Explain the difference between grey iron and white iron. What is mottled iron ?

The carbon in cast iron could exist at room temperature as either iron carbide, or as graphite which is the more stable form. Irons containing carbon as graphite are soft, easily machinable and are called "grey irons". Irons with carbon present as iron carbide are extremely hard, difficult to machine and are called "white" irons.
Irons with fairly equal proportions of graphite and iron carbide have intermediate hardness and are called "mottled" irons.