The Iron–Carbon System

The Iron–Carbon System

Of all binary alloy systems, the one that is possibly the most important is that for iron and carbon. Both steels and cast irons, primary structural materials in every technologically advanced culture, are essentially iron–carbon alloys. This section is devoted to a study of the phase diagram for this system and the development several of the possible microstructures.
This system of the type 3 (tow substances are completely soluble in each other in the liquid state but are only partially soluble in the solid state )
A portion of the iron–carbon phase diagram is presented in Figure (9-24). Pure iron, upon heating, experiences two changes in crystal structure before it melts. At room temperature the stable form, called ferrite, or ?-iron, has a BCC crystal structure .Ferrite experiences a polymorphic transformation to FCC austenite, or ?-iron912?C. This austenite persists to 1394 ?C, at which temperature the FCC austenite reverts back to a BCC phase known as ? ferrite , which finally melts at 1538 ?C
at6.70 wt% C concentration the intermediate compound iron carbide, or cementite (Fe3C), is formed ,Figure(9-24) shows three horizontal lines which indicate isothermal reactions (723,1131,1492) ?C