Question

(ii) Explain the reason(s) qualitatively behind the "C" shape of a typical TTT curve, i.e. explain why a typical TTT curve has a "C" shape. What is the reason, for some steels, the TTT curve has a double-nosed shape? (iii)

Answer 1

2.)  In A TTT diagram close to the eutectoid temperature, the undercooling decreases most of the time so that the driving force for the transformation is small. However, as the undercooling increases transformation accelerates until the maximum rate is obtained at the “nose” of the curve. Below this temperature the driving force for transformation continues to increase but the reaction impeded by slow diffusion. This is why TTT curve takes on a “C” shape with most rapid overall transformation at some intermediate temperature.

3.)  A large number of alloy steels have two noses of the TTT curves, i.e., two minimums for the stability of super-cooled austenite-upper (higher temperature) one for pearlitic, and the lower one for bainitic transformation. TTT diagrams of plain carbon steels and some alloy steels have single knee. Actually, single knee curve is a particular case where super-imposition of temperature intervals for pearlitic and bainitic transformations takes place. So, there is no particular reason that why for some steel TTT curve has a double-nosed shape.