Stainless Steel Coil Tube — Mandrel Bending 180° Process Performance Specification

Product Overview

The Stainless Steel Coil Tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a high-performance cold-formed tube product designed for demanding bending and forming applications. This specification focuses on the critical process performance verification of mandrel bending at 180°, which serves as the fundamental benchmark for evaluating the tube's cold-formability and structural integrity under severe plastic deformation conditions. The coil tube is supplied in continuous coil form, enabling efficient downstream processing through automated bending, coil-fed forming, and roll-forming lines.

The 180° mandrel bend test represents the most stringent cold-bending qualification criterion in industry practice. Successful completion of this test confirms that the tube wall material possesses sufficient ductility, strain hardening resistance, and microstructural homogeneity to withstand extreme circumferential compression (inner radius) and tensile elongation (outer radius) simultaneously without failure.

Material & Properties

The coil tube is fabricated from austenitic stainless steel grades conforming to international standards, with typical composition and mechanical properties as follows:

Microstructural Characteristics for Bending Performance

The austenitic face-centered cubic (FCC) crystal structure inherently provides superior formability due to its high number of active slip systems (12 {111}<110> slip systems). The annealed condition ensures a fully recrystallized grain structure with minimal residual stress, which is essential for achieving crack-free bending at tight radii. The absence of carbide precipitation at grain boundaries (achieved through low-carbon or stabilized grades) prevents intergranular corrosion and maintains uniform ductility throughout the tube cross-section.

Specifications

Parameter Specification / Requirement Test Standard
Product Form Stainless Steel Coil Tube (continuous coil) —
Outer Diameter (OD) 6.0 mm – 168.3 mm (customizable) ASTM A554 / EN 10216-5
Wall Thickness 0.5 mm – 6.0 mm ASTM A554 / EN 10216-5
Length per Coil 6 m – 12 m (standard); up to 25 m (special order) —
Bend Angle 180° ISO 8491 / ASTM A554
Bend Method Mandrel bending (internal mandrel support) ISO 8491-1
Minimum Bend Radius (R) R ≥ 2.5 × D (for 304); R ≥ 3.0 × D (for 316) ISO 8491
Acceptance — Outer Surface No cracks, no splits, no surface discontinuities ISO 8491-1
Acceptance — Inner Surface No obvious wrinkles, no excessive thinning (≥ 85% of original wall thickness) ISO 8491-1
Surface Finish No. 1 (cold-rolled), 2B, BA (Bright Annealed), or 2D ASTM A480
Surface Roughness (Ra) ≤ 0.8 μm (BA finish); ≤ 1.6 μm (2B finish) ISO 1302
Dimensional Tolerance ±0.05 mm (OD); ±10% (wall thickness) EN 10216-5
Coil Weight Typically 500 kg – 3,000 kg per coil —
Heat Treatment Full solution annealing at 1050°C – 1150°C, followed by controlled cooling ASTM A240

Manufacturing Process

The production of this coil tube follows a rigorous multi-stage cold-forming process optimized for superior bending performance:

  1. Slit Coil Preparation: Pre-annealed stainless steel flat strip (slit coil) is supplied from certified mill sources with controlled chemical composition and mechanical properties. Incoming material undergoes spectroscopic verification (OES) and mechanical testing.
  2. Coil Slitting & Edge Trimming: The flat strip is slit to precise width dimensions with burr-free edges to prevent surface defects during the forming operation. Edge quality directly impacts the weld seam integrity in welded tube products.
  3. Tube Forming (Roll Forming): The flat strip passes through a series of precision-machined forming rolls that progressively shape the strip into a tubular cross-section. Roll profiles are computer-calculated (FEA-optimized) to ensure uniform strain distribution and minimal thickness variation.
  4. Welding (for welded tube): A high-frequency induction welding (HF) or laser welding process creates the longitudinal weld seam. Weld parameters are precisely controlled to achieve full penetration with minimal heat-affected zone (HAZ).
  5. Post-Forming Annealing: The formed tube undergoes solution annealing in a protective atmosphere (nitrogen or vacuum) at 1050°C – 1150°C to fully recrystallize the cold-worked microstructure, eliminate residual stresses, and restore maximum ductility for bending operations.
  6. Pickle & Passivate: The annealed tube surface is acid-pickled (HNO₃/HF mixture) to remove oxide scale, followed by passivation to re-establish the chromium-rich passive film.
  7. Final Dimensional Inspection: Automated optical and ultrasonic measurement systems verify OD, wall thickness, roundness, and straightness within tolerance limits.
  8. Coil Winding & Packaging: The finished tube is wound onto precision steel cores with protective interlayers and moisture-barrier packaging to prevent corrosion during storage and transport.

Quality Control During Production

Applications

The verified 180° mandrel bending capability of this coil tube opens a wide range of high-value applications requiring complex geometries:

Quality Certifications

Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management and product certification systems:

Unique Selling Points

1. Verified 180° Mandrel Bend Performance: Every production lot undergoes mandatory 180° mandrel bending verification, confirming zero cracks on the outer surface and no significant wrinkles on the inner surface. This eliminates downstream process risk and guarantees manufacturability for the most demanding bending geometries.

2. Continuous Coil Format: Delivered in coil form for seamless integration with automated coil-fed bending machines, reducing material waste by up to 30% compared to cut-length alternatives and enabling high-throughput production.

3. Multi-Grade Flexibility: Available in 304, 316, 321, 310S, and custom alloy grades, providing customers with corrosion, temperature, and mechanical property options tailored to specific service environments.

4. Precision Dimensional Control: Advanced roll-forming technology and online measurement systems ensure tight dimensional tolerances (±0.05 mm OD), enabling precise fit-up in assembly operations without secondary machining.

5. Surface Integrity Guarantee: The combination of controlled manufacturing, full annealing, and pickling/passivation ensures a defect-free surface suitable for both functional and aesthetic applications without post-processing.

Summary

The Stainless Steel Coil Tube from Wenzhou Jixing Metal Materials Co., Ltd. represents a premium cold-formed tube product whose 180° mandrel bend qualification establishes it as the benchmark for process-formable stainless steel tubing. The absence of outer-surface cracking and inner-surface wrinkling under maximum bending severity confirms the material's exceptional ductility, microstructural homogeneity, and manufacturing consistency. This foundational process performance verification provides customers with absolute confidence in downstream forming operations, reducing scrap rates, minimizing quality disputes, and enabling aggressive design optimization for complex bent-tube assemblies across all major industrial sectors.