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Mechanical Properties of Gray Iron

Microscopically, all gray irons contain flake graphite dispersed in a silicon-iron matrix. How much graphite is present, the length of the flakes and how they are distributed in the matrix directly influence the properties of the iron.

The basic strength and hardness of the iron is provided by the metallic matrix in which the graphite occurs. The properties of the metallic matrix can range from those of a soft, low carbon steel to those of hardened, high carbon steel. The matrix can be entirely ferrite for maximum machinability but the iron will have reduced wear resistance and strength. An entirely pearlitic matrix is characteristic of high strength gray irons, and many castings are produced with a matrix microstructure of both ferrite and pearlite to obtain intermediate hardness and strength. Alloy additions and/or heat treatment can be used to produce gray iron with very fine pearlite or with an acicular matrix structure.

Graphite has little strength or hardness. It decreases these properties of the metallic matrix,however, the presence of the graphite provides several valuable characteristics to cast iron. These include:

The ability to produce sound castings economically in complex shapes such as water cooled engine blocks.
Good machinability even at wear resisting hardness levels and without burring.
Dimensional stability under differential heating such as in brake drums and disks.
High vibration damping as in power transmission cases.
Borderline lubrication retention as in internal combustion engine cylinders.

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Atlas Foundry Company, Inc.
601 N. Henderson Avenue
Marion, IN 46952-3348
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