This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Metallography”.
1. Removal of hard sample material is done by _________
a) Manual hacksawing
c) Abrasive cutoff wheel
d) Oxy-fuel burning
Explanation: To conduct a microscopic study, a sample must be removed from an area close to the failure. If the material is soft, manual hacksawing is done to cut the sample. If the material is hard, however, abrasive cutoff wheels are employed.
2. What can be done to make the specimen easy to handle?
a) Surface grinding
b) Cylindrical grinding
c) Rough grinding
d) Creep-feed grinding
Explanation: Whenever possible, it is preferred that the size of the specimen must be such that it is easy to handle. For this purpose, the soft and hard specimen may be rough-ground on a belt sander. The rough grinding is continued until the surface is flat and free from irregularities.
3. Which thermosetting resin is used in the mounting of the specimen in metallography?
d) Urea formaldehyde
Explanation: The most common thermosetting resin used for mounting is bakelite. It is available in a variety of colors and helps easily identify the specimen.
4. How much pressure is applied for molding of Bakelite during mounting?
a) 1000 psi
b) 2000 psi
c) 4000 psi
d) 6000 psi
Explanation: Bakelite, along with the specimen, is placed in a mounting press for the mounting process. The temperature is gradually increased to 150oC and a molding pressure of 4000 psi is applied at the same time.
5. Which thermoplastic resin is used for mounting a specimen in metallography?
d) Acetal copolymer polyoxymethylene
Explanation: Lucite is a completely transparent thermoplastic resin which is used for mounting. This transparency helps to observe the exact section that is being polished. Here, these resins do not undergo any curing at the molding temperature.
6. What is the effect of removing the mount while it is still hot?
b) Low strength
c) Rough finish
d) Opaque appearance
Explanation: After the heating coil is removed, the cooling fins are inserted which cool the mount below 75oC in about 7 minutes. Ejecting the mold while it is hot or allowing it to cool slowly causes the mount to turn opaque.
7. Which material is used for intermediate polishing of soft materials?
a) Emery paper
b) Silicon carbide
d) Acetal copolymer polyoxymethylene
Explanation: After mounting, the specimen is polished using emery paper, which is usually done dry. For soft materials, however, silicon carbide is used due to its higher rate of removal.
8. Which of the following is not used for fine polishing of the mount?
a) Aluminum oxide
b) Diamond paste
c) Bauxite tailing
d) Cerium oxide
Explanation: The gamma form of aluminum oxide is widely preferred for fine polishing of ferrous and copper-based materials. Cerium oxide is also used for fine polishing of aluminum, magnesium, and their alloys. Other abrasives used are a diamond paste, chromium oxide, and magnesium oxide.
9. Which polishing cloth is mostly used for polishing of materials?
Explanation: Polishing cloths are chosen carefully based on the types of material and nature of the study. Among synthetic polishing cloths, Gamal and Microcloth are two of the most widely used varieties.
10. What is the final stage of intermediate polishing using emery paper?
Explanation: After mounting, the specimen is polished using emery paper containing fine abrasives. It starts by using No.1 paper, and then progresses to 1/0, 2/0, 3/0, and finally 4/0. Black spots found on magnification are oxide impurities of the specimen or mount.
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