Casting quality includes inner quality, outer quality and service quality of casting. Its specific requirements, generally in the parts drawing and related technical documents are clearly stipulated. In order to ensure the quality of casting, every link of casting production, especially after cleaning, should be inspected. Any defective casting, after repair can meet the requirements, do not affect the user should be repaired.

1. Inspection of casting defects

1) During the inspection and inspection of appearance defects, observe and find the defects exposed on the appearance of the casting, such as cracks, surface pores and shrinkage holes, and sticky sand; Use measuring tools, templates and working platforms to check whether the size of the casting conforms to the requirements of the drawing, and use various weighing tools to check whether the weight of the casting is within the permissible deviation range.

2) Surface defect inspection

① Fluorescence detection method: the casting is soaked in fluorescent liquid, because of the capillary phenomenon, the fluorescent liquid infiltrates into the shortcomings of the surface of the casting, and then take out the casting and wipe it, and put it under ultraviolet (mercury quartz lamp) irradiation. By using the principle of luminescence of fluorescent liquid irradiated by ultraviolet light, the defects such as surface cracks can be judged.

(2) Coloring method: The use of liquid infiltration properties, the surface of the casting is coated with a layer of infiltration good coloring liquid (such as kerosene, pigment, etc.), to be liquid into the surface shortcomings, wipe off the surface of the coloring liquid, spray a layer of white zinc and other white display powder, then the residual in the shortcomings of the pores of the coloring agent and be sucked to the surface display powder, showing the shape of the shortcomings.

3) Internal defect inspection

(1) X-ray inspection: internal defects of the casting can be found, such as pores, shrinkage holes, cracks, inclusions, etc. Figure 2-10 shows the X-ray examination. For important castings, such as Class I castings used in the defense industry, 100 percent radiographic inspection is generally required. Commonly used rays are: X-rays and g rays. The former is less penetrating but more sensitive than the latter, and these rays can penetrate metal and sensitize the negative. As rays pass through an object, they interact with atoms in the object, and their power is gradually diminished by absorption and scattering. The density of holes and inclusions in castings is much lower than the density of metals. When rays interact with these defects, the attenuation is small, and the ability of the negative is larger than that of the parts without defects. Therefore, the shape of defects can be shown on the negative.

②*** inspection: can also find the internal defects of the casting, such as pores, cracks, inclusions, shrinkage and so on. For steel casting, this method can be used to detect a large wall thickness (1000mm). *** The working frequency used for inspection is usually above 1MHz. *** is reflected at the interface as it travels from one medium to another. In particular, as much as 99 percent of the ultrasound is reflected back from the interface when it travels from metal to air. *** inspection is the application of this property to find the internal defects of castings. When the head moves slowly on the casting, if the casting has no defects, only T wave reflected on the detection surface and B wave reflected on the bottom surface will appear on the oscilloscope screen. If there is a defect on a part of the casting, there will be F wave reflected by the defect, as shown in Figure 2-11. The greater the defect, the greater the height of F wave, 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, valves, etc., pressure test is needed to check whether there are holes in the castings. When testing, there will be a certain pressure of air (or water, oil, etc.) into the sealed cavity of the casting, such as casting through cracks, holes, there will be leakage, so as to find the location of the shortcomings. The test pressure is usually more than 30~50 percent of the working pressure of the casting, which is also a strength assessment of the casting. When the casting is not easy to form a sealed cavity, so that the pressure test cannot be carried out, the density of the casting can be tested by means of kerosene infiltration.

2. Repair of casting defects

Casting defects do not mean casting scrap, according to the design requirements of castings, in the case of meet the use, can be repaired to repair. There are three common methods of casting repair: repairing with putty and epoxy resin, patching and infiltration patching.

1) Repair with putty and epoxy resin: holes and holes found on the parts that are not very important to the casting but have decorative significance can be repaired by preparing putty according to the color of the casting. For cast iron, can use iron powder, water glass and cement preparation; For cast aluminum parts, can use aluminum powder, adhesive preparation; For the defects on the non-machining surface, static surface and non-main position of the casting, as well as the shortcomings of oil leakage in the oil tank and oil pool, epoxy resin adhesive can be used to repair.

2)** repair: This is a common method of casting repair. Arc welding is mainly used for welding steel casting defects. Generally, it can be welded without preheating, but the important parts of the casting, or the structure is very complex, and more metal needs to be welded. In order to prevent and remove the internal stress, improve the plasticity of surfacing metal and reduce hardness, it is necessary to preheat and annealing before welding. Gas welding is mostly used to repair cast iron and non-ferrous metal castings. Cast iron is brittle, in order to prevent internal stress and cracks during repair welding, it is often necessary to slow preheat to 500~550°C, and slow cooling to 50~100°C after welding. Cast iron with complex shape needs to be annealed at 700~800°C after welding. In order to prevent oxidation, argon arc welding is often used in the repair welding of aluminum magnesium alloy castings, and there are strict requirements on the perfect process, otherwise, there will be shortcomings in the repair welding process.

3) Infiltration repair welding: infiltration is a new technology to solve the problem of casting leakage. It is a gelatinous infiltrating agent into the pores of the casting, and then make the infiltration agent hardening, and the inner wall of the porosity of the casting into one, so as to achieve the purpose of plugging. The commonly used impregnating agent is water glass type and synthetic resin type two categories. At present, vacuum pressure method is generally used to repair castings. The castings were put into a special cage and then put into the infiltration tank. After vacuum-pumping for 1.5~2min(the pressure is less than 5KPa), the castings were flooded with leaching liquid to make the infiltrating agent penetrate into the defective pores. Then, 0.5~0.7MPa compressed air was injected and the pressure was maintained for 20min to further enhance the infiltration effect. Take out the casting, clean it, and wait for the soaking solution to cure.