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 that 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 behavior 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.
Specialty Castings produces gray iron castings in tensile strengths from 30,000 to 50,000 psi.