# Heat Treatment of Metals and Alloys Questions and Answers – Steels – Kinetics of Pearlitic Transformation

This set of Heat Treatment of Metals and Alloys Multiple Choice Questions & Answers (MCQs) focuses on “Steels – Kinetics of Pearlitic Transformation”.

1. Which one of the following statements is correct at a lower critical temperature?
a) Free energy of austenite is less than that of pearlite
b) Free energy of austenite is more than that of pearlite
c) Free energy of austenite is equal to that of pearlite
d) Free energy of austenite cannot be determined

Explanation: At lower critical temperature, free energy of austenite is equal to the free energy of pearlite. This means that at this temperature, austenite to pearlite transformation can take place in infinite time.

2. Below the lower critical temperature ____________
a) free energy of austenite is less than that of pearlite
b) free energy of austenite is more than that of pearlite
c) free energy of austenite is equal to that of pearlite
d) free energy of austenite cannot be determined

Explanation: Below lower critical temperature, free energy of austenite is more than free energy of pearlite. This is the reason why pearlite is thermodynamically more stable below lower critical temperature.

3. How does the rate of pearlite transformation change?
a) Increases with increasing transformation temperature
b) Increases with decreasing transformation temperature
c) Increases with decrease in undercooling
d) Does not depend on transformation temperature or undercooling

Explanation: Since at lower critical temperature, free energy of austenite is equal to free energy of pearlite therefore the rate of pearlite transformation is zero. That is why we need undercooling. So, lower the transformation temperature or larger the undercooling more will be the rate of reaction.

4. The rate of diffusion of carbon atoms from austenite to pearlite varies as __________
a) increased degree of undercooling increases the rate of diffusion of carbon atoms
b) increased degree of undercooling decreases the rate of diffusion of carbon atoms
c) increased degree of undercooling does not affect the rate of diffusion of carbon atoms
d) increased temperature increases the rate of diffusion of carbon atoms

Explanation: As the degree of undercooling increases, the transformation temperature decreases. Therefore, at lower temperatures, rate of diffusion of carbon atoms decreases.

5. Which one of the following statements is correct?
a) The maximum rate of crystal growth is greater than the maximum rate of nucleation for pearlitic transformation
b) The maximum rate of crystal growth is lower than the maximum rate of nucleation for pearlitic transformation
c) The maximum rate of crystal growth is equal to the maximum rate of nucleation for pearlitic transformation
d) The maximum rate of crystal growth may be greater or lower than the maximum rate of nucleation for pearlitic transformation

Explanation: The maximum rate of crystal growth is greater than the maximum rate of nucleation for pearlitic transformation. It can be seen in the following diagram

6. Which one of the following elements does not decrease the rate of nucleation of pearlite?
a) Nickel
b) Cobalt
c) Titanium

Explanation: Almost all alloying elements be it ferrite or austenite stabilizer decrease the rate of nucleation of pearlite. Cobalt is an exception.

7. As compared to carbon, alloying elements diffuse quickly.
a) True
b) False

Explanation: As compared to carbon, alloying elements diffuse quickly. This statement is false. In reality, alloying elements diffuse very slowly as compared to carbon.

8. Generally, stable carbides possess higher metal to carbon ratio.
a) True
b) False

Explanation: Generally, stable carbides possess higher metal to carbon ratio. This is the reason why the formation of stable carbides is feasible only at high transformation temperatures.

Sanfoundry Global Education & Learning Series – Heat Treatment of Metals and Alloys.