Some key points about ductile cast iron

The differing solidification patterns of ductile iron and gray cast iron are caused by their distinct growth mechanisms. In hypoeutectic gray cast iron, graphite begins to precipitate along the edges of primary austenite, with both sides of the graphite flakes subsequently surrounded by austenite. These flakes absorb graphite from the austenite, causing them to thicken, while the sharp tips of the flakes continue growing by incorporating graphite directly from the liquid phase.

　　In ductile iron, since the graphite is spherical, the graphite spheres, after settling, begin to absorb the surrounding graphite, causing the surrounding liquid to As the carbon content (w(C)) decreases, solid austenite forms around the graphite spheres. Because the graphite spheres are enclosed by austenite, their ability to absorb carbon from the surrounding matrix is restricted. Meanwhile, carbon from the liquid phase slowly diffuses through the solid austenite and into the graphite spheres, further limiting the growth of the graphite nodules. Consequently, even though ductile iron typically has a carbon equivalent far higher than gray cast iron, achieving proper graphitization in ductile iron remains challenging. This lack of sufficient graphitization expansion prevents it from effectively counteracting the shrinkage that occurs during solidification. As a result, ductile iron is prone to forming shrinkage cavities.

　　Additionally, the thickness of the austenite layer surrounding the graphite spheres typically equals the diameter of the graphite spheres. 1.4 times—that is, the larger the graphite sphere, the thicker the austenite layer becomes, making it more difficult for carbon in the liquid to diffuse through the austenite and reach the graphite sphere.

　　The fundamental reason why low-silicon ductile iron is prone to white spots also lies in its solidification behavior. As mentioned earlier, due to the difficulty of graphitization in ductile iron, the latent heat of crystallization released during graphitization isn't sufficiently transferred to the mold, thereby increasing the degree of supercooling and leading to the formation of cementite before graphite precipitates. Additionally, the rapid inoculation and subsequent decay characteristic of ductile iron further contribute to its susceptibility to supercooling.

　　From the perspective of the solidification characteristics of ductile iron, achieving riser-free casting of ductile iron is not difficult. Under the same conditions, tiny graphite particles readily dissolve into the molten iron and are less likely to grow; however, as graphite begins to develop, its growth rate accelerates. Therefore, inducing the formation of primary graphite in the molten iron before the eutectic reaction occurs can significantly promote the graphitization process during eutectic solidification. Molten iron with hypereutectic compositions can meet this requirement, but excessively high levels The CE value enables graphite to grow before eutectic solidification; however, once the graphite reaches a certain size, it begins to float upward, leading to graphite floatation defects. At this point, the volume expansion caused by graphitization only results in an increase in the molten iron's liquid level—something that not only fails to aid feeding in the casting but also reduces the carbon content (w(C)) in the molten iron during eutectic solidification. As a result, insufficient eutectic graphite is formed to counteract the shrinkage associated with eutectic solidification. Practical experience has shown that maintaining the CE value within the range of 4.30% to 4.50% is generally ideal.

　　Experiments show that the pouring temperature of ductile iron ranges from From 1350 to 1500, the shrinkage porosity volume of castings showed no significant change, though the porosity gradually shifted from being concentrated to more dispersed. As the pouring temperature increased, the size of graphite spheres grew larger while their number decreased. Therefore, there is no need to insist on excessively low pouring temperatures—provided the mold strength is sufficient to withstand the hydrostatic pressure of the molten iron, the pouring temperature can actually be higher. By preheating the mold with molten iron, we can reduce the degree of supercooling during the eutectic solidification process, thereby allowing ample time for graphitization to occur. However, the pouring speed should be as fast as possible to minimize temperature differences between the molten iron and the mold itself.