2. Answer the questions to the text.

1. What does "Sigma" mean?

2. When was this process developed?

3. What general methods is this process applied by?

4. What is the difference between inert-gas metal-arc welding and inert-gas tungsten-arc welding?

5. What gases are used in sigma-welding?

6. How is melting action accomplished?

7. What requirements must the inert-gas torch meet?

8. What can you say of the sigma-equipment?

9. What conditions are very essential for making sound and strong welds?

10. What are the main advantages of this process?

11. Where does this process find its application?

12. Is there a sigma-equipment in your laboratory?

3. Give the short summary of the text.

Unit 2. Text 1.

Atomic-hydrogen welding

New words and word combinations.

1. to obtain – получать

2. worn surface – изношенная поверхность

3. to disconnect – разъединять

4. to be surrounded – быть окруженным

5. to be reduced – восстанавливаться

6. underside of the weld – обратная сторона сварного шва

1. Read the text and try to understand it.

Atomic-hydrogen welding is an arc welding process in which unusuall, clean welds can be obtained.

The process is particularly applicable to the welding of sheet metal thin and moderate wall tubing, and to repair welding of the worn surfaces. The process may be used with or without filler rods. An advantage in repair welding is that sound dense deposits with properties of the parent metal can be produced with proper selection of filler material.

In the atomic-hydrogen process the heat of an arc is used indirectly as a source of welding heat. An arc is established between two tungsten electrodes using alternating current. At the same time, streams of hydrogen gas are passed along the electrodes, through the arc, and toward the base metal to be welded. The arc heats the hydrogen to a high temperature, perhaps 9000°F. Leaving the arc, the hydrogen strikes the metal to be welded, raising its temperature and melting it, using the additional heat of combination. At high temperatures molecules of hydrogen fall apat. Each molecule of hydrogen becomes two separate hydrogen atoms. To disconnect the hydrogen atoms in a molecule requires heat, which is provided by the arc. When the hydrogen atoms cool below the temperature at which they fall apart. the atoms L.-connect together in pairs again with the liberation of heat.

The relatively cool surface of the metal to be welded provides the low temperature necessary for the atoms to join to form molecules. The heat liberated by the junction raises the temperature of the metal. The arc in atomic hydrogen welding heats the metal not only by imparting its termal energy, but also by the heat of combination of hydrogen atoms. The hydrogen also serves as a shielding gas to protect the molten metal from the atmosphere. The metal being welded is surrounded by hot hydrogen. Small amounts of iron oxide on the surface of the steel are reduced to iron by the hydrogen. No flux ordinarily is required, for there is no oxygen and consequently no oxide. The oxides of some metals, such as aluminium and chromium, are not rapidly reduced to metal by hydrogen even at high temperatures. For these metals, that is, aluminium, magnesium and stainless steel, flux is used particularly on the underside of the weld to which the stream of hydrogen cannot penetrate.

Notes to the text.

1. for – в течение, для, ибо.

2. that is – то есть.