The principle and characteristics of the magnetic method and the penetration method in the maintenance of diesel generators.

2021-11-12Font scaling:

        When the diesel generator is overhauled, physical quantities such as electricity, magnetism, light, sound, and heat are used to physically inspect the generator to find internal defects caused by components and conduct internal defects detection. This method is generally used in conjunction with instrument and tool inspection, and will not damage the parts, separate or damage. Now, people generally call it nondestructive testing. Predict the degree of development of the defect and decide to repair or discard it.

1. Magnetic force method   

         This method is also called magnetic flaw detection, which uses the condition shown by the magnetic lines of force in the ferromagnetic material to determine whether there are defects such as cracks, voids, and uneven organization in the part. The principle is to use a strong direct current to induce a magnetic field to magnetize parts. As the magnetic field passes through the magnetic material, the lines of magnetic force will pass through the shortest straight line. If the internal organization of the workpiece is uniform, the direction of the magnetic wire passing through the workpiece is consistent. If there is a defect inside the component, a larger magnetic resistance is formed at the defect site, and the magnetic field lines will change direction, bypass the defect, gather around the defect, and cause a leakage magnetic field similar to the defect on the surface of the part. When the surface of the workpiece is evenly sprinkled with iron powder, the iron powder will be adsorbed on the edge of the defect, revealing the position and size of the defect.
     This method is characterized by high sensitivity and fast operation, but it is only suitable for parts that are easily magnetized, and if there are deep defects on the surface of the workpiece, it is not easy to find. Magnetic detection is widely used in production units. Common flaw detection equipment has two types: machine tool type and hand-held type.
  Before the magnetic inspection, the surface of the workpiece should be cleaned up to block the parts that may flow into the magnetic powder. The parts are magnetized first and then inspected. The degaussing treatment should be carried out after the inspection, the purpose is to eliminate the residual magnetic field in the parts, to avoid affecting the normal operation of the parts after installation.

2. Infiltration method

   This method is to apply a highly penetrating penetrant to the surface of the cleaned part. Through wetting, make it penetrate the defect, then remove it from the surface, and then evenly coat a thin layer of developer (commonly used MgO2.Si02 white powder). The penetrant remaining in the defect is sucked into the surface under the capillary action of the developer and thus appears as a defect.
      The penetration method can be divided into the coloring method and the fluorescence method. The dyeing method is to add a red dye with a strong display ability to the penetrating liquid, and the developer is prepared with chalk powder so that the penetrating liquid can be displayed in the white developer after being sucked out. The law of fluorescence is to add a yellow-green fluorescent substance to the penetrant. The imaging agent should be specially formulated. When the absorbent is absorbed, it will emit obvious fluorescence after being irradiated with near-ultraviolet light, thereby showing the position and shape of the defect.
     The imaging agent of the present invention is composed of zinc white fire brocade glue, benzene, acetone, xylene, anhydrous alcohol, and the like.
     The dyeing method is used to detect the surface cracks of the parts, and the parts that are difficult to detect by the magnetic inspection and the fluorescence method. The method itself is not limited by the magnetic or non-magnetic material and is mainly used for surface defect detection of non-magnetic materials.

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