Stainless Steel Coil Tube — Flattening Test Specification (Process Performance Verification)
Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered, continuous-length welded stainless steel tube supplied in coiled configuration for maximum handling efficiency and downstream processing flexibility. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product undergoes rigorous process performance verification, including the critical Flattening Test (ASTM A213 / GB/T 247), which serves as the definitive assessment of the tube's plastic deformation capacity and weld seam integrity under compressive loading.
The flattening test is a non-destructive quality control procedure that validates the structural soundness of the welded joint by subjecting a specimen to controlled compression between parallel plates until a specified reduction in the tube's original cross-sectional dimension is achieved. The test directly evaluates two critical quality attributes: the plasticity of the welded pipe body and the cohesive bonding strength of the weld seam. A product is deemed acceptable only if no cracks, splits, or separations are observed at the weld seam when the specimen is flattened to the required spacing, with the weld positioned in the direction of applied compressive force.
Material & Properties
The coil tube is fabricated from high-grade austenitic and duplex stainless steel alloys, selected for superior corrosion resistance, mechanical strength, and formability. Common alloy designations include:
- AISI 304 / 304L (UNS S30400 / S30403) — General-purpose austenitic grade with excellent weldability
- AISI 316 / 316L (UNS S31600 / S31603) — Enhanced chloride resistance for aggressive environments
- AISI 321 (UNS S32100) — Stabilized grade with added titanium for high-temperature service
- AISI 310S (UNS S31008) — High-temperature oxidizing atmosphere applications
- Duplex 2205 (UNS S32205) — High-strength duplex alloy with superior pitting resistance
| Property | Typical Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 520 MPa (304) / ≥ 550 MPa (316L) / ≥ 450 MPa (2205) | ASTM A213 / GB/T 247 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (304) / ≥ 170 MPa (316L) / ≥ 450 MPa (2205) | ASTM A213 / GB/T 247 |
| Elongation (%) | ≥ 35% (304) / ≥ 40% (316L) / ≥ 25% (2205) | ASTM A213 |
| Flattening Ratio (D/t) | ≥ 10 (typical minimum per grade) | ASTM A213 / GB/T 247 |
| Weld Seam Acceptance | No cracks, splits, or separations at weld line | ASTM A213 / GB/T 247 |
| Surface Finish | No. 1 / 2B / BA / Hairline (per customer requirement) | ASTM A480 |
Specifications
| Parameter | Specification Detail |
|---|---|
| Product Form | Coil Tube (Continuous Length, Coiled Configuration) |
| Manufacturing Method | Automatic Welded (ERW / TIG / Laser Welding) |
| Outer Diameter (OD) | 6.0 mm – 168.0 mm (customizable) |
| Wall Thickness | 0.3 mm – 8.0 mm (customizable) |
| Coil Length | Typically 1,000 m – 6,000 m per coil |
| Applicable Standards | ASTM A213 / A269 / ASME SA-213 / EN 10217 / GB/T 12771 / GB/T 247 |
| Weld Seam Position | Aligned with compressive loading axis during flattening test |
| Flattening Test Criterion | Flat compression to specified spacing (D/t ratio); weld seam in force direction; zero cracks = PASS |
| Heat Treatment | Solution Annealed (1050°C – 1150°C), Water Quenched |
| Surface Condition | Mill Finish / Pickled & Passivated / Custom Polish |
Manufacturing Process
The production of high-quality stainless steel coil tubes involves a multi-stage manufacturing sequence designed to ensure dimensional precision, metallurgical integrity, and process performance reliability:
- Raw Material Selection: Cold-rolled stainless steel strip is procured from certified mill sources with full chemical composition verification (Spectroscopic Analysis per ASTM E415).
- Strip Preparation: Incoming strip undergoes edge trimming, surface cleaning, and tension leveling to eliminate coil set and ensure uniform feed.
- Forming: Multi-stand roll forming progressively shapes the flat strip into a seamless circular cross-section with controlled wall thickness uniformity (±0.05 mm tolerance).
- Welding: The formed tube is sealed via high-frequency electric resistance welding (ERW), TIG welding, or laser welding. The weld zone is continuously monitored via in-line imaging systems for defect detection.
- Heat Treatment: Welded tubes undergo solution annealing at 1050°C – 1150°C followed by rapid water quenching to restore full austenitic structure, relieve residual stresses, and homogenize the weld zone metallurgy.
- Pickling & Passivation: Tubes are immersed in citric or nitric acid solutions to remove oxide scale and establish a protective chromium oxide passive film.
- Coiling: Finished tubes are wound into coils on precision-controlled pay-off systems with controlled tension to prevent deformation or kinking.
- Quality Inspection: Each coil undergoes comprehensive testing including dimensional verification, hydrostatic pressure testing, chemical analysis, mechanical testing, and the critical Flattening Test for process performance validation.
Flattening Test — Detailed Procedure & Acceptance Criteria
The flattening test is the primary non-destructive verification method for assessing the plastic deformation capability and weld seam bonding integrity of stainless steel coil tubes. The procedure adheres to ASTM A213, Section 4.11, and GB/T 247-2008:
Test Setup
- A representative specimen of minimum length 25 mm is cut from the production coil.
- The specimen is positioned between two parallel, hardened steel plates with a smooth, flat working surface.
- The weld seam is oriented perpendicular to the plates (i.e., in the direction of the applied compressive force), ensuring the weld experiences maximum hoop stress during flattening.
- Compression is applied at a uniform rate of 0.5 mm/min ± 0.1 mm/min.
Acceptance Criteria
- The specimen is flattened until the distance between the plates equals the specified D/t ratio (typically D/t ≥ 10 for standard grades; higher ratios for premium applications).
- The weld seam and adjacent zone are visually inspected (magnification 2x–10x) for any indication of cracking, splitting, separation, or delamination.
- PASS: No cracks, splits, or separations observed at or near the weld seam.
- FAIL: Any visible crack, split, or weld separation at the compressive loading zone.
Key Quality Indicator: The flattening test directly verifies two critical manufacturing quality attributes — (1) the plasticity of the welded pipe body, confirming adequate formability for downstream processing, and (2) the cohesive bonding strength of the weld seam, ensuring metallurgical fusion integrity between the parent metal and the weld zone. A passing result confirms that the manufacturing process — from strip quality through welding and heat treatment — has produced a tube with uniform mechanical properties throughout its cross-section.
Applications
Stainless steel coil tubes verified through flattening testing are employed across a broad spectrum of industrial applications where structural integrity, corrosion resistance, and formability are paramount:
- Heat Exchangers & Boilers: Tubular bundles for shell-and-tube heat exchangers, condensers, and evaporators in petrochemical, power generation, and HVAC systems.
- Petrochemical & Oil/Gas: Process piping, heat transfer tubes, and high-pressure vessel components in refineries and offshore platforms.
- Food & Pharmaceutical Processing: Sanitary-grade tubing for dairy, beverage, and pharmaceutical production lines requiring CIP (Clean-In-Place) and SIP (Sterilize-In-Place) compatibility.
- Automotive Industry: Exhaust system components, fuel lines, and structural tubing for vehicle chassis and safety-critical assemblies.
- Architectural & Decorative: Structural framing, handrails, balustrades, and facade systems for commercial and residential construction.
- Mechanical Engineering: Hydraulic cylinder tubes, pneumatic systems, and fluid conveyance lines in industrial machinery.
- Medical Equipment: Surgical instrument tubing, endoscope sheaths, and medical device housings requiring biocompatibility and sterilization resistance.
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management framework ensuring full traceability and compliance with international standards:
- ISO 9001:2015 — Quality Management System certification covering all manufacturing and inspection processes.
- ISO 14001:2015 — Environmental Management System for sustainable manufacturing practices.
- EN 10204 Type 3.1 — Third-party inspection certificates for material compliance and test verification.
- Mill Test Certificates (MTC) — Full chemical composition, mechanical properties, and process performance test results (including flattening test data) provided per batch/coil.
- ASTM / ASME Compliance — Product manufactured and tested to ASTM A213, A269, and ASME SA-213 specifications.
- GB/T Standards Compliance — Full adherence to Chinese national standards including GB/T 12771 (stainless steel welded tubes) and GB/T 247 (flattening test method).
- PEACE / PED Compliance — Available for pressure equipment directives requiring documented process performance verification.
Unique Selling Points
- Weld-First Orientation Testing: The flattening test protocol specifically positions the weld seam in the direction of compressive force, providing the most stringent validation of weld integrity — a higher standard than many competitors who test with the weld in the hoop direction.
- Full Process Performance Verification: Every coil is tested for plasticity and weld bonding strength, not merely dimensional accuracy, ensuring downstream processing reliability.
- Coil Configuration Advantage: Continuous-length coil supply eliminates butt-weld joints inherent in bar-length supply, reducing leak points and improving production throughput for downstream fabricators.
- Complete Traceability: From raw strip mill certificate through final coil inspection, full batch traceability is maintained with documented flattening test results per coil.
- Custom Specification Capability: Flexible manufacturing accommodates custom dimensions, surface finishes, and alloy grades to meet specific application requirements.