How is magnetic particle testing performed on pressure vessels?

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Magnetic particle testing is widely used in the non-destructive testing of pressure vessels. The testing method is as follows: 01 Magnetic particle testing is another important method for detecting surface defects such as cracks in steel materials. Its principle is as follows: when a magnetic field is applied to the test specimen, strong ferromagnetic materials such as steel can be strongly magnetized by that field. If there are cracks on the specimen and the direction of the cracks is perpendicular to the magnetization direction, magnetic poles will appear at the cracks, generating a leakage magnetic field. When the fine particles of magnetic powder enter the leakage magnetic field, they are attracted and remain there. Since the leakage magnetic field is wider than the crack, the accumulated magnetic powder can be easily seen with the naked eye. When the magnetization intensity is high enough, a clearly visible leakage magnetic field can be generated even with very small cracks. If a fluorescent magnetizing fluid is used, under ultraviolet light, surface defects on the test piece such as tiny cracks, folds, pores, and inclusions appear as yellow-green lines or dotted lines. 02 Magnetic particle inspection procedures include preprocessing, magnetization, applying magnetic particles, observation and recording, as well as post-processing (including demagnetization). When magnetizing the test specimen, the direction of the crack being inspected should be taken into account; the magnetic field should be applied in a direction perpendicular to that of the crack. The magnetization method in practical applications is shown in the figure below, where the direction of the magnetic field is indicated by magnetic field lines (dashed lines). Since the direction of cracks is often unpredictable, a composite magnetization method that generates mutually perpendicular magnetic fields is commonly used in practice. Image 03: The surface magnetic field strength can be calculated based on the dimensions of the specimen and the current value. 04 Magnetic particle testing has a high degree of accuracy; it can detect surface cracks as well as other defects that are shallow beneath the surface. However, it is unable to determine the depth of these defects, nor can it be used to detect internal defects. 05 There are many magnetization methods, including axial current application, direct current application, electrode penetration method, coil method, inter-pole method, and flux penetration method.