Gray Cast Iron
Gray cast iron is the most commonly used of all cast irons. It is strong and hard, but brittle under tension.
Although gray cast iron is black, it's called "gray," because when it fractures, the fracture surfaces look gray. China started making cast iron plows in the 3rd century BC -- many centuries before the first cast iron foundry was built in Britain.
Gray cast iron is easy to cast -- to pour into a mold -- but it can't be reheated or reworked after casting.
White Cast Iron
White cast iron has a light appearance. It has carbon only as a "carbide," which makes white cast irons extremely hard and resistant to abrasion.
However, white cast iron is even more brittle than gray cast iron. According to Professor Carl Koch of North Carolina State University's Materials Science Engineering Department, white cast iron is used for rolling machines for paper manufacturing.
Ductile Cast Iron
Ductile cast iron is the most versatile of cast irons, according to Professor Serdar Z. Elgun of Farmingdale State College's Mechanical Engineering Department.
It is strong and tough, easy to machine, and less expensive than other cast irons. Ductile cast iron is used by automobile manufacturers to make crankshafts, truck axles, engine rods and many other vehicle parts.
Ductile cast iron is also used by the modern cast iron pipe industry.
Malleable Cast Iron
Malleable cast iron is made from reheating, then cooling, white cast iron, according to Professor Elgun. Malleable cast iron has superior machinability and is used to make connecting rods, transmission gears and valve parts for trains, ships and other heavy-duty applications.