Eddy Current Testing ECT requirements for stainless steel tubes under ASME E309 GBT 7735 EN 10839-2

1. Scope and Material Coverage

• ASTM/ASME (e.g., ASTM E309, ASME Sec. V):
  • Focuses on non-ferromagnetic stainless steel (e.g., 300 series).
  • Applies to seamless/welded tubes in aerospace, oil/gas, and power industries.
• GB (e.g., GB/T 7735):
  • Targets austenitic stainless steel tubes (e.g., 304, 316) for industrial and construction applications.
  • Explicitly covers welded joints and surface defects.
• EN/ISO (e.g., EN ISO 15549, ISO 10893-2):
  • Applies to all non-magnetic stainless steel (including duplex steel grades if non-ferromagnetic).
  • Emphasizes tubes for food processing, chemical plants, and EU pressure equipment.

**2. Frequency and Probe Requirements

• ASTM/ASME:
  • Frequency range: 10 kHz–2 MHz, selected based on wall thickness and defect type.
  • Probes: Absolute or differential coils; array probes allowed for complex geometries.
• GB:
  • Strict frequency bands: 1–125 kHz (e.g., 64 kHz for thin-walled tubes).
  • Mandates encircling coils for full-body inspection of stainless steel tubes.
• EN/ISO:
  • Frequency guided by skin depth (e.g., 100–500 kHz for surface cracks).
  • Encourages array probes (PA-ECT) for high-speed inspection of long tubes.

**3. Calibration and Reference Defects

• ASTM/ASME:
  • Uses artificial defects (e.g., transverse/axial notches, drilled holes).
  • Notch depth: 5–20% of wall thickness (e.g., 0.1 mm for 2 mm wall).
• GB:
  • Requires certified reference tubes with 0.5–1.5 mm diameter through-holes and 0.2–0.5 mm deep notches.
  • Calibration must align with GB/T 5121 (non-ferrous metal standards).
• EN/ISO:
  • Follows ISO 15548 for calibration: EN-specific reference blocks with 0.3–2.0 mm defects.
  • Traceability to national/international standards (e.g., NIST, BAM).

**4. Defect Evaluation Criteria

• ASTM/ASME:
  • Signal amplitude and phase angle determine defect severity.
  • Acceptance thresholds often defined by industry codes (e.g., ASME B31.3: max defect depth ≤10% wall thickness).
• GB:
  • Strict quantitative limits:
    • Surface defects: ≤0.3 mm depth for food-grade stainless tubes.
    • Through-wall defects: rejected outright.
  • Mandatory defect mapping for critical applications.
• EN/ISO:
  • Uses impedance plane analysis for defect characterization.
  • Acceptance criteria tied to application standards (e.g., EN 10246-14 for pressure tubes: max defect length ≤30 mm).

**5. Reporting and Personnel Certification

• ASTM/ASME:
  • Requires raw signal data, calibration records, and ASNT SNT-TC-1A Level II/III certification.
• GB:
  • Reports must include defect location maps, instrument settings, and CNAS/CMA accreditation.
  • Inspectors require GB-specific certification (e.g., NDT Level II).
• EN/ISO:
  • Documentation must comply with ISO 9712 (personnel certification) and ISO 9001 (quality systems).
  • Full traceability from raw data to final report.

Key Differences Summary

Practical Implications

• Export to the EU: EN/ISO standards prioritize traceability and impedance plane analysis.
• Chinese Compliance: GB’s strict defect thresholds dominate domestic projects.
• Aerospace/Oil & Gas: ASTM/ASME offers flexibility for high-performance stainless tubing.