The methods listed in table 1 are all electric signal detection except magnetic particle method, which is easy to realize automation, but it is only used for parts or profiles with simple shapes, with low sensitivity and great limitations, so magnetic particle method is still widely used. Magnetic particle method is to apply ferromagnetic powder to the magnetized inspected part, and the magnetic powder is attracted by the leakage magnetic field to show the position and shape of the defect. The process sequence of magnetic powder method is surface preparation, magnetization, magnetic powder addition, inspection, demagnetization and post-cleaning. When the magnetic field is perpendicular to the defect direction, the leakage magnetic field is the strongest and the defect display is the clearest. Therefore, it is often necessary to magnetize the inspected parts twice in circumferential direction and longitudinal direction or compound magnetization to find defects in all directions. The commonly used magnetization methods are shown in Table 2. magnetic testing
The types of magnetizing current include DC, AC, half-wave rectification and full-wave rectification. Magnetic powder which is easy to find near-surface defects by direct current includes non-fluorescent magnetic powder and fluorescent magnetic powder, and the methods of applying magnetic powder include dry method and wet method. Dry method is to directly sprinkle magnetic powder on the surface of the inspected parts; Wet method is to suspend magnetic powder in liquid medium and then coat it on the inspected parts. The concentration of suspended magnetic powder should be appropriate: it should not be too light to avoid missing tiny defects; Not too thick, it will cause bad backing.
Applying magnetic powder to the detected part under the action of external magnetic field is called continuous method; Applying magnetic powder after removing external magnetic field is called remanence method. The continuous method is suitable for any ferromagnetic material; The remanence method is only suitable for materials with strong remanence induction and coercivity.
Magnetic flaw detection can be used for the inspection of raw materials and semi-finished products such as plates, pipes, profiles and forging blanks, and can also be used for the inspection of working procedures and finished products of forgings, weldments, castings and turning parts. It can also be used for the maintenance of aircraft, trains, tractors and other machinery, high-pressure containers, oil storage tanks and other important equipment. Defects such as cracks, hairlines, wrinkles, white spots, delamination, pores and inclusions can be found.
Magnetic particle method is almost not limited by the size and shape of parts, and has the advantages of fast detection speed, intuitive display, low cost and high sensitivity, and can find micron-sized defects. The buried depth of detectable near-surface defects depends on the size of the defects, and generally does not exceed 1 ~ 2mm. Its disadvantages are that non-ferromagnetic materials can not be detected, and coatings such as paint and electroplating have obvious influence on detection sensitivity, so it is difficult to realize automation of complex parts; The inability to detect internal defects of parts is also one of its limitations.