This set of Heat Treatment of Metals and Alloys Multiple Choice Questions & Answers (MCQs) focuses on “Steels – Martensitic Transformation Reversibility, Bain Distortion Model and Hardness of Martensite”.
1. Martensitic transformation is irreversible.
a) True
b) False
View Answer
Explanation: Martensite formation is irreversible. This statement is incorrect. In reality, martensite can be reversed to austenite on heating above a certain temperature.
2. Martensite can be converted to austenite by which of the following process?
a) Heating martensite above Ms Temperature
b) Heating martensite above Mf Temperature
c) Cooling martensite below Mf temperature
d) Heating martensite above nose temperature
View Answer
Explanation: Martensite can be reverted back to austenite on heating above Ms Temperature. If heating is done below Ms Temperature it won’t make any difference. Also, there is no absolute theory which support the role of nose temperature for this as austenite is stable at nose temperature.
3. Which one of the following conditions is essential for reversibility of martensite?
a) No change in chemical composition of martensite during heating
b) No change in chemical composition of martensite during cooling below Mf
c) Change in chemical composition of martensite during heating
d) Change in chemical composition of martensite during cooling below Mf
View Answer
Explanation: It is true that martensite can be reverted back to austenite. But it is not true for all types of martensitic steels. Only that type of martensite can be reverted back in which no change in chemical composition takes place during heating. The reason is that martensite is supersaturated solid solution of carbon in alpha iron which can decompose easily on heating.
4. What is the maximum c/a ratio of BCT structure of martensite?
a) 1
b) 1.4
c) 1.08
d) 1.8
View Answer
Explanation: The c/a ratio is 1 for BCC structure. The BCT structure has a c/a ratio of 1.4. But martensite is a special case in which the c/a ratio cannot be more than 1.08. It has a c/a ratio much closer to BCC.
5. Which one of the following statements is incorrect?
a) Bain distortion model does not involve shear
b) Bain distortion model does not involve orientation relationship
c) Bain distortion model does not involve the concept of habit planes
d) Undistorted plane is available in Bain distortion model
View Answer
Explanation: Bain distortion model has many drawbacks. It does not involve shear, orientation relationship and concept of habit planes. Along with this Bain distortion model does not have undistorted plane. Hence it is not possible to explain invariant plane strain associated with martensitic transformation.
6. How does the hardness of martensite depend on carbon content?
a) Increases with increasing carbon content
b) Increases with decreasing carbon content
c) May increase or decrease with increasing carbon content
d) Does not depend on carbon content
View Answer
Explanation: Hardness of martensite increases with increasing carbon content. The relationship between hardness and carbon content in nonlinear. Some minimum carbon content is essential to have reasonable hardness.
7. The amount of retained austenite is less in high carbon steels.
a) True
b) False
View Answer
Explanation: The amount of retained austenite is less in high carbon steels. This statement is incorrect. In reality, high carbon steels have lower Ms and Mf temperature than medium and low carbon steels. This is the reason why the amount of retained austenite is higher for high carbon steels.
8. Which one of the following statements is correct?
a) As the amount of retained austenite increases, hardness increases
b) As the amount of retained austenite increases, hardness decreases
c) As the amount of retained austenite increases, hardness may increase or decrease
d) Hardness is independent on the amount of retained austenite
View Answer
Explanation: As the amount of retained austenite increases, hardness decreases. Austenite is a soft phase, therefore as the amount of austenite increases the hardness decreases as softness increases.
Sanfoundry Global Education & Learning Series – Heat Treatment of Metals and Alloys.
To practice all areas of Heat Treatment of Metals and Alloys, here is complete set of Multiple Choice Questions and Answers.
If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]
