2. Answer the questions to the text.

1. Where is the welding used?

2. What technology and welding processes do the nuclear power plants and spaceships require?

3. What welding process was the basic method just a few yers ago?

4. How many welding processes are used in Russia?

5. What welding processes are widely used in our country?

6. What methods of welding do our scientists pay great attention to?

7. Where are the scientific and technical aspects of welding studied?

8. What role will welding play in the future world?

9. What welding methods are used for joining refractory metals?

10. Have you read anything about welding?

11. What do you know about your future speciality?

12. Is there welding equipment in the laboratory of your institute?

3. Give the short summary of the text.

Text 2.

TECHNIQUE OF ARC WELDING.

New words and word combinations.

1. in a molten condition – в расплавленном состоянии

2. in a blacksmith shop – в кузнечном цехе

3. a carbon arc – угольная дуга

4. a source of energy – источник энергии

5. a welding circuit – сварочная цепь

6. terminal – зажим, клемма

7. «straight» or standart polarity – прямая или стандартная полярность

8. a polarity switch – переключатель полярности

9. reverse polarity – обратная полярность

10. lead cable – сварочный провод; провод, идущий к электроду

11. for tapping into – для впуска

12. output – мощность

13. penetrating power – проникающая способность

14. a molten deposit – расплавленный осадок; слой осажденного металла

15. bridging gap - зазор непровара корня шва

16. fit-up – сборка (соединение под сборку)

17. iron powder – железный порошок

18. weld appearance – внешний вид шва

19. dipping – погружение, окунание

20. extrusion – нанесение электродного покрытия методом опрессовки

21. winding – обмотка, наматывание

22. high fatigue – большая усталость

23. backing – подкладка, опора

1. Read the text and try to understand it.

Welding in its original meaning was the hammering together of two or more pieces of metal while they were sufficiently heated to be in a molten condition, a condition between the cold and the fusion states of the metal. This was the kind of forge welding that was done in blacksmith shops.

History. The History of electric arc welding begins with experiments conducted on electric arc during the first half of the 19th century. V.V. Petrov and H. Davy investigated the possibility of utilizing the electric arc for welding purposes. In 1842, the carbon arc flame was suggested for soldering. In 1882, the Russian scientist N.N. Benardos experimented with a carbon arc, obtaining patent in 1886. Electric metal-arc welding, widely employed today, is known to be developed by N.G. Slavyanov in 1889 and patented in Russia, France, Germany and England. Slavyanov replaced the carbon electrode by a metal rod which, upon striking an arc, gradually melted and added fused drops of metal to the weld.

What is metal-arc welding? Metal-arc welding is a process of joining two pieces of metal by heating, melting and fusing the edges or surfaces under the temperature of 5400-6000°F. In arc welding, the increase heat, required to reduce metal to a liquid state, is created by an electric arc. The arc is formed between the work to be welded and a metal wire called an electrode. The electrode, held in a suitable holder, is brought close to metal to be welded, forming an arc between the tip of the electrode and the work.

The arc is a means for transforming electrical energy into heat. The tremendous heat, highly concentrated at the tip of the electrode melts a small pool of metal. The pieces to be joined thus become liquid and fuse together in this molten pool called the crater. At the same time, the end of the electrode at the arc melts and this molten metal is carried over by the arc to the molten pool on the workpiece. With a consumable electrode, once the arc is obtained, it is necessary to move the electrode uniformly toward the workpiece, thus compensating for the loss of metal at its end. As the areas solidify, the metals are joined into one solid, homogeneous piece. Metal-arc welding is normally a manual rather, than¹ mechanized operation. If the pieces to be joined are relatively thick, it may be necessary to make two or more passes in order to complete the weld.

Current. The arc welding process requires a continuous supply of electric current, sufficient in amount (amperes) and of proper voltage to maintain an arc. This current may be either alternating (AC) or² direct (DC). The source of energy must provide a range of voltage across the arc from 17, which is the minimum for starting an arc, to approximately 45 volts. Welding current may vary from 10 amperes to as high as³ 1500 and over for automatic welding.

Polarity. If DC is used. the polarity of the welding circuit is important to correct procedure. DC welding machines have positive and negative terminals. When the electrode is connected to the negative terminal, 'straight' or standard polarity is used. This means⁴ that the current is flowing from the work to the electrode. By changing the lead or polarity switch the current will flow in the opposite direction from electrode to work. This known as 'reverse' polarity. Most welding machines are equipped with a polarity switch for reversing the flow of current without changing the lead cables at the terminals.

Power sources. Several different types of welding machines are available for producing satisfactory welding current. Direct current is produced in either electricmotor-generator sets or engine-driven generator sets. Practically all AC arc-welding machines are transformers which take current from an outside source and convert into welding current. Combination welders, producing both AC and⁵ DC current, are the most versatile of all types of welders. They are basically a transformer and rectifier with means⁶ provided for tapping into the either AC or DC output.

The arc Shielding. One of the difficulties was the fact that.in welding molten metal in the arc stream, as well as in the molten crater in the workpiece, was exposed to the atmosphere. Metal, when exposed to the atmosphere, oxidized. The oxides that formed in the weld reduced its ductility. The arc, between a bare electrode and work, was unstable. In addition, the bare wire arc was less concentrated and, therefore, had less penetrating power. These factors, chemical and electrical, were the reasons for coating electrodes. It was necessary to protect the molten metal from the air. It was necessary to stabilize the arc and make more effective use of the arc energy.

One of the most important advantages of modern metal-arc welding lies in the excellent protection that this method provides for the molten deposit. All arc welding metal electrodes are coated with a specially prepared flux which protects the deposit from the atmosphere, provides a protective shield and stabilizes the arc.

In welding alloy steels some alloing elements, such as, chromium, molybdenum, titanium and columbium are partially lost by oxidation in passing through⁸ the extremely hot arc. To compensate for serious losses, electrodes contain additional alloys, either in the rod or in the coating or both.

By adding oxides of certain refractory metals, such as titanium, the action of the arc is modified to give an arc that is softer and less penetrating. This arc is needed for welding sheet metal or bridging gaps where metal fit-up is poor. Flux or slag, coating is used to remove impurities from the molten metal, some electrodes are made to produce heavy slags around the arc and crater for the same purpose. It is through the coating, also, that electrodes have been made for satisfactory operation with alternating current.

The latest development in the evolution of electrode coatings has been the addition of iron powder. When this is added in relatively large amounts, the speed of welding is increased and the weld appearance improved. The iron powder is extra metal available for deposition, in addition to that of the core wire. The presence of the iron powder in the coating also makes efficient use of the arc energy in melting that metal.

The coatings may be applied to bare wire by dipping, extrusion or winding. The coatings not only produce a protecting shield of non-oxidizing or reducing atmospheres around the arc, but also control the fluidity of the metal, penetration, shape of beads physical properties of the deposit, and may control the composition of the deposit by addition of various metals and alloys.

Thus, we see that welding depends upon many factors, and welds produced under carefully controlled conditions⁹ are usually strong, very tough and ductile. Moreover, they possess a high fatigue and wear resistance and meet all the requirements¹⁰ of modern production.

Having used the main principles of arc welding the scientists have developed such welding processes as atomic-hydrogen welding, submerged-arc welding, inert-gas metal-arc welding, electro-slag welding and other processes.

Transmission of current. Welding current is conducted from the source of power to the arc by a flexible insulated copper or aluminium cable. A somewhat less flexible, but equally wear-resistant cable is used for grounding to complete the circuit.

Electrode holders. An electrode holder is simply a clamping device for holding the electrode and is provided with an insulated handle for the operator's hand. The clamping device should be so designed as to hold¹¹ the electrode securely in position when welding, but permit easy removal and replacement of electrode, also providing good electrical contact. It should also be light in weight, but mechanically strong.

Ground clamps. A ground clamp is a device for connecting the work or ground lead to the work. It must furnisha strong positive connection, yet be quickly and readily movable from one part of the work to another.

In addition to the equipment outlined above, there should be available: steel scratch brushes for cleaning welds, tools for removing scale and slag from welds, carbon and copper blocks for backing, and other shop equipment.

Notes to the text.

1. rather than — a нe, вместо того, чтобы

2. either ... or - или ... или, как ... так

3. as high as — вплоть дo

4. to mean — означать, значить

5. both ... and — кaк ... тaк и, и то и другое

6. means — средство, способ

7. both - оба

8. through —при помощи, посредством, через

9. under conditions —при условиях

10. meet requirements —отвечать требованиям

11. as to hold — чтобы держать