The Basic Elements Of Ductile Iron

- Jun 26, 2017 -

Ductile iron chemical composition mainly includes carbon, silicon, manganese, sulfur, phosphorus five common elements. For some of the organization and performance of the special requirements of the casting, but also includes a small amount of alloying elements. Unlike ordinary gray cast iron, in order to ensure the graphite ball, ductile iron must also contain trace amounts of residual spheroidal elements.

1, carbon and carbon equivalent selection principle: carbon is the basic element of ductile iron, carbon high contributes to graphitization. As the graphite has a spherical effect on the mechanical properties of the graphite has been reduced to a minimum, ductile iron carbon content is generally high, between 3.5 to 3.9%, carbon equivalent between 4.1 to 4.7%. Casting wall thin, large amount of spherical elements or inadequate birth to take the upper limit; the contrary, take the lower limit. Ductile Iron Casting It is not only to improve the fluidity of the molten iron in the vicinity of the eutectic point. For the ductile iron, the increase of the carbon equivalent can also improve the self-filling ability of the molten iron due to the increased graphitization of the cast iron during solidification. However, the carbon content is too high, will cause graphite floating. Therefore, the upper limit of carbon equivalent in ductile iron does not appear to float on the principle of graphite.

2, the principle of silicon selection: silicon is a strong graphite element. In ductile iron, silicon can not only effectively reduce the white mouth tendencies, Ductile Iron Casting increase the amount of ferrite, but also has a refined eutectic group, to improve the role of graphite ball roundness. However, silicon to improve the ductile ductile ductile transition temperature, reduce the impact toughness, so the silicon content should not be too high, especially when the cast iron in the manganese and phosphorus content is high, the more stringent control of silicon content. Ductile iron in the final silicon is generally 1.4-3.0%. After the selection of carbon equivalent, the general use of high carbon and low silicon to strengthen the principle of birth. The lower limit of silicon is free from the principle of free cementite.

Ductile iron in the carbon and silicon content determined, can be used to test Figure 2. If the carbon and silicon content in the shadow area in the figure, the composition of the basic design appropriate. If higher than the best area, prone to graphite floating phenomenon. If below the optimum area, shrinkage defects and free carbides are prone to occur.

3, the choice of manganese principle: As the ductile iron sulfur content has been very low, do not need too much manganese to neutralize sulfur, ductile iron in the role of manganese is mainly in the growth of pearlite stability, promote the formation of (Fe , Mn) 3C. These carbides segregated at grain boundaries and have a great influence on the toughness of nodular cast iron. Ductile Iron Casting Manganese also increases the ductile ductile transition temperature of ferritic nodular cast iron, with a 0.1% increase in manganese content and a brittle transition temperature of 10 to 12 ° C. Therefore, the nodular cast iron manganese content is generally the lower the better, even if the pearlitic ductile iron, manganese content should not exceed 0.4 ~ 0.6%. Except for the purpose of improving the wear resistance of the manganese ductile iron and bainitic ductile iron exception.

4, the principle of phosphorus selection: phosphorus is a harmful element. It is very low in the solubility of cast iron, when its content is less than 0.05%, the solution in the matrix, the mechanical properties of almost no effect. When the content is greater than 0.05%, the phosphorus is easily segregated to 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, 0.01% for phosphorus content, and 4 ~ 4.5 ℃ for ductile brittle transition temperature. Therefore, the proportion of ductile iron in the lower phosphorus the better, Ductile Iron Casting under normal circumstances should be less than 0.08%. For more important castings, the phosphorus content should be less than 0.05%.

5, the principle of sulfur selection: sulfur is a kind of anti-spheroidizing elements, it and magnesium, rare earth and other spheroidal elements have a strong affinity, the presence of sulfur will consume a large amount of iron in the spheroidal elements, the formation of magnesium and rare earth Of the sulfide, causing slag, porosity and other casting defects. Ductile iron in the sulfur content of the general requirements of less than 0.06%.

6, the choice of spheroidal elements of the principle: the current use of the ball in the industry are mainly spherical and rare earth. Magnesium and rare earth elements can neutralize the role of sulfur and other anti-spheroidizing elements, so that the growth of graphite by spherical. The residual amount of magnesium and rare earth should be determined according to the content of counter-sparing elements such as sulfur in molten iron. Under the premise of ensuring that the ball is qualified, the residual amount of magnesium and rare earth should be as low as possible. Magnesium and rare earth residues are too high, will increase the white ink tendons tend to, and because of their segregation in the grain boundary and affect the mechanical properties of castings.