Прочитайте текст The properties of a metal can be further improved by use of heat treatment. Heat tr...: ответ на тест 439344 - 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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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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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?

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Machine-tools are machines designed for cutting metal by means of a cutting tool. The machine-tool comprises the principle manufacturing equipment in a machine shop. It is the original source of every manufactured article we use or touch. It can not only produce itself but it is the only machine which can create other machines.

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The lathe is a machine-tool in which work is held so that it can be rotated about the axis. The cutting tool is traveled past the work from one end to the other. It is designed primarily for turning and boring operations. However, in addition to turning and boring, many other operations may also be performed on a lathe. The lathe is considered to be the oldest but still the most important of all machine-tools. Any shop, containing machines or machine-tools, contains a lathe.