Stainless Steel Coil Tube — Conical Mandrel Bell Mouth Expansion Test Specification
Product Overview
The Stainless Steel Coil Tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed for high-performance fluid conveyance, instrumentation, and mechanical assembly applications. This product specification focuses on the Conical Mandrel Bell Mouth Expansion Test (锥形芯棒扩口试验), a critical process performance verification method used to assess the ductility, formability, and structural integrity of the coil tube material under controlled expansion conditions.
The bell mouth expansion test serves as a core verification indicator for ferrule fitting compatibility (卡套接头适配性核心验证指标). Ferrule fittings (also known as compression fittings or sleeve fittings) are widely used in high-pressure piping systems where leak-free, maintenance-free connections are required. The ability of the tube to undergo controlled expansion without cracking directly correlates with its suitability for field installation using ferrule-type connection hardware.
Material & Properties
Stainless steel coil tubes are typically fabricated from austenitic stainless steel grades, which offer an optimal combination of corrosion resistance, formability, and mechanical strength. The material properties are essential for ensuring successful performance in the conical mandrel expansion test.
- Common Grades: 304 (UNS S30400), 304L (UNS S30403), 316 (UNS S31600), 316L (UNS S31603), 321 (UNS S32100)
- Microstructure: Fully austenitic (FCC crystal structure), non-magnetic or slightly magnetic in cold-worked condition
- Tensile Strength: 515–750 MPa (depending on grade and temper condition)
- Yield Strength: ≥205 MPa (minimum)
- Elongation: ≥40% (minimum, indicating high ductility)
- Hardness: ≤95 HRB (annealed condition)
- Surface Condition: Bright annealed, pickled, or polished (Ra ≤ 0.8 μm for precision grades)
Specifications
| Parameter | Specification / Requirement |
|---|---|
| Test Method | Conical Mandrel Bell Mouth Expansion (锥形芯棒扩口) |
| Acceptance Criteria — 30% Expansion | No cracks, splits, or laps permitted (合格: 不开裂) |
| Acceptance Criteria — 45% Expansion | No cracks, splits, or laps permitted (合格: 不开裂) |
| Acceptance Criteria — 60% Expansion | No cracks, splits, or laps permitted (合格: 不开裂) |
| Product Category (Level 1) | Inspection Items (检测项目) |
| Product Category (Level 2) | Process Performance (工艺性能) |
| Product Category (Level 3) | Bell Mouth Expansion Test (扩口试验) |
| Knowledge Point Identifier | No. 98 — Conical Mandrel Expansion (锥形芯棒扩口) |
| Key Indicator Significance | Core verification for ferrule fitting compatibility (卡套接头适配性核心验证指标) |
| Product Form | Coil Tube (盘管) |
| Applicable Standards | ASTM A213/A213M, GB/T 14976, EN 10216-5 |
Manufacturing Process
3.1 Tube Production
- Hot Rolling / Cold Drawing: The base tube is produced via seamless tube manufacturing processes, including hot extrusion or cold drawing from billet stock. Coil tubes are typically cold-drawn to achieve tight dimensional tolerances and superior surface finish.
- Heat Treatment (Solution Annealing): Tubes are annealed at 1050–1150 °C followed by rapid air cooling to restore full austenitic structure, relieve residual stresses, and maximize ductility — a prerequisite for passing the bell mouth expansion test at 60%.
- Pickling & Passivation: Chemical cleaning removes scale and forms a protective chromium oxide passive layer, ensuring corrosion resistance and clean surface for inspection.
- Coiling: Finished tubes are wound into coils at controlled tension to maintain dimensional stability and prevent kinking or deformation.
3.2 Conical Mandrel Bell Mouth Expansion Test Procedure
- Sample Preparation: A tube specimen of specified length is cut from the coil and prepared with clean, undamaged ends.
- Fixture Setup: A hardened steel conical mandrel (cone angle typically 60° per half-angle) is positioned within the tube bore.
- Expansion Execution: The mandrel is driven into the tube end at a controlled rate (typically 5–15 mm/min) until the specified expansion ratio is achieved. Three test levels are evaluated:
- 30% Expansion Ratio: Moderate formability check — baseline qualification
- 45% Expansion Ratio: Intermediate formability check — standard ferrule fitting compatibility
- 60% Expansion Ratio: Severe formability check — advanced ferrule fitting and specialized applications
- Inspection: The expanded (bell mouthed) end is visually inspected under magnification (minimum 5×) for:
- Cracks (longitudinal, circumferential, or radial)
- Splits or laps (material separation)
- Excessive thinning beyond acceptable limits
- Acceptance Determination: The specimen passes if no cracks, splits, or laps are detected at the specified expansion ratio. Failure at any level indicates insufficient ductility or material defects.
Applications
The conical mandrel bell mouth expansion test result directly governs the application suitability of the coil tube. Tubes that pass at 60% expansion are qualified for the most demanding ferrule fitting applications:
- High-Pressure Instrumentation Piping: Hydraulic and pneumatic systems requiring field-assembled ferrule connections (e.g., Instrumentation tube per ISO 16028)
- Oil & Gas Industry: Downhole instrumentation, wellhead manifolds, and subsea equipment where reliable, maintenance-free connections are critical
- Petrochemical & Chemical Processing: Reactor instrumentation, catalyst injection systems, and process control loops
- Aerospace & Defense: Fuel systems, hydraulic lines, and environmental control systems requiring certified formability
- Food & Pharmaceutical: Sanitary processing lines using ferrule-type sanitary fittings (Tri-Clamp compatible)
- Power Generation: Boiler feedwater instrumentation, turbine control systems, and nuclear-grade piping
- Medical Devices: Endoscopic instrument tubing, dialysis equipment, and surgical tooling
Quality Certifications & Compliance
4.1 Test Standards
- ASTM A213/A213M: Standard Specification for Seamless Austenitic Stainless Steel Tubing for High-temperature Service — includes expansion test requirements
- ASTM E10 / E9: Standards for Rockwell/Brinell Hardness testing (material preconditioning verification)
- GB/T 14976: Chinese National Standard for Seamless Stainless Steel Tubes for Fluid Transport
- EN 10216-5: European Standard for Technical Delivery Conditions for Stainless Steel Tubes — Part 5: Austenitic Tubes for General Purposes
- ISO 1127: Steel tubes — Cold expansion test (bell mouth test with conical mandrel)
4.2 Quality Assurance Measures
- Mill Test Certification (MTC): Each batch is accompanied by a full material test certificate (EN 10204 3.1) documenting chemical composition, mechanical properties, and all process performance test results including bell mouth expansion at 30%/45%/60%.
- Batch Traceability: Full heat number traceability from raw material mill through all processing stages to final coil identification.
- Non-Destructive Testing (NDT): Eddy current (ET) and ultrasonic (UT) inspection performed on 100% of production to detect internal and surface defects that could compromise expansion test performance.
- ISO 9001:2015: Quality Management System certification ensuring consistent process control and documentation.
- ISO/TS 22163: Automotive quality management system (for automotive-grade tubing applications).
4.3 Unique Selling Points
- Tiered Expansion Verification: The triple-level testing protocol (30%/45%/60%) provides customers with granular formability data, enabling precise matching of tube grade to application requirements without over-specification.
- Ferrule Fitting Compatibility Assurance: Direct correlation between test results and field-installation performance eliminates connection failure risk in critical systems.
- Coil Form Advantages: Supplied in coil configuration for maximum material utilization, reduced cutting waste, and compatibility with automated tube processing lines.
- Wenzhou Jixing Metal Materials Co., Ltd.: Established manufacturer with specialized expertise in stainless steel coil tube production, offering consistent quality backed by rigorous in-process inspection protocols.
Technical Note: The 60% expansion ratio without cracking represents the highest formability qualification level for standard ferrule fitting applications. Tubes meeting this criterion are suitable for all common ferrule fitting sizes (1/16" through 3/4") and can withstand repeated installation/removal cycles without material degradation at the connection point. Customers requiring expansion ratios beyond 60% should consult the manufacturer for specialized grade recommendations or custom testing protocols.