Solidification process of gray cast iron

Iron carbon equilibrium phase diagram is indispensable for the foundation of cast iron, which has theoretical guiding significance for the production of cast iron castings. When studying the crystallization process of gray cast iron, the iron carbon dual phase diagram is needed. As shown in the figure, the solid line represents the Fe-Fe3C phase diagram (white cast iron), and the dotted line represents the Fe-G phase diagram (gray cast iron). Under suitable solidification conditions, graphite can be crystallized from Fe-C alloy with carbon content higher than 2.14%. For example, at a certain temperature, carburization during heat treatment will decompose into austenite and graphite; Austenite in cast iron will be transformed into graphite and ferrite under the condition of eutectoid temperature holding or slow cooling.

From the perspective of thermodynamics, Fe3C has relatively high free energy and will decompose into austenite and graphite with low energy under certain conditions. The transformation process of austenite plus Fe3C is balanced, so Fe-Fe3C phase diagram is metastable and Fe-G phase diagram is stable. From the perspective of crystallization kinetics, the formation of graphite in molten iron requires the aggregation of C and the outward diffusion of Fe. Compared with the formation of Fe3C (interstitial compound), its conditions require higher requirements.

The eutectic and eutectoid transformations in the Fe-C (graphite) phase diagram are as follows:

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