This set of Heat Treatment of Metals and Alloys Multiple Choice Questions & Answers (MCQs) focuses on “Steels – Decomposition of Austenite”.
1. Which one of the following statements is correct according to following diagram?
a) Austenite is stable above A3 temperature for hypereutectoid steel
b) Austenite is stable above A1 temperature for hypoeutectoid steel
c) Austenite is stable above Acm temperature for eutectoid steel
d) A mixture of ferrite and cementite is thermodynamically stable below A1 temperature
View Answer
Explanation: As we can see in the diagram austenite is stable above Acm, A1 and A3 temperatures for hypereutectoid, eutectoid and hypoeutectoid steels respectively. Below A1 temperature we have a thermodynamically stable mixture of ferrite and cementite.
2. What is the structure of martensite?
a) BCC
b) FCC
c) BCT
d) HCP
View Answer
Explanation: The structure of martensite in Body Centered Tetragonal (BCT). Austenite which has an FCC structure undergoes diffusionless transformation to form martensite. The chemical composition remains the same only the structure changes.
3. What is the structure of bainite?
a) Lamellar
b) Accicular
c) Sphereodized
d) Plate like
View Answer
Explanation: The structure of bainite is accicular i.e. needle like. Pearlite has lamellar structure. Plate like structure is found in martensite to relieve the excessive strain.
4. Which one of the following statements is correct about bainite transformation?
a) Clear cur delineation is seen in TTT diagram of all types of steels
b) The final structure is lamellar
c) It involves diffusion of both carbon and iron
d) It involves diffusion of carbon atoms only
View Answer
Explanation: In most of the steels, the temperature range of bainite overlaps with that of other products. The structure of bainite is accicular. The transformation of bainite involves diffusion of carbon only unlike that of pearlite which involves transformation of both iron and carbon atoms.
5. The austenitic decomposition process is a diffusion controlled process.
a) True
b) False
View Answer
Explanation: The austenite decomposes to form a ferrite-cementite aggregate. It is a diffusion controlled process. It is driven by nucleation and growth mechanism.
6. Which one of the following statements is correct for the homogeneity of austenite?
a) Higher the homogeneity of austenite, higher is the probability of getting an accicular structure
b) Higher the homogeneity of austenite, higher is the probability of getting a sphereodized structure
c) Higher the homogeneity of austenite, higher is the probability of getting a lamellar structure
d) Higher the heterogeneity of austenite, higher is the probability of getting a lamellar structure
View Answer
Explanation: Higher the homogeneity of austenite, higher is the probability of getting a lamellar structure. Greater the heterogeneity, greater is the probability of getting a sphereodized structure.
7. Which one of the following statements is correct about the transformation temperature?
a) Higher the transformation temperature, higher is the chance of getting a harder structure
b) Higher the transformation temperature, higher is the chance of getting a finer structure
c) Lower the transformation temperature, higher is the chance of getting a harder and finer structure
d) Lower the transformation temperature, higher is the chance of getting a coarser structure
View Answer
Explanation: As the transformation temperature decreases, we get a finer and harder aggregate of ferrite and cementite. At lower temperatures, we get a very hard structure of martensite.
8. Which one of the following is not the product of the decomposition of austenite?
a) Martensite
b) Pearlite
c) Bainite
d) Delta ferrite
View Answer
Explanation: The various microstructures that are formed as a result of austenite decomposition are Allotriomorphic ferrite, Widmanstatten ferrite, and intragranular ferrite, Cementite, Pearlite, Bainite and Martensite.
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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