2. Answer the questions to the text.

1. What cutting processes are used for cutting metals?

2. What current and polarity are applied in cutting?

3. What does the insulating covering permit?

4. What materials are the electrodes made of?

5. Is it posible to cut nonferrous metals?

6. Where does arc-oxygen cutting find application?

7. What requirements must the underwater arc cutting electric holders meet?

3. Give the short summary of the text.

Unit 3. Text 1.

RESISTANCE WELDING

New words and word combinations.

1. spot welding – точечная (контактная) сварка

2. flash upset welding – стыковая сварка оплавлением с осадкой под током

3. projection welding – рельефная сварка

4. seam welding – роликовая (шовная) сварка

5. flash butt welding – стыковая сварка оплавлением

6. upset butt welding – стыковая сварка сопротивлением

7. to be confined – быть ограниченным

8. the flow of the welding current – поток сварочного тока

9. a die (clamping dies) – зажимные губки (машины для стыковой сварки)

10. to result in – привести к

11. arcing surfaces – оплавленные поверхности

12. incandescent particles – раскаленные частицы

13. high carbon and low carbon steels – высокоуглеродистые и низкоуглеродистые стали

14. electric circuit – электрическая цепь

15. to occur – происходить

16. to prevent arcing – предотвратить оплавление

17. upsetting – осадка

18. appreciable pressure – ощутимое давление

19. intermittent weld – прерывистый шов

20. a stich effect – эффект стежка

21. water-hight weld – водонепроницаемый шо

1. Read the text and try to understand it.

Resistance welding is an unusually broad term as it includes spot welding, projection welding, seam wеlding, flаsh and upset butt welding and other processes. The required heat at the joints to be welded is generated by resistance offered by the parts to a low voltage, high density electric current. Except in the case of flash butt welding force is always applied before, during and after the application of current to fuse the heated parts together. Resistance welding of the low alloy heat treatable steels is usually limited to spot and flash welding but all the types of resistance welding are applied to the high strength low alloy constructional steels.

Spot welding. Spot welding consists of clamping two or more sheets of metal between two copper or copper alloy tips and passing a current through the pieces to weld them together. As resistance to the current is highest the interface of the sheets and as the copper electrodes conduct heat better than steel, fusion occurs first at the interface of the sheets. By regulating current density, Pressure of the electrode and the time the current flows, the weld is confined to a relatively small area spot.

The equipment consists of a machine designed with a transformer, electrodes for clamping the work and transmitting the current, a suitable mechanism for applying pressure to the parts to be joined. Spot welding machines are now available with electronic devices for controlling, regulating and timing the flow of the welding current. The process is especially suitable for welding parts of cars, airplanes, refrigerators, chasses and cabinets.

Flash welding. Flash butt welding is an electrical resistance welding process that can be applied to bars, tubing, plate, strip and other forms of products. In flash butt welding the material is heated by rapidity recurring shod circuits of high electrical currents and the pieces joined by pressure. The welds are made in a flash welding machine equipped with dies for holding the parts.

A high electric current flowing between the two lightly touching surfaces results in an extreme localization of heat at these surfaces caused by a series of short circuiting contacts. The points of metal contacting are repeatedly raised to the melting point and are blown out in the form of incandescent particles or sparks. This action generates sufficient heat to cause a small zone back of the molten surface to become plastic, and the metal is blown out at the arcing surfaces the pieces are moved together to bring new points into contact. When the plastic zone extends the correct distance back of the molten surface the pieces are suddenly butted together with upsetting force, causing a forging action. The forging action pushes out the molten metal and extrudes part of the plastic metal.

Until recent years flash welding in the ferrous field was confined mainly to the low carbon unalloyed steels. The process has now been extended to higher carbon and low alloy steels.

Upset butt welding. Upset butt welding consists of placing two pieces of metal in contact so that their juncture forms the place of highest resistance in an electric circuit. When an electric current of sufficient magnitude is applied, heating and subsequent fusion occur at the joint. The pressure, which is applied during the entire operation, is usually just sufficient to prevent arcing at the start and is increased when fusion occurs until it is high enough to cause upsetting. At this point the current is cut off.

The principal difference between flash welding and upset butt welding is that in flash welding there is very light contact or even a slight gap between the pieces, which causes arcing when the current is applied, with incandescent particles being thrown out, while in upset butt welding, appreciable pressure is applied at all times, so that arcing does not occur. Although flash butt welding has displaced upset butt welding to a great extent, the latter process is still used in wire working industries.

Seam welding. Seam welding, frequently known as resistance or line welding, is similar to spot welding in principle. Instead of using two electrodes in marking one weld at a time, a roller is applied on both electrodes and the work is fed between these, as a continual series of intermittent welds is produced. Various machines employ different methods of producing this intermittent effect, but is nearly all cases the resultant weld is a series

Notes to the text.

1. the latter — последний (из двух)

2. as — в то время, так как.

3. nearly - почти