sementit

Sementit is a term in metallurgy commonly used to refer to cementite, the iron carbide Fe3C. Cementite is a hard, brittle intermetallic compound that forms in steel and cast iron and plays a crucial role in determining mechanical properties. Its chemical formula is Fe3C, and it contains about 6.67% carbon by weight.

In the crystal structure, cementite is orthorhombic. In steel, it appears as discrete particles or lamellae within a ferritic matrix, most famously forming the alternating layers of ferrite and cementite in pearlite. In cast irons with higher carbon content, cementite can exist as nodules or networks, contributing to hardness and wear resistance.

Formation and transformation: In steels with near-eutectoid composition (about 0.76% carbon), austenite transforms at about 727°C into pearlite, a mixture of ferrite and cementite. Upon further cooling, cementite can precipitate, increasing hardness. Heating steel to high temperatures can dissolve cementite back into austenite; rapid cooling (quenching) can trap carbon and form martensite.

Significance: Cementite influences hardenability and brittleness. Its distribution affects tensile strength, toughness, and wear resistance, so controlling cementite through alloying and heat treatment is central to steel processing.

See also: Pearlite, Bainite, Martensite, Iron-carbon phase diagram.