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    On casting quality inspection and defect repair
    Release Time:2019-05-17 16:59:23      Hit Count:1070

    Casting quality includes inner quality, appearance quality and service quality. The specific requirements are generally specified in detail drawings and relevant technical documents. In order to ensure casting quality, all links of casting production, especially after cleaning, should be carried out quality inspection. All defective castings can be repaired to meet the requirements without affecting the user.

    1. Inspection of casting defects

    1) visually inspect casting defects, such as cracks, surface pores and shrinkage holes, and sand sticking, by observing and looking for defects exposed on casting surface with naked eyes or with magnifying glass, tip hammer and other tools; Use gauges, templates and work platforms to check casting dimensions as per drawing requirements, and use various weighing tools to check casting weight within the allowable deviation range.

    2) surface defect inspection

    (1) fluorescence inspection method: the casting soaked in fluorescence liquid, because of the capillary phenomenon, fluorescence liquid penetration into the casting surface defects, and then remove the casting and clean, placed in the ultraviolet (mercury quartz lamp) irradiation. By using the principle of ultraviolet irradiation of fluorescence solution, the defects such as surface cracks can be judged.

    (2) staining method: using the permeability properties of the liquid, in the inspection of castings surface coated with a layer of liquid permeability good coloring, such as kerosene, a mixture of acetone, paint, etc), liquid penetrate the surface defect, wipe the surface coloring liquid, powder spraying a layer of white on white display liquid, then the residual defects in pore of colorants and sucked upon the surface shows powder, presents the shape of the defect.

    3) internal defect inspection

    (1) X-ray examination: casting internal defects can be found, such as pores, shrinkage, cracks, inclusions, etc. Figure 2-10 shows the schematic diagram of the X-ray examination. Important castings, such as class I castings used in the national defense industry, are generally subject to 100% radiographic inspection. Commonly used rays are: X rays and g rays. The former is not as penetrating as the latter, but is more sensitive. When a ray penetrates an object, it interacts with the atoms in the object, and the energy of the ray is continuously absorbed and scattered and gradually attenuated. The higher the density of the object, the faster the energy attenuation. The density of the holes and inclusions in the casting is much lower than that of the metal. When the ray ACTS on these defects, the attenuation is smaller.

    Ultrasonic examination: casting internal defects can also be found, such as pores, cracks, inclusions, shrinkage porosity. For steel castings, this method can be used to detect the maximum wall thickness (1000mm) of a method. The working frequency of ultrasonic examination is usually above 1MHz. Ultrasonic waves travel from one medium to another and reflect off the interface, especially when they travel from metal to air. Ultrasonic inspection is the application of this feature to find casting internal defects. When the probe moves slowly on the casting, if there is no defect in the casting, only T wave reflected from the detection surface and B wave reflected from the bottom surface will appear on the oscilloscope screen. If there is a defect in a part of the casting, the f-wave reflected by the defect will also appear, as shown in FIG. 2-11. The larger the defect is, the higher the f-wave height will be, so as to determine the location and severity of the defect in the casting.

    (3) pressure test: for high pressure and vacuum castings, such as pump body, valve, etc., the pressure test should be done to check whether there are holes in the casting. During the test, air (or water, oil, etc.) with a certain pressure will pass into the inner cavity of the sealed casting. If there are cracks or holes in the casting, leakage will occur, and thus the location of defects will be found. The test pressure usually exceeds 30~50% of the working pressure of the casting, which is also a kind of strength examination for the casting. When the casting is not easy to form a sealed cavity, so the pressure test cannot be carried out, the method of permeating kerosene can be used to test the density of the casting.

    2. Repair casting defects

    Defects in castings do not mean that castings are scrapped. According to the design requirements of castings, those that can be repaired shall be repaired if they meet the requirements for use. There are three common casting repair methods: repair with putty and epoxy resin, welding repair and infiltration repair.

    1) repair with putty and epoxy resin: for the hole defects found on the parts which are not very important but have decorative significance, the putty can be made according to the color of the casting to be repaired. For cast iron, iron powder, sodium silicate and cement can be used. For cast aluminum parts, aluminum powder and high-efficiency adhesive can be used. Defects on the non-machined surface, stationary surface and non-main position of the casting, as well as oil leakage defects in the oil tank and oil pool, etc., can be repaired with epoxy resin adhesive.

    2) welding repair: this is the most common casting repair method. Arc welding is mainly used for repairing defects in steel castings. Generally, welding repair can be done without preheating. However, for important parts of castings or complicated structures that need more metal welding, preheating and post-annealing are required to prevent and eliminate the internal stress, improve the plasticity and reduce the hardness of the surfacing metal. Gas welding is mostly used for repairing cast iron and nonferrous metal castings. Cast iron is crisp, to prevent internal stress and cracks when the repair welding, often need to preheat to 500 ~ 550 ° C, slow slow cooling after welding to 50 ~ 100 ° C, complex shape of cast iron parts, after welding still need annealing under 700 ~ 800 ° C. In order to prevent oxidation, argon arc welding is often used for the repair welding of al-mg alloy castings.

    3) infiltration repair welding: infiltration is a new technology to solve the problem of casting leakage. It is to infiltrate the colloidal impregnation agent into the pores of the casting, and then make the impregnation agent harden and connect with the inner wall of the pores of the casting, so as to achieve the purpose of plugging up the leakage. The commonly used infiltrating agents are sodium silicate type and synthetic resin type. At present, the vacuum pressurization method is generally used in the whole casting repair in China. The casting is put into a special cage and then put into the infiltration tank. The vacuum is pumped for 1.5-2min (the pressure is lower than 5KPa), and the infiltration liquid is injected to submerge the casting, so that the infiltration agent infiltrates into the defect pores. Then the compressed air of 0.5-0.7mpa is injected and the pressure is maintained for 20min to further enhance the infiltration effect. Remove the casting, clean and let soak and solidify.

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