Gray Iron - Classifications

Classifications

In the United States, the most commonly used classification for gray iron is ASTM International standard A48. This classifies gray iron into classes which corresponds with its minimum tensile strength in thousands of pounds per square inch (ksi); e.g. class 20 gray iron has a minimum tensile strength of 20,000 psi (140,000 kPa). Class 20 has a high carbon equivalent and a ferrite matrix. Higher strength gray irons, up to class 40, have lower carbon equivalents and a pearlite matrix. Gray iron above class 40 requires alloying to provide solid solution strengthening, and heat treating is used to modify the matrix. Class 80 is the highest class available, but it is extremely brittle. ASTM A247 is also commonly used to describe the graphite structure. Other ASTM standards that deal with gray iron include ASTM A126, ASTM A278, and ASTM A319.

In the automotive industry the SAE International (SAE) standard SAE J431 is used to designate grades instead of classes. These grades are a measure of the tensile strength-to-Brinell hardness ratio.