T-T-T & C-C-T Diagrams

TTT diagram is a time temperature transformation diagram- that you obviously know.

This diagram would show you that at a given temperature (Y axis of the graph) if the metal is held for a given time, how the microstructural transformation will take place with the % constition of various phases. It is an isothermal transformation.

If we talk abour steel, obviously above Eutectoid temperature it is all Austenite. Say you hold the metal at 600 deg C then you will need about say 1 minute for the austenite to transform to pearlite.

For 650 deg C however it may need about 15 minutes (austenite is more stable)

and at 500 deg c it will transform to bainite and not pearlite.

Obviousy this is critical in view of the holding time in the furnace for the full transformation to take place.

The CCt (continuous Cooling Curve/Transformation curve) is the transformations that is achieved when the metal is cooled at a constant rate. These curves will show the constituents when the metal is cooled continuosly.

Thus at a cooling rate of say 50 deg c per hour one may land up in ferrite+pearlite wheras at fast rate (as in quenching) from the same temperature these are pure martensites.

One curve is for holding in furnace and other is for the rate of quenching.

BTW- the expert on this is Milo.