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 and undergoes eutectic transformation; the molten iron is hypoeutectic When the crystal composition is low, silicon is enriched in the primary austenite during solidification. During eutectic transformation, silicon is enriched in the early crystallized eutectic austenite, which inhibits carbon and iron from forming cementite, enhances the diffusion rate of carbon in austenite, and promotes the precipitation of carbon in the form of eutectic graphite; During eutectoid transformation, silicon dissolved in austenite still inhibits carbon and iron from forming cementite, enhances the diffusion rate of carbon in austenite, and promotes the precipitation of carbon in the form of eutectoid graphite.
In gray cast iron, nodular cast iron, vermicular graphite cast iron and black heart malleable cast iron, carbon and silicon are the main elements affecting the shape and quantity of graphite. It is white-heart malleable cast iron that basically does not contain graphite. During its decarburization annealing process, silicon promotes the diffusion of carbon in 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 content of oxygen and nitrogen in it, thus indirectly enhancing the effect of silicon on graphitization.