This phenomenon involves the transformation of austenite (a face-centered cubic phase of iron) into ferrite (a body-centered cubic phase) during thermomechanical processes such as rolling and forging.

For the hypereutectoid composition shown by the vertical green bar the material enters the two phase austenite + cementite zone prior to cooling ... When the alloy reaches the eutectoid temperature ...

Before transforming to pearlite, this material phases through the two-phase zone (yellow) between austenite and ferrite. In cooling through this zone, ferrite is precipitated on the austenite grain ...