The difference between the solidification methods of nodular cast iron gray cast iron is caused by the different growth methods of nodular cast iron flake ink.

After the precipitation of graphite in the hypoeutectic gray iron at the edge of the primary austenite, the two sides of the graphite sheet are surrounded by the austenite absorb the graphite the austenite to thicken, the tip of the graphite sheet is in the liquid Absorb graphite grow.

In spheroidal graphite cast iron, because graphite is spherical, the graphite spheres begin to absorb graphite around after precipitation, the surrounding liquid becomes solid austenite surrounds the graphite spheres because the amount of w(C) decreases; because the graphite spheres are in Surrounded by austenite, the carbon that can only be absorbed austenite is relatively limited, while the carbon in the liquid diffuses through the solid to the graphite ball very slowly, being surrounded by austenite limits its growth; so Even if the carbon equivalent of ductile iron is much higher than that of gray cast iron, graphitization of ductile iron is more difficult, so there is enough graphitization expansion to offset solidification shrinkage; therefore, ductile iron is prone to shrinkage.

In addition, the thickness of the austenite layer surrounding the graphite spheres is generally 1.4 times the diameter of the graphite spheres, which means that the larger the graphite spheres, the thicker the austenite layer, the more difficult it is for the carbon in the liquid to be transferred to the graphite spheres through the austenite Big.
The fundamental reason why low-silicon ductile iron is prone to whiteness is also the solidification method of ductile iron. As described above, due to the difficulty of graphitization of nodular cast iron, insufficient latent heat of crystallization caused by graphitization is released into the mold to increase the degree of supercooling, graphite is too late to precipitate to form cementite. In addition, the rapid decay of nodular cast iron is also one of the factors that are prone to undercooling.