Grey cast iron expands on cooling because of its tendency to expand during the solidification process. Riser material ( also known as a feeder) is not required during the casting process in Cast iron.
The reason for the expansion during freezing is the presence of a high amount of carbon in the form of graphite flakes.
The solubility of carbon at higher temperatures in Iron is more, which means the iron atom can hold a large amount of carbon within its crystal lattice at higher temperatures.
