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ТОП 10 на сайтеПриготовление дезинфицирующих растворов различной концентрации
Техника нижней прямой подачи мяча.
Франко-прусская война (причины и последствия)
Организация работы процедурного кабинета
Смысловое и механическое запоминание, их место и роль в усвоении знаний
Коммуникативные барьеры и пути их преодоления
Обработка изделий медицинского назначения многократного применения
Образцы текста публицистического стиля
Четыре типа изменения баланса
Задачи с ответами для Всероссийской олимпиады по праву
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ЗНАЕТЕ ЛИ ВЫ?
Влияние общества на человека
Приготовление дезинфицирующих растворов различной концентрации
Практические работы по географии для 6 класса
Организация работы процедурного кабинета
Изменения в неживой природе осенью
Уборка процедурного кабинета
Сольфеджио. Все правила по сольфеджио
Балочные системы. Определение реакций опор и моментов защемления
Read and translate the following sentences with Participle II.
1) Metals have served man to create new machines.
2) The first definition of metals was given by M.V. Lomonosov.
3) Metals have found wide application because of their properties.
4) There are some classifications of metals based on their properties and composition.
5) An alloy is a simple metal combined with some other elements.
6) Some metals are called refractory metals.
7) Metals are found in the form of ores.
8) There are many methods used to separate metals from ore.
9) Impurities are separated and removed as slag.
10) About two thirds of all elements found in the earth are metals.
11) Cast iron is produced in cupolas.
12) Ore is a metal combined with some impurities.
13) Metals used in industry are called engineering metals.
Read the additional text about metals and retell it in Russian.
There are many superstitions (суеверий) about iron. The Romans (римляне) believed, if you drew (рисовать, проводить) three circles in the air around anyone and three on the ground with a bit (кусочек) of iron, that person would be bewitched (заколдован). It is interesting to note that wedding rings (обручальные кольца) in Ancient Rome were made of iron.
We find a lot of superstitions among the Creeks and the Egyptians. But we do not have to go back to Roman days for curious superstitions and beliefs (поверий). We can find them nowadays (today). There is still a superstition among some people to-day about horseshoes (подковы). They believe that horseshoes bring good luck or bad luck. Though people were superstitions about iron it turned out (оказалось) to be a much better metal for making tools than bronze. And so the Bronze Age gave ways to the Age of Iron.
SUPPLEMENTARY TEXTS FOR READING
Read and translate the following texts using a dictionary. Write down unknown words with their translation. Learn them.
HARDENING AND TEMPERING
Hardening means making harder. Steel which contains more than 0.75 per cent carbon becomes very hard and very brittle when hardened. High carbon steel is hardened by carrying out the following operations. The steel is first slowly heated to a proper temperature. Having been heated to red color, it is then rapidly cooled in oil, water, brine or some other liquid. The process of cooling is called quenching the steel, thus the hardening operation consists of heating and quenching. Alloy steels are usually hardened by special ways. The hardness produced by heat treatment depends upon the: 1) amount of carbon in steel; 2) temperature of heated steel; 3) speed of cooling.
The critical temperature, or critical point, is the temperature at which a piece of steel is properly hardened. When steel is heated to the critical temperature, the grain becomes very fine, that is the crystals get smaller, some other changes in the physical properties of steel take place, too, in this condition. The critical temperature is different for different kinds of steel. It depends upon the amount of carbon in steel. The more carbon the steel contains, the less it should be heated for hardening. In other words, the more carbon the steel contains, the lower is the critical point. The critical temperature may be tested with a magnet. Steel is magnetic until it reaches the critical temperature, then it is non-magnetic. Thus, a piece of steel to be hardened may be heated until it becomes non-magnetic; it should be then rapidly cooled (quenched).
Temper is the hardness, toughness and brittleness of a metal. Tempering, also called drawing the temper, means taking some of the hardness and brittleness out of hardened steel so that it could do good work. Having been tempered, the steel becomes stronger because its grain gets finer; as we already know, steel with a coarse grain is weaker than steel with a fine grain. Hardened steel is too hard and too brittle for many tools. A hardened cutting tool will break easily while cutting with it; it is therefore better to have a cutting tool tough and not too hard. Tempering brings about this condition.
Having hardened the steel, it is polished and heated. The different temperatures will show in the form of different colors on the polished surface. Each temperature or color is a different hardness or temper. The temperatures for tempering are from 400 to 600 degrees of Fahrenheit. When the right color appears, the steel is cooled (the speed of cooling is not important here). Thus the steel is a little softened and will be tougher and less brittle.
STRUCTURE OF METALS
Strength together with plasticity is the combination of properties that makes metals so valuable in industry. In addition to strength and plasticity, metals have many favourable characteristics, such as resistance to corrosion, electrical and heat conductivity, etc.
The characteristics of metals are due to two structural factors: first, the atoms of which the metallic state is composed; and second, the way in which these atoms are arranged.
When a metal becomes solid, it crystallizes. The crystallization or solidification is accompanied by a complete change in the atomic arrangement of the metal. The atoms of liquid metal become arranged into a definite pattern, forming small solid bodies of regular geometric shape such as cubes, etc., when crystallization occurs.
If liquid is cooled slowly in a crucible, nuclei form at the temperature of freezing, and these nuclei continue to grow until the liquid has changed to solid.
Knowing that metals are composed of many crystals or grains, and that each grain in turn is composed of a great many atoms all arranged in some pattern, how can we understand the plastic flow that must take place when metals are deformed during a bending operation or during the drawing out of a piece of a metal? This deformation may be understood as shearing; that is, when a metal is subjected to stresses exceeding its elastic limit, the crystals of the metal elongate by an action of slipping or shearing which takes place within the crystals and between the crystals. If deformation of the metal continuous, the crystals become remarkably elongated. This plastic flow of the metal, resulting in permanent deformation of the crystals is accompanied by marked changes in the physical properties of the metal. The tensile strength, yield point and particularly hardness are increased, but not the scratch hardness or ductility of cutting, as in machine operations in a lathe. The stiffness remains about the same, though in some cases it may be increased as much as 3 per cent. With the increase in hardness and strength, the plasticity or formability is reduced. Ultimately, if deformation of the crystals is continued, the metal becomes brittle. This process changing the physical properties of a metal is called work-hardening.
If the temperature of work-hardened metal is raised above normal, the deformation begins to disappear and the metal returns to the normal condition of structure and properties.
Nonferrous forgings are metal shapes produced by hot-working nonferrous metals, subjecting them to hammering and pressing operations. The result is the compression, bending, twisting or extrusion of the metal so that various parts of the forging are formed by pressure against dies.
Die-pressed or hammered forgings offer an efficient and economical method of producing irregularly-shaped metal parts from slugs cut from ingots and rods. The result is a strong, dense metal part, closely resembling the shape and size of the finished product, thus insuring a minimum of scrap metal in the final procession operation. Such forgings are freed of excess material and are ready for machining or further finishing operations.
Grain structure is uniform and dense, eliminating the disadvantages of porosity and rough surface finish. Nonferrous forgings also have high tensile strengths; the great strength and nonporosity often permit reduction in weight of parts previously produced by other processes. Fewer finishing operations are necessary, and the required machining may be performed with maximum speed. However, greater strength naturally reduces machinability.
Many nonferrous alloys are readily adaptable to the forging process and have been successfully used. Among them are forging brass, nickel, silver, leaded brasses, aluminum bronze, manganese bronze, silicon bronze and several aluminum alloys.
At present, nonferrous forgings are used for many purposes: for electrical and chemical equipment, in welding and in many other cases. Brass forgings have an important part in air compressors, compressed gas valves, gas and water meters, oil burner equipment, etc.
Nonferrous forgings can be provided in a great variety of finishes such as bright polished, plated with nickel, chromium, copper or other metals.
COLD WORKING OF STEEL
Steel is hot worked when it is in a homogeneous or heterogeneous austenitic condition. Hot working is the working of metals above the annealing temperature, so that the deformed metal becomes annealed before cooling to room temperature, and therefore has a normal grain structure with normal ductility and toughness. Cold working passes under quite different conditions; it is generally accompanied by many annealings of the metal being cold-worked.
Cold rolling, for example, like hot rolling may be carried out on a two-high or four-high rolling mill, or in a continuous rolling mill. Cold rolling is continued until the rolled section becomes too hard to continue the process, or until it reaches its final size. It may become necessary to anneal the metal after several passes through the cold rolling mill, in order to keep it in a workable condition. If annealing is carried out in an open furnace, pickling again is necessary before re-rolling to remove the scale and clean the metal. Today annealing of cold-rolled products is usually carried out in special furnaces that complete the annealing without the formation of scale.
Cold drawing is carried out by drawing the metal through a succession of conical tapering holes in a die plate. Die materials are: steel, cast iron, tungsten carbide and diamond. Shapes varying in size from the finest wire to those having a cross-sectional area of several square inches are commonly drawn. Due to the difficulty of making dies and small need for any other form, the fine sizes are drawn only to a round cross-section; larger sizes may be drawn to square, round, or irregular cross-sections, the larger sections being drawn on a drawbench. Metals can be formed to much closer dimensions by drawing than by rolling, and for this reason large quantities of steel and brass are cold drawn.
Multiple die machines are often used to produce wire. In these machines the wire passes through one die, around a capstan, through a second die and round another capstan, etc. As many as twelve dies may be used in a machine. Having gone through each die, the wire of course, is greatly elongated. The speed of drawing in multiple die machines may reach 10 000 feet per minute on fine wire. The die, or drag plate as it is often called, may be made of a number of materials, tungsten carbide having largely replaced other die materials because of its great ability to retain its shape during the drawing operation.
In drawing seamless tubing the metal is forced between the die and the mandrel, and in this way the wall thickness, the outside diameter, and the inside diameter may be controlled. As in any cold-working operation, the metal should be free from scale and other defects before it is cold-drawn.
Cold-working operations may be divided into two large classes:
1. Where the cold working is carried out for the purpose of shaping the articles only; and where the hardening effect is not desired and must be removed at various stages of plastic shaping as well as from the finished article by annealing.
2. Where the object of cold working is not only to obtain the required shape but to harden and strengthen the metal, and where the final annealing operation is not carried out.
In order to shape metals by cold working they must be annealed at proper intervals. Otherwise deformation must be carried out at temperatures where annealing is simultaneous with hardening, as in the hot working of metals. The selected method will depend on the individual metal as well as on the desired product. Metals vary greatly in the ease with which they deform. Copper, for example, may be worked readily at room temperatures, whereas some steels can only be worked at a red heat. Practically all metals and alloys become brittle very near their melting points and hence must not be worked at too high temperatures. There are metals that can be worked only in certain temperature ranges without cracking. Thus zinc must be worked at 200 to 300 degrees of Fahrenheit; nearly pure iron must not be worked in the blue heat range. Brass must not be heated too near its melting point in annealing, also, or it becomes «burnt».
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