5 Selection Principles of Ductile Iron

As a ductile iron supplier, I will share it you.

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, in order to ensure graphite spheroidization, nodular cast iron must contain trace residual spheroidizing elements.

Ductile Iron Valve Plate

1. The Selection Principle of Carbon in Ductile Iron

Carbon is the basic element of nodular cast 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%. Too high carbon content will cause graphite to float. Therefore, the upper limit of carbon equivalent in ductile iron is based on the principle that no graphite floats occur.

2. Selection Principle of Silicon in Ductile Iron

Silicon is a strong graphitizing element. In spheroidal graphite cast 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 cluster and improving the roundness of graphite spheres. Silicon increases the ductile-brittle transition temperature of cast iron and reduces the impact toughness. Therefore, the content of silicon should not be too high, especially when the content of manganese and phosphorus in cast iron is high, it is necessary to strictly control the content of silicon. The final silicon content in ductile iron is generally 1.4% to 3.0%. After selecting the carbon equivalent, generally adopt the principle of high-carbon and low-silicon intensified inoculation. The lower limit of silicon is based on the principle that free cementite does not appear.

3. Selection Principle of Manganese in Ductile Iron

Since the content of sulfur in ductile iron is already very low, there is no need for excessive manganese to neutralize sulfur. The effect 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. The Selection Pinciple of Phosphorus in Ductile Iron

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 to form binary, ternary or composite phosphorus eutectic, which reduces 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. The Selection Principle of Sulfur in Ductile Iron

Sulfur is a kind of despheroidizing element, it has a strong affinity with magnesium, rare earth and other spheroidizing elements. The presence of sulfur will consume a large amount of spheroidizing elements in molten iron, forming magnesium and rare earth sulfides, causing slag inclusion , Porosity and other casting defects. The sulfur content in ductile iron is generally required to be less than 0.06%.

HEBEI YOGEM is a supplier of OEM castings. The materials are mainly involved in grey iron/ductile iron, aluminum alloy, stainless steel, duplex/supper duplex, alloy steel, heat-resistant steel etc. We produce them by lost wax (silica gel; water glass), pre-coated sand and resin sand.

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