Spheroidized elements usually have the following common properties:
(1) In gray cast iron, the solubility of elements in molten iron is low, and the segregation tendency is obvious during solidification.
(2) There are one or two valence electrons in the outermost electron layer of the element, and there are two electrons in the secondary inner layer. This electronic structure will make carbon, sulfur and oxygen have strong affinity with elements, and the properties of the generated compounds are stable, which will significantly reduce the oxygen and sulfur in gray cast iron molten iron.
(3) Although carbon has affinity with spheroidized elements, spheroidized elements are difficult to dissolve in graphite lattice. According to the above characteristics, it can be found that yttrium (heavy rare earth element) and cerium (light rare earth element) in rare earth are effective spheroidizing elements. Rare earth reacts with sulfur, oxygen and other elements to form rare earth oxides and sulfides. These compounds belong to high solubility compounds and can be used as external core to promote graphite nucleation.
However, rare earth is generally not used as a spheroidizing agent alone. Research shows that the relative atomic mass of rare earth is generally large, and the consumption of desulfurization and deoxidation is larger than that of some other spheroidizing elements, such as magnesium and copper. At the same time, the density of rare earth compounds is large, which is not easy to remove from molten iron, Therefore, rare earth is generally used as spheroidizing agent in combination with other elements, such as rare earth magnesium alloy, rare earth magnesium calcium alloy and so on. At the same time, the residual amount of rare earth in gray cast iron needs to be controlled, because when the amount of rare earth is too large, it will increase the tendency of white cast iron and inhibit the growth of eutectic grains. In serious cases, it will distort the graphite morphology of gray cast iron and appear anisotropic graphite. The literature points out that for thin-walled gray cast iron, The optimum residual content is for molten iron with sulfur content, the residual content of rare earth can be; When cupola is used to produce large-scale ferritic nodular cast iron, the elongation of the thicker part of gray cast iron can be increased if the residual rare earth content is times that of sulfur.