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Standards and Specifications
The most common standard defining gray cast iron specifications used in the United States is that published by ASTM. ASTM A48 and A48M specifically deal with gray cast iron for engineering purposes where the material's physical properties would be exploited for some application. Other ASTM standards related to gray cast iron include A126, which deals with valves and pipe applications; A278, which involves high temperature high pressure applications; and A319, which defines specifications for high temperature applications without pressure.
The SAE International standard J431 is used often in the automotive industry for gray iron specifications, but more commonly to define grades of cast iron instead of classes. Each of these standards specifies details regarding the chemical composition, physical properties and microstructure of gray cast iron materials.
Chemical Composition
While the specific chemical composition of gray cast iron will depend on the application it's intended for, all grades share similarities. Gray cast iron contains between 2.5 percent and 4 percent carbon in the form of graphitic flakes. The only other major nonferrous element is silicon, which is present in the range of 1 percent to 3 percent.
The high carbon content of gray cast iron is critical for its high compressive strength and durability. Specifications insist on this chemical composition to ensure that the physical behavior of the material stays constant no matter who manufactures it.
Microstructure
The other factor that determines the physical properties of gray cast iron is the material's microstructure. Specifications for gray cast iron call for a ferritic matrix where graphite flakes are intermingled with iron grains. This is typically achieved through heat treatments after casting. Specifications for gray cast iron do not identify what manufacturing processes are required to achieve the ferritic matrix, only the physical properties resulting from it. Rapid cooling suppresses the mixing of graphite flakes and iron grains, and must be avoided. Some grades of gray cast iron call for pearlitic microstructures instead of a ferritic matrix to achieve a high tensile-strength-to-hardness ratio.
Physical Properties
The specific physical properties of gray case iron depend on the class and grade of the material, which is one reason why several standard specifications exist. The grade and class specifies the tensile strength, compressive strength, tensile modulus and hardness of the material. Gray cast iron is more brittle than steel, but exhibits superior mechanical damping capacity to absorb undesirable vibrations.
One reason gray cast iron is used so regularly is its easy machinability and high resistance to wear or sliding friction. The specific values of these properties can be tweaked by subtly altering the microstructure of the material and changing how well interspersed the graphitic flakes are in the iron grain matrix. The goal of gray cast iron specifications is to define which set of physical properties is best suited to specific applications.