SAND CASTING EQUIPMENT.

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. Xinyuanzhu Group summed up the following experience in the casting process.

1. The selection principle of carbon and carbon equivalent
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%, and the carbon equivalent is between 4.1% and 4.7%. 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. Choosing the carbon equivalent near the eutectic point can not only improve the fluidity of the molten iron, but for ductile iron, the increase in the carbon equivalent will also increase the self-feeding ability of the molten iron due to the increase of the graphitization expansion during the solidification of the cast iron. Too high carbon content will cause graphite to float. Therefore, Xinyuanzhu Group recommends that the upper limit of carbon equivalent in ductile iron is based on the principle that no graphite floats.

2. The selection principle of silicon
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 group and improving the roundness of graphite spheres. Silicon increases the toughness and brittleness transition temperature of cast iron and reduces 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, it is necessary to strictly control the silicon content. 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.
After the carbon and silicon content in ductile iron is determined, the composition design is basically appropriate. If it is higher than the optimal area, graphite floating is likely to occur. If it is lower than the optimal area, shrinkage defects and free carbides are prone to appear.

3. Selection principle of manganese
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 nodular cast iron. For every 0.1% increase in manganese content, the brittle transition temperature will increase by 10 to 12°C. Therefore, Xinyuanzhu Group recommends that 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 principle of 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 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, Xinyuanzhu Group recommends that the lower the phosphorus content in ductile iron, the better, and generally it should be less than 0.08%. For more important castings, the phosphorus content should be less than 0.05%.

5. The selection principle of sulfur
Sulfur is a kind of despheroidizing element. It has a strong affinity with spheroidizing elements such as magnesium and rare earth. 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%.

6. The selection principle of spheroidization elements
The spheroidizing elements currently used in industry are mainly magnesium and rare earths. Magnesium and rare earth elements can neutralize the despheroidizing elements such as sulfur and make graphite grow in a spherical shape. The residual amount of magnesium and rare earth 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 rare earth should be as low as possible. Too high residual amounts of magnesium and rare earths will increase the whitish tendency of molten iron and affect the mechanical properties of castings due to their segregation on the grain boundaries.

Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.