Cementite

A compound of iron and carbon, known chemically as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure and is composed of 93% iron and 7% carbon. When it occurs as a phase in steel, the chemical composition will be altered by the presence of manganese and other carbide-forming elements. Cementite is brittle, hard and falls under the ceramic classification.

Since it tends to be unstable thermodynamically, it is eventually converted to graphite and ferrite.

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