Introduce Magnetic particle Inspection (MPI) of Non-destructive testing (NDT)
What is Magnetic particle Inspection (MPI)?
Magnetic particle Inspection, a non-destructive method of detecting defects on or near the surface of ferromagnetic materials by the accumulation of magnetic particles in the leakage field near the defect. Let's find out more about it.
I. Principles and Introduction
Magnetic particle Inspection is a surface flaw detection method. Suitable for detecting surface and near-surface defects in objects made of magnetic materials such as steel. The basic principle is that after the magnetization of ferromagnetic materials and workpieces, due to the existence of discontinuities, local distortion of the magnetic lines on the surface and near the surface of the workpiece to produce leakage magnetic field, adsorption of magnetic particles applied to the surface of the workpiece, the formation of visually visible magnetic traces under suitable light, thus showing the location, shape and size of the discontinuity.
How much kinds of Magnetic particle Inspection (MPI)?
II. Types of Magnetic particle Inspection (MPI)
1、According to the different magnetization direction of the workpiece, can be divided into circumferential magnetization method, longitudinal magnetization method, composite magnetization method and rotary magnetization method.
2、According to the different magnetization currents used, it can be divided into DC magnetization method, half-wave DC magnetization method, and AC magnetization method.
3、According to the different preparation of the magnetic powder used in the detection, can be divided into dry powder method and wet powder method.
4、According to the time difference of applying magnetic particle on the workpiece, it can be divided into continuous method and residual magnetic method.
How to do Magnetic particle Inspection (MPI)?
III. Operational process and evaluation
1, Pre-treatment: All materials and test pieces should be free of grease and other impurities that may affect the normal distribution of magnetic particles, the density, characteristics and clarity of magnetic particle accumulation.
2. Magnetization: Magnetic particle detection shall be subject to the satisfactory detection of any harmful defects in any respect. (b) To cause the magnetic line to cross, as far as practicable, any defect that may be present in the specimen.
3、Apply magnetic powder or magnetic suspension: Magnetic suspension should be stirred well before each application. When the magnetic powder suspension is poured on the surface of the specimen, defective magnetic traces can be seen on the surface of the specimen in daylight, and these traces will be more visible under ultraviolet light, especially under black light. Manual spraying, continuous detection method, i.e., the magnetization of the examined workpiece, the application of magnetic powder and the observation of magnetic traces should be completed within the magnetization power-up time, the power-up time is 1s to 3s, stop applying magnetic suspension for at least 1s before stopping the magnetization.
4、Magnetic trace observation and recording: The observation of defective magnetic trace should be carried out immediately after the formation of magnetic trace. All other magnetic trace indications shall be treated as defects except where it can be ascertained that the magnetic trace is due to local unevenness of the workpiece material or improper handling. To identify small magnetic traces, a 2 to 10 magnifying glass should be used. The magnetic trace display records were recorded photographed and marked on a photograph.
5、Demagnetization treatment: the workpiece can use AC electromagnetic yoke for local demagnetization or the use of winding coil segment demagnetization, the demagnetization effect of the workpiece is generally measured by magnetic field intensity meter, not more than 0.3mT (240A/m).
6. Evaluation of results
6.1 No cracks or white spots are allowed.
6.2 Quality classification
6.3 Composite rating
When multiple defects exist simultaneously in a circular defect rating area, a composite rating shall be made. Each type of defect is graded separately, with the lowest level of quality taken as the level of the composite rating; when the levels of each type of defect are the same, the level of the composite rating is reduced by one level.
