Gray Cast Irons

Cast Irons are alloys of iron, carbon, silicon in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. In gray cast irons, the carbon that exceeds the solubility in austenite precipitates as flake graphite. Gray cast irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of manganese, depending on the desired microstructure. Sulfur and phosphorus are also present in low percentages as residual impurities.

Classes of Gray Cast Iron:
A simple and convenient classification of the gray cast irons is found in ASTM specification A 48, which classifies the various types in terms of tensile strength, expressed in ksi. In many applications strength is not the major criterion for the choice of grade.

For example, for parts such as clutch plates and brake drums, where resistance to heat checking is important, low-strength grades of iron are the superior performers. In machine tools and other parts subject to vibration, the better damping capacity of low-strength irons is often advantageous.

The following properties of gray cast irons can be considered to increase with increasing tensile strength from grade 20 to grade 60.
• All strengths, including strength at elevated temperature
• Ability to be machined to a fine finish
• Modulus of elasticity
• Wear resistance

On the other hand, the following properties decrease with increasing tensile strength, so that low-strength irons often perform better than high-strength irons when these properties are important.
• Machinability
• Resistance to thermal shock
• Damping capacity
• Ability to be cast in thin sections

Applications:
Gray cast irons is used for many different types of parts in a very wide variety of machines and structures. Like parts made from other metals and alloys, parts intended to be produced as gray iron castings must be evaluated for the specific service conditions before being approved for production.

Often a stress analysis of prototype castings helps establish the appropriate class of gray iron as well as any proof test requirements or other acceptance criteria for production parts.