Question

The incubation period at nose temperature region of TTT diagram of eutectoid steel is shorter than below and above nose regions. This is due to:

• A. Slow cooling rate
• B. More diffusion
• C. Optimum nucleation and diffusion
• D. Less nucleation

Hint:

The nose of the TTT curve represents the point of fastest transformation due to the perfect balance between nucleation and diffusion — both processes are effective here.

Answer 1

The Correct Option is C

The Time-Temperature-Transformation (TTT) diagram for eutectoid steel shows how austenite transforms into other microstructures like pearlite or bainite depending on time and temperature.
At the nose of the TTT curve, the transformation happens most rapidly. This is because both nucleation and diffusion rates are at their optimum levels.
- Nucleation increases as the temperature decreases below the eutectoid point. - Diffusion, however, slows down with decreasing temperature. Hence, at the nose temperature, there's a balance — optimum nucleation and sufficient diffusion — resulting in the shortest incubation time.
Too high or too low temperatures disturb this balance, either by reducing nucleation (at high temperature) or slowing down diffusion (at low temperature).