ddooley

The key point here is the difference in structure between austenite and ferrite. Austenite has its iron atoms arranged in a face centered cubic atomic structure (fcc) while in ferrite, the iron atoms are arranged in a structure called body centered cubic (bcc). In pure iron, austenite changes to ferrite below 1666 F (i.e. goes from fcc to bcc). The addition of carbon, however, makes things more interesting since carbon atoms fit more easily in the fcc structure of austenite than then can in bcc ferrite. As such, much more carbon can be dissolved in austenite than in ferrite. Because of this difference, the addition of carbon to iron allows austenite to remain stable at lower and lower temperatures. It's like adding salt to water: since salt can more easily dissolve in liquid water than in ice, it lowers the melting point. This is why the A3 line slopes downward - higher carbon contents allow austenite to remain stable at lower temperatures. The limit to this is the eutectoid point. Austenite of this composition (0.8% C) will transform to pearlite below 1333 F. Note that pearlite is not a single substance, but rather a fine mixture of ferrite (essentially pure iron) and cementite/iron carbide (a chemical compound of iron and carbon). Under the microscope these two components or phases are arranged in alternating plates. When polished and etched with dilute acid, pearlite appears iridescent like a pearl - hence its name.