In the production of precision castings for mining machinery parts, controlling the cooling rate is crucial to preventing cracks. Cracks often originate from temperature gradients that form within the precision castings during cooling, leading to uneven shrinkage in different areas and generating thermal stress. When this stress exceeds the tensile strength of the material, cracks form. Therefore, rationally controlling the cooling rate to ensure uniform cooling of the casting is an important means of reducing crack defects and improving casting quality.
Excessive cooling rate is one of the common causes of casting cracks. When the surface of the casting cools rapidly, the internal metal remains at a high temperature. This temperature difference creates a large temperature gradient, hindering surface shrinkage and generating tensile stress. If the metal's strength is low at this point, it cannot withstand this stress, and cracks will initiate on the surface or inside. This is especially true for thin-walled parts or castings with complex shapes, where excessively rapid cooling is more likely to cause cracks. Therefore, in casting process design, the cooling rate must be rationally set according to the material, shape, and size of the casting to avoid localized overheating or undercooling.
The choice of cooling medium and temperature control directly affect the cooling rate. Commonly used cooling media include water, oil, and air, each with significantly different cooling capacities. Water has high thermal conductivity and a fast cooling rate, but it can easily lead to excessive temperature differences between the inside and outside of the casting. Oil and air have relatively slower cooling rates, but offer more uniform temperature distribution. In actual production, a suitable cooling medium can be selected based on the requirements of the casting, and the cooling rate can be controlled by adjusting the medium temperature. For example, for alloy castings prone to cracking, preheating media or segmented cooling can be used to gradually reduce the casting temperature and decrease thermal stress.
Optimizing the design of the cooling system is also a crucial aspect of controlling the cooling rate. A reasonable cooling channel layout ensures that the cooling medium uniformly covers the casting surface, avoiding localized excessively fast or slow cooling. The shape and size of the cooling channels need to be designed based on the geometry and thermal conductivity characteristics of the casting to improve the flow rate and cooling effect of the cooling medium. Furthermore, using thermal analysis simulation software to simulate the solidification process and temperature distribution of precision castings can predict potential crack risks and optimize the design of the cooling system, thereby reducing crack initiation at the source. Adjusting casting process parameters is equally crucial for controlling the cooling rate. Parameters such as pouring temperature, mold temperature, and casting speed all affect the cooling process of the casting. Higher pouring temperatures prolong the solidification time of precision castings and increase the temperature gradient during cooling; while lower pouring temperatures may lead to casting defects due to insufficient metal fluidity. Therefore, the pouring temperature must be set appropriately according to the casting's material and shape to ensure sufficient metal filling of the mold cavity while avoiding excessively high temperatures that could cause cracks. Furthermore, the mold temperature must be matched with the cooling rate to avoid excessively slow cooling due to excessively high mold temperatures, or excessively rapid cooling due to excessively low mold temperatures.
Improving post-treatment processes plays a positive role in eliminating internal stress in castings and reducing the risk of cracking. Heat treatment is one of the commonly used post-treatment methods. Through processes such as preheating, holding, and quenching, the grain structure and properties of castings can be improved, reducing internal stress and defects. For example, appropriate annealing can eliminate residual stress generated during the cooling process of castings, improving their crack resistance. Furthermore, cutting forces and vibrations during machining can also cause cracks. Therefore, it is necessary to plan the machining path carefully and avoid over-machining in areas of stress concentration.
The quality and maintenance of the mold are equally important. Wear and deformation of the mold directly affect the forming quality of the casting, leading to uneven stress distribution during cooling and subsequently cracking. Therefore, the mold must be inspected and maintained regularly to ensure that its dimensional accuracy and surface quality meet requirements. In addition, the preheating and cooling of the mold must be coordinated with the cooling rate of the casting to avoid uneven cooling of the casting due to excessively high or low mold temperatures.
