This set of Engineering Materials and Metallurgy Assessment Questions and Answers focuses on “Sintering and Compacting, Pig Iron and its Manufacture”.
1. In powder metallurgy, how is the powder for brittle materials prepared?
Explanation: Pulverization is a process of preparation of powders for metallurgical operations. It can be done using a variety of machines like hammering, counter-rotating plates, or ball milling. Depending on the type of metal and the grain size, an appropriate pulverization technique can be chosen.
2. What is the disadvantage of atomization?
a) Electrical resistivity
c) Poor mechanical strength
d) Coarse grains
Explanation: This process is mostly similar to pulverization but requires special nozzles at controlled temperature and pressure. It has a disadvantage of oxidation of molten metal when in contact with air.
3. Electrolysis usually used for materials like _________
a) Zinc, cadmium
b) Aluminum, nickel
c) Silver, tin
d) Silicon, antimony
Explanation: Since atomization results in oxidation of molten metal as it comes in contact with air, it is only used for materials like zinc and cadmium. Electrolysis is a standard electrolytic process which is used for materials like silver and tin. It is advantageous as it produces oxidation resistant powders.
4. In gravity compaction, the die is made of _________
Explanation: In gravity compaction, the die is filled with loose powder which is then sintered. The die is usually made of an inert material such as graphite. Since pressure is not used, the parts are more porous.
5. Sintering is performed ___________
a) At room temperature
b) Below melting point
c) Above boiling point
d) At cryogenic temperature
Explanation: The sintering process is usually carried out at a temperature below the highest melting constituent. In some cases, the temperature is high enough to form a liquid constituent. In other cases, no melting of any constituent takes place.
6. For cemented carbide cutting tools, the compact is heated at __________
a) 650 F
b) 1200 F
c) 1989 F
d) 2750 F
Explanation: In the production of cemented carbide cutting tools, a mixture of carbides of tungsten, tantalum, and titanium with cobalt as a binder is compacted and pre-sintered. These can be cut, machined, and ground into the final shape. The compact is then subjected to a high-temperature sintering operation at 2750 F.
7. _________ is the process done to reduce the void space in the sintered part.
Explanation: The sintered part is repressed in the die to reduce the void space and impart the required density. This is known as coining. Sizing operation improves the surface finish of the component at the cost of change in density.
8. Impregnation of sintered parts is usually done for _________
a) 2-5 minutes
b) 10-20 minutes
c) 30-50 minutes
d) 80-120 minutes
Explanation: Sintered parts may be impregnated with oil, grease, wax, or other lubrication materials. The parts are immersed in the lubricant and heated to about 93oC. The porous structure gets completely impregnated in 10-20 minutes.
9. The size of pig iron is _________ (in inches).
Explanation: The oldest method of pig casting in sand beds was superseded by pig casting machines. Machine cast pigs are cleaner than sand cast pigs and have no adhering sand to contaminate the remelt process. The size of the pig (iron) for foundry is about 20*9*4 inches in size.
10. How much carbon does pig iron contain?
Explanation: Pig iron is the raw material for all iron and steel products. It is composed of 3-4% carbon, 1-3% silicon, 0.3-0.7% phosphorus, 0.1-1.0% manganese, and less than 1% sulfur. The remaining composition is that of iron.
11. Why is sulfur in pig iron kept low?
a) Steel impurity
b) Electrical resistivity
c) Demagnetizing effect
d) Low coefficient of expansion
Explanation: Basic pig iron must be low in sulfur of about 0.04%. This is since sulfur is an active impurity in steels and is not eliminated in refining furnaces.
12. How much manganese does ferromanganese contain?
Explanation: Ferromanganese and ferrosilicon are two ferroalloys of pig iron, which are used as additives. Ferromanganese contains about 74-82% manganese, whereas ferrosilicon contains 5-17% silicon.
13. The smelting zone of the furnace is called as _________
Explanation: The Bosh is the smelting zone and the hottest part of the furnace. It extends upwards from the tuyere level and has an outward slope of 20 degrees with a height of about 3.6 meters.
14. Which is the iron ore processed in blast furnaces?
Explanation: The iron ore processed in blast furnaces is usually in the form of an oxide. This is either hematite or magnetite. Hematite contains 50-65% iron and is used in a greater proportion than magnetite. The ore pieces are usually less than 100 mm in diameter.
15. _____________ is a flux used in iron smelting.
Explanation: Fluxes must form slags of low viscosity and low density so that the slag will settle freely down through the charge and float in a distinct layer of iron in the hearth. Limestone or dolomite usually acts are the flux in iron smelting. The slag also helps in control of chemical properties of iron.
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