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Cementite
Cementite, also known as iron carbide, is an interstitial compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure.〔.〕 It is a hard, brittle material,〔 normally classified as a ceramic in its pure form, though it is more important in ferrous metallurgy. While in everyday's life iron carbide is present in most of steels and cast irons 〔.〕, it is produced as a raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking technologies
==Metallurgy==
In the iron–carbon system (i.e. plain-carbon steels and cast irons) it is a common constituent because ferrite can contain at most 0.02wt% of uncombined carbon. Therefore, in carbon steels and cast irons that are slowly cooled a portion of the element is in the form of cementite.〔.〕 It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering. An intimate mixture with ferrite, the other product of austenite, forms a lamellar structure called pearlite.
While cementite is thermodynamically unstable, eventually being converted to ferrite and graphite at higher temperatures, it does not decompose on heating at temperatures below the euctectoid temperature of the metastable Iron-Carbon phase diagram, which is of the order of 1000 K.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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