The presence of trace elements, the addition of alloying elements, the modification of solidification behaviour, and heat treatment after solidification are used to change the microstructure of cast irons to produce the desired mechanical properties in the following common types of cast iron.

White Iron

White Iron is fully carbidic in its final form. The presence of different carbides, produced by alloying, makes White Iron extremely hard and abrasion resistant but very brittle.

Gray Iron

Gray Iron is by far the oldest and most common form of cast iron. As a result, it is assumed by many to be the only form of cast iron and the terms "cast iron" and "gray iron" are used interchangeably. Gray Iron, named because its fracture has a gray appearance, consists of carbon in the form of flake graphite in a matrix consisting of ferrite, pearlite or a mixture of the two. The fluidity of liquid gray iron, and its expansion during solidification due to the formation of graphite, have made this metal ideal for the economical production of shrinkage-free, intricate castings such as motor blocks.

The flake-like shape of graphite in Gray Iron exerts a dominant influence on its mechanical properties. The graphite flakes can act as stress raisers which may prematurely cause localized plastic flow at low stresses, and initiate fracture in the matrix at higher stresses. As a result, Gray Iron exhibits no elastic behaviour and fails in tension without significant plastic deformation. The presence of graphite flakes also gives Gray Iron excellent machinability, damping characteristics and self-lubricating properties.

Malleable Iron

Unlike Gray and Ductile Iron, Malleable Iron is cast as a carbidic or white iron and an annealing or "malleablizing" heat treatment is required to convert the carbide into graphite. The microstructure of Malleable Iron consists of irregularly shaped nodules of graphite called "temper carbon" in a matrix of ferrite and/or pearlite. The presence of graphite in a more compact or sphere-like form gives Malleable Iron ductility and strength almost equal to cast, low-carbon steel. The formation of carbide during solidification results in the conventional shrinkage behaviour of Malleable Iron and the need for larger feed metal reservoirs, causing reduced casting yield and increased production costs.