This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Iron-Carbon System and their Alloys”.
1. Alloys containing 2.0-6.7% carbon are considered as __________
Explanation: Due to availability, low-cost, and their properties, iron and steel are used in more than 90% of the total usage of metals in the world. Steels are those alloys containing 0.06-2.0% carbon, whereas cast-irons contain 2.0-6.7% carbon.
2. The existence of two or more crystal structures for any substance, depending on temperature, is known as _________
Explanation: Allotropy is defined as the possibility of the existence of two or more different crystal structures for a substance depending on temperature. This phenomenon is known as polymorphism. Pure iron is an allotropic metal.
3. Pure iron exists in ________ allotropic forms.
Explanation: Allotropy is defined as the possibility of the existence of two or more different crystal structures for a substance depending on temperature. Pure iron is an allotropic metal that exists in three forms. These allotropic forms are α iron, ϒ iron, and δ iron.
4. What is the crystal structure of ϒ iron?
a) Body-centered cubic
b) Face-centered cubic
c) Hexagonal closely packed
d) Body-centered tetrahedral
Explanation: Pure iron exists in three allotropic phases of α iron, ϒ iron, and δ iron. α iron and δ iron appear as body-centered cubic, whereas ϒ iron is a face-centered cubic that is stable between 908oC and 1535oC.
5. At what temperature range is δ iron stable?
a) Up to 908oC
Explanation: Pure iron exists in three allotropic phases of α iron, ϒ iron, and δ iron. δ iron is a body-centered cubic that is stable around 1388-1535oC. α iron and δ iron are stable up to 908oC and between 908-1535oC respectively.
6. Which of the following is Not applicable to α iron?
d) Can be hot worked
Explanation: α iron is one of the three available allotropic forms of pure iron. It is also known as a ferrite. This form of iron is soft, ductile, highly magnetic, and can be extensively cold worked.
7. Which of the following is a nonmagnetic iron?
Explanation: Austenite, commonly known as ϒ iron, is an allotropic form of pure iron. It is soft, tough ductile, and nonmagnetic (paramagnetic), but has a better electrical resistance and coefficient of expansion than that of ferrite.
8. Cementite is magnetic up to ________
Explanation: Cementite (Fe3C) is an intermetallic compound of iron containing 6.69% of carbon. It is hard, brittle, and is magnetic below 250oC.
Explanation: Sorbite is the microstructure containing ferrite and cementite. This is produced by tempering martensite above 450oC. Sorbite steels are also known as toughened steels owing to their tensile and yield strengths being high.
10. ___________ is a mixture of radial lamellae of ferrite and cementite.
Explanation: Troostite is a microstructure containing ferrite and finely divided cementite. This is produced by tempering martensite below 450oC. In other words, it is a mixture of radial lamellae of ferrite and cementite. It differs from pearlite only due to the degree of fineness.
Explanation: Pearlite is the eutectoid mixture containing ferrite in 87.5% and cementite in 12.5%. It forms due to the decomposition of austenite during cooling. This pearlite is strong, hard, and brittle.
12. ___________ is a eutectic mixture of α iron and Fe3C.
Explanation: Ledeburite is known as the eutectic mixture of α iron (austenite) and Fe3C (cementite) containing 4.3% carbon. It forms at 1140oC in the pure iron-carbon alloy.
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