In cast iron, silicon is an alloying element that promotes graphitization, and its ability to promote graphitization is 3 times that of nickel and 5 times that of copper. And no matter in liquid or solid cast iron, silicon combines with iron stronger than carbon.
Liquid cast iron contains silicon, which reduces the solubility of carbon. The higher the silicon content in the molten iron, the lower the carbon content correspondingly, the more carbon will be expelled.
It is not difficult to understand from the iron-carbon phase diagram: when the molten iron is a hypereutectic composition, the silicon content is high, and during the solidification process, more carbon is precipitated in the form of primary graphite, until the remaining molten iron reaches the eutectic composition. After the eutectic transformation occurs; when the molten iron is a hypoeutectic component, during the solidification process, silicon is enriched in the primary austenite. During the eutectic transformation, silicon is enriched in the eutectic austenite crystallized in the early stage, which inhibits the synthesis of cementite by carbon and iron, enhances the diffusion rate of carbon in austenite, and promotes the precipitation of carbon in the form of eutectic graphite; During the eutectoid transformation, the silicon dissolved in the austenite still inhibits the formation of cementite between carbon and iron, enhances the diffusion rate of carbon in the austenite, and promotes the precipitation of carbon in the form of eutectoid graphite.
In gray cast iron, ductile iron, vermicular graphite iron and black-core malleable iron, carbon and silicon are the main elements that affect the shape and quantity of graphite. It is a white-core malleable cast iron that basically does not contain graphite. In the process of decarburization and annealing, silicon promotes the diffusion of carbon in the austenite, which also plays an important role in the decarburization of this malleable cast iron.
In addition, both oxygen and nitrogen in cast iron have the effect of stabilizing carbides. The silicon contained in cast iron can reduce the oxygen and nitrogen content in it, thus indirectly enhancing the effect of silicon on graphitization.
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