Cementite or iron carbide, is very hard, brittle intermetallic compound of iron & carbon, as Fe3C, contains 6.67 % C. It is the hardest structure that appears on the diagram, exact melting point unknown.
In some articles the value of 1223ºC is given according to the iron-carbon diagram, but it is actually metastable.
In general before melting, it will decompose into ferrite and graphite over 723ºC