Английский язык для инженеров сварочного производства. Гричин С.В - 41 стр.

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41
4 Read the text and answer the questions.
1. What is a weld?
2. How can the heat be supplied for welding?
3. Is pressure employed in solid-phase processes?
4. What does an arc column consist of?
5. How is heat applied during welding?
6. What is the role of inert atmospheres?
7. What can make a joint brittle while welding?
8. What does the weld metal comprise in arc welding?
9. What is the base metal influenced by?
10. How can residual stress in welded structures be controlled?
Basic Principles of Welding
A weld can be defined as a coalescence of metals produced by heating
to a suitable temperature with or without the application of pressure, and with
or without the use of a filler material.
In fusion welding a heat source generates sufficient heat to create and
maintain a molten pool of metal of the required size. The heat may be
supplied by electricity or by a gas flame. Electric resistance welding can be
considered fusion welding because some molten metal is formed.
Solid-phase processes produce welds without melting the base
material and without the addition of a filler metal. Pressure is always
employed, and generally some heat is provided. Frictional heat is developed
in ultrasonic and friction joining, and furnace heating is usually employed
in diffusion bonding.
The electric arc used in welding is a high-current, low-voltage
discharge generally in the range 10–2,000 amperes at 10–50 volts. An arc
column is complex but, broadly speaking, consists of a cathode that emits
electrons, a gas plasma for current conduction, and an anode region that
becomes comparatively hotter than the cathode due to electron bombardment.
Therefore, the electrode, if consumable, is made positive and, if
nonconsumable, is made negative. A direct current (dc) arc is usually used,
but alternating current (ac) arcs can be employed.
Total energy input in all welding processes exceeds that which is
required to produce a joint, because not all the heat generated can be
effectively utilized. Efficiencies vary from 60 to 90 percent, depending on the
process; some special processes deviate widely from this figure. Heat is lost
by conduction through the base metal and by radiation to the surroundings.

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