cementite

Cementite, also known as iron carbide, is an intermetallic compound with the chemical formula Fe3C. It is a hard, brittle phase that occurs in steel and cast iron. It contains about 6.67 weight percent carbon and has an orthorhombic crystal structure. In steel, cementite forms during cooling from the austenite region and as a product of certain heat treatments.

In the iron–carbon phase diagram, cementite is a constituent of key microstructures. At the eutectoid composition of plain carbon steel (about 0.77 wt% C), austenite decomposes upon cooling at 727°C into ferrite and cementite, producing pearlite. In cast irons with higher carbon content, cementite participates in the eutectic reaction around 4.3 wt% C, forming ledeburite at about 1147°C. Cementite can also occur as dispersed particles in tempered martensite and as lamellar or spheroidal precipitates depending on heat treatment and processing.

The presence and morphology of cementite influence mechanical properties: it increases hardness and wear resistance but reduces ductility. Cementite is commonly observed as lamellae in pearlite, as aggregates in spheroidized structures, or as networks along grain boundaries or within a matrix. Its stability and distribution are central to heat-treatment outcomes in steels and cast irons.