Where's the Martensite in your diagram?If I remember right, martensite forms as a result of rapid cooling, and doesn't appear on the "steady state" iron-carbon phase diagram.I know it has to be there, because I had to drill some out of a broken-off stud in an exhaust manifold with a carbide cutter today...
-Eric
Yes, the equilibrium phase diagram shows those phases which are thermodynamicly stable. As steel is cooled slowly and equilibrium is maintained, austinite transforms to pearlite as carbon precipitates out of the solid solution and forms a "rod" or "parallel-plate" like structure. However, if steel is quenched rapidly, carbon atoms become trapped within the lattice and austinite is transformed to martensite rather than pearlite. Martinsite has a more "needlelike" microstructure when etched and magnified.