Austenite: This is the term used to describe the structure of iron which exists above its upper critical temperature. Above this temperature, iron has a face centered cubic (FCC) structure which can dissolve a maximum of 2% carbon in solid solution. Austenite is a hard, non- magnetic substance. Cementite: This material is a chemical compound of iron and carbon. The chemical formula for cementite is Fe3C and it is a very hard, brittle material. As the amount of Cementite in steel increases, its hardness and brittleness increase. Pearlite: At room temperature, a steel with less than .87% carbon will contain regions of ferrite and pearlite. The regions of Pearlite consist of layers of ferrite and Cementite. Each unit of Pearlite contains .87% carbon. Ferrite: This is the term used to describe iron which contains a maximum of .02% carbon dissolved in solid solution. Ferrite has a body centered cubic (BCC) structure. Martensite: This is the structure which is produced when austenite is quenched rapidly. It is a very hard brittle structure. The lower critical temperature is the temperature at which carbon steel must be heated to before changes will occur in its structure. This temperature is 723 ºC. The term allotropy refers to the ability for some elements to exist in more than one crystalline structure. Steel can exist as a body centered structure when cold or as a Face centered cubic structure or austenite when heated above its upper critical temperature. For a 0.4 % carbon steel this temperature would be about 880 º Celsius. A eutectic reaction occurs at the eutectic point this means that the material has changed from a liquid to a solid . This occurs at approx. 115 ºC for a 4% carbon steel. Slow cooling of 0.4% carbon steel from 1000 ºC At 1000ºC a 0 .4% carbon steel consists of a solid solution of carbon dissolved in FCC iron. As the steel cools, it reaches its upper critical temperature 880 º C :Ferrite begins to form at the grain boundaries of the austenite. As ferrite has a BCC structure, the carbon can't dissolve in it so the carbon remains dissolved in the FCC austenite.
