This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Iron and Iron Carbide System”.
1. The eutectic point in the iron-iron carbide phase diagram occurs at __________ weight % composition of carbon.
Explanation: The iron-iron carbide equilibrium diagram is constructed by plotting the composition of carbon against the temperature. At 4.3 weight % of carbon, eutectic point is formed.
2. At what temperature does δ ferrite melt?
a) 1674 F
b) 1990 F
c) 2541 F
d) 2800 F
Explanation: δ ferrite is formed as a BCC phase due to heating of austenite at 2541 F. This δ ferrite finally melts at 2800 F and these changes are plotted on the iron-iron carbide equilibrium diagram.
3. What is the solubility of α ferrite at 0oC?
Explanation: α ferrite is an interstitial solid solution of carbon in a BCC crystal lattice. It has a solubility of 0.02% at 723oC and decreases to 0.005% at 0oC.
4. What is the crystal structure of austenite upon heating?
a) Body-centered cubic
b) Face-centered cubic
c) Hexagonal closely packed
d) Body-centered tetrahedral
Explanation: At room temperature, ferrite exists as a BCC crystal as seen on the iron-iron carbide equilibrium diagram. Upon heating, this changes into austenite in FCC form at 1674 F.
5. How much iron does cementite contain?
Explanation: Cementite is an intermetallic compound of iron-carbon (Fe3C). It has a solubility that is negligible with a composition of 6.67% C and 93.3% Fe.
6. What is the maximum solubility of carbon in austenite at 1148oC?
Explanation: Austenite is an FCC crystal with a solubility of 2.08% at 1148oC which decreases to 0.8% at 723oC. δ ferrite has a maximum solid solubility of 0.09% at 1465.
7. At what temperature does peritectic reaction occur?
Explanation: In a peritectic reaction, 0.53% C of liquid combines with 0.09% C of ferrite at 1495oC. This results in the formation of ϒ austenite (0.17% of C).
8. Peritectic reactions are applicable for carbon contents up to _________
Explanation: In a peritectic reaction, 0.53% C of liquid combines with 0.09% C of ferrite at 1495oC. This results in the formation of ϒ austenite (0.17% of C). This reaction affects only solidification of steels with less than 0.55% carbon.
9. How much austenite does eutectic reaction form?
Explanation: In a eutectic reaction, 4.7% C of liquid at 1148oC forms ϒ austenite (2.08% of C) and Fe3C (6.67% of C). This reaction holds great importance for cast irons.
10. At what temperature does eutectoid reaction occur?
Explanation: In a eutectoid reaction, 0.8% C of ϒ austenite at 723oC forms α ferrite (0.02% of C) and Fe3C (6.67% of C). This reaction holds great importance for heat treatment of steels.
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