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Grain size has a significant impact on the mechanical properties of materials. Refining the grain structure not only enhances the strength and hardness of the material, but also improves its ductility and toughness. Grain refinement is the only method of strengthening materials that can improve both strength and toughness. So how can grains be refined? From the perspective of metal crystallization, each grain forms through nucleation and then grows. Therefore, the size of the grains depends on the nucleation rate and the growth velocity. The higher the nucleation rate, the greater the number of primary nuclei, and the finer the grains. The slower the grain growth rate, the more nuclei will form during the grain growth process, and the resulting grains will be finer. It can be seen that any factor capable of promoting nucleation and suppressing grain growth can refine the grains of the material. During the crystallization of metals, grain refinement can be achieved through the following methods: 1. Controlling the degree of supercooling. Both the nucleation rate and growth rate are related to the degree of supercooling. Increasing the supercooling degree leads to an increase in both the nucleation rate and the growth rate, but the nucleation rate increases more rapidly; therefore, under normal conditions, increasing the supercooling degree results in finer grains. Figure 12: Modification treatment. Increasing the supercooling degree can refine the grains, but for components and castings with a large volume, although the surface supercooling degree is high, it is not possible to achieve the same cooling rate in the core as at the surface; therefore, the grains in the core cannot be effectively refined by increasing the supercooling degree. Modification treatment can avoid this deficiency. The so-called modification treatment involves adding a nucleating agent, also known as a modifier, to the liquid metal before pouring it, i.e., before the molten metal is poured. These nucleators promote massive heterogeneous nucleation of the liquid metal, thereby achieving grain refinement. This is the principle behind adding alloying elements such as Ti and V to steel. 3. Vibration, stirring. When a metal is about to solidify, vibrating or stirring it can promote nucleation and cause the dendrites to break apart, thereby achieving the goal of refining the grain structure. Mechanical vibration, ultrasonic vibration, electromagnetic stirring, and the like can all be used to achieve grain refinement. Additionally, through hot rolling or forging processes, by controlling the degree of deformation as well as the temperatures during hot rolling and forging, it is also possible to refine the grain structure; that is, by controlling rolling and cooling.