Supercritical Region
At temperatures where complete transformation to austenite has just occurred a fine austenite grain size is produced
- on cooling this transforms back to a fine ferrite/carbide array
If the metal temperature rises above ~1200oC rapid austenite grains growth occurs
- coarser structure produced on cooling
Structure depends on cooling rate
- Fast - martensite or ferrite with M-A-C
- Slow - ferrite and pearlite