# Heat Treatment of Metals and Alloys Questions and Answers – Steels – Importance of Austenitic Grain Size

This set of Heat Treatment of Metals and Alloys Multiple Choice Questions & Answers (MCQs) focuses on “Steels – Importance of Austenitic Grain Size”.

1. Which one of the following equations correctly represents the Hall-Petch equation?
a) σ0=kyD-1/2
b) σ0=kyD1/2
c) σl=kyD-1/2
d) σl=kyD1/2

Explanation: σ0=kyD-1/2 correctly represents the Hall Petch equation. Here, σ0 represents the yield stress, σ1 represents the frictional stress, and D is the average grain diameter.

2. As the grain size increases ___________
a) the impact transition temperature decreases
b) the impact transition temperature increases
c) the steel becomes more resistant to brittle fracture
d) there is no impact on brittle fracture

Explanation: As the grain size increases, the transition temperature increases. Therefore, as the transition temperature increases steel becomes more prone to brittle fracture.

3. Which of the following statements is correct?
a) Above the equicohesive temperature fine grained steel have more creep strength than coarse grained steels
b) Below equicohesive temperature, coarse grained steel has more creep
c) Cast steels have more creep strength than forged steels
d) Equicohesive temperature is independent of the fracture strength of a metal

Explanation: Above the equicohesive temperature, fine grained steels have less creep strength than coarse grained steels. Below this temperature, fine grained steels have more strength. Cast steels have more creep strength than forged steels. The reason behind this is the coarse grain size of cast steels.

4. Fine grained steels have better hardenability than coarse grained steels.
a) True
b) False

Explanation: Fine grained steels have lesser hardenability than coarse grained steels. The reason behind this is that coarse grained steels have lesser grain boundaries than fine grained steels which are responsible for diffusion.

5. In coarse grained steels, preferential formation of martensite from austenite takes place.
a) True
b) False

Explanation: Coarse grained steels have lesser grain boundary area. At grain boundaries, rate of diffusion is high. Pearlitic formation is a diffusion controlled process and martensitic transformation is a diffusionless process. Therefore, with lesser grain boundary area, as in the case of coarse grained steels, preferential formation of martensite from austenite takes place.

6. According to hall petch equation, which of the following equations can be derived?
a) Yield stress is independent of grain size
b) Yield stress is directly proportional to grain size
c) Yield stress is inversely proportional to grain size
d) Higher the yield stress, finer will be the grain size

Explanation: According to hall petch equation, σ0=kyD-1/2. So, as the grain size increases, kyD-1/2 factor will decrease. Thus, the yield stress will decrease but it will not decrease proportionally.

7. Which one of the following statements is correct?
a) Cast steel have lower creep strength than forged steel
b) Silicon deoxidized steels have higher creep strength than Aluminium deoxidized steel
c) Coarse grained steels have higher fatigue strength than fine grained steels
d) There exists a mathematical relationship between creep strength and grain size

Explanation: Because of the presence of coarse grains, cast steels have higher creep strength than forged steels. Same reason is for higher creep strength of Silicon deoxidized steels over Aluminium deoxidized steel. If the temperature is not so high that creep becomes a predominant phenomenon, fine grained steels have higher fatigue strength than coarse grained steels. Change in grain size with increasing creep strength is always accompanied with other changes, which is why a mathematical relationship between these two is not possible.

8. Which one of the following statements is true for coarse grained steels?
a) They have lower hardenability than fine gained steels
b) They have greater grain boundary area
c) They have greater toughness than fine grained steels
d) They have greater machinability than fine grained steels

Explanation: Because of lesser grain boundaries, coarse grained steels have higher hardenability than fine grained steels. However, they have lower toughness than fine grained steels but higher machinability than fine grained steels. In coarse grained structure during cooling, chip formation occurs which is responsible for machinability.

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