Unit Two

1. Study and try to memorize the following words and word combinations:

1. Brittle - непрочный

2. hot short - нетеплостойкии

3. to embitter - портить

4. high-strength joint - место стыка повышенной прочности

5. heat-affected area - зона термического влияния

6. to anneal - прокаливать

7. adjacent - прилежащий, соседний

8. to vaporize - испарять, превращать в пар

9. to elevate- повышать, поднимать

2. Read and translate the text Welding Copper-Base Alloys

Copper and copper-base alloys have specific properties which make them widely used. Their high electrical conductivity makes them widely used in the electrical industries and corrosion resistance of certain alloys makes them very useful in the process industries. Copper alloys are also widely used for friction or bearing applications.

Copper shares some of the characteristics of aluminum. Attention should be given to its properties that make the welding of copper and copper alloys different from the welding of carbon steels.

Copper alloys possess properties that require special attention when welding. They are:

l.High thermal conductivity.

2.High thermal expansion coefficient.

3.Relatively low melting point.

4.It is hot short, i.e., brittle at elevated temperatures.

5.The molten metal is very fluid.

6.It has high electrical conductivity.

7.It owes much of its strength to cold working.

Copper has the highest thermal conductivity of all commercial metals and the comments made concerning thermal conductivity of aluminum apply to copper, to an even greater degree. Copper has a relatively high coefficient of thermal expansion, approximately 50% higher than carbon steel, but lower than aluminum. One of the problems associated with copper alloys is the fact that some of them, such as aluminum bronze, have a coefficient of expansion over 50% greater than that of copper. This creates problems when making generalized statements about the different copper-based alloys. The melting point of the different copper alloys varies over a relatively wide range, but is at least 538°C lower than carbon steel. Some of the copper alloys are hot short. This means that they become brittle at high temperatures. This is because some of the alloying elements form oxides and other compounds at the grain boundaries, embittering the material.

Copper does not exhibit heat colors like steel and when it melts it is relatively fluid. This is essentially the result of the high preheat normally used for heavier sections. Copper has the highest electrical conductivity of any of the commercial metals and this is a definite problem in the resistance welding processes.

All of the copper alloys derive their strength from cold working. The heat of welding will anneal the copper in the heat-affected area adjacent to the weld and reduce the strength provided by cold working. This must be considered when welding high-strength joints.

There is one other problem associated with the copper alloys that contain zinc. Zinc has a relatively low boiling temperature, and under the heat of an arc will tend to vaporize and escape from the weld. For this reason the arc processes are not recommended for the alloys containing zinc.