Stainless Steel Coil Tube — Rotating Probe Eddy Current Testing (ET) Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered austenitic stainless steel tubing product supplied in coiled configuration. This product is distinguished by its rigorous quality assurance protocol, which incorporates Rotating Probe Eddy Current Testing (Rotating Probe ET) — an advanced non-destructive testing (NDT) methodology that delivers high-resolution volumetric and surface defect detection with precise circumferential defect localization capability.
Rotating Probe Eddy Current Testing represents a state-of-the-art evolution of conventional axial and circumferential probe techniques. By mounting the sensing coil on a mechanically or electromagnetically rotating assembly, this method achieves omnidirectional interrogation of the tube wall with superior spatial resolution. The resulting detection signature permits accurate determination of both the axial position and circumferential orientation of subsurface and surface-breaking discontinuities — a critical advantage for weld integrity assessment and fitness-for-service evaluation.
2. Material & Properties
2.1 Base Material
- Primary Alloy Grades: AISI 304 (UNS S30400), AISI 316 (UNS S31600), AISI 321 (UNS S32100), AISI 347 (UNS S34700)
- Equivalent Standards: EN 1.4301, EN 1.4401, EN 1.4541, EN 1.4550; JIS SUS304, SUS316
- Microstructure: Fully austenitic, solution-annealed condition
- Electrical Resistivity (ρ): 0.72 × 10⁻⁶ Ω·m (typical for 304 grade at 20 °C)
- Magnetic Permeability (μᵣ): ≈ 1.00–1.02 (essentially non-magnetic)
2.2 Key Mechanical Properties (Annealed Condition)
| Property | Unit | Minimum Requirement | Typical Value |
|---|---|---|---|
| Tensile Strength (UTS) | MPa | ≥ 520 | 550–700 |
| Yield Strength (Rp0.2) | MPa | ≥ 205 | 220–280 |
| Elongation (A50) | % | ≥ 40 | 45–60 |
| Hardness (HBW) | — | — | 110–140 |
| Specific Heat Capacity | J/(kg·K) | — | 500 |
| Thermal Conductivity | W/(m·K) | — | 16.3 |
2.3 Chemical Composition (Typical — Grade 304)
| Element | C | S | P | Cr | Ni | Mn | Si | N |
|---|---|---|---|---|---|---|---|---|
| Specification | ≤ 0.08 | ≤ 0.030 | ≤ 0.045 | 18.0–20.0 | 8.0–10.5 | ≤ 2.0 | ≤ 0.75 | ≤ 0.10 |
3. Specifications
3.1 Dimensional Parameters
| Parameter | Range | Tolerance | Applicable Standard |
|---|---|---|---|
| Outer Diameter (OD) | 3.0 – 323.9 mm | ± 0.10% OD | ASTM A213 / ASME SA-213 |
| Wall Thickness (WT) | 0.25 – 12.0 mm | ± 10% WT | ASTM A213 / ASME SA-213 |
| Coil Length | Custom (typically 50–300 m per coil) | — | Manufacturer specification |
| Coil Inner Diameter | 200 – 600 mm | ± 25 mm | Manufacturer specification |
| Surface Finish | Pickled & Annealed (PA), Bright Annealed (BA), No. 4 Brushed | Ra ≤ 0.4 μm (BA) | ASTM A478 |
3.2 Eddy Current Testing (ET) Parameters — Rotating Probe Configuration
| Parameter | Specification |
|---|---|
| Testing Method | Rotating Probe Eddy Current (RP-ET) |
| Probe Configuration | Single or dual rotating differential coil assembly, axial orientation |
| Operating Frequency | 10 kHz – 500 kHz (adaptive to wall thickness) |
| Standard Penetration Depth (δ) | 0.05 – 2.0 mm (depending on frequency and material) |
| Rotation Speed | 50 – 3000 rpm (configurable per tube OD) |
| Axial Scan Speed | 0.5 – 5.0 m/min |
| Defect Localization Accuracy | Circumferential: ± 5°; Axial: ± 1.0 mm |
| Minimum Detectable Defect | 0.05 mm deep × 0.1 mm wide (surface); 0.03 mm deep × 0.1 mm wide (subsurface) |
| Reference Standards | ASTM E309, ASTM E213, ASTM E709, ISO 15548, EN ISO 10148 |
4. Manufacturing Process
4.1 Production Workflow
- Hot-Rolled Billet Preparation: Ingot casting and homogenization of austenitic stainless steel stock.
- Piercing & Extrusion: Production of seamless tube blank via hot piercing or extrusion route.
- Pilger Milling / Floating Roll Expansion: Cold reduction to achieve precise OD and wall thickness tolerances.
- Solution Annealing: Heating to 1050–1150 °C followed by controlled cooling to restore full austenitic microstructure and relieve residual stresses.
- Pickling & Passivation: Removal of oxide scale via acid immersion (HNO₃/HF blend) to establish a clean, corrosion-resistant chromium oxide passive layer.
- Coiling: Continuous coiling onto a mandrel with controlled tension to prevent wrinkling or inter-coil contact damage.
- Rotating Probe Eddy Current Inspection (RP-ET): Full-length inline volumetric examination with high-resolution circumferential defect mapping.
- Final Dimensional Verification: Optical and mechanical gauge checks for OD, WT, and roundness.
- Packaging & Dispatch: Moisture-protected wrapping, wooden or steel cradles for coil support.
4.2 Rotating Probe ET — Technical Detail
The rotating probe eddy current system employed for this product line represents a significant advancement over static axial or circumferential probe arrangements. The core principle is as follows:
A differential eddy current coil assembly is mounted on a precision rotary mechanism that spins the probe around the tube axis while the tube simultaneously traverses axially through the probe bore. This dual-motion configuration generates a scanning vector field that interrogates every point on the tube's inner and outer surfaces, enabling detection of defects irrespective of their circumferential orientation.
Signal Processing: The received eddy current response is demodulated using phase-sensitive detection techniques, separating in-phase (ΔR) and quadrature (ΔX) components. Defect signatures are classified by their characteristic impedance locus on the complex plane — axial cracks, transverse cracks, and circumferential weld defects each produce distinguishable signal trajectories. Advanced signal processing algorithms, including wavelet denoising and neural-network-based classification, are applied to minimize false indications from geometric features such as weld seams and coil transitions.
4.3 Key Performance Advantages of Rotating Probe ET
- High Spatial Resolution: Sub-millimeter defect localization in both axial and circumferential directions, achieved through the high-frequency interrogation and rapid angular sampling of the rotating coil.
- Circumferential Defect Positioning: Uniquely capable of reporting the angular position of detected discontinuities — a capability not available from conventional non-rotating axial probes.
- Weld Seam Evaluation: Precise discrimination of weld-related indications (undercut, lack of fusion, porosity) from manufacturing artifacts, enabling quantitative fitness-for-service assessment.
- Small-Diameter Tube Suitability: Optimized probe geometries allow effective inspection of tubes with OD as small as 3.0 mm, where conventional probe configurations suffer from signal-to-noise degradation.
- Continuous Inline Integration: Compatible with high-speed production lines, enabling 100% inspection of coiled product without interruption to throughput.
5. Applications
Stainless steel coil tubes inspected by rotating probe ET are deployed in demanding environments where corrosion resistance, dimensional precision, and metallurgical integrity are paramount:
- Chemical Processing: Heat exchanger tubes, reactor piping, and distillation column internals in petrochemical and pharmaceutical plants.
- Food & Beverage Industry: Hygienic process piping, CIP (Clean-in-Place) compatible tubing, and sanitary heat exchangers compliant with FDA/3-A standards.
- Energy Sector: Boiler tubes, superheater elements, and nuclear-grade steam generator tubes (where applicable grades are specified).
- Automotive Industry: Exhaust system components, turbocharger manifolds, and fuel injection lines.
- Medical Devices: Endoscope guide tubes, implant manufacturing stock, and laboratory instrumentation tubing.
- Aerospace: Hydraulic lines, fuel system tubing, and structural micro-tubes in composite repair applications.
6. Quality Certifications & Compliance
6.1 Product Standards
| Standard | Title / Scope |
|---|---|
| ASTM A213 / ASME SA-213 | Specification for Austenitic Stainless Steel Seamless, Cold-Drawn or Cold-Rolled Heat-Exchanger, Heat-Treater, and Similar Tubes |
| ASTM A554 | Specification for Stainless Steel Tubing, Seamless and Welded, for Instrumentation and Similar Applications |
| EN 10216-5 | Steel Tubes for Technical Purposes — Non-Alloy and Alloy Cold-Drawn Tubes — Part 5: Technical Delivery Conditions for Stainless Tubes |
| ISO 4200 | Steel Tubes for Technical Purposes — Non-Alloy and Alloy Tubes — General Technical Delivery Conditions |
| GB/T 14976 | Stainless Steel Tubes for Fluids Transport (Chinese National Standard) |
6.2 NDT Method Standards
| Standard | Title / Scope |
|---|---|
| ASTM E309 | Standard Practice for Eddy Current Examination of Tubular Products Using Rotating Probes |
| ASTM E213 | Standard Practice for Eddy Current Examination of Tubular Products Using Non-Rotating Probes |
| ASTM E709 | Standard Practice for Eddy Current Testing of Tubular Products Using Rotating Probes — Detailed Procedure |
| ISO 1 |