key points in solidification process of gray cast iron and NODULAR CAST IRON(1)

The final properties of iron castings are mainly determined by the structure formed during solidification. For example, the thermal properties of gray iron are affected by the shape, size, and quantity of graphite in its structure. The mechanical properties depend on the number of primary Austenite Dendrites, the morphology of graphite and the size of the eutectic group, while the Mechanical Properties of nodular cast iron rely on the number and morphology of graphite spheres and the characteristics of the Matrix structure.

The solidification process of gray cast iron and nodular cast iron includes precipitation of primary phase (Austenite, graphite), eutectic transformation and solidification of residual liquid.
At the end of the eutectic transformation, the eutectic and eutectic grains and primary austenite dendrites are connected, and the remaining low melting point liquid is at the grain boundary between the grains. Although the volume fraction of this residual liquid in cast iron is tiny, it is rich in a variety of segregation elements and inclusions, and its solidification state can cause a variety of grain boundary defects in cast iron, such as phosphorus eutectic, Grain Boundary Carbide, grain boundary non-metallic inclusions, deformed graphite, inter-grain shrinkage porosity and so on. It has a great impact on the quality of castings. Many factors affect the properties of the residual liquid in the production process, such as the selection of the chemical composition of cast iron, the quality of various raw materials for melting, the control of melting process, the post-treatment process of the liquid iron and so on. Therefore, to discuss the solidification of residual liquid, by no means one or two paragraphs can be clear, here can only be pressed for time not to say.
So far, our understanding of the solidification process of cast iron is still insufficient; it is necessary to explore further and study.

Cast iron is a high carbon content of Fe-C alloy, in addition to carbon, but also contains a variety of other alloy elements. The carbon in low alloy cast iron can be precipitated as graphite or FE3C.
In high-temperature molten iron, the free energy of graphite is much lower than that of Fe3C, and it is easier to separate directly from molten iron. Of course, the carbon in cast iron can also be separated from the solid austenite. From the thermodynamic analysis, the binary phase diagram of ‘Fe-graphite’ system is a stable equilibrium state, so it is called the stable system of FE-C alloy. Comparatively, the binary phase diagram of FE-FE3C is the metastable system of FE-C alloy.
To understand the solidification process of cast iron, of course, reference to the FE-C alloy phase diagram. We usually see books, FE-C binary alloy phase diagram, generally used dotted line for the stable system (Fe-graphite), solid line for the metastable system (FE-FE3C). In recent years, it has been suggested that it may be more appropriate to represent the stable system (Fe-graphite) with the solid line and the metastable system (FE-FE3C) with the dotted line in the phase diagram of FE-C alloy.
In this paper, only the solidification of ordinary gray cast iron and nodular cast iron is concerned with the precipitation of graphite. It is hoped that no FE3C will be precipitated during the solidification.

Under equilibrium conditions, the EUTECTIC temperature of the stable system Teg (1153 °C) is only 6 °c higher than that of the metastable system TEC (1147 °c). During the solidification process of cast iron, the cooling rate is a little higher and the undercooling degree is a little higher, then the cast iron will be transformed by the metastable system. Under the actual production condition, the cooling rate of cast iron is higher and the undercooling degree is bigger. If the cast iron is pure Fe-C alloy, it is easy to appear white. In order to produce gray iron and ductile iron castings, the carbon cannot be precipitated as FE3C during solidification, so the carbon must be changed according to the stable system.

The addition of silicon into Fe-C alloy can increase the EUTECTIC temperature of the stable system, but this effect is not obvious. However, silicon can decrease the EUTECTIC temperature of the metastable system, and enlarge the temperature difference between TEG and TEC. This role of silicon is shown in figure 2.

Therefore, in both gray and ductile iron, silicon is an indispensable and important alloy element, which can promote the precipitation of carbon in graphite form and effectively inhibit the formation of Fe3C.

There are many factors that affect the solidification process of cast iron under the actual production conditions like there are many kinds of alloy elements in cast iron, not only FE-C alloy; There are a lot of fine non-metallic inclusions in molten iron. The conditions of crystallization and nucleation during solidification are complex. In order to ensure the quality of the casting, it is better to study the cooling curve obtained from the thermal analysis to master the actual solidification characteristics of molten iron under the specific conditions of this unit.