Методичка МТ-7 Шевченко Сварка (Методичка по сварке), страница 2

Welding Versus Other ProcessesSince 1920, welding has competed with riveting, bolting,casting. Without it, neither high pressure boilers nor nuclearpower plants would be economically possible. A welded jointpermits no play; nor, if continuous, will it allow leakage of fluid.The demand for sheet metal products, including items too thin incross section to be cast, has increased markedly with the development of resistance welding. The cost of fixtures and of cleaning surfaces in preparation for finishing is generally less forwelding than for casting, especially if the production quantity islow. Yet welding a given product can cost more in labor thancasting it.

Sometimes the expense of strain relief precludes awelded design.In a number of cases, steel weldments can replace gray ironcastings on an economical basis, but good engineering designpractice would suggest that a valid decision could only be madeif a casting redesign were carried out at the same time.

A goodexample of superiority of weldments would be motor generatorbases for dc power supplies or air conditioning units. Suchwelded steel bases are lighter and less expensive for about thesame rigidity. On the other hand, most machine tool bases arecast if they are complex and if there are more than a few to bebuilt.The strength of cast components is equivalent to that of weldments provided equivalent designs and alloys are compared.

Intension, gray iron is weaker and less stiff than steel; but in compression, the opposite is true; therefore, when there are combined stresses, as there are in most engineering applications, thestrength advantage of steel is considerably reduced. Thus, it is9wise to analyze each design application carefully before deciding which production method is superior.On balance, the designer of a product must consider both theproperties of materials and the characteristics of availableequipment.

While welding is more costly than casting, bending,or cold forging in some cases, it is often the most useful –particularly if the material in question is easily welded and ifsuitable welding equipment has already been installed.IX. Answer the following questions.1. What has welding competed with since 1920?2.

What has increased markedly with the development of resistance welding?3. What would good engineering casting practice suggest?4. Is the strength of cast components equivalent to that ofweldments provided equivalent designs and alloys are compared?5. What must the designer of a product consider?X. Explain why welding is thought of as one of the most effectivemethods for joining metals.10UNIT 2I. Learn the words below:adaptor – адаптер; переходникblindness – слепота; ослеплениеconsumable – плавящийся; non-consumable – неплавящийся(об электроде)core – электродный стерженьdesignation – обозначениеhard facing – наплавка твердым сплавомferrous – железистый; ferrous metals – черные металлы;nonferrous – цветнойlight-gage – тонколистовой (о материале)ground – заземленный проводvolumetric loss – объемные потериocclusion – 1) окклюзия; 2) газовые включения; 3) механическое удержание газов твердыми металлами или расплавамиoffshore – прибрежный, береговойpenetration – проплавление, проплав; глубина проплавленияmolten pool – сварочная ваннаto shield – ограждатьspeciality (specialty) – особенность; спецификаsunburn – солнечный ожогtorch – сварочная горелкаarc spot weld – дуговой точечный шовshielded metal arc welding – дуговая сварка (плавящимся) покрытым электродомgas tungsten arc welding – дуговая сварка вольфрамовымэлектродом в среде защитного (инертного) газа11II.

Read and translate the text.Text 2A. Arc Welding – Consumable ElectrodesSHIELDED METAL ARC WELDINGManual arc welding is widely used in the construction andfabrication of metal sheets, plates, and roll formed products. Theequipment includes a source of direct or alternating electric current, a ground, an electrode holder, and proper safety equipment.The latter consists of a helmet with dark eye protection, longsleeves, and a leather apron. Safety precautions should be takenlest the ultraviolet light from the welding arc should cause theequivalent of sunburn or snow blindness.A conventional electrode forms a molten pool in the jointarea.

A gaseous shield and slag protect the weld deposit fromoxidation and rapid loss of heat. Unskilled operators find thedrag-type electrode, with large amounts of iron powder in theelectrode coating, much easier to use. The iron powder increasesthe rate of deposition, but reduces the penetration and permitsthe core to burn away so that the coating can drag along the surface, with the arc length staying constant. Thereby a good deposit can be made by an operator with relatively little skill.In shielded metal arc welding the arc is started by momentarily striking the electrode against the base metal and quicklywithdrawing to form an arc. The arc must not be too long, as thisgives an opportunity for contamination by the atmosphere, itbeing more difficult to control its application to the joint.

Thecurrent and voltage must be under close control, they being governed by the quality of equipment and its inherent regulatingcharacteristics.The arc being formed between the base metal and the electrode, the immediate surface is melted, and, with the use of anelectrode that cannot conduct the heat away rapidly, some of themetal is vaporized. These droplets and the vaporized metal flow12along the stream of the arc path to the base metal where theycondense, build up, and solidify. (Motion-picture studies of thisaction have been made and are available from leading weldingequipment suppliers.) Therefore, the arc process is primarily alocalized casting process that is influenced by the action of theelectrode, current, flux, and operator. In the liquid and gaseousstate, it is essential that no harmful chemical action (such asoxidation and forming of nitrides) occur, that gas occlusions escape, that flux inclusions be avoided, and that the material coolwithout tearing or cracking.Shielded metal arc welding is versatile and can be performedwith relatively inexpensive equipment, making it well suited toshop jobs and field work.

It is also often used in underwaterwelding in the construction and repair of ships, offshore platforms, pipelines, and in space. But it has the following disadvantages: weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residuefrom the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though speciality electrodes have made possible the weldingof cast iron, nickel, aluminum, copper, and other metals.III.

Give the corresponding English equivalents from the text:ручная дуговая сварка; источник постоянного и переменного электрического тока; последний (из названных);разнородные металлы; шлем с защитными очками; строительство; изношенные и поврежденные штампы; локализованный процесс отливки; скорость наплавки; поддающийсясварке, сваривающийся; остаток от флюса; изделия проката;электрод с толстой обмазкой или с толстым покрытием.IV. Answer the following questions.1.

Where is manual arc welding widely used?2. What does the equipment include?133. What does proper safety equipment consist of?4. What can the ultraviolet light from the welding arc cause?5. What protects the weld deposit from oxidation and rapidloss of heat?6. What increases the rate of deposition but reduces the penetration?7. What is the arc process influenced by?V. Translate the sentences with the Objective Infinitive Construction.1. The material is raised to a temperature that causes it actually to melt and, under pressure, it is fused or forged together.2.

By making a series of spot welds in rapid succession, theoperator finds slight variations in contact pressure, surface conditions, and electrode contact resistance to result in a betterweld.3. The iron powder increases the rate of deposition, but reduces the penetration and permits the core to burn away so thatthe coating can drag along the surface, with arc length stayingconstant.VI. Translate the sentences with the Predicative Infinitive Construction.1. Automatic welding is reported to have been made possibledue to the development of welding heads that strike the arc, feedthe electrode, and maintain an arc of proper length and current.2. A manually operated submerged arc welder is known tohave been developed for the flexibility of hand operation and forthe advantages of automatic welding.3.

Copper is known to be the best electrode material for general application.144. Formerly a carbon electrode was also used, but in the1950s the tungsten inert gas (TIG) process proved to be economically superior.5. The operator is sure to be much more comfortable becauseof the absence of smoke, spatter, and visible arc rays.VII. Read and translate the text.Text 2B. Arc Welding – Non-consumable ElectrodesGAS TUNGSTEN ARC WELDINGIn this process, the arc usually passes between a tungstenelectrode and the metal joint. High temperature (up to10,000 °F) is concentrated at the end of the arc, where a smallpool of molten metal is formed. The arc passes from the electrode to the work and is shielded by an inert gas such as heliumor argon or a mixture of the two.

Formerly a carbon electrodewas also used, but in the 1950s the tungsten inert gas (TIG)process proved to be economically superior. The TIG name isstill used in the shop but the AWS1 designation is technicallygas tungsten arc welding.TIG welding was originally developed for joining magnesium alloys, but it is now used for all alloys. It is particularlyadapted to welding dissimilar metals and for hard facing worn ordamaged dies. It can also be adapted to welding light-gagesheet.

It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications.In general, an ac power source is best for TIG welding nonferrous alloys except deoxidized copper. For ferrous alloys, thedc power source with straight polarity (electrode negative) isbetter for gas tungsten arc welding because it greatly reduces thevolumetric loss from the tungsten electrode. For example, a TIGtorch that has a rating of 250 A when used with straight polarity15(dcsp) must be de-rated to 15 to 25 A when used with reversepolarity (dcrp-electrode positive).Arc spot welds can also be made with a TIG torch fitted withspecial adaptors. In this case a 1/8 in. electrode is often used fora total cycle time of 1/2 to 3 s depending on the alloy and itsthickness.Note:1AWS (American Welding Society) – Aмериканское общество специалистов по сваркеVIII. Answer the following questions.1.

What gas is used in TIG welding?2. What process was TIG welding originally developed for?3. What is it particularly adapted to now?4. What power sources are best for TIG welding nonferrousand ferrous alloys?5. How can arc spot welds also be made?IX. Speak about the difference between the consumable electrode arc welding and non-consumable electrode arc welding.16UNIT 3I. Learn the words below:series arc – косвенная дуга (между двумя электродами)welding gun – 1) сварочный пистолет; сварочная горелка;2) сварочные клещиreverse polarity – обратная полярностьstraight polarity – прямая полярностьspatter – 1) брызги (при сварке); всплески (при плавке);2) разбрызгиваниеtooling – технологическая оснастка; инструментальная оснасткаpressure vessel – сосуд высокого давления; камера высокогодавленияgas metal arc welding – дуговая сварка металлическим (плавящимся) электродом в среде защитного газаmetal active gas (MAG) welding – дуговая сварка плавящимся электродомmetal inert gas (MIG) welding – дуговая сварка плавящимсяэлектродом в среде инертного газаsubmerged arc welding – дуговая сварка под флюсомII.