demanding than single-direction bending because each reversal of the bending direction subjects the outer fiber to alternating tensile and compressive stresses. This creates a fatigue loading condition that accelerates crack initiation and propagation. A tube that passes a single 180° bend test may fail catastrophically under even 10–15 alternating cycles. Therefore, the alternating bending test serves as the definitive qualification criterion for coil tubes intended for production applications involving repeated forming, coil winding/unwinding, or in-service flexing.
Applications
Stainless steel coil tubes that pass the reverse bending test are qualified for demanding industrial applications:
- Automotive Industry: Exhaust system tubing, turbocharger piping, fuel injection lines — where tubing undergoes repeated thermal cycling and mechanical flexing
- Heat Exchanger Manufacturing: Coil winding for air-to-air and air-to-liquid heat exchangers, refrigeration coils, and HVAC systems requiring tight bending radii
- Hydraulic & Pneumatic Systems: High-pressure tubing that experiences cyclic pressure loading and mechanical vibration
- Food & Pharmaceutical Processing: Sanitary tubing for CIP (Clean-In-Place) systems where repeated assembly/disassembly bending is required
- Construction & Architecture: Decorative handrails, structural coil applications requiring repeated forming during fabrication
- Medical Equipment: Tubing for fluid delivery systems requiring repeated flexing without failure
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality assurance systems ensuring full traceability and compliance with international standards:
- ISO 9001:2015 — Quality Management System certification covering all production and testing processes
- ISO 14001:2015 — Environmental Management System for sustainable manufacturing practices
- GB/T 247-2008 — Chinese national standard for reverse bending tests on stainless steel tubes
- ASTM E290 — Standard test method for reverse bending of stainless steel wire/tube
- EN 10217-7 — European standard for seamless and welded austenitic stainless steel tubes
- JIS G 3459 — Japanese Industrial Standard for austenitic stainless steel tubes
- Mill Test Certificates (MTC): EN 10204 Type 3.1 / Type 3.2 with full chemical analysis, mechanical test results, and reverse bending test documentation
- Third-Party Inspection: Available upon request (SGS, BV, TUV, Lloyd's Register)
Unique Selling Points
- Dedicated Coil Tube Testing Protocol: Unlike general-purpose tube manufacturers, Wenzhou Jixing implements a specialized alternating bending test regime specifically calibrated for coil tube fatigue performance — providing customers with verified proof of cyclic deformation resistance.
- Full Recrystallization Guarantee: Post-forming solution annealing ensures complete grain recovery, maximizing the material's capacity for repeated deformation without work-hardening accumulation.
- Continuous Coil Supply: Eliminates butt welds inherent in straight-length tube supply, reducing failure points in coil-winding applications and improving production efficiency.
- Multi-Grade Availability: From economical 304 to high-corrosion-resistance 316L, all grades are tested to the same rigorous reverse bending criteria.
- Custom Cycle Requirements: Customers with specialized fatigue loading conditions can request increased cycle counts (up to 200 cycles) during qualification testing.