Basic concept of cementite
Cementite is a metal compound formed by iron and carbon and has a chemical formula of Fe3C. The carbon content of the cementite is ωc = 6.67% and the melting point is 1227 °C. Its crystal lattice is a complex orthorhombic lattice with a high hardness of HBW=800, plasticity and toughness are almost zero, and the brittleness is very large.
There are different forms of cementite in iron-carbon alloys, and their quantity, morphology and distribution have a direct impact on the properties of iron-carbon alloys.  Divided into primary cementite (precipitated from the liquid phase), secondary cementite (precipitated from austenite) and tertiary cementite (precipitated from ferrite).