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Question
TTT diagram
TTT diagram  
In about 500 degree C why the time increases again though due to change of temperature should be decreasing?
Thank you sir.
I Really need the answer

Answer
Sorry for the late reply!

Referring to the TTT diagram that you have attached, the rate of austenite to pearlite transformation is highest around 550 degrees Celsius. As the temperature decreases below 550 C, the driving force for the transformation increases but the diffusivity of carbon (for the simple case of Fe-C binary system) becomes smaller and smaller. As the temperature increases above approximately 550 C, on the other hand, although carbon diffusivity progressively increases (C becomes very mobile), the thermodynamic driving force decreases as the equilibrium transformation temperature (i.e. the temperature at which the driving force drops to zero) is approached.

In summary, at temperatures around 550 C, the combined effect of the thermodynamic driving force and the diffusivity of elements is such that the transformation takes place in a very short time. Below that, the low diffusivity decelerates the transformation and above that the low thermodynamic driving force slows down the transformation.

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