Shrinkage cavity and porosity are shrinkage casting defects produced in the solidification process of nodular cast iron. The solidification characteristics of nodular cast iron are different from most alloys. Its eutectic solidification temperature range is wide and belongs to paste solidification; Compared with gray cast iron, there are more eutectic clusters and larger eutectic expansion. Due to the eutectic crystallization stage of nodular cast iron and the result of magnesium treatment, the graphite ball core will be surrounded by austenite when it grows to a certain size in the liquid phase, and the austenite shell can hinder the diffusion of carbon atoms from the liquid metal to nodular cast iron, so as to slow down the growth rate of nodular cast iron; The eutectic reaction is completed not only by the existing eutectic head, but also by the precipitation of new crystal nuclei. Therefore, the temperature range of eutectic reaction transformation is wide, resulting in the long coexistence state of solid-liquid two phases in the casting and paste solidification.
Shrinkage casting defect is a cavity defect formed by the volume reduction of liquid with the decrease of temperature, which occurs in the primary shrinkage stage. In view of the thin solidification layer on the surface of nodular cast iron and the low strength of the shell, the atmospheric pressure is easy to concave the thin shell (because there is vacuum in the cavity), resulting in surface depression and local hot spot depression. If the strength of the solidified layer is enough to resist atmospheric pressure, the internal vacuum will precipitate the gas (CO2, N2, etc.) dissolved in the molten iron from the molten iron, and one or more large pores will accumulate in the process of rising to the top. This large shrinkage casting defect may be all inside or can extend to the surface.
Shrinkage porosity is a more common casting defect in nodular cast iron, which mainly occurs in the process of secondary shrinkage. Because nodular cast iron belongs to paste solidification, the strength of the shell is poor, and the solidification expansion pressure of nodular cast iron is greater than that of gray prayer iron. During the secondary expansion, the shell neck can not stand the graphitization expansion force during a large amount of solidification inside the casting, which makes the casting expand outward, thus increasing the shrinkage and porosity tendency. After the final stage of solidification, the possibility that the internal pressure of nodular iron castings is less than the atmospheric pressure is greater than that of gray iron castings. When the internal pressure of the casting is lower than atmospheric pressure, it will cause internal vacuum, and the direct cause of any shrinkage cavity casting defect is that the internal liquid pressure is lower than atmospheric pressure. The vacuum part often appears in the small solution pools separated by dendrites, mostly at the hot spot of final solidification.