Isothermal transformation diagram

They are generated from percentage transformation-vs time measurements, and are useful for understanding the transformations of an alloy steel at elevated temperatures.

Isothermal transformation (IT) diagram or the C-curve is associated with mechanical properties, microconstituents/microstructures, and heat treatments in carbon steels.

The rate of nucleation increases and the rate of microconstituent growth decreases as the temperature decreases from the liquidus temperature reaching a maximum at the bay or nose of the curve.

Thereafter, the decrease in diffusion rate due to low temperature offsets the effect of increased driving force due to greater difference in free energy.

However, greater undercooling by rapid quenching results in formation of martensite or bainite instead of pearlite.

Time-Temperature-Transformation diagram for two steels: one with 0.4% wt. C (red line) and one with 0.4% wt. C and 2% weight Mn (green line). P = pearlite, B = bainite and M = martensite.
TTT diagram of isothermal transformations of a hypoeutectoid carbon steel; showing the main components obtained when cooling the steel and its relation with the Fe-C phase diagram of carbon steels.