Тексты по профессиональному английскому языку
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WHAT IS ENGINEERING?
Is general, engineering is a science that deals with design, construction and operation of structures, machines, engines and other devices. Engineer is a person who has received technical education and has a basic knowledge of other engineering fields, because most engineering problems are complex and interrelated. The term engineering is difficult to translate into Russian because it has a lot of meanings. Most often it is translated as: инженерное дело, техника, машиностроение, строительство. There exist the following main branches of engineering:
Civil Engineering (Гражданскоестроительство)
Civil engineering deals with the design of large buildings, roads, bridges, dams, canals, railway lines, airports, tunnels and other constructions. A civil engineer must have a thorough knowledge of the properties and mechanics of construction materials, the mechanics of structures and soils, and of hydraulics and fluid mechanics. Among the main subdivisions in this field are constructions engineering (строительство), transports engineering (дорожный транспорт) and hydraulic engineering (гидротехника).
Mechanical Engineering (Машиностроение)
Engineers in this field design, test, build, and operate machinery of all types. The field is divided into:
(1) Machine — tools, mechanisms, materials, hydraulics and pneumatics
(2) heat as applied to engines, work and energy, heating, ventilation, and air conditioning. A mechanical engineer must be trained in mechanics and hydraulics, metallurgy and machine design. A mechanical engineer designs not only the machines that make products but the products themselves.
Electrical and Electronics Engineering (ЭлектротехникаиЭлектроника)
This is the widest field of engineering, concerned with systems and devices that use electric power and signals. Among the most important subjects in the field are electric power and machinery, electronic circuits, control systems, computer design, superconductors, solid — state electronics, and fibre optics.
Electrical engineering can be divided into four main branches: electric power and machinery, electronics, communications and control, and computers.
Electric Power and Machinery(Энергетикаиэнергомашиностроение)
Engineers working in this field design and operate systems for generating, transmitting, and distributing electric power. Several important developments appeared in this field. One of these is the ability to transmit power at extremely high voltages in both the direct current (DS) and alternating current (AC) modes, reducing power losses. Another is the real — time control of power generation, transmission and distribution, using computers.
Electronic engineering (Электроника)
Electronic engineering deals with the research, design and application of circuits and devices used in the transmission and processing of information.
The revolution in electronics is the trend towards integrating electronic devices on a single tiny chip of silicon or some other semi conductive material. Much of the research in electronics is directed towards creating even smaller chips, faster switching of components, and three — dimensional integrated circuits.
CommunicationsandControl (Техника средств связи и управление)
Engineers in this field work on control systems and communication systems that are used widely in aircraft and ships, in power transmission and distribution, in automated manufacturing and robotics.
Major developments in this field are the replacement of analogue systems with digital systems and copper cables with fibre optics lowers interference, has large carrying capacity, and is extremely light and inexpensive to manufacture.
Computers engineering (Компьютернаятехника)
Computer engineering is now the most rapidly growing field. Computer engineers design and manufacture memory systems, central processing units and peripheral devices. Major developments in this field are microminiaturization (design of Very Large Scale Integration (VLSI) chips) and new computer architectures. Using VLSI, engineers try to place greater numbers of circuit elements onto smaller chips. Another trend is towards increasing the speed of computer operations through the use of parallel processors and superconducting materials.
Aeronautical and Aerospace Engineering (Авиакосмическаятехника)
Aeronautics deals with the whole field of design. Manufacture, maintenance, testing, and use of aircraft for both civil and military purposes. It involves the knowledge of aerodynamics, structural design, propulsion engines, navigation, communication, and other related areas.
Aerospace engineering is closely connected with aeronautics, but is concerned with the flight of vehicles in space, beyond the earth’s atmosphere, and includes the study and development of rocket engines, artificial satellites, and spacecraft for the exploration of outer space.
Naval Engineering (Кораблестроение)
Naval architects are engineers who design and built so that they are safe, stable, strong, and fast enough to perform the type of work intended for them. A naval architect must be familiar with the variety of techniques of modern shipbuilding.
Marine engineering is a specialized branch of mechanical engineering devoted to the design and operation of systems, both mechanical and electrical, needed to propel a ship. Engineers in this field develop diesel engines and steam turbines that provide enough power to move the ship at the required speed.
Chemical Engineering(Химическоемашиностроение)
This branch of engineering is concerned with the design, construction, and management of factories in which the essential processes consist of chemical reactions. The task of the chemical engineer is to select and specify the design that will best meet the particular requirements of production and the most appropriate equipment for the new applications.
Nuclear Engineering (Ядернаятехника)
This branch of engineering is concerned with the design and construction of nuclear reactors. In addition to designing nuclear reactors that yield specified amounts of power, nuclear engineers develop the special materials necessary to withstand the high temperatures and radioactivity. Nuclear engineers also develop methods to shield people from the harmful radiation by nuclear reactors.
Safety Engineering (Техникабезопасности)
This field of engineering has as its object the prevention of accidents. Safety engineers develop methods and procedures to safeguard workers of hazardous occupations. They also assist in designing machinery, factories, ships, and roads, suggesting alterations and improvements to reduce accidents.
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Modern Engineering Trends
Among various recent trends in the engineering profession computerization is the mopst 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 mace 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.
George Stephenson
George Stephenson was a British inventor and engineer. He is famous for building the first practical railway locomotive.
Stephenson was born in 1781 in England. During his youth he worked as a fireman and later as an engineer in the coal mines of Newcastle. He invented one of the first miner’s safety lamps independently of the British inventor Humphrey Davy. Stephenson’s early locomotives were used to carry loads in coal mines, and in 1823 he established a factory at Newcastle for their manufacture. In 1829 he designed a locomotive known as the Rocket, which could carry both loads and passengers at a greater speed than any locomotive constructed at that time. The success of the Rocket was the beginning of the construction of locomotives and the laying of railway lines.
Robert Stephenson
Robert Stephenson, the son of George Stephenson was a British civil engineer. He is mostly well — known for the construction of several notable bridges.
He was born in 1803. He was educated in Newcastle and at the University of Edinburgh. In 1829 he assisted his father in constructing a locomotive known as the Rocket, and four years later he was appointed construction engineer of the Birmingham and London Railway, completed in 1838.
Stephenson built several famous bridges, including the VictoriaBridge in England, the BritanniaBridge in Wales, two bridges across the Nice in Egypt and the VictoriaBridge in Montreal, Canada.
James Watt
James Watt, a famous inventor and mechanical engineer, was born on January 19, 1736, in Scotland. He worked as a mathematical-instrument maker from the age of 19 and soon became interested in improving the steam engine, which was used at that time to pump out water from mines.
Watt is known for his improvements of the steam engine. He determined the properties of steam, especially the relation of its density to its temperature and pressure, and designed a separate condensing chamber for the steam engine that prevented large losses of steam in the cylinder. For this device and other improvements on steam engine Watt received his first patent in 1769.
Watt continued his research and patented rotary engine for driving various types of machinery; the double-action engine, in which steam is admitted alternately into both ends of the cylinder; and the steam indicator, which records the steam pressure in the engine. He retired from the firm manufacturing steam engines in 1800 and devoted himself entirely to research work.
In 1788 Watt invented the centrifugal or flyable governor, which automatically regulated the speed of an engine. It uses the feedback principle of a servomechanism, linking output to input, which is the basic concept of automation. The watt, the unit of power, was named in his honor.
Questions:
What was James Watt?
What were the inventions made by him for which he received patents?
James Prescott Joule
James Prescott Joule, famous British physicist, was born in 1818, in England.
Joule was one of the most outstanding physicists of his time. He is best known for his research in electricity and thermodynamics. In the course of his investigations of the heat emitted in an electrical circuit, he formulated the law, now known as Joule’s law of electric heating. This law states that the amount of heat produced each second in a conductor by electric current is proportional to the resistance of the conductor and to the square of conservation of energy in his study of the conversion of mechanical energy into heat energy.
Joule determined the numerical relation between heat and mechanical energy, or the mechanical equivalent of heat, using many independent methods. The unit of energy, called the joule, is named after him. It is equal to 1 watt-second. Together with the physicist William Thomson (Baron Kelvin), Joule found that the temperature of a gas falls when it expands without doing any work. This phenomenon. which became known as the Joule-Thomson effect, lies in the operation of modern refrigeration and air -conditioning systems.
Famous Russian Scientists
M.V. Lomonosov (1711-1765)
Mikhail Vasilyevich Lomonosov was a famous Russian writer, chemist and astronomer who made a lot in literature and science.
Lomonosov was born November 19, 1711, in Denisovka (now Lomonosov), near Archangelsk, and studied at the University of the Imperial Academy of Sciences in St. Petersburg. After studying in Germany at the Universities of Marburg and Freiberg, Lomonosov returned to St. Petersburg in 1745 to teach chemistry and built a teaching and research laboratory there four years later.
Lomonosov is often called the founder of Russian science. He was an innovator in many fields. As a scientist he rejected the phlogiston theory of matter commonly accepted at the time and he anticipated the kinetic theory of gases. He regarded heat as a form of motion, suggested the wave theory of light, and stated the idea of conservation of matter. Lomonosov was the first person to record the freezing of mercury and to observe the atmosphere of Venus.
Interested in the development of Russian education, Lomonosov helped to found MoscowStateUniversity in 1755, and in the same year he wrote a grammar that reformed the Russian literary language by combining Old Church Slavonic with modern language. In 1760 he published the first history of Russia. He also revived the art of Russian mosaic and built a mosaic and colouredglass factory. Most of his achievements, however, were unknown outside Russia. He died in St. Petersburg on April 15, 1765.
D.I.Mendeleyev (1834-1907)
Dmitry Ivanovich Mendeleyev is a famous Russian chemist. He is best known for his development of the periodic table of the properties of the chemical elements. This table displays that elements’ properties are changed periodically when they are arranged according to atomic weight.
Mendeleyev was born in 1834 in Tobolsk, Siberia. He studied chemistry at the University of St. Petersburg, and in 1859 he was sent to study at the University of Heidelberg. Mendeleyev returned to St. Petersburg and became Professor of Chemistry at the Technical Institute in 1863. He became Professor of General Chemistry at the University of St. Petersburg in 1866. Mendeleyev was a well — known teacher, and, because there was no good textbook in chemistry at that time, he wrote the two volume «Principles of a Chemistry» which became a classic textbook in chemistry. In this book Mendeleyev tried to classify the elements according to their chemical properties. In 1869 he published his first version of his periodic table, in which he left gaps for elements that were proved later when three predicted elements: gallium, germanium, and scandium were discovered.
Mendeleyev investigated the chemical theory of solution. He found that the chemical theory of and water in vodka is 40%. He also investigated the thermal expansion petroleum.
In 1893 he became director of the Bureau of Weights and Measures in St. Petersburg and held this position until his death in 1907.
Questions:
What is main principle of periodic table of elements?
What was the reason of writing the textbook «Principles of Chemistry»?
How Materials React To External forces
Materials Science and Technology is the study of materials and how they can be fabricated to meet the needs of modern technology. Using the laboratory techniques and knowledge of physics, chemistry, and metallurgy, scientists are finding new ways of using metals, plastics and materials.
Engineers must know how materials respond to external forces, such as tension, compression, torsion, bending, and shear. All materials respond to these forces by elastic deformation. That is, the materials return their original size and form when the external force disappears. The materials may also have permanent deformation or they may fracture. The results of external forces are creep and fatigue.
Compression is a pressure causing a decrease in volume. When a material is subjected to a bending, shearing, or torsion (twisting) force, both tensile and compressive forces are simultaneously at work. When a metal bar is bent, one side of it is stretched and subjected to a tensional force, and the other side is compressed.
Tension is a pulling force; foe example, the force in cable holding a weight. Under tension, a material usually stretches, returning to its original length if the force does not exceed the material’s elastic limit. Under larger tensions, the material does not return completely to its original condition, and under greater forces the material ruptures.
Fatigue is the growth of cracks under stress. It occurs when a mechanical part is subjected to a repeated or cyclic stress, such as vibration. Even when the maximum stress never exceeds the elastic limit, failure of the material can occur even after a short time. No deformation is seen during fatigue, but small localized cracks develop and propagate through the material until the remaining cross-sectional area cannot support the maximum stress of the cyclic force. Knowledge of tensile stress, elastic limits, and the resistance of materials to creep and fatigue are of basic importance in engineering.
Creep is a slow, permanent deformation that results from steady force acting on a material. Materials at high temperatures usually suffer from this deformation. The gradual loosening of bolts and the deformation of components of machines and engines are all the examples of creep. In many cases the slow deformation stops because deformation eliminates the force causing the creep. Creep extended over a long time finally leads to the rupture of the material.
Properties of Materials
Density (specific weight) is the amount of mass in a unit volume. It is measured in kilograms per cubic meter. The density of water is 1000kg/m3 but most materials have a higher density and sink water. Aluminum alloys, with typical densities around 2800 kg/m3 are considerably less dense than steels, which have typical densities around 7800 kg/m3. Density is important in any application where the material must not be heavy.
Stiffness (rigidity) is a measure of the resistance to deformation such as stretching or bending. The Young modulus is a measure of the resistance to simple stretching or compression. It is the ratio of the applied force per unit area (stress) to the fractional elastic deformation (strain). Stiffness is important when a rigid structure us to be made.
Strength is the force per unit area (stress) that a material can support without failing. The units are the same as those of stiffness, MN/m2, but in this case the deformation is irreversible. The yield strength is the stress at which a material first deforms plastically. For a metal the yield strength may be less than the fracture strength, which is the stress at which it breaks. Many materials have a higher strength in compression than in tension.
Ductility is the ability of a material to deform without breaking. One of the great advantages of metals is their ability to be formed into the shape that is needed, such as car body parts. Materials that are not ductile are brittle. Ductile materials can absorb energy by deformation but brittle materials cannot.
Toughness is the resistance of a material to breaking when there is a crack in it. For a material of given toughness, the stress at which it will fail is inversely proportional to the square root of the size if the largest defect present. Toughness is different from strength: the toughest steels, for example, are different from the ones with highest tensile strength. Brittle materials have low toughness: glass can be broken along a chosen line by first scratching it with a diamond. Composites can be designed to have considerably greater toughness than their constituent materials. The example of a very tough composite is fiberglass that is very flexible and strong.
Creep resistance is the resistance to a gradual permanent change of shape, and it becomes especially important at higher temperatures. A successful research has been made in materials for machine parts that operate at high temperatures and under high tensile forces without gradually extending, for example the parts of plane engines.
Composite Materials
The combinations of two or more different materials are called composite materials. The usually have unique mechanical and physical properties because they combine the best properties of different materials. For example, a fibre-glass reinforced plastic combines the high strength of thin glass fibres with the ductility and chemical resistance of plastic. Nowadays composites are being used for structures such as bridges, boat-building etc.
Composite materials usually consist of synthetic fibres within a matrix, a material that surrounds and is fightly bound to the fibres. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibres made of a ceramic material such as carbon or glass embedded in a plastic matrix. Usually Composites with metal matrices or ceramic matrices are called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.
Continuous-fibre composites are generally required for structural applications. The specific strength (strength to-density ratio) and specific stiffness (elastic mobulusto-density ratio) of continuous carbon fibre PMCs, for example, can be better than metal alloys have. Composites can also have other attractive properties, such as high thermal or electrical conductivity and a low coefficient of thermal expansion.
Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMCs and other composite also have some disadvantages. For example, PMCs and other composite materials tend to be highly anisotropic-that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the composite material. Foe example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction. The designer, who uses composite materials in structures subjected to multidirectional forces, must take these anisotropic properties into account. Also, forming strong connections between separate composite material components is difficult.
The advanced composites have high manufacturing costs. Fabricating composite materials is a complex process. However, new manufacturing techniques are developed. It will become possible to produce composite materials at higher volumes and at a lower cost than is now possible accelerating the wider exploitation of these materials.
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 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 1per cent) are often added to a pure metal. This is called alloying (легирование) and it changes the grain structure and properties if 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 ofter 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 milt 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 used in cutting tools, surgical instruments, razor blades and springs. Too steel, also called silver steel, contains about 1 per cent carbon and is strengthened and toughened by questing 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 snail magnets. The addition of chromium gives extra strength and corrosion resistance, so we can get rust-proof 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 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 color 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.
Hot working of steel
An important feature of hot working is that it provides the improvement of mechanical properties of metals. Hot working (hot-roiling 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 other 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 work piece 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.
Metalworking
Metals are important in industry because they can be easily deformed into useful shapes. A lot of metalworking processes have been developed for certain applications. They can be divided into broad groups:
- rolling,
2.extrusion,
- drawing,
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forging,
-
sheet-metal forming.
During the first four processes metal is subjected to large amounts of strain (deformation). But if deformation goes at a high temperature, the metal will recrystallize — that is, new strain-free grains will grow instead of deformed grains. For this reason metals are usually rolled, extruded, drawn, or forged above their recrystallization temperature. This is called not working. Under these conditions there is no limit to the compressive plastic strain to which the metal can be subjected. Other processes are reformed below the recrystallization temperature. These are called cold working. Cold working hardens metal and makes the part stronger. However, there is a limit to the strain before a cold part cracks.
Rolling
Rolling is the most common metalworking process. More than 90 percent of the aluminum, steel and copper produced are rolled at least once in the course of production. The most common rolled product is sheet. Rolling can be done either hot or cold. If the rolling is finished cold, the surface will be smoother and the product stronger.
Extrusion
Extrusion is pushing the billet to flow through the orifice of a die. Products may have either a simple or a complex cross section. Aluminum window frames are the examples of complex extrusions.
Tubes or other hollow parts can also be extruded. The initial piece is a thick-walled tube, and the extruded part is shaped between a die on the outside of the tube and a mandrel held on the inside.
In back-extrusion (штамповка выдавливанием) the work piece is placed in the bottom of a hole and a loosely fitting ram is pushed against it. The ram forces the metal to flow back around it, with he gap between the ram and they die determining the wall thickness. The example of this process is the manufacturing of aluminum beer cans.
Technological processes
Drawing consists of pulling metal through a die. An example of drawing is wire drawing. The diameter reduction that can be achieved in one die is limited, but several dies in series can be used to get the desired reduction.
Sheet metal forming (штамповка листового метала) is widely used when parts if certain shape and size are needed. It includes forging, bending and shearing. One characteristic of sheet metal forming is that the thickness of the sheet changes little in processing. The metal is stretched just beyond its yield point (2 to 4 percent strain) in order to retain the new shape. Bending can be done by pressing between two dies. Shearing is a cutting operation similar to that used for cloth.
Each of the processes may be used alone, but often all there are used on one part. For example, to make the roof of an automobile from a flat sheet, the edges are gripped and the piece pulled in tension over a lower die. Next an upper die is pressed over the top, finishing the forming operation, and finally the edges are sheared off to give final dimensions.
Forging is the shaping of a piece of metal by pushing with open or closed dies. It is usually done hot in order to reduce the required force and increase the metal’s plasticity.
Open-die forging is usually done by hammering a part between two flat faces. It is used to make parts that are too big to be formed in a closed die or in cases where only a few parts are to be made. The earliest forging machines lifted a large hammer that was then dropped on the war piece, but now air or steam hammers are used, since they allow greater control over the force and the rate of forming. The part shaped by moving or turning it between blows. The process starts with a rod or bar cut to the length needed to fill the die. Since large, complex shapes and large strains are involved, several dies may be used to go from the initial bar to the final shape. With closed dies, parts can be made to close tolerances so that little finish machining is required.
Two closed-die forging operations are given special names. They are upsetting and coining. Coining takes its name from the final stage of forming metal coins, where the desired imprint is formed on a metal disk that is pressed in a closed die. Coining involves small strains and is done cold. Upsetting involves a flow of the metal back upon itself. An example of this process is the pushing of a short length of a rod through a hole, clamping the rod, and then hitting the exposed length with a die to form the head of a nail or bolt.
Welding
Welding is a process when metal parts are joined together by the application of heat, pressure or a combination of both. The processes of welding can be divided into two main groups:
- pressure welding, when the weld is achieved by pressure and
-
heat welding, when the weld is achieved by heat. Heat welding is the most welding process used today.
Nowadays welding is used instead of bolting and riveting in the construction of many types of structures, including bridges, buildings, and ships. It is also a basic process in the manufacture of machinery and in the motor and aircraft industries. It is necessary almost in all productions where metals are used.
The welding process depends greatly on the properties of the metals, the purpose of their application and the available equipment. Welding processes are classified according to the sources of heat and pressure used: gas welding, arc welding, and resistance welding. Other joining processes are laser welding, and electron-deem welding.
Gas Welding
Gas welding is a non-pressure process using heat from a gas flame. The flame is applied directly to the metal edges to be joined and simultaneously to a filler metal in the form of wire on rod, called the welding rod, which is melted to the joint. Gas welding has the advantage of using equipment that is portable and does not require an electric power source. The surfaces to be welded and the welding rod are coated with flux, a fusible material that shields the material from air, which would result in a defective weld.
Arc welding
Arc-welding is the most important welding process for joining steels. It requires a continuous supply of either direct or alternating electrical current. This current is used to create an electric arc, which generates enough heat to melt metal and create a weld.
Arc welding has several advantages over other welding methods. Arc welding is faster because the concentration of heat is high. Also, fluxes are not necessary in certain methods 9of arc welding. The most widely used arc — welding processes are shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc.
Resistance welding.
In resistance welding, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to great pressure, and a heavy current. This resistance causes heat, which melts the metals creates the weld. Resistance welding is widely employed in many fields of sheet metal or wire manufacturing and is often used for welds made by automatic [«semI»Ltq’mxtI] machines especially in automobile industry.