Read and translate the text. BASIC PRINCIPLES OF WELDING

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 non-consum­able, 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.

Most metals, when heated, react with the atmosphere or other nearby metals. These reactions can be extremely detrimental to the properties of a welded joint. Most metals, for example, rapidly oxidise when molten. A layer of oxide can prevent proper bonding of the metal. Molten-metal droplets coated with oxide become en­trapped in the weld and make the joint brittle. Some valu­able materials added for specific properties react so quickly on exposure to the air that the metal deposited does not have the same composition as it had initially. These problems have led to the use of fluxes and inert atmospheres.

In fusion welding the flux has a protective role in fa­cilitating a controlled reaction of the metal and then pre­venting oxidation by forming a blanket over the molten material. Fluxes can be active and help in the process or inactive and simply protect the surfaces during joining.

Inert atmospheres play a protective role similar to that of fluxes. In gas-shielded metal-arc and gas-shielded tungsten-arc welding an inert gas—usually argon—flows from an tube surrounding the torch in a continuous stream, displacing the air from around the arc. The gas does not chemically react with the metal but simply pro­tects it from contact with the oxygen in the air.

The metallurgy of metal joining is important to the functional capabilities of the joint. The arc weld illus­trates all the basic features of a joint. Three zones result from the passage of a welding arc: (1) the weld metal, or fusion zone, (2) the heat-affected zone, and (3) the unaf­fected zone. The weld metal is that portion of the joint that has been melted during welding. The heat-affected zone is a region adjacent to the weld metal that has not been welded but has undergone a change in microstructure or mechanical properties due to the heat of weld­ing. The unaffected material is that which was not heated sufficiently to alter its properties.

Weld-metal composition and the conditions under which it freezes (solidifies) significantly affect the abil­ity of the joint to meet service requirements. In arc weld­ing, the weld metal comprises filler material plus the base metal that has melted. After the arc passes, rapid cool­ing of the weld metal occurs. A one-pass weld has a cast structure with columnar grains extending from the edge of the molten pool to the centre of the weld. In a multipass weld, this cast structure maybe modified, depending on the particular metal that is being welded.

Here are some of the notices in the fun palace and safari park. Can you fill in the gaps with passive verbs?

1. Visitors ...... into the house and grounds between 9.00 am and 5.30 pm. (allow) 2. A bell ........ fifteen minutes before closing time, (ring) 3. Visitors ......not to feed the animals, (request)

4. People ......... not to leave their cars while touring the sa­fari park, (advise) 5. Dogs ......... in the safari park, (not allow) 6. You ............ not to touch the furniture, (ask) 7. The staff.........to accept tips, (not permit) 8. Lunch ......... in the cafeteria from 11.00 to 2.30. (serve)

Rewrite the sentences using the passive form of the verbs in italics.

Вариант 8.