The Iron Carbon Phase Diagram

There are two iron-carbon equilibrium diagrams: - stable iron-graphite Fe-Gr - metastable iron-cementite Fe-Fe3C The stable condition usually takes a very long time to develop. The metastable diagram is of more interest. Fe3C iron carbide called cementite because it is hard. Following phases exist on Fe-Fe3C diagram: - liquid solution of iron and carbon (L) - ferrite (α) – an interstitial solid solution of carbon in Feα (bcc). At room temperature ferrite is ductile but not very strong. - austenite - an interstitial solid solution of carbon in Feγ (fcc). - cementite (Fe3C) hard and brittle compound with chemical formula Fe3C. It has metallic properties. On a base of Fe-Fe3C diagram we can divide iron-carbon alloys into: - steels, - cast steels, - cast irons. Steel is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content up to 2% intended for wrought products or semi products. Cast iron is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content over 2% intended for castings. Now, we consider only a part of Fe-Fe3C diagram referring to steel. Perlite is a structure (i.e. consists of two phases) consists of alternate layers of ferrite and cementite in the proportion 87:13 by weight. Perlite is formed from austenite at eutectoid temperature (A1) 727°C upon slow cooling. There are three groups of steels according to carbon content: - hypoeutectoid steels containing less than 0.76% C - eutectoid steel with carbon content about 0.76% - hypereutectoid steels contain more than 0.76% C (up to 2% C).