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The Material Composition of Cast Iron


Cast iron is actually an iron-carbon-silicon alloy containing between 3 percent and 5.5 percent carbon, 1 percent to 3 percent silicon, plus traces of other elements such as sulfur, magnesium and potassium totaling about 0.5 percent. By contrast, steels contain less than 2 percent carbon. According to the Mat Base website, cast iron's main advantages are low price and the capability to be molded into complex shapes in a single production step. Cast iron is a ferrous metal--but one that has a reasonably strong resistance to rust

Cast iron is weaker than steel when under tension, but is as strong or stronger than steel when under compression, according to the Mat Base website. It also is more brittle than steel and can crack or break under shock. The strength of cast iron depends on the morphology of the carbon. In gray cast iron, the carbon is present as plates of pure graphite. This is the weakest form. In ductile cast iron, the carbon is present as graphite spheres. This is the strongest form of cast iron.

Other Types
Other forms of cast iron include white iron. This is actually iron carbide. It is a very hard but very brittle metal. Often, parts of an iron casting that will be subject to wear are chilled to convert that area to white iron. Then there is malleable cast iron, which is white iron that has been subjected to a two-stage heat treating process that produces irregular carbon grains. It is easily machined and can be selectively hardened after machining.

Engineering Material
Cast iron can be engineered to serve many purposes, according to the Machine Design website. It can be machined to close tolerances, resists warping, can be heat treated to impart desired characteristics such as surface hardness, can be cast with inserts of other materials, and can produce highly complex shapes and section sizes ranging from a few ounces to over 100 tons.

Cast iron also can be alloyed with other metals to impart desirable characteristics impossible to achieve with cast iron alone, according to Machine Design. Cast iron typically is alloyed with chromium and/or nickel with anywhere from 3 percent to 30 percent or more of the alloying metal. For instance, high chromium iron (up to 16 percent) combines wear resistance and corrosion resistance. High-nickel iron (over 35 percent) is dimensionally stable under high heat, nonmagnetic and very rust resistant.


Gray cast iron, according to Machine Design, is used in industry for engine blocks, light-duty gears, flywheels, brake discs or drums, and machine bases. Its ability to dampen vibration makes it valuable for precision machinery. It's also found in the home in items such as cookware, ornamental objects and toys. Ductile cast iron is used for engine crankshafts, heavy-duty gears and auto door hinges. White iron is used for applications requiring abrasion resistance, such as railroad brake shoes, mill liners and sandblasting or shot-blasting equipment. Malleable iron castings are used for bearing surfaces in trucks, construction equipment, railroad rolling stock and other extreme-wear service.





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