Grain coarseness of castings refers to the defect that the grain structure is too coarse to be suitable for application when the machinist or the fracture inspection is carried out. This kind of grain coarseness may occur in the whole casting or in the local part of the casting. In essence, coarse grain defect is a kind of metallurgical defect. According to the production practice for many years and referring to the relevant data, the author discusses the causes of the coarse grain defects in castings and the preventive measures.
1) If the section difference of the casting is too large, the grain will be coarse because of the slow cooling of the thicker section. Gray cast iron and other metals which are very sensitive to cross-section change are more likely to produce such defects.
The effective way to prevent this kind of defect is to avoid the excessive disparity of casting section size, but this way is sometimes beyond the ability of foundry workers. Therefore, in terms of casting itself, we can reduce the occurrence of such problems and the severity of such defects by setting the chill, controlling the pouring temperature or selecting the appropriate pouring system. Using cold iron can speed up the cooling speed of the thicker section of the casting. If the pouring temperature is too high, this kind of problem will be more serious and should be avoided. By adjusting and modifying the design of the pouring system, the molten metal with low temperature will be located in the thicker section of the casting, and the most effective riser will be designed at the thick section of the casting, so as to reduce the size of the riser as much as possible.
(2) For the castings with holes, the process designer sometimes does not use the core which helps to reduce the effective cross-section size, so that the cross-section without core is too thick to produce this defect. Therefore, in the process design, the sand core should be set in the thicker cross-section as much as possible.
(3) In some cases, the section of the casting is not too thick, but the result is the same as that of the thick section because of the formation of the heat sink section in the casting due to a narrow depression or core. For example, a core may need to be set at a cylindrical navel in a deeper part of the casting, which will cause slow cooling. In the case that the design cannot be modified, unless the metal temperature can be reduced or the gate can be replaced, the best solution is to set cold iron at the core or the section of the mold.
(4) In the process design, if the machining allowance is too large, it will not only increase the cost of machining, but also cut off the surface layer of the denser casting, and expose the loose part with slower central cooling. This design has no merit, because it is unreasonable from the perspective of casting or machining. The solution is to change the design of casting. If it is not allowed to change the design, the correct method is to use cold iron, control the pouring temperature and adjust the pouring system.
(5) Improper core design, incorrect core support, or other technology causing core deviation at the thick section will result in the change of the section of the casting, resulting in the coarseness of the grain.