Significance of magnetic method and penetration method for component maintenance.

    When the diesel generator is overhauled, physical quantities such as electrical, magnetic, light, sound, and heat will be used to physically inspect the generator, and to find and detect defects. This method is generally used in conjunction with instrument and tool inspection. Because it will not cause parts to be separated or damaged, it is generally called non-destructive testing.

    The maintenance of diesel generators is mainly to carry out regular inspection and in-operation inspection of diesel generator parts. If a defect is found, it is necessary to predict the degree of development of the defect according to the type, shape, size, generation location, stress level, stress direction, etc. of the defect, and repair or discard the parts. Now, magnetic method, penetration method, ultrasonic method, ray method, etc. have been widely used in production testing.

1. Magnetic method
    Also known as magnetic flaw detection, its principle is to use a strong direct current to induce a magnetic field to magnetize parts. As the magnetic field passes through the magnetically permeable material, the magnetic field lines pass through in the shortest straight line. If the inside of the part is uniform, the direction of the magnetic line passing through the workpiece is the same. If there is a defect inside the part, a large magnetoresistance is formed at the defect site. The magnetic field lines change direction, bypass the defect, and gather around the defect. When the surface of the workpiece is evenly sprinkled with iron powder, the iron powder will be adsorbed on the edge of the defect, so that the position and size of the defect will be revealed.

    This method is characterized by high sensitivity and fast operation, but it is only suitable for parts that are easily magnetized. If the defect is deep, it is difficult to find. Magnetic detection is widely used in production. Common flaw detection equipment has two types: machine type and hand-held. Degaussing treatment should be carried out after testing to eliminate the residual magnetic field in the parts, to avoid affecting the normal operation of the parts after installation.

2. Penetration
    This method is to apply a highly penetrating penetrant to the surface of the cleaned part. Make it penetrate the defect, then remove it from the surface, and then evenly apply a thin layer of developer (commonly used MgO2.SiO2 white powder). The penetrant remaining in the defect is drawn into the surface by the capillary action of the developer, thereby revealing the defect.
    The penetration method can be divided into the coloring method and the fluorescence method. The dyeing method is to add red dye with a strong display ability to the permeate, and the developer is prepared with chalk powder so that after the permeate is sucked out, it can be seen in the white developer. The law of fluorescence is to add yellow-green fluorescent substances to the permeate, and the imaging agent should be specially formulated. When the absorber absorbs it and then irradiates it with near-ultraviolet light, it will emit obvious fluorescence, thereby showing the position and shape of the defect.

    The dyeing method is used to detect surface cracks of parts, and parts that are difficult to be detected by magnetic inspection and fluorescence methods. The method itself is not limited by magnetic or non-magnetic materials and is mainly used for surface defect detection of non-magnetic materials.
    The equipment used is simple, the operation is convenient, and it is not limited by the material and the shape of the parts. Compared with other methods, it has obvious advantages and is still a commonly used method.