Methods of steel heat treatment

ВАРИАНТ №19

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MODERN ENGINEERING TRENDS

Among various recent trends in the engineering profession computerization is the most widespread. Computers are widely used for solving complex problems as well as for handling, storing, and generating the enormous volume of data modern engineers must work with.

Engineers in industry work not only with machines but also with people, to determine, for example, how machines can be operated most efficiently by workers. This is called ergonomics. The aim of ergonomics is to make the working place more comfortable and the work itself easier. A small change in the location of the controls of a machine or of its position with relation to other machines or equipment, or a change in the muscular movements of the operator, often results in greatly increased production. Ergonomics looks for criteria for the efficient design of large, complicated control panels that monitor nuclear reactor operation.

Metals

Metals are materials most widely used in industry because of their properties. The study of the production and properties of metals is known as metallurgy.

The separation between the atoms in metals is small, so most metals are dense. The atoms are arranged regularly and can slide over each other. That is why metals are malleable (can be deformed and bent without fracture) and ductile (can be drawn into wire). Metals vary greatly in their properties. For example, lead is soft and can be bent by hand, while iron can only be worked by hammering at red heat.

The regular arrangement of atoms in metals gives them a crystalline structure. Irregular crystals are called grains. The properties of the metals depend on the size, shape, orientation, and composition of these grains. In general, a metal with small grains will be harder and stronger than one with coarse grains.

Heat treatment controls the nature of the grains and their size in the metal. Small amounts of other metals (less than l per cent) are often added to a pure metal. This is called alloying (легирование) and it changes the grain structure and properties of metals.

All metals can be formed by drawing, rolling, hammering and extrusion, but some require hot-working. Metals are subject to metal fatigue and to creep (the slow increase in length under stress) causing deformation and failure. Both effects are taken into account by engineers when designing, for example, airplanes, gas-turbines, and pressure vessels for high-temperature chemical processes. Metals can be worked using machine-tools.

The ways of working a metal depend on its properties. Many metals can be melted and cast in moulds, but special conditions are required for metals that react with air.

STEEL

The most important metal in industry is iron and its alloy - steel. Steel is an alloy of iron and carbon. It is strong but corrodes easily through rusting, although stainless and other special steels resist corrosion. The amount of carbon in steel influences its properties considerably. Steels of low carbon content (mild steels) are quite ductile and are used in the manufacture of sheet iron, wire and pipes. Medium-carbon steels containing from 0.2 to 0.4 per cent carbon are tougher and stronger and are used as structural steels. Both mild and medium-carbon steels are suitable for forging and welding. High-carbon steels contain from 0.4 to 1.5 per cent carbon, are hard and brittle and are used in cutting tools, surgical instruments, razor blades and springs. Tool steel, also called silver steel, contains about 1 per cent carbon and is strengthened and toughened by quenching and tempering.

The inclusion of other elements affects the properties of the steel. Manganese gives extra strength and toughness. Steel containing 4 per cent silicon is used for transformer cores or electromagnets because it has large grains acting like small magnets. The addition of chromium gives extra strength and corrosion resistance, so we can get rust-proof steels. Heating in the presence of carbon or nitrogen-rich materials is used to form a hard surface on steel (case-hardening). High-speed steels, which are extremely important in machine-tools, contain chromium and tungsten plus smaller amounts of vanadium, molybdenum and other metals.

METHODS OF STEEL HEAT TREATMENT

Quenching is a heat treatment when metal at a high temperature is rapidly cooled by immersion in water or oil. Quenching makes steel harder and more brittle, with small grains structure.

Tempering is a heat treatment applied to steel and certain alloys. Hardened steel after quenching from a high temperature is too hard and brittle for many applications and is also brittle. Tempering, that is re-heating to an intermediate temperature and cooling slowly, reduces this hardness and brittleness. Tempering temperatures depend on the composition of the steel but are frequently between 100 and 650°C. Higher temperatures usually give a softer, tougher product. The colour of the oxide film produced on the surface of the heated metal often serves as the indicator of its temperature.

Annealing is a heat treatment in which a material at high temperature is cooled slowly. After cooling the metal again becomes malleable and ductile (capable of being bent many times without cracking).

All these methods of steel heat treatment are used to obtain steels with certain mechanical properties for certain needs.

НОТ WORKING OF STEEL

An important feature of hot working is that it provides the improvement of mechanical properties of metals. Hot-working (hot-rolling or hot-forging) eliminates porosity, directionality, and segregation that are usually present in metals. Hot-worked products have better ductility and toughness than the unworked casting. During the forging of a bar, the grains of the metal become greatly elongated in the direction of flow. As a result, the toughness of the metal is greatly improved in this direction and weakened in directions transverse to the flow. Good forging makes the flow lines in the finished part oriented so as to lie in the direction of maximum stress when the part is placed in service.

The ability of a metal to resist thinning and fracture during cold-working operations plays an important role in alloy selection. In operations that involve stretching, the best alloys are those which grow stronger with strain (are strain hardening) — for example, the copper-zinc alloy, brass, used for cartridges and the aluminum-magnesium alloys in beverage cans, which exhibit greater strain hardening.

Fracture of the workpiece during forming can result from inner flaws in the metal. These flaws often consist of nonmetallic inclusions such as oxides or sulfides that are trapped in the metal during refining. Such inclusions can be avoided by proper manufacturing procedures.

The ability of different metals to undergo strain varies. The change of the shape after one forming operation is often limited by the tensile ductility of the metal. Metals such as copper and aluminum are more ductile in such operations than other metals.

II.Переведите на русский язык следующие английские словосочетания:

1) crystalline structure 6) arrangement of atoms

2) heat treatment 7) lead is soft

3) complex problems 8) the efficient design

4) most efficiently 9) the ability to resist

5) the aim of ergonomics 10) good forging

III.Найдите в тексте английские эквиваленты следующих словосочетаний:

1) свойства металлов 6) пригодны для ковки и сварки

2) расстояние между атомами 7) нержавеющая сталь

3)низкое содержание углерода 8)необработанная отливка

4) конструкционные стали 9) правильное расположение

5)расстояние между атомами 10)улучшение механических

свойств металла

IV.Найдите в тексте слова, имеющие общий корень с данными словами. Определите, к какой части речи они относятся, и переведите их на русский язык:

Engineer, computer, wide, treatment, product, hard, weak, strong, tough, brittle.

V.Задайте к выделенному в тексте предложению все типы вопросов: общий, альтернативный, разделительный, два специальных: а) к подлежащему, б) к любому члену предложения.

VI.Выполните анализ данных предложений, обратив внимание на следующие грамматические явления: формы глаголовto be, to have; оборот there is/are; степени сравнения прилагательных и наречий; множественное число существительных; Present, Past, Future Simple Active/Passive; модальные глаголы.

1. All these methods of steel heat treatment are used to obtain steels with certain mechanical properties for certain needs.

2. Higher temperatures usually give a softer, tougher product.

3. The most important metal in industry is iron and its alloy - steel

4. For example, lead is soft and can be bent by hand, while iron can only be worked by hammering at red heat.

5. In general, a metal with small grains will be harder and stronger than one with coarse grains.

VII.Ответьте на вопросы по текстам: