the role of silicon in cast iron
The role of silicon in cast iron is multifaceted. Among them, we are most concerned about the first two items: "promoting graphitization" and "solid solution strengthening":
1.The role of silicon in promoting graphitization in cast iron. Silicon in cast iron is the strongest alloy element that promotes graphitization. The ability of silicon to promote graphitization is three times that of nickel and five times that of copper,the liquid cast iron contains silicon will reduce the solubility of carbon, the higher the content of silicon in the molten iron, the lower the carbon content, and more carbon will be squeezed out.During the solidification process,the silicon content in the molten iron is high more carbon is precipitated in the form of primary graphite, until the remaining molten iron reaches the eutectic composition and the eutectic transformation occurs.
In gray cast iron, spheroidal graphite cast iron, vermicular graphite cast iron and black core malleable cast iron, carbon and silicon are the main elements that affect the shape and quantity of graphite. It is the white core malleable cast iron that basically does not contain graphite. During the decarburization annealing process, 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, the 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.
2.The solid solution strengthening effect of silicon in ferrite. Silicon atoms and iron atoms can be combined into silicon-containing ferrite with strong covalent bonds, which not only promotes the formation of ferrite, but also strengthens ferrite. In cast iron, the solid solution strengthening effect of silicon can be used to reduce or eliminate alloying elements such as copper, nickel, tin, molybdenum, and chromium that increase strength.
According to data analysis, the following observations can generally be established:
(1) In nodular cast iron, silicon is solid-dissolved in ferrite, has the effect of inhibiting pearlite and promoting ferrite, and it is solid-soluble in ferrite silicon Can strengthen ferrite.
(2) For ductile cast iron with a pearlite-ferrite mixed structure and a tensile strength of 600M Pa and lower than this value, the silicon content can be increased to 3.8% to 4.4%, and a full ferrite structure can be obtained. Under the condition that the tensile strength meets the requirements, the yield strength and elongation have been greatly improved. At the same time, the obtained castings have uniform hardness and significantly improved processing performance.
(3) If the tensile strength is required to be above 700MPa, the ferritic nodular cast iron strengthened by silicon solid solution cannot meet the requirements.
(4) The fatigue limit of silicon solid solution strengthened ductile iron is better than that of conventional ductile iron.
(5) Silicon solid solution strengthened ductile iron has a high brittleness transition temperature: when measured with V-notch and U-notch samples, the brittleness transition temperature is higher than room temperature and above 60℃; use unnotched specimens During the measurement, it was also 10°C or higher. Therefore, silicon solid-solution-strengthened nodular cast iron is not suitable for manufacturing castings with stress concentration points on the structure, and especially not suitable for castings that bear impact loads under low temperature conditions.
(6) Above the brittle transition temperature, the impact toughness of silicon solid-solution-strengthened ductile iron is better than that of conventional ductile iron.
In addition, silicon has more important functions. Here, I briefly mention the following two points:
(1) Silicon dissolved in liquid cast iron greatly enhances the oxidation resistance of molten iron, and silicon can also reduce the solubility of nitrogen in molten iron. It is precisely because of this effect of silicon that cast iron can be smelted under strong oxidizing and nitrogen-rich conditions. Among various casting alloys, only cast iron can be smelted in an oxygen-rich and nitrogen-rich atmosphere with melting equipment such as cupolas and oxygen rotary furnaces.
(2) Increasing the silicon content in cast iron to over 3.5%, the oxidation resistance and thermal growth resistance of cast iron are greatly improved. In the early days, there were silicon-based heat-resistant cast iron grades in the standards of heat-resistant cast iron in various countries. In recent years, due to the consideration of energy saving, various internal combustion engines have increased the temperature of exhaust gas. In the automobile industry of various countries, the application of heat-resistant silicon-molybdenum ductile iron castings has been attached great importance.
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