The time-temperature transformation

Time-Temperature-Transformation (TTT) Curves

TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition.
TTT diagram indicates a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.
The aims of TTT diagrams is determined type of structure for and portion in the curve and to obtained on specific properties .
It is also called isothermal transformation diagram
Pearlite
The eutectoid reaction is fundamental to the development of microstructures in steel alloys.
? (0.76 wt% C) ? ? (0.022 wt% C) + Fe3C (6.70 wt% C)
Pearlite is the microstructural product of this transformation.-
-Above eutectoid temperature: only austenite exists
Below eutectoid temperature: nucleation + growth-
-The percentage of the transformation product is related to the holding temperature and holding time.
-The thickness of the ferrite/cementite layers in pearlite depends on the temperature. With decreasing temperature, the layers become progressively thinner.
-At temperatures just below eutectoid ? relatively thick layers ? coarse pearlite.
In the vicinity of 540?C ? relatively thin layers ? fine pearlite-
Martensite
-Martensite is formed when austenitized Fe-C alloys are rapidly cooled (or quenched) to a relatively low temperature .
-Non-equilibrium single phase.
Transformation of FCC to BCT (body-centered tetragonal).-
-The martensite grains nucleate and grow at a very rapid rate .

(BCT)
Martensite (FCC) Austenite phase
(body-centered tetragonal)
The time-temperature transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite.