The chemical composition of ductile iron mainly includes five common elements: carbon, silicon, manganese, sulfur and phosphorus. For some castings with special requirements for structure and performance, a small amount of alloying elements are also included. Different from ordinary gray cast iron, nodular cast iron must contain trace residual spheroidizing elements in order to ensure graphite spheroidization.
1.The principle of carbon selection: Carbon is the basic element of ductile iron, and high carbon contributes to graphitization. Since the influence of graphite on mechanical properties has been minimized after the graphite is spherical, the carbon content of ductile iron is generally higher, between 3.5% and 3.9%. The upper limit is taken when the casting wall is thin, the residual amount of spheroidizing elements is large or the incubation is insufficient; otherwise, the lower limit is taken. Too high carbon content will cause graphite to float. Therefore, the upper limit of the carbon equivalent in ductile iron is based on the principle that no graphite floats occur.
2.Principles for selecting silicon: Silicon is a strong graphitizing element. In ductile iron, silicon can not only effectively reduce the white mouth tendency and increase the amount of ferrite, but also has the function of refining the eutectic group and improving the roundness of graphite spheres. However, silicon will increase the ductile-brittle transition temperature of cast iron and reduce the impact toughness. Therefore, the silicon content should not be too high, especially when the content of manganese and phosphorus in the cast iron is high, the silicon content must be strictly controlled. The final silicon content in ductile iron is generally 1.4-3.0%. After selecting the carbon equivalent, generally adopt the principle of high-carbon and low-silicon strengthening inoculation. The lower limit of silicon is based on the principle that free cementite does not appear.
3.Manganese selection principle: Since the sulfur content in ductile iron is already very low, there is no need for excessive manganese to neutralize sulfur. The function of manganese in ductile iron is mainly to increase the stability of pearlite and promote the formation of (Fe, Mn). ) 3C. These carbides are segregated in grain boundaries and have a great influence on the toughness of ductile iron. Manganese will also increase the ductile-brittle transition temperature of ferritic ductile iron. For every 0.1% increase in manganese content, the brittle transition temperature will increase by 10-12°C. Therefore, the manganese content in ductile iron is generally as low as possible. Even in pearlitic ductile iron, the manganese content should not exceed 0.4 to 0.6%. Only medium manganese ductile iron and bainite ductile iron for the purpose of improving wear resistance are exceptions.
4.Principles for selecting phosphorus: Phosphorus is a harmful element. Its solubility in cast iron is extremely low. When its content is less than 0.05%, it is solid-dissolved in the matrix and has almost no effect on the mechanical properties. When the content is greater than 0.05%, phosphorus is easily segregated at the boundary of the eutectic group, forming binary, ternary or composite phosphorus eutectic, reducing the toughness of cast iron. Phosphorus increases the ductile-brittle transition temperature of cast iron. For every 0.01% increase in phosphorus content, the ductile-brittle transition temperature increases by 4 to 4.5°C. Therefore, the lower the content of phosphorus in ductile iron, the better, and should be less than 0.08% in general. For more important castings, the phosphorus content should be less than 0.05%.
5.Sulfur selection principle: Sulfur is a kind of despheroidizing element, and it has a strong affinity with magnesium,RE and other spheroidizing elements. The presence of sulfur will consume a large amount of spheroidizing elements in molten iron, forming magnesium and RE sulfide It can cause casting defects such as slag inclusion and pores. The sulfur content in ductile iron is generally required to be less than 0.06%.
6.Principles for the selection of spheroidizing elements: The spheroidizing elements currently used in industry are mainly magnesium and REs. Magnesium and RE elements can neutralize the despheroidizing elements such as sulfur and make graphite grow in a spherical shape. The residual amount of magnesium and RE should be determined according to the content of despheroidizing elements such as sulfur in the molten iron. On the premise of ensuring qualified spheroidization, the residual amount of magnesium and RE should be as low as possible. Too high residual amounts of magnesium and REs will increase the whitish tendency of molten iron and affect the mechanical properties of castings due to their segregation on the grain boundaries.
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