Isothermal transformation of austenite to bainite.
This heat treatment offers increased ductility / notch toughness at a given hardness. It also minimizes distortion and reduces risks of quench cracks.
It represents an ideal alternative to conventionnal hardening (quench & temper) for parts needing high fatigue resistance or a combination of high hardness / high toughness.
A variation of this process (although the formation of ausferrite rather than bainite is desired) allows the heat treatment of ductile iron to the five (5) A.D.I. ASTM A897-90 grades. (See Table).
P.S. Gray iron may also be austempered.