This set of Heat Treatment of Metals and Alloys Multiple Choice Questions & Answers (MCQs) focuses on “Steels – Kinetics of Martensitic Transformation”.
1. The increased amount of martensite at a lower temperatures is due to which of the following factors?
a) Growth of existing platelets of martensite
b) Formation of new platelets of martensite
c) Both due to growth of existing platelets of martensite and formation of new platelets of martensite
d) Neither due to growth of existing platelets of martensite nor formation of new platelets of martensite
View Answer
Explanation: By lowering the transformation temperatures, more austenite gets transformed to martensite. This is due to formation of new platelets of martensite rather than growth of existing platelets of martensite.
2. What happens at Mf temperature?
a) All the austenite gets transformed to martensite
b) All the pearlite gets transformed to martensite
c) Formation of martensite is finished
d) Martensite formation is started
View Answer
Explanation: Mf temperature is the end temperature to martensitic transformation. Further cooling will not result in any more martensite. It is not practically possible to convert all austenite to martensite.
3. Which one of the following statements is incorrect?
a) Amount of martensite formed is dependent on transformation temperature
b) Speed of transformation is dependent on transformation temperature
c) Speed of transformation is very fast
d) Speed of transformation is independent on transformation temperature
View Answer
Explanation: The amount of martensite formed is dependent on transformation temperature but its speed of transformation is independent of transformation temperature and is very fast. Quenching process is used for martensitic process.
4. Which one of the following elements increases the Ms Temperature?
a) Carbon
b) Molybdenum
c) Aluminium
d) Nickel
View Answer
Explanation: Almost all alloying elements lower the Ms temperature. Exceptions are Cobalt and Aluminium which increase the Ms temperature.
5. Which one of the following is correct?
a) Ms temperature can be raised by increasing the cooling rate
b) Ms temperature can be raised by decreasing the cooling rate
c) Ms temperature can be changed by varying the cooling rate
d) Ms temperature can neither be raised nor lowered by increasing the cooling rate
View Answer
Explanation: Ms temperature can neither be raised nor lowered by increasing the cooling rate. This temperature is independent of the cooling rate and can be changed by alloying elements.
6. Martensitic transformation is independent of holding time.
a) True
b) False
View Answer
Explanation: Martensitic transformation is independent of holding time. There are some exceptions to this statement where transformation is done isothermally.
7. Austenising temperature affects which one of the following?
a) Ms temperature only
b) Chemical composition only
c) Austenite grain size only
d) Ms temperature, chemical composition and austenite grain size
View Answer
Explanation: Austenising temperature to which heating of steel is done prior to transformation changes the Ms temperature. It affects both the composition and austenite grain size.
8. Which one of the following statements is correct?
a) The higher the austenising temperature, higher will be the Ms temperature
b) The higher the austenising temperature, lower will be the Ms temperature
c) The higher the austenising temperature, the Ms temperature may increase or decrease
d) Ms temperature is independent of austenising temperature
View Answer
Explanation: As the austenising temperature increases, greater will be the probability of dissolution of carbide. Thus, the Ms temperature decreases. But higher austenising temperature increases the austenitic grain size, which increases the Ms temperature. Thus, the increases or decreases in Ms temperature depends on the predominating factor.
9. Which one the following statement is correct regarding Ms and Mf temperature?
a) Ms temperature depends on cooling rate
b) Mf temperature depends on cooling rate
c) Both Ms and Mf temperature depends on cooling rate
d) Both Ms and Mf temperature are independent of cooling rate
View Answer
Explanation: Unlike Ms temperature, Mf temperature depends on the cooling rate. Thus it can be varied by varying the cooling rate.
10. How does cooling rate affect the Mf temperature?
a) Mf temperature increases with increasing cooling rate
b) Mf temperature decreases with increasing cooling rate
c) Mf temperature may increase or decrease with increasing cooling rate
d) Mf temperature is independent of cooling rate
View Answer
Explanation: Slower cooling rates result in a lowering of Mf temperature. This is the reason why Mf temperature is not indicated in many TTT diagrams. It cannot be measured precisely at such a low temperature because of very slow rate of reaction.
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.
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