Grey iron is a form of cast iron. There are many different kinds of cast iron, depending on the particular materials they contain.
Grey iron is a cast iron that contains carbon in the form of graphite, plus silicon, manganese and phosphorus. The fractured surface of a cast iron with graphite appears grey, hence the name. It is brittle and cannot absorb shocks. It resists heat and corrosion, and can be cast into many different shapes. It is used for many components.