Why is grey cast iron expand during the process of casting and in which phase of cooling?

Why is grey cast iron expand during the process of casting and in which phase of cooling?

Grey cast iron is rich in carbon content and the carbon present in grey cast iron is in graphite form. The graphite form of carbon has lesser density to volume than liquid form of carbon. Thus, cast iron expands on cooling. 

This cooling starts when sensible cooling begins in molten metal and continues till partial liquid state is present in the casting material and gets stopped whole metal solidify. 

Thus, there is no expansion during sensible cooling of grey cast iron in solid state. In fact, the grey cast iron itself contracts like all other metals once it acquires completely solid state.

That’s why cast iron too has positive shrinkage allowance like all other metals even though it expands on cooling while others contract.