After the ferromagnetic material workpiece is magnetized, due to the existence of discontinuity, the magnetic field lines on the surface and near the surface of the workpiece are locally distorted to generate a leakage magnetic field, which adsorbs the magnetic powder applied on the surface of the workpiece and forms a visible magnetic field under suitable illumination. marks, thereby showing the location, size, shape and severity of the discontinuity. Also known as magnetic particle inspection or magnetic particle inspection, it is one of the five conventional methods of non-destructive testing.
1. Wet and dry methods
Magnetic powder suspension in oil, water or other liquid medium is called wet method. In the process of detection, the magnetic suspension is evenly distributed on the surface of the workpiece, and the magnetic powder is attracted by the flow of the carrier liquid and the leakage magnetic field. The shape and size of the defect. In wet detection, due to the dispersion and suspension properties of the magnetic suspension, the magnetic powder particles that can be used are smaller. Therefore, it has high detection sensitivity. It is especially suitable for detecting small surface defects, such as fatigue cracks, grinding cracks, etc. Wet methods are often used with stationary equipment, but also with mobile and portable equipment. The magnetic suspension for wet method can be recycled.
The dry method is called the dry powder method. In some special occasions, when the wet method cannot be used for detection, the special dry magnetic powder is directly applied to the magnetized workpiece according to the program, and the defect of the workpiece shows magnetic marks. Dry inspection is mostly used for local area inspection of large castings, forging blanks, large structural parts and welding parts, and is usually used in conjunction with portable equipment.
2. Continuous method and remanence method
(1) Continuous method The continuous method, also known as the accessory magnetic field method or the current magnetic method, is to apply magnetic powder or magnetic suspension to the workpiece for magnetic particle inspection under the action of an external magnetic field. The observation and evaluation of the workpiece can be carried out under the action of an external magnetic field or after the magnetic field is interrupted.
(2) Residual magnetization method The residual magnetization method is to first magnetize the workpiece, and then pour the suspension on the workpiece, and observe it after the magnetic powder is gathered. This is a method of detecting the residual magnetism of the material, so it is called the residual magnetism method.
Magnetic particle inspection
Magnetic particle inspection is called magnetic particle inspection. It is one of the most commonly used methods because of its intuitive display, simple operation, and people willing to use it.
Magnetic flux leakage detection
Without magnetic particle display, it is customarily called magnetic flux leakage detection. It often uses induction coils, magneto-sensitive tubes, Hall elements, etc. to reflect defects. It is more hygienic than magnetic particle detection, but not as intuitive as the former. Because magnetic flaw detection mainly uses magnetic powder to display defects, people sometimes refer to magnetic particle flaw detection directly as magnetic flaw detection, and its equipment is called magnetic flaw detection equipment.