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Answer: For examination under microscope metal surface should be free from scratches. It should have mirror finish. This is achieved by polishing the surface in stages. This consists of polishing by belt grinder followed by manual polishing using different grades (0, 00, 000)of emery papers from coarse to fine. For hard metals like steel dry polishing is good enough. For soft metals like aluminum you need wet emery paper. Take care to wash the sample after each stage. If the sample is too small to grip by hand it may be mounted (fixed) in Bakelite or sample holder. After emery paper finish move over to wet polishing on cloth mounted on rotating disc. The cloth is kept soaked in water having fine suspension of polishing powder. After this if you look under microscope you would see a shining flat surface. Inclusions (or graphite in cast iron) if present may be visible. You need to etch the sample to see other fine details like grain structure. Use nital (2% HNO3 in alcohol)to etch steel.

Answer: For examination under microscope metal surface should be free from scratches. It should have mirror finish. This is achieved by polishing the surface in stages. This consists of polishing by belt grinder followed by manual polishing using different grades (0, 00, 000)of emery papers from coarse to fine. For hard metals like steel dry polishing is good enough. For soft metals like aluminum you need wet emery paper. Take care to wash the sample after each stage. If the sample is too small to grip by hand it may be mounted (fixed) in Bakelite or sample holder. After emery paper finish move over to wet polishing on cloth mounted on rotating disc. The cloth is kept soaked in water having fine suspension of polishing powder. After this if you look under microscope you would see a shining flat surface. Inclusions (or graphite in cast iron) if present may be visible. You need to etch the sample to see other fine details like grain structure. Use nital (2% HNO3 in alcohol)to etch steel.

Answer: Primary cementite forms in iron carbon system when carbon content is more that ~4.3% (eutectic point). Since steel has much less carbon it can not have any primary cementite. Secondary cementite forms as the steel / cast iron cools from eutectic temperature ~1130C to eutectoid temperature ~723C. Most commercial steels have carbon content much lower than eutectoid composition. Therefore they too will not have any secondary cementite. Tertiary cementite precipitates as the steel / cast iron cools from eutectoid temperature to room temperature. This occurs because of drop in solubility of carbon in ferrite from ~0.02 to 0.002%C.Since the amout of carbon that comes out of ferrite is very small the amount of tertiary cementite in steel will also be very small (<<1%).Bulk of the cementite is steel is present within Pearlite as Cm plates. %Cm in Pearlite is ~14%.

Answer: The reason lies in the way the atoms are arranged in the respective lattice. In BCC packing density is 68% as against 74% in FCC. In FCC the octahedral and tetrahedral sites are enclosed by equilateral triangles whereas in BCC it is not so. Please refer to lecture 3 & 4 for the methods of estimating the size of the gaps. The octahedral site in BCC is asymmetric. The minimum ratio is 0.155. However the tetrahedral site is symmetric and the ratio is 0.291. In FCC these are 0.414 for octahedral & 0.155 for tetrahedral sites.

Answer: You may refer to Engineering Physical Metallurgy by Y Lakhtin, Mir Publishers avlable in India through CBS Publishers & Distributors.