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The Main Factors Affecting the Degree of Graphitization of Cast Iron

Because the crystal structure of iron is very different from that of graphite, and the crystal structure of iron and cementite is closer, ordinary cast iron can only get white cast iron under normal casting conditions, but gray cast iron is not easy to obtain. Therefore, it is necessary to promote the graphitization of cast iron by adding alloying elements and improving the casting process to form gray cast iron.

1. Influence of Chemical Composition

Carbon, silicon, manganese, sulfur, and phosphorus have different effects on graphitization. Among them, carbon, silicon, and phosphorus are elements that promote graphitization, and manganese and sulfur are elements that hinder graphitization. In actual production, adjusting the content of carbon and silicon is one of the basic measures to control the structure and properties of cast iron. Under general casting conditions, higher carbon content in cast iron is a necessary condition for graphitization, and ensuring a certain amount of silicon is a sufficient condition for graphitization. The higher the content of carbon and silicon, the easier it is for graphitization. If the content of carbon and silicon is too low, white mouth is easy to appear, and the mechanical properties and casting properties are poor; but if the content of carbon and silicon is too high, the graphite will be large and large, the amount of ferrite in the matrix will be large, and the mechanical properties will be reduced. . Therefore, the carbon and silicon content in general gray cast iron is controlled within the following ranges: 2.8%~3.5%C (mass fraction), 1.4%~2.7% Si (mass fraction).

2. Influence of Temperature and Cooling Rate

In addition to the chemical composition of the carbon graphitization process in cast iron, it is also affected by the cooling rate of the casting during the casting process. After ensuring the necessary and sufficient conditions for the content of carbon and silicon in terms of composition, if the cooling rate is too fast, graphitization is still impossible or even impossible to proceed. This is because regardless of the first or second stage of graphitization, the diffusion condition of carbon becomes a restrictive factor. Under the condition of high temperature and slow cooling, due to the high diffusion ability of atoms, it is usually carried out according to the Fe-C phase diagram, and the carbon in the cast iron is precipitated in a free state (graphite phase). When the cooling rate is faster, cementite instead of graphite is precipitated from the liquid state. This is because the carbon mass fraction (6.69%) of cementite is closer to the carbon mass fraction (2.5%~4.0%) of the alloy than graphite (100%). Therefore, generally the slower the cooling rate of the casting, the more fully the graphitization proceeds; The cooling rate is fast, carbon atoms are difficult to diffuse, and graphitization is difficult to proceed.