This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Transformation Diagrams”.
1. Non-equilibrium phases are shown for their time and transformation using _________
a) Fe-Fe3C diagram
b) TTT diagram
c) CCT diagram
d) TTT and CCT diagram
View Answer
Explanation: Ferrite, cementite, pearlite, and austenite are equilibrium phases which are based on the iron-iron carbide equilibrium diagram. For other non-equilibrium phases like martensite and bainite, the Fe-Fe3C diagram cannot be used. In such cases, we use the TTT and CCT diagrams.
2. The CCT or the TTT diagrams are used for _________
a) One steel of specific composition
b) A family of various steels
c) Alloy system of various compositions
d) Combination of all alloys and steels with various compositions
View Answer
Explanation: An individual equilibrium diagram like the Fe-Fe3C can be used to depict an entire alloy system of various compositions. However, a TTT or a CCT diagram can only be used for one steel of a specified composition.
3. What does CCT diagram stand for?
a) Constant-critical-temperature
b) Constant-cooling-temperature
c) Continuous-cooling-transformation
d) Continuous-creep-transformation
View Answer
Explanation: Martensite and bainite are non-equilibrium phases which cannot be depicted on a Fe-Fe3C equilibrium diagram. In such cases, either the time-temperature-transformation or the continuous-cooling-transformation diagrams can be used.
4. __________ is used to predict quenching reactions in steels.
a) Isothermal transformation diagram
b) Iron-iron carbide equilibrium diagram
c) Continuous cooling transformation diagram
d) Logarithm scale
View Answer
Explanation: An isothermal transformation (IT) is a tool used by heat treaters to calculate quenching reactions in steels. The IT diagram can also be called as a time-temperature-transformation diagram.
5. Which of the following is not an alternative name for the TTT diagram?
a) S curve
b) C curve
c) Adiabatic curve
d) Bain’s curve
View Answer
Explanation: Owing to the shape of the diagram, TTT diagrams may be called S curves or C curves. Due to their nature and type of work, they may also be known as isothermal transformation diagrams or Bain’s curves.
6. The first step in constructing a TTT diagram involves _________ the sample.
a) Annealing
b) Normalising
c) Quenching
d) Austenising
View Answer
Explanation: To construct a TTT diagram, a large number of the small specimen are collected and austenised in a furnace. Next, heat treatment and quenching stages are carried out. After each stage, the temperature and time are plotted as curves.
7. Austenising of samples for TTT diagram is done __________ temperature.
a) At room temperature
b) Below melting point
c) Above eutectoid temperature
d) Above boiling temperature
View Answer
Explanation: A large number of small samples of the same material are collected and austenized in a furnace above the eutectoid temperature. Then it is rapidly cooled at a desired temperature below the eutectoid temperature.
8. Examination of transformation time after quenching is done ___________
a) At room temperature
b) Below melting point
c) Above eutectoid temperature
d) Above boiling temperature
View Answer
Explanation: Austenising of the sample is done above eutectoid temperature, whereas quenching is done below the eutectoid temperature. After each transformation time, the microstructure is examined at room temperature and plotted in the form of curves.
9. Isothermal transformations of eutectoid steel between 723oC and 550oC produces __________ microstructure.
a) Pearlite
b) Bainite
c) Ferrite
d) Cementite
View Answer
Explanation: Isothermal transformations of eutectoid steel between 723oC and 550oC bring into being a pearlitic microstructure. The pearlite changes from coarse structure to fine structure as the transformation is decreased in this range.
10. Rapid quenching of eutectoid steel ___________ transforms the austenite into martensite.
a) At room temperature
b) Below 320oC
c) At 550oC
d) Above 723oC
View Answer
Explanation: An isothermal transformation of eutectoid steel between 723oC and 550oC gives a pearlitic microstructure. Rapid quenching above 723oC transforms the austenitic condition into martensitic condition.
11. Hot-quenching of eutectoid steels in austenitic condition results in formation of ___________
a) Pearlite
b) Bainite
c) Ferrite
d) Cementite
View Answer
Explanation: If eutectoid steels in austenitic condition are hot-quenched in a 550oC to 250oC range, they are isothermally transformed into bainite. This bainite formed is an intermediate structure between pearlite and martensite.
12. Bainite in iron-carbon alloys has a ___________ structure.
a) Dendritic
b) Non-lamellar
c) Linear
d) Hexahedral
View Answer
Explanation: Bainite in iron-carbon alloys can be defined as an austenitic decomposition product. It has a non-lamellar eutectoid structure containing α ferrite and cementite (Fe3C).
13. Lower bainite is formed at ___________ temperature range.
a) 750-550oC
b) 550-350oC
c) 350-250oC
d) 250-150oC
View Answer
Explanation: For eutectoid steels, bainite exists in two forms known as upper and lower bainite. Upper bainite is formed by the isothermal transformation between 550-350oC. Lower bainite is formed between 350-250oC which has much finer cementite particles.
14. Which of the following factors do not affect the critical cooling rate?
a) Chemical composition
b) Hardening temperature
c) Number or nature of grains
d) Purity of steel
View Answer
Explanation: The slowest rate of cooling of austenite that results in 100% martensite transformation is called a critical cooling rate. This depends on the chemical composition, hardening temperature, and metallurgical nature (purity) of steel.
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