6 Selection Principles Of Ductile Iron Parts

Nov. 09, 2021

Cast Iron Pump Casing

Ductile iron chemical composition mainly includes 5 common elements of manganese, carbon, phosphorus, sulfur, and silicon. For some castings with special requirements on organization and performance, a small amount of alloying elements are also included. Different from ordinary gray cast iron, in order to ensure the spheroidization of graphite, ductile iron must also contain a trace of residual spheroidized elements.


1. The principle of manganese selection


As the content of sulfur in ductile iron is already very low, do not need too much manganese to neutralize sulfur, the role of manganese in ductile iron is mainly in increasing the stability of pearlite, promoting the formation of (Fe, Mn) 3C. these carbides bias in the grain boundary, the impact on the toughness of ductile iron is great. Manganese will also improve the toughness of ferrite ductile iron brittle transition temperature, manganese content of each 0.1% increase, brittle transition temperature increased by 10 ~ 12 ℃. Therefore, the manganese content in ductile iron is generally the lower the better, even if the pearl ductile iron, manganese content should not exceed 0.4% to 0.6%. Only to improve wear resistance for the purpose of manganese ductile iron and bainite ductile iron exception.


2. The principle of carbon and carbon equivalent selection


Carbon is the basic element of ductile iron, high carbon helps graphitization. As the graphite is spherical, the effect of graphite on mechanical properties has been reduced to a minimum, the carbon content of ductile iron is generally high, between 3.5% and 3.9%, and the carbon equivalent is between 4.1% and 4.7%. Castings with thin walls, large residual spheroidal elements or insufficient breeding take the upper limit; conversely, take the lower limit. The carbon equivalent is selected near the eutectic point not only to improve the fluidity of the molten iron, for ductile iron, the increase in carbon equivalent will also improve the graphitization expansion of the cast iron during solidification to improve the self-replenishing shrinkage capacity of the molten iron. Excessive carbon content will cause graphite to float. Therefore, the upper limit of carbon equivalent in ductile iron is based on the principle of no graphite floating.


3. The principle of phosphorus selection


Phosphorus is a harmful element. It is extremely low solubility in cast iron when its content is less than 0.05%, solid solution in the matrix, almost no effect on the mechanical properties. When the content is greater than 0.05%, phosphorus is very easy to bias in the eutectic group boundary, the formation of binary, ternary, or composite phosphorus eutectic, reducing the toughness of cast iron. Phosphorus increases the toughness of cast iron brittle transition temperature, phosphorus content increases by 0.01%, the toughness of the brittle transition temperature increases by 4 ~ 4.5 ℃. Therefore, the lower the phosphorus content of ductile iron, the better, in general, should be less than 0.08%. For the more important castings, phosphorus content should be less than 0.05%.


4. The principle of silicon selection


Silicon is a strong graphitizing element. In ductile iron, silicon can not only effectively reduce the tendency of white mouth and increase the amount of ferrite, but also has the role of refining eutectic clusters and improving the roundness of graphite spheres. Silicon increases the toughness and brittleness transition temperature of cast iron and reduces the impact toughness, so the silicon content should not be too high, especially when the manganese and phosphorus content in cast iron is high, it is necessary to strictly control the silicon content. Ductile iron in the final silicon amount is generally 1.4% to 3.0%. After the selected carbon equivalent, generally take the principle of high carbon and low silicon reinforced breeding. The lower limit of silicon to the principle of not appearing free carburized body.                                


After the carbon and silicon content in ductile iron is determined, the composition design is basically suitable. If it is higher than the optimal area, the graphite floating phenomenon is likely to occur. If it is lower than the optimal region, it is prone to shrinkage defects and free carbides.


5. The principle of sulfur selection


Sulfur is an antispheroidal element, which has a strong affinity with magnesium, rare earth, and other spheroidal elements. The presence of sulfur will consume a large number of spheroidal elements in the ferrofluid and form sulfides of magnesium and rare earth, causing slag, porosity, and other casting defects. The content of sulfur in ductile iron is generally required to be less than 0.06%.


6. The principle of spheroidal elements selection


Currently in the industrial use of spheroidizing elements are mainly magnesium and rare earth. Magnesium and rare earth elements can neutralize the effect of sulfur and other anti-spheroidizing elements so that the graphite grows according to the spherical shape. The residual amount of magnesium and rare earth should be determined according to the content of anti-spherical elements such as sulfur in the iron solution. The residual amount of magnesium and rare earth should be as low as possible under the premise of ensuring qualified spheroidization. The residual amount of magnesium and rare earth is too high, it will increase the tendency of the white mouth of the iron liquid, and will affect the mechanical properties of the casting due to their segregation on the grain boundary.


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