2. Answer the questions to the text.

.

1. What processes does resistance welding include?

2. How is the welding heat generated?

3. Is pressure required in resistance welding?

4. What steels are welded by resistance welding?

5. What operations does spot welding consist of? Where is the highest resistance to the current?

6. Does fusion occur at the interface of the sheets?

7. What units does the spot welding equipment consist of?

8. What electronic devices are now available and used in modern spot welding machines?

9. Where is spot welding especially suitable?

10. Where is flash welding used?

11. How are the pieces heated in flash welding?

12. What does a high electric current result in?

13. At what moment are the pieces of metal butted together?

14. What action makes the plastic metal be extruded in flash welding?

15. What is the main difference between flash and butt welding?

16. Is any pressure applied in butt welding?

17. Why does not arcing occur in upset butt welding?

18. Does upset butt welding have a wide application?

19. What is seam welding?

20. Is seam welding similar to spot welding?

21. What is the difference between these two processes?

22. Where is seam welding used?

23. Have you seen the machines for seam and butt welding?

24. Did you work on spot welding machines?

3. Give the short summary of the text.

Text 2.

BRAZING

New words and word combinations.

1. melting point – точка плавления

2. soldering – пайка мягким припоем

3. alloying – сплавление

4. oxyacetilene torch – ацетилено-кислородная горелка

5. ample strenght – прочность с запасом

1. Read the text and try to understand it.

Dip brazing is used in two different ways. It is produced by heating in a molten chemical or metal bath and by using a nonferrous filler metal having a melting point above 800°F, but below that of the base metals. When a metal bath is used the bath provides the filler metal. Brazing is a group of metal joining processes wherein the filler metal is a nonferrous metal or alloy whose melting point is higher than 1000°F, but is lower than that of the metals or alloys being joined. Brazing, unlike welding, does not require that the surfaces of the metal be welded; it differs from soldering in that the latter method uses filler metals which melt below 700°F.

During brazing, the base metal of the two pieces to be joined is not melted. Some diffusion or alloying of the filler metal with the base metal takes place.

The brazing process is widely used because a great many metals can be effectively joined in that manner. Simple carbon steels as well as cast iron; nickel, bronze, stainless steels, high-temperature alloys may be joined by brazing.

Brazing materials commonly used fall into three broad groups: silver- and copper-base alloys, nickel-base alloys, and gold-base alloys. The melting points of these alloys range from about 1150 to 2100°F.

The surface to be joined must be free of oxide and grease to obtain a good bond. It is therefore necessary either to protect the surfaces from the oxidation, which may occur during heating, or to use a flux which will remove oxide by chemical means. The flux chosen should be highly fluid at the brazing temperature, be readily floated to the surface of the brazing metal and, of course, must react with the metal oxide. Common fluxes are chlorides mixtures of borax and bond acid, fluorides, and berates. Flux may be applied to the surface beging joined before brazing or as powder with the brazing metal. After the flux is added, the joint is heated to the proper brazing temperature. Solid filler metal may be preplaced on the metal pieces and thus is melted as the metal pieces after brazing temperature is reached.

Heating for brazing may be done by oxyacetylene torch, in furnace, by dip technique or by electric means. The latter may be provided by the carbon arc, electric resistance or induction methods.

In arc brazing, heat is obtained from an electric arc formed between the base-metal and an electrode, or between two electrodes. Joints made by the induction heating method are usually designed with the brazing alloy preplased. Heating is uniform and readily controlled. It works very well where it is desirable to keep the heating localized. Resistance heating has been in use for many years. Heat is obtained by passing an electric current through the area to be brazed. There are two general types of resistance heating: one in which heat is generated in electrodes and transferred to the joint by conduction and another where heat s generated in the joint members.

Joint design and preparation are of prime importance. It is advisable to keep clearances within 0,002-0,005 in for maximum strength, the exact distance between surfaces to be joined depending on the heating means, the type of brazing alloy selected, and the design of the part. When metals are correctly brazed, the joint will have ample strength and ductility for a great many applications. It is understandable, therefore, that brazed joints are finding increasing use for new applications, as for example, the use of gold brazing corrosion-resistant metals, and alloys working under high-temperature conditions.

Soldering.

Similar to brazing, soldering by definition, employs a filler metal which melts, however, below 700°F. The usual filler metal is solder, an alloy of tin and lead in various proportions. Small quantities of certain other metals may be added. Since solder melts at a relatively low temperature, it is easier to apply than the filler metal in brazing. Ordinary gas flames, and electric resistance-heated soldering irons may be used to supply the required heat. The fluxes used with soldering include rosin, zinc chloride and other substances. Most metals, such as iron, steel, copper and copper alloys can be joined by soldering. The strenghth of soldered joints is relatively low and is determined by the strength of the soldering metal.

Soldering is most commonly used to ensure good electrical contact in joining wires and small parts.

Notes to the text.

1. unlike – в отличие от

2. readily — легко