There are many factors that affect the quality of castings, one of which is the design process of castings. When designing, in addition to determining the geometric shape and size of the casting based on working conditions and metal material properties, the rationality of the design must also be considered from the perspective of casting alloy and casting process characteristics, that is, obvious size effects and problems such as solidification, shrinkage, stress, etc., in order to avoid or reduce the occurrence of defects such as component segregation, deformation, and cracking in the casting. The second requirement is to have a reasonable casting process. Based on the structure, weight, and size of the casting, the characteristics of the casting alloy, and production conditions, appropriate parting surfaces, shapes, and core making methods are selected, and casting bars, cold iron, risers, and gating systems are reasonably set. To ensure the acquisition of high-quality castings. The third is the quality of the raw materials used for casting. The quality of metal furnace materials, refractory materials, fuels, fluxes, modifiers, as well as casting sand, sand binders, coatings, and other materials that do not meet standards can cause defects such as porosity, pinholes, slag inclusions, and sand sticking in castings, affecting the appearance and internal quality of castings. In severe cases, castings can be scrapped. The fourth is process operation, which requires the development of reasonable process operation procedures, the improvement of workers’ technical level, and the correct implementation of process procedures.
In casting production, it is necessary to control and inspect the quality of castings. Firstly, it is necessary to establish process guidelines and technical conditions for the control and inspection of raw materials, auxiliary materials, and each specific product. Strictly control and inspect each process according to the process rules and technical conditions. Finally, conduct quality inspection on the finished castings. Reasonable testing methods and appropriate testing personnel should be equipped. Generally, the appearance quality of castings can be judged by comparing sample blocks to determine the surface roughness of castings; Minor cracks on the surface can be inspected by coloring and magnetic particle methods. The internal quality of castings can be inspected and judged using methods such as audio, ultrasound, eddy current, X-ray, and gamma ray.
Defects in sand casting castings include cold shut, insufficient pouring, porosity, sand sticking, sand inclusion, sand holes, and sand swelling.
1) Insufficient liquid metal filling ability or poor filling conditions during cold sealing and pouring will cause the metal liquid to stop flowing before the mold cavity is filled, resulting in insufficient pouring or cold sealing defects in the casting. Insufficient pouring can prevent the casting from obtaining a complete shape; During cold insulation, although the casting can obtain a complete shape, the mechanical properties of the casting are severely damaged due to incomplete fusion joints.
Prevent insufficient pouring and cold insulation: increase pouring temperature and pouring speed.
2) Pore like defects formed in castings due to the failure of gas to escape in a timely manner before the metal liquid crust is formed. The inner wall of the pores is smooth, bright, or slightly oxidized. After the formation of pores in castings, their effective bearing area will be reduced, and stress concentration will be caused around the pores, which will reduce the impact resistance and fatigue resistance of the castings. Pores can also reduce the density of castings, resulting in the scrapping of certain castings that require hydrostatic testing. In addition, pores also have a negative impact on the corrosion and heat resistance of castings.
Preventing the formation of pores: reducing the gas content in the molten metal, increasing the permeability of the sand mold, and adding gas outlet risers at high positions in the mold cavity.