Silicon in the role of cast iron is multifaceted, Silicon of which we are most concerned about the first "to promote the graphitization" and "solid solution strengthening" two, Silicon in addition, there are many important silicon, here, simple To mention the following two points:
(1) soluble in liquid cast iron in the silicon, so that iron oxide antioxidant capacity greatly enhanced, and silicon can also make nitrogen in the molten iron solution decreased. Silicon It is precisely because of this role of silicon, cast iron can be strong oxidizing, nitrogen-rich conditions of melting. All kinds of cast alloys, only cast iron can use cupola, oxygen rotary furnace such smelting equipment, Silicon in the oxygen-rich, nitrogen-rich atmosphere of smelting.
(2) the silicon content of cast iron increased to 3.5% or more, cast iron antioxidant capacity, thermal growth performance are greatly improved. Early, the national standards for heat-resistant cast iron, there are silicon-resistant refractory cast iron grades. In recent years, for energy-saving considerations, Silicon a variety of internal combustion engine to improve the exhaust temperature, Silicon the automotive industry, are very concerned about the application of heat-resistant silicon-molybdenum ductile iron.
The role of silicon in the promotion of graphitization in cast iron
Silicon in cast iron is the strongest alloying element to promote graphitization. Silicon The ability of silicon to promote graphitization is three times that of nickel and five times that of copper.
Whether in liquid or solid cast iron, Silicon silicon and iron combined with the role of stronger than carbon.
Liquid silicon contains silicon, which will reduce the solubility of carbon. The higher the silicon content in the molten iron, the lower the carbon content is, Silicon and the more carbon will be squeezed out.
When the molten iron is hypereutectic, the silicon content is high. Silicon During the solidification process,Silicon more carbon is precipitated in the form of primary graphite until the remaining iron liquid reaches the eutectic component and the eutectic transition occurs.
When the iron is a hypereutectic component, Silicon the silicon is enriched in the primary austenite during the solidification process.
In the eutectic transition, the silicon is enriched in the eutectic austenite in the early crystallization, suppresses the carbon and ferric synthetic cementite, enhances the diffusion rate of carbon in the austenite, Silicon and causes the carbon to precipitate in the form of eutectic graphite.
Eutectic transformation, the silicon dissolved in austenite, still inhibit the formation of carbon and iron cementite, and enhance the diffusion rate of carbon in austenite, Silicon and promote the precipitation of carbon in the form of eutectoid graphite.
In the gray cast iron, ductile iron, vermicular graphite cast iron and black heart malleable cast iron, carbon and silicon are the main elements that affect the graphite form and quantity. Silicon Is a graphite-free white heart malleable cast iron, in its decarburization annealing process, Silicon silicon to promote the diffusion of carbon in austenite, for this malleable iron decarburization also has an important role.
In addition, oxygen and nitrogen in cast iron have stabilized carbides. The silicon contained in the cast iron can reduce the oxygen and nitrogen content, thus indirectly enhancing the effect of silicon on the graphitization.
In solid cast iron, silicon is almost entirely solid in austenite and ferrite, not into the carbide. Silicon atoms and iron atoms can be combined into a strong covalent bond containing silicon ferrite, not only to promote the formation of ferrite, but also to strengthen the role of ferrite strong.
In order to understand the ability of silicon to strengthen ferrite, to avoid the impact of graphite and other alloying elements, Silicon in the 1950s, foreign people in the carbon content of 0.1%, Silicon without other alloying elements of steel, Silicon adding different amounts of silicon to Comparison of the effect of silicon on mechanical properties