Casting Properties and Casting Process Characteristics of Ductile Iron

Oct 11th, 2025

Casting Properties and Casting Process Characteristics of Ductile Iron

Due to its relatively high carbon and silicon content, ductile iron, like gray cast iron, exhibits excellent fluidity and self-feeding capacity. However, differences in ladle treatment processes and solidification processes lead to significant disparities in casting properties between ductile iron and gray cast iron, resulting in distinct casting processes for the two materials.

I. Fluidity and Pouring Process of Ductile Iron

The addition of nodulizers during the nodularization process has two main effects: on one hand, it reduces the temperature of the molten iron; on the other hand, elements such as magnesium and rare earths form inclusions in the ladle and gating system. Consequently, the fluidity of molten iron decreases after nodularization. Meanwhile, if these inclusions enter the mold cavity, they will cause casting defects such as inclusions, pinholes, and rough casting surfaces.

To address the above issues, the following points must be noted in the casting process of ductile iron:

1.Thoroughly remove the dross floating on the surface of molten iron in the ladle; preferably, use a teapot spout ladle.

2.Strictly control the residual magnesium content, ideally keeping it below 0.06%.

3.The gating system must have sufficient dimensions to ensure that molten iron fills the mold cavity as quickly as possible while minimizing turbulence.

4.Adopt a semi-enclosed gating system. According to data recommended by the American Foundry Society (AFS), the ratio of the sprue, runner, and ingate should be 4:8:3.

5.Locate the ingate as close to the bottom of the mold as possible.

6.Installing a filter screen in the gating system helps remove inclusions.

7.Appropriately increase the pouring temperature to improve the mold-filling capacity of molten iron and prevent the formation of carbides. For molten iron treated with rare earths, refer to relevant Chinese technical handbooks for the recommended pouring temperature. For molten iron treated with magnesium, AFS-recommended data specifies:

(1)When the casting wall thickness is 25mm, the pouring temperature should not be lower than 1315℃;

(2)When the casting wall thickness is 6mm, the pouring temperature should not be lower than 1425℃.

II. Solidification Characteristics and Feeding Process Characteristics of Ductile Iron

Ductile iron differs significantly from gray cast iron in terms of solidification characteristics, mainly reflected in the following aspects:

1.Wide eutectic solidification range: During the eutectic solidification of gray cast iron, the ends of flake graphite remain in contact with molten iron, allowing the eutectic solidification process to proceed rapidly. In contrast, for ductile iron, graphite nodules are surrounded by an austenite shell in the later stage of growth. Their growth relies on the diffusion of carbon atoms, resulting in a slower solidification process. To sustain eutectic solidification, new graphite nuclei must form on new graphite heterogeneous cores under a larger degree of supercooling. As a result, ductile iron has a wide liquid-solid coexistence zone on its cross-section during solidification, exhibiting a "mushy solidification" characteristic. This makes feeding during the solidification of ductile iron more challenging.

2.Abundant graphite nuclei: After nodularization and inoculation treatment, ductile iron has far more graphite nuclei than gray cast iron, leading to much finer eutectic cell sizes compared to gray cast iron.

3.High eutectic expansion force: During the eutectic solidification of ductile iron, graphite is quickly surrounded by an austenite shell. The expansion caused by the volume increase of graphite during growth cannot be transmitted to the molten iron, resulting in a high eutectic expansion force. If the mold rigidity is insufficient, this eutectic expansion will cause shrinkage porosity defects.

4.Three stages of volume change during solidification:

Liquid shrinkage: Occurs after molten iron is poured into the mold and cooled to the eutectic temperature;

Volume expansion: Caused by the precipitation of graphite nodules during eutectic solidification;

Solid shrinkage: Takes place when the solidified molten iron cools down.


Given the above solidification characteristics, from the perspective of feeding, the casting process of ductile iron has the following features:

1.High mold compaction: The mold must have sufficient compaction to ensure it has enough rigidity to resist the eutectic expansion force of ductile iron during eutectic solidification. It should be noted that appropriate measures must be taken to improve the mold’s gas permeability while minimizing the moisture content in the molding sand to prevent "metal penetration" (a defect caused by the rapid vaporization of mold moisture).

2.Rational placement of risers and gates: The risers for ductile iron differ from those for ordinary steel and white cast iron. The key to rational riser design for ductile iron lies in ensuring the riser can fully compensate for the liquid shrinkage of molten iron. When the molten iron enters the eutectic expansion stage, the gating system and riser neck should solidify promptly, allowing the casting to achieve self-feeding using the expansion from graphite precipitation.

3.Sufficient flask rigidity: The flask must have adequate rigidity, and a secure clamping device should be installed between the upper and lower flask halves.


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nico@zyqfoundry.com

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