The influence of five elements of ductile iron on casting

The chemical composition of ductile iron mainly includes carbon, silicon, manganese, sulfur, phosphorus five common elements. For some castings with special requirements for organization and performance, including a small amount of alloy elements. Different from ordinary gray cast iron, in order to ensure graphite sphetization, ductile iron must also contain trace amounts of residual globtization elements.
1. Selection principle of carbon and carbon equivalent: carbon is the basic element of ductile iron, and high carbon is conducive to graphitization. Because the influence of graphite on mechanical properties has been reduced to the lowest degree, the carbon content of ductile iron is generally high, between 3.5 and 3.9%, and 4.1% between 4.7% and 4.7%. Take the upper limit for thin wall, large residue of globtized elements or insufficient gestation; take the lower limit. Choosing carbon equivalent near the cocrystal point can not only improve the fluidity of iron, but also the increase of ductile iron, but also improve the self-complement and shrinkage ability of iron due to the improvement of graphitization expansion of cast iron during solidification. However, a high carbon content can cause graphite to float. Therefore, the upper limit of carbon equivalent in ductile iron is based on the principle of no graphite floating.
2, the selection principle of silicon: silicon is a strong graphitized element. In ductile iron, silicon can not only effectively reduce the white tendency and increase the amount of ferrite, but also have the effect of refining the cocrystal group and improving the degree of graphite sphere. However, silicon improves the brittleness transition temperature of cast iron (Figure 1) to reduce the impact toughness, so the silicon content should not be too high, especially when the content of manganese and phosphorus in cast iron is high, it is more necessary to strictly control the content of silicon. The amount of final silicon in ductile iron is generally 1.4- -3.0%. After selecting carbon equivalent, the principle of high carbon and low silicon enhancement is generally adopted. The lower limit of silicon is based on the principle of no free carburization body.
3. Selection principle of manganese: Since the content of sulfur in ductile iron is very low, there is no need for too much manganese to neutralize sulfur, the role of manganese in ductile iron is mainly reflected in increasing the stability of the body and promoting the formation of (Fe, Mn) 3C. These carbide are seaggregated at the grain boundary and have a great influence on the toughness of ductile iron. Manganese will also increase the brittle transition temperature of ferrite diductile iron, increasing by 0.1% by 10 to 12℃. Therefore, the manganese content in the ductile iron is generally the lower the better, even if the bead light body ductile iron, the manganese content should not exceed 0.4~0.6%. Only iron medium manganese and ferrite for the purpose of improving wear resistance.
4. Selection principle of phosphorus: Phosphorus is a harmful element. It has a very low solubility in cast iron, when its content is less than 0.05%, solid soluble in the matrix, with little effect on the mechanical properties. When the content is greater than 0.05%, phosphorus is easy to differentiate at the boundary of the common crystal group, forming binary, ternary or composite phosphorus co-crystal, and reducing the toughness of cast iron. Phosphorus increases the brittle transition temperature of cast iron, increasing the phosphorus content by 4 to 4.5℃. Therefore, the lower the content of phosphorus in ductile iron, the better, generally should be less than 0.08%. For the more important castings, the phosphorus content should be less than 0.05%.
5, the selection principle of sulfur: sulfur is a kind of anti-globation element, it has a strong affinity with globation elements such as magnesium, rare earth, the existence of a large number of sulfur