2. Answer the questions to the text.

1. What was welding in its original meaning?

2. When does the history of electric arc-welding begin?

3. What did Petrov and Davy investigate?

4. Who was a founder of metal-arc welding?

5. When did he develop metal-arc welding?

6. What is metal-arc welding?

7. How is the welding heat created?

8. Describe briefly the metal-arc welding process.

9. What current is used in the metal-arc?

10. What factors are required for maintaining the arc?

11. How do the voltage and ampere of the welding current range?

12. What machines is direct current produced in?

13. What does «straight polarity» mean?

14. How is «reverse» polarity obtained?

15. What is the AC arc-welding machine?

16. What welding machines are the most versatile types of machines?

17. What chemical and electrical factors are the reasons for coating electrodes?

18. What is the characteristic feature of the bare-electrode arc?

19. What materials are sometimes contained in the electrode coating?

20. What is the main purpose of addition of iron powder?

21. What requirements do the welds produced under carefully controlled conditions meet?

22. Where have our scientists used the main principles of arc welding?

23. How is the current transmitted in arc welding?

24. What is an electrode holder?

25. What requirements must the electrode holder meet?

26. What metal-arc equipment is there in your laboratory?

27. Did your work as a welding operator?

3. Give the short summary of the text.

Text 3.

SHIELDED INERT-GAS METAL-ARC WELDING.

New words and word combinations.

1. heat resistant stainless steel – жаропрочная нержавеющая сталь

2. SIGMA (shielded inert-gas metal-arc) welding process – дуговая сварка металлическим (плавящимся) электродом в среде инертного газа

3. a non-consumable tungsten electrode – неплавящийся вольфрамовый электрод

4. a hose – шланг, рукав

5. an automatic welding head – автоматическая сварочная головка

6. a reel – катушка, барабан, бобина

7. lb (от лат. libra) – фунт (453,6 г)

1. Read the text and try to understand it.

A more recent development in the field of arc welding aluminium, copper, magnesium, their alloys; and heat resistant stainless steel has been inert-gas metal-arc welding process (sigma welding process) in which the filler wire is fed continuously through the arc shielded by the inert gas.

This process is known to be applied by the two general methods. One method employs a non-consumable tungsten electrode; the other method makes use of a consumable metal electrode. To protect the molten metal in the weld pool from the harmful effects of oxygen and nitrogen in the air, a soft stream of an inert gas either helium or argon, or a combination of the two, flows around the electrode and forms a gaseous protective cover over the weld pool. In this process melting action is accomplished by the heat generated by an electric arc established between a consumable or a non-consumable electrode and the metal to be welded.

Power may be supplied from standard 200- or 300-amp welding generators, provided they employ sensitive adjustment of current. Welding current is conducted through a water-cooled power cable to the torch and electrode. The inert-gas torch must be light, easily manipulated and adjusted. Two hoses and a water-cooled power cable supply the shielding gas, cooling water and direct or alternating current for the operation of the torch. Direct current with reversed polarity is preferred in sigma welding.

The equipment developed for this process includes both a manual gun and an automatic welding head. With the manual welding gun, welding may be performed in all positions, i.e., flat, horizontal, vertical and overhead.

Sigma process provides the best, and perhaps, the only means¹ of shielding the weld from the atmospheric contamination which makes the weld joint hard and brittle.

Inert-gas metal-arc welding. This is also known as "Consumable-electrode inert-gas arc welding". Filler metal wire of desired composition is automatically and continuously fed from a reel. It passes through the electrode holder and is deposited by the arc; which is completely surrounded by an inert gas. The filler metal wire is usually from 0.035 to 0.062 in. in dia., and the speed at which it is fed may range from 100 to 300 inches per minute. Welding speeds may be high, ranging from 10 to 14 lb of filler metal deposited per hour.

This welding process is useful for welding thick pieces (above 3/16 inch in thickness) rather than² thin pieces. Metal pieces with thicknesses up to 2 inches have been welded with complete penetration. Consumable-electrode inert-gas arc welding may be done either manually or with mechanized equipment.

Inert-qas metal-arc welding. With the exception that the electrode is non-consumable, this welding process is similar to the consumable-electrode inert-gas arc welding process previously described. The non-consumable electrode may also be of carbon or some other materials. The arc is obtained between the non-consumable electrode and the worcpiece. Filler metal may be fed from aside. Any metal can be welded with this process, although it is best suited for welding relatively thin pieces of metal.

For either manual or mechanized inert-gas tungsten-arc welding, as well as for inert-gas consumable-electrode welding, several conditions are essential for making a sound, strong and ductile weld joint. It is important that the surfaces to be welded, and the filler rod, if used, should be clean and free from scale; the molten weld-metal pool and the adjacent heated metal must have a complete and continuous cover of inert-gas; the shielding gas, which may be either argon or helium, or a mixture of argon and helium, must be of high purity.

The process has a number of³ advantages:

1. Highly concentrated heat application minimizes distortion resulting in deep penetration and smooth, dense deposits.

2. High quality welds can be made at high speeds with no slag or spatter because the weld area is blanketed by inert gas which forms a completely inert-gas shielding.

3. The concentrated arc permits close control of the weld deposit in all positions.

4. One of the principal features of this process is the use of very high current densities, twelve times⁴ those used in ordinary metal-arc welding, and high rates of filler metal feed.

All these advantages are of great importance when using the sigma-process in welding ocean and aircraft lines, atomic power stations, railroad cars and even buildings.

Notes to the text.

1. the only means — единственное средство

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

3. a number of - ряд

4. twelve times — в двенадцать раз (дольше)