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Gray iron , or grey cast iron , is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite. It is used for housings where the stiffness of the component is more important than its tensile strength , such as internal combustion engine cylinder blocks , pump housings, valve bodies, electrical boxes, and decorative castings.
Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors. A typical chemical composition to obtain a graphitic microstructure is 2.
Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. A moderate cooling rate forms a more pearlitic matrix, while a fast cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed. The graphite takes on the shape of a three-dimensional flake.
In two dimensions, as a polished surface, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids.
The tips of the flakes act as preexisting notches at which stresses concentrate and it therefore behaves in a brittle manner. Grey iron also has very good damping capacity and hence it is often used as the base for machine tool mountings. 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.
These grades are a measure of the tensile strength-to- Brinell hardness ratio. Gray iron is a common engineering alloy because of its relatively low cost and good machinability , which results from the graphite lubricating the cut and breaking up the chips. It also has good galling and wear resistance because the graphite flakes self lubricate. The graphite also gives gray iron an excellent damping capacity because it absorbs the energy and converts it into heat.
Gray iron also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increased fluidity when casting. From Wikipedia, the free encyclopedia. Retrieved Categories : Cast iron Ferrous alloys Iron. Namespaces Article Talk. Views Read Edit View history. In other projects Wikimedia Commons.
SAE J431 G3500
Subscribers can view annotate, and download all of SAE's content. Automotive Gray Iron Castings. View Details. Steel, Highly-Stressed Springs 0.
Material Notes:. If microstructure. Key Words:. UNS F, ferritic, pearlitic, soft iron castings No vendors are listed for this material. Please click here if you are a supplier and would like information on how to add your listing to this material.