Austenite

Austenite is the Face Centered Cubic form of iron that is stable for
iron-carbon alloys above the Eutectoid temperature of 727 C. The
carbon atoms form an interstitial solid solution in the fcc iron matrix.

Because Austenite is a single phase solid, it is relatively ductile and
has a low work-hardening rate.

Below 727 C, Austenite is thermodynamically unstable and the alloy converts to the stable
low temperature Pearlite. This is a mixture of two phases, alpha-iron and iron carbide.

Alpha-iron has a Body Centered Cubic crystal structure and carbon
can form an interstitial solid solution in the matrix. It is relatively ductile.

Iron carbide is a compound Fe₃ C, and is a brittle ceramic-like
material. The presence of these two phases in Pearlite makes this
material less ductile, and it has a higher work hardening rate than the single phase Austenite.