2. Divide the text into logical parts, and make up a plan of each for giving a summary of the whole of the text.

3. Make up an abstract of the text «Hardening» in writing.

4. Prepare a report on the topic «The Heat Treatment Proc­esses» taking in Units 25-28. Use the word combinations from Chapter 3, Appendix 4 «English for Scientific Discussions».

Unit 29

POWDER METALLURGY

The traditional methods of shaping metals by casting or cold working are difficult and sometimes impossible to ap­ply to many metals. Such refractory metals include tungsten (m. p. 3380 °C), molybdenum (2622 °C) and tantalum (2996 °C) whose melt­ing points are too high to enable them to be melted by conventional means. In these cases an alternative procedure has emerged based on the fact that metals in powdered form may be caused to adhere together without being melted by employment of high pressure. The technique known as powder metallurgy consists in subjecting the powdered metal contained in a mould or die of the shape desired to a high pres­sure followed by sintering at a suitable temperature. The method pro­vides either finished metal components or compact blocks of metal for subsequent mechanical working. Having been applied first to the re­fractory metals the method has been extended to many of the more tractable metals.

The technique came into being more than a century ago. The preparation of compact platinum from the then infusible metal by W.H. Woiiaston in 1830 represents one of the earliest applications of powder metallurgy.

The production of powder of the requisite properties is an im­portant stage in the procedure or powder metallurgy. Powders of met­als and alloys may be produced by mechanical methods such as grind­ing, machining and milling; other metal powders can be obtained by reduction of the metal oxide by hydrogen or carbon. Copper, iron, co-

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bait, molybdenum and tungsten can also be so prepared in a powder form. Electrolysis is also used, and aluminium, tin, and lead are trans­formed into the powder form by atomization, molten metal being poured through an orifice into a chamber and sprayed with a high pressure jet of inert gas, the instantaneous chilling converting the metal into a finely di­vided dust.

At present powder metallurgy is mainly used in making large numbers of identical components usually of relatively small size, such as permanent magnets, coins, medals, small gear wheels, and brushed for motors and dynamos. A novel extension of its application is in the manufacture of the oil-less bearing, which can be impregnated with oil and made self-lubricating. Such bearings are designed to retain within their strucnire a sufficient amount of oil to last for several years.

Exercises

1. Answer the following questions

1. Why are the traditional methods of shaping metals sometimes impossible to apply?

2. What do the refractory metals include?

3. Is it possible to melt them by conventional means? Why?

4. What is the alternative procedure based upon?

5. What does the technique of powder metallurgy consist in?

6. What does this method provide in the long run?

7. When was the technique of powder metallurgy applied first?

8. When did the technique come into being?

9. What is important in the procedure of powder metallurgy?

10. How can the powder of metals and alloys be produced or ob­tained?

11. What other means are used to prepare powders?

12. Where is powder metallurgy mainly used at present?

2. Find in the text «Powder Metallurgy» synonyms for the following

1. Hard to melt, fuse or work

2. To make able (or possible) to do sth.

3. Ordinary

4. Use

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5. To make its origin or rise

6. Manufacture

7. Necessary; desired

8. To change; to transform

9. Today; nowadays

10. The same; exactly similar

11. Of a new kind; new

12. Enough

3. Give the names to the following definitions

1. The art of producing metal powders and utilizing metal pow­ders for the production of massive materials and hard objects.

2. Metals which are hard (difficult) to melt, fuse for work.

4. Retell the text «Powder Metallurgy» using the answers to the questions of 1 as a plan.

5. Read the following text

1. In recent years there has been a marked extension in the ap­plication of hydrometallurgical processes to low grade ores due mainly to the introduction of new techniques such as ion-exchange, solvent extraction and high temperature and high pressure methods. Ion-exchange used for the softening of water has found employment in the purification and enrichment of the solution resulting from the acid leach of uranium ore. Solvent extraction widely employed in the petro­leum industry is now used in the separation and recovery of uranium, tantalum, etc. High pressure and high temperatures have long been util­ized in the chemical industry, but it is only guite recently that their use­fulness has been recognized in connection with hydrometallurgical processes. The first plant employing high pressure and temperature was initiated in Canada at Fort Saskatchewan in 1953 for the treatment of nickel-copper-cobalt sulphide ore, since then it has been followed by similar plants in America and Cuba.

2. Tungsten (m. p. 3380 °C) is another metal which was devel­oped through the advent of powder metallurgy. As obtained by reduc­tion of its oxide, tungsten is a grey brittle powder. Because of its high Welting point, it had for long been recognized that the metal would be eminently suitable for electric lamp filaments.

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