Прочитайте текст The properties of a metal can be further improved by use of heat treatment. Heat tr...: ответ на тест 607568 - Английский язык

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The properties of a metal can be further improved by use of heat treatment. Heat treatment is the term given to number of different procedures in which the properties of metals and alloys are changed. It usually consists of heating the metal or alloy to a selected temperature below its melting point and then cooling it at a certain rate to obtain those properties which are required. For example, hardening is used to make metals harder. Tempering makes them softer and less brittle. Annealing is carried out to make a metal soft so that it can be machined more easily. In this way, metallic materials can be produced to meet every kind of engineering specification and requirement.
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What makes metals softer and less brittle?

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The properties of a metal can be further improved by use of heat treatment. Heat treatment is the term given to number of different procedures in which the properties of metals and alloys are changed. It usually consists of heating the metal or alloy to a selected temperature below its melting point and then cooling it at a certain rate to obtain those properties which are required. For example, hardening is used to make metals harder. Tempering makes them softer and less brittle. Annealing is carried out to make a metal soft so that it can be machined more easily. In this way, metallic materials can be produced to meet every kind of engineering specification and requirement.
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What makes metals softer and less brittle?

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Steel, copper and aluminium are used in making engineering products.
Steel is a grey plastic material. It is heavier than aluminium but it is the hardest and the strongest of these three metals.
Copper is a red metal. It is harder and stronger than aluminium and more flexible than steel. But copper is the heaviest and the most expensive metal. The lightest and the most flexible of these materials is aluminium. It is a white hard metal.
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What kind of metal is aluminium?

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We use metals for a variety of engineering purposes. Some metals have ductile properties. If a metal is ductile, it can be pressed into a new shape. Copper is an example of this. It is a ductile metal because it can be stretched into another shape. Lead can be also pressed into a new shape.
Copper is a good conductor of electricity. We often use it for electrical conductors. And lead can be used for electrical cables because it resists corrosion.
Cast iron is unlike copper and lead. Its shape cannot be altered and it is not able to resist corrosion.
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What kind of conductor is copper?

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Many people have just returned from enjoyable holiday in the South West of England. But as well as being a major tourist attraction, this region also contains a healthy industry which provides a wide range of machining services.
The company set up in 1999 is a family-run business. It specializes in the machining of various precision parts.
Two years ago the company bought a new machining centre. ‘Much of our work involves machining different sizes of parts and producing large numbers of holes’, says the managing director. ‘This machine centre will help us become more efficient and reduce the level of skill required for certain key operations.’
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When was the company set up?

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The Egyptian pyramids are huge structures which are almost 4600 years old. The Pyramid of Cheops consists of 2300000 stones. Each of the stones weighs 2.5 tonnes.
According to the usual explanation large blocks of stone were cut in the mountains and then transported with the help of the most primitive equipment. So about 100 thousand people had to take part in the construction.
But the modern experiments show that the stones of pyramids were made synthetically. The Egyptians produced some special solution. Later they mixed it with one of the minerals. It took them several hours to transform this mixture into very hard rock. This rock was produced just on the construction sites and so there was no need of transporting heavy stones.
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How much does each of the stones weigh?