Welding processes

Welding is the process of joining together metallic parts by bringing them into intimate proximity and heating the places of contact to a state of fusion or plasticity. This leads to interpenetration of the atoms of the metals in the weld zone, and a strong inseparable joint is formed after the metals have cooled.

Welding finds widespread application in almost all the industries and construction. The wide use of welding at the present time is due to its economic advantages over other methods of joining metals, such as riveting, casting, etc.

Fig. 1. Diagram of welding processes

Welding processes may be classified according to the source of energy employed for heating the metals and the state of the metal at the place being welded (see Fig. 1). In fusion welding the welding area is heated by a concentrated source of heat to a molten state and filler metal must be added to the weld. In accordance with the method applied for feeding the filler metal to the weld, welding procedures are classified as manual, semiautomatic or automatic welding. Pressure welding processes involve the heating of the metallic pieces only to a plastic or slightly fused state and forcing them together with external pressure. Pressure welding is applied to metals which are capable of being brought to a plastic state by heating or due to the action of external forces. It has been found that in this process the most weldable metals prove to be those that have higher thermal conductivity.

The weld structure is shown schematically in Fig. 2. The deposited layer in zone 2 is obtained by melting the filler metal and its mixing with the parent metal 1 in the narrow fusion zone (3 — fusion line). In the fusion zone the parent metal is brought to the molten state so that it is rapidly penetrated by the deposited metal and a sound weld is formed. A heat-affected zone (referred to as HAZ) 4 is formed in the parent metal of all welds. In this zone only the structure of the metal is changed by the rapid heating and cooling during the welding process, its chemical composition remaining unchanged.

Fig. 2. Schematic weld structure.

Welded structures are assembled by butt, lap, corner and T joints. They are shown below (Fig. 3).

Butt joint

Lap joint

T joint

Corner joint

Fig. 3. Types of welded joints.

5.8. Read the text Gas Welding and number the sentences after it in the correct order.

Gas welding

In gas welding, it is necessary to use a mixture of two gases. To create a hot enough flame, a combustible gas must be mixed with oxygen. Hydrogen or petroleum gases can be used as a combustible gas.

Oxygen can be stored at very high pressure (120 atmospheres). It is dangerous to compress a combustible gas in the same way and so it is dissolved under pressure at a much lower pressure than oxygen (about 15 atmospheres). To create a suitable flame, the gases must be supplied to the welding torch at low pressure. Pressure regulators are therefore used to regulate the gas flow from the cylinders. They are screwed into the top of each cylinder.

Gas welding is normally used to join steel to steel. To make a very strong joint, the workpieces must be composed of the same metal. Welding rods are used to provide filler metal. In gas welding, these rods are generally composed of steel. Bronze or brass filler metal may sometimes be used. When bronze or brass filler metal is used the process is called brazing.

Notes

combustible gas – горючий газ welding torch – сварочная горелка adjusting valve – регулирующий клапан

workpiece n – свариваемая деталь welding rod – сварочный электрод brazing n – пайка твердым припоем

1. Welding rods are used to provide filler metal.

2. In gas welding, it is necessary to use a mixture of two gases.

3. Pressure regulators are used to regulate the gas flow from the cylinders.

4. Oxygen can be stored at very high pressure.

5. A combustible gas must be mixed with oxygen to create a hot flame.

6. To create a suitable flame, the gases must be supplied to the welding torch at low pressure.

5.9. Examine the table below and make similar PRECAUTION sentences from the following STATEMENTS using to or so that.

Statement
Long sleeves will protect your arms from burns.
Precaution
	to protect your arms from burns.
Long sleeves should be worn
	so that your arms are protected from burns.

1. Thick shoes will protect your feet from sparks.

2. Goggles will protect your eyes from sparks.

3. Dry clothes will prevent injuries from electrical equipment.

4. A face mask will protect your eyes from the arc light.

5. A metal container will protect the floor from burns.

6. CO₂ extinguishers will prevent damage from electrical fires.

5.10. What do you know about nanotechnology? Ask five questions about this new science and technology, then read and translate the text and see if it answers your questions. Why is the text entitled Hard to Define?