The chemical composition directly affects the matrix structure and mechanical properties of nodular cast iron castings. Reasonable chemical composition is the basis for manufacturing castings meeting the service requirements, and the optimized chemical composition is the basic condition for producing high-performance castings. The selection of main chemical elements of nodular cast iron should not only meet the requirements of graphite spheroidization, but also make the castings have good casting properties. In the process of melting, pouring and solidification of nodular cast iron, the contents of major elements such as C, Si, s and trace alloying elements have great influence on the physical and chemical properties and casting properties of
Element C: a certain content of C element will affect the number of graphite balls. If the carbon content is high, the number of graphite particles will be more, the size of ball diameter will be small, and the roundness will be increased [. Many studies have shown that when the carbon content is controlled in the range of 3.2 ~ 3.8 wt.%, the properties of nodular cast iron are not affected. Proper increase of carbon content can increase the amount of ferrite in the matrix structure of ferrite nodular cast iron, which is beneficial to improve its plasticity and toughness; appropriate increase of carbon content can reduce the shrinkage cavity volume and shrinkage porosity area, and make the casting body structure compact. Generally, when determining the composition of nodular cast iron, the carbon equivalent CE is preferred to be about 4.3% – 4.6% of eutectic carbon equivalent. For Ferritic Ductile Iron, the C content is generally recommended to be 3.6-4.0 wt.%, so as to ensure its good mold filling fluidity and structural compactness.
Si element: Si element promotes graphitization. Si in nodular cast iron has a good inoculation effect. Si can effectively reduce the tendency of white cast iron, refine graphite ball and increase ferrite in matrix structure. The tensile strength RM and yield strength Rp0.2 of Ferritic Ductile iron can be effectively improved by Si content within a reasonable range, but the high Si content will cause abnormal shape distortion of graphite morphology and reduce the toughness of ductile iron. For ferritic nodular cast iron, the Si content is generally recommended to be 2.4-2.8 wt.%
S element: s element in the molten iron of nodular cast iron is an anti spheroidizing element. The S element in the molten iron will combine with the nodularized element to form sulfide or sulfur oxide, so as to increase the usage of nodularizing agent and accelerate the spheroidizing decline. The sulfide and sulfur oxide formed by chemical reaction will form serious casting defects in the form of slag inclusion and inclusion. Therefore, it is necessary to control the s content in the molten iron of nodular cast iron. Some foreign manufacturers require that the s content in the original molten iron be controlled to less than 0.02 wt.%.
Other trace alloying elements: in recent years, in the market trend of increasingly fierce market competition with low manufacturing cost, increasingly strict environmental protection requirements, and high prices of raw materials such as cast pig iron, the use of low-cost scrap steel instead of cast pig iron has become an important cost reduction measure and research direction. However, the alloying elements brought in by various scrap have a great negative impact on the properties and spheroidization of ductile iron, which are usually called interference elements. Among the interfering elements, Mn, Cr, Mo, B and other elements form carbides at the grain boundary of the final solidification of the thick section. The content of Ti, Al, P and other elements increases, which is easy to form segregation.