What Is the Composition of Cast Iron?


Cast iron is actually an iron-carbon-silicon alloy containing between 3% and 5.5% carbon, 1% to 3% silicon, plus traces of other elements such as sulfur, magnesium and potassium totaling about 0.5%. By contrast, steels contain less than 2% carbon. According to MatBase.com, 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 Characteristics

  • Cast iron is weaker than steel when under tension, but is as strong or stronger than steel when under compression, said MatBase.com. 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 Cast Iron 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, said the MachineDesign.com 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.

Can Be Alloyed

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

Cast Iron Uses

  • Gray cast iron, said MachineDesign.com, 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 things like 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 shotblasting 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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