Magnetic particle inspection of black seamless pipe

What is magnetic particle inspection

Magnetic particle inspection is a method of observing defects using magnetic particles as a display medium. According to the type of magnetic powder medium applied during magnetization, the detection method is divided into wet method and dry method; according to the time when magnetic powder is applied to the workpiece, the inspection method is divided into continuous method and residual magnetization method.

After the ferromagnetic material workpiece is magnetized, due to the existence of discontinuity, the magnetic field lines on the surface and near surface of the workpiece are locally distorted, resulting in a leakage magnetic field, which absorbs the magnetic powder applied to the surface of the workpiece and forms a visible magnetic field under appropriate lighting. traces, showing the location, size, shape and severity of discontinuities. Also known as magnetic particle inspection or magnetic particle flaw detection, it is one of the five conventional methods of non-destructive testing.

Principle of magnetic particle testing of black seamless pipes

Magnetic particle testing magnetizes black seamless pipes, causing leakage magnetic fields to be generated near and on the surface of the pipes, adsorbing magnetic particles on the pipes, and revealing the shape, size, location and severity of discontinuities, which can be used to determine whether there are defects inside the pipes. Typically, this technology is used for surface and surface inspection of pipe welds only. When using magnetic particle inspection, the inspection pipeline needs to be magnetized transversely and longitudinally to ensure the quality of magnetic particle inspection.

Work flow of magnetic particle inspection of black seamless pipes

The magnetic particle inspection operations of black seamless pipes include: pretreatment, magnetization, application of magnetic particles, observation, and recording. Post-processing, etc.

1. Pretreatment: Remove grease, paint, rust, etc. from the surface of the black seamless pipe to avoid preventing magnetic particles from adhering to defects. When using dry magnetic powder, the surface of the pipe should also be dry. In addition, the assembled components must be disassembled one by one and then inspected for defects.

2. Magnetization: Select the appropriate magnetization method and magnetizing current value. Then turn on the power and magnetize the black seamless tube.

3. Apply magnetic powder: add dry powder or magnetic suspension according to the selected dry method or wet method. The spraying time of magnetic powder is divided into two application methods: continuous method and residual magnet method. The continuous method is to spray magnetic powder while magnetizing the pipe, and the magnetization continues until the magnetic powder is applied. The residual magnetization method applies magnetic powder after magnetizing the black seamless pipe.

4. Observation and judgment of magnetization: The tube rubbing of magnetic marks is carried out after applying magnetic powder. When using non-fluorescent magnetic particle flaw detection, the tube rub should be carried out in a brightly lit place with natural sunlight and lighting; while when using fluorescent magnetic particle flaw detection , use an ultraviolet lamp to wipe the tube in a dark place such as a dark room. In magnetic particle inspection, the accumulation of magnetic particles visible to the naked eye is referred to as magnetic marks. But not all magnetic marks are defects. There are many reasons for the formation of magnetic marks, so the magnetic marks must be analyzed and judged to eliminate false magnetic marks. Sometimes it is necessary to re-detect the flaw using other flaw detection methods for verification.

5. Recording: In order to record magnetic traces, you can use photography or transparent tape to tape down the magnetic traces and prepare them. Such records have the advantages of simplicity and intuitiveness.

6. Post-processing: After the flaw detection is completed, the pipeline should be demagnetized, removed magnetic particles and anti-rust as needed. The reason for demagnetization is that residual magnetism may cause obstruction of pipeline operation and increase wear of parts.

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