Certain materials, such as carbon steel alloys, can have
their mechanical properties changed by heat treatment.
This is important in the manufacture of tools where the
material may be shaped in the soft condition and then
heat treated to bring it to the required working hardness
for long tool life.

The diagram shows constant cooling rate paths for a
1040 carbon steel sample initially heated to 750 C to
form Austenite. For cooling rates greater than 140 C/s,
the material forms Martensite upon cooling to room
temperature. Between 35 C/s and 140 C/s, the room temperature microstructure is a mixture of Martensite and Pearlite, and for cooling rates less than 35 C/s the material has a Pearlitic structure. Martensite makes the material hard and brittle. Pearlite makes it less hard and its ductility is increased as the size of the Pearlite grains increase as a result of lower cooling rates from the Austinite state.