This set of Heat Treatment of Metals and Alloys Multiple Choice Questions & Answers (MCQs) focuses on “Steels – Martensitic Transformation”.
1. Which one of the following transformations takes place in martensitic transformation?
a) Delta iron to alpha iron
b) Delta iron to gamma iron
c) Gamma iron to alpha iron
d) Gamma iron to delta iron
View Answer
Explanation: During martensitic transformation, gamma iron changes to alpha iron. The chemical compositions will same as that of gamma iron.
2. What is the structure of martensite?
a) BCC
b) FCC
c) BCT
d) HCP
View Answer
Explanation: Martensite has BCT structure. Austenite has FCC structure. Epsilon carbide has HCP structure. Ferrite has BCC structure.
3. What is martensite?
a) It is a supersaturated interstitial solid solution of carbon in alpha iron
b) It is an interstitial solid solution of carbon in gamma iron
c) It is an interstitial solid solution of carbon is delta iron
d) It is an interstitial solid solution of carbon in epsilon carbide
View Answer
Explanation: Martensite is a supersaturated interstitial solid solution of carbon in alpha iron. It has a tetragonal structure. Austenite is known as gamma iron and has an FCC structure.
4. Which process is involved in martensite formation?
a) Annealing
b) Normalizing
c) Quenching
d) Spheroidizing
View Answer
Explanation: Martensite is formed by quenching austenised steel. It involves rapid cooling without giving any time for diffusion. Quenching results in high hardness values. Brittleness also increases.
5. Which one of the following is correct?
a) The driving force for martensite transformation increases with increasing transformation temperature
b) The driving force for martensite transformation increases with decreasing transformation temperature
c) The driving force for martensite transformation is independent of transformation temperature
d) The driving force for martensite transformation decreases with decreasing transformation temperature
View Answer
Explanation: Free energy change provides for the driving force for martensitic transformation. This driving force increases with decreasing transformation temperature.
6. Martensitic transformation is a diffusionless process.
a) True
b) False
View Answer
Explanation: Martensitic transformation is a diffusionless process. Pearlitic and bainitic transformation are diffusion controlled processes.
7. The driving force required for martensitic transformation is provided by the free energy change accompanying the transformation.
a) True
b) False
View Answer
Explanation: A large driving force is required for martensitic transformation. This driving force is provided by the free energy change accompanying the transformation.
8. Which one of the following statements is correct?
a) If austenite phase is ordered, then martensitic phase will be disordered
b) If austenite phase is ordered, then martensitic phase will be ordered
c) If austenite phase is ordered, then martensitic phase may be ordered or disordered
d) Austenite and martensite are independent of each other
View Answer
Explanation: Since there is no compositional change during austenite to martensite transformation, therefore if austenite phase is ordered, then martensitic phase will be ordered.
9. Which one the following statement is correct regarding martensitic transformation speed?
a) It occurs at a very slow rate
b) It occurs at speed of light
c) It occurs at speed of sound
d) It occurs at speed slower than that of bainite
View Answer
Explanation: Martensitic transformation occurs at speed of sound. The reason behind this is the cooperative movement of large group of atoms.
10. What is the final structure after martensitic transformation of austenite without carbon?
a) FCC
b) BCC
c) BCT
d) HCP
View Answer
Explanation: The final structure in this case is BCC. The martensite formed undergoes 20% contraction in c axis and 12% expansion of an axis so as to keep volume constant resulting in a BCC structure.
11. Which one of the following has the highest hardness?
a) Coarse pearlite
b) Bainite
c) Martensite
d) Tempered martensite
View Answer
Explanation: Martensite has the highest hardness. For 0.8% carbon steel, its hardness is 65Rc. For coarse pearlite, bainite and tempered martensite it is 16Rc, 45Rc and 55Rc respectively.
12. Which one of the following has highest tensile strength?
a) Coarse pearlite
b) Bainite
c) Martensite
d) Tempered martensite
View Answer
Explanation: When we do tempering of martensite, dual phase mixture of pearlite and martensite is obtained and brittleness gets decreased thus I can withstand high tensile stress.
13. Which one of the following has the highest toughness?
a) Coarse pearlite
b) Bainite
c) Martensite
d) Tempered martensite
View Answer
Explanation: Although martensite has higher hardness, tempered martensite has higher toughness because of the presence of pearlite which has unstrained crystal structure.
14. Which one of the following statements is correct?
a) The rate of decrease of Mf temperature with increasing carbon content is greater than that for Ms Temperature
b) The rate of decrease of Mf temperature with increasing carbon content is lower than that for Ms Temperature
c) The rate of decrease of Mf temperature with increasing carbon content equal to that for Ms Temperature
d) The rate of decrease of Mf temperature with increasing carbon content may be greater than or lower than that for Ms Temperature
View Answer
Explanation: The rate of decrease of Mf temperature with increasing carbon content is greater than that for Ms Temperature. At Ms Temperature, distortion occurs only at some crystals but at Mf temperature, complete crystal distortion occurs. So if C content increases, then there will be greater difficulty in BCT transformation.
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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