Question

In a TTT diagram, increasing alloying elements generally causes:

• A. Leftward shift of C-curve
• B. Rightward shift of C-curve
• C. No change
• D. Steeper C-curve

Hint:

A rightward shift of the TTT C-curve directly increases the critical cooling rate's margin, making it easier to fully harden thick sections of steel without forming soft pearlite.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks how the addition of alloying elements (excluding cobalt) affects the position of the transformation start curves (C-curves) on a Time-Temperature-Transformation (TTT) diagram of steel.

Step 2: Detailed Explanation:

• TTT Diagram C-Curve: The C-curve on a TTT diagram represents the start and finish times of diffusional phase transformations (such as austenite to pearlite or bainite) at different isothermal temperatures.
The "nose" of the C-curve corresponds to the minimum time required for these transformations to begin.

• Effect of Alloying Elements: Alloying elements (such as Chromium, Nickel, Manganese, and Molybdenum) dissolve in austenite and stabilize it.
These solute atoms slow down the diffusion of carbon, which is required for pearlite and bainite nucleation and growth.
Because diffusion is hindered, the time required for these phase transformations to start increases significantly.
This causes the C-curves to shift to the right (toward longer times).

• Hardenability Significance: A rightward shift of the C-curve means that slower cooling rates can be used during quenching to bypass the pearlite/bainite nose and form martensite.
This significantly increases the hardenability of the steel.

• Exceptions: Cobalt is a notable exception; it increases the rate of nucleation of pearlite and shifts the C-curve to the left. However, almost all other common alloying elements shift it to the right.


Step 3: Final Answer:
Thus, increasing alloying elements generally causes a rightward shift of the C-curve, which is Option (B).