Austenitizing is the stage of a heat treatment process where the steel is heated to above the transformation range (A₁ line), where it transforms to austenite.

Austenite is fully formed when the steel is heated completely above A₃ or A_cm temperatures.

Hypoeutectoid Steel

Hypoeutectoid steels (i.e. containing <0.77 %C) have ferrite + pearlite structure at room temperature. When heated above the A₁ temperature, the pearlite transforms to austenite, and above A₃, the ferrite transforms to austenite to give a fully austenitic matrix.

Hypereutectoid Steel

Hypereutectoid steel (>0.77 % C) also have a ferrite + pearlite structure at room temperature. However, on heating above the A₁ temperature, the pearlite and ferrite transform into the two-phase area of austenite and cementite (Fe₃C). Upon further heating above the A_cm temperature, the Cementite transforms to austenite to give a fully austenitic matrix.