I. Visual Inspection and Magnification (Preliminary Screening)
Observation Characteristics: Cracks typically appear as straight, spiral, or network-like cracks on the inner and outer surfaces of the steel pipe. They have sharp edges, sometimes penetrating the pipe wall, and are dark in color without oxidation or decarburization.
Inspection Methods: Rotate the steel pipe under sufficient light, using a flashlight at an angle to enhance shadow contrast. Use a magnifying glass or portable microscope to examine fine hairline cracks, paying particular attention to weld areas, pipe ends, and stress concentration points.
Applicable Scenarios: Suitable for rapid on-site identification of obvious cracks, but cannot detect internal or minute defects.
II. Non-Destructive Testing Technology (Precise Judgment)
1. Magnetic Particle Inspection (MT)
Principle: After magnetizing the steel pipe, surface or near-surface cracks create a leakage magnetic field, attracting magnetic particles and revealing magnetic traces.
Advantages: High sensitivity to surface cracks in ferromagnetic materials (such as carbon steel and alloy steel), capable of detecting microcracks as small as 0.1 μm in width.
Operating Points: Applicable to wet or fluorescent methods. Ensure the magnetization direction is perpendicular to the crack to avoid missed detections.
2. Ultrasonic Testing (UT)
Principle: Utilizes the reflection of ultrasonic waves encountered by defects as they propagate through the material. The location and size of the crack are determined by the echo signal.
Advantages: Can detect deep defects such as internal cracks, inclusions, and delamination. Detection depth can reach several millimeters to tens of millimeters.
Standard Basis: Should comply with GB/T 5777-2019 "Ultrasonic Testing Method for Seamless Steel Pipes".
3. Eddy Current Testing (ET)
Principle: Based on electromagnetic induction, cracks disturb the eddy current distribution, causing changes in the impedance of the detection coil.
Advantages: Fast detection speed, suitable for online batch testing, sensitive to surface and near-surface cracks and folds.
Standard Basis: Complies with GB/T 7735-2022 "Eddy Current Testing Method for Seamless Steel Pipes".
4. Penetrant Testing (PT)
Principle: A penetrant is applied to the surface, penetrating into open cracks. After cleaning, a developer is applied to reveal the defect.
Advantages: Suitable for non-ferromagnetic materials (such as stainless steel and copper pipes); simple operation and low cost.
Limitations: Can only detect surface cracks; not suitable for closed or internal defects.
III. Comprehensive Judgment Recommendations
Preferably use a combination of MT + UT: Magnetic particle testing for surface inspection and ultrasonic testing for internal inspection, achieving comprehensive coverage.
Acceptance Standards: Follow the relevant national standards according to the product's application, such as GB 5310 for boiler tubes and API 5CT for oil casing pipes.
Third-Party Re-inspection: For pipes used in critical operating conditions, it is recommended to commission a professional organization to issue a test report to ensure the data is authoritative and reliable.
✅ Tip: If a suspicious crack is found, it should be marked immediately and use should be stopped to prevent it from expanding under pressure or vibration, leading to failure.
