Put 7 questions according to the Text.

Put down the underlined words to the dictionary.

Retell the Text in English.

Свариваимость

Exercise 1. Read the text

Coalescence of metals

1. A weld can be defined as a coalescence of metals. 2. It isproduced by heating to a suitable temperature with or without the applscation of pressure. 3. In fusion welding a heat source generates sufficient heat. 4. It creates and mainteins a molten pool of metal of the required size. 5.The heat may be supplied by tltctricity or by a gas flame. 6. Frictional heat is developed in friction joining. 7. Most metals, when heated, react with the atmosphere or other nearby metals. 8. Most metals rapidly oxidise when molten. 9. A layer of oxide can prevent proper bonding of the metal. 10. Molten metal droplets cjated with oxide make the joint bruttle.

Exercise 2. Translate the text in writing

Exercise 3. Match the sides

Exercise 4. Make up the sentences

1.Where, they, do, work?

2. It create, does, not, good, a joint?

3. I, my, profession, like?

4. Most, react, metals, with, armosphere the?

5. Popular, is, welding, very, now?

Home assignment

WELDING DEFECTS

(деффекты сварки)

1. Read the following Text:

Common welding defects include lack of fusion, lack of penetration or excess penetration, porosity, inclusions, cracking, undercut, lamellar tearing. Any of these defects are potentially disastrous as they can give rise to high stress intensities which may result in sudden unexpected failure below the design load. To achieve a good quality joint it is essential that the fusion zone extends to the full thickness of the sheets being joined. Thin sheet material can be joined with a single pass and a clean square edge will be a satisfactory basis for a joint. However, thicker material will normally need edges cut at a V-angle and may need several passes to fill the V with weld metal. Where both sides are accessible one or more passes may be made along the reverse side to ensure the joint extends to the full thickness of the metal. Lack of fusion results from too little heat input and / or too rapid traverse of the welding torch (gas or electric). Excess penetration or burning through arises from too high a heat input and / or too slow traverse of the welding torch. It is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal. Porosity occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal, or from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and the workpiece is carefully cleaned and degreased prior to welding. Inclusions occur when several runs are made along a V-joint when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run. Cracking can occur due to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with a carbon content greater than 0.2 % are being welded, self-cooling may be rapid enough to cause some brittle martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating may be needed, and after welding a slow controlled post-cooling in stages will be required. This can greatly increase the cost of welded joints, but for high strength steels, such as those used in petrochemical plants piping, there may well be no alternative. Solidifying cracking is also called centerline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulfur and phosphorus, are more likely to occur in higher carbon steels. Solidification cracks are normally distinguishable from other types of cracks by the following features: 1) they occur only in the weld metal – although the parent metal is almost always the source of the low melting point contaminants associated with the cracking; 2) they normally appear in straight lines along the centerline of the weld bead, but may occasionally appear as transverse cracking; 3) solidification cracks in the final crater may have a branching appearance; 4) as the cracks are open they are visible to the naked eye. On breaking open the weld the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries. There may be evidence of segregation associated with the solidification boundary. The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses as the weld pool solidifies. Factors which increase the risk include insufficient weld bead size or inappropriate form, welding under excessive restraint, material properties, such as a high impurity content or a relatively large shrinkage on solidification. Joint design can have an influence on the level of residual stresses. Large gaps between components will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking. In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese can help to reduce the risk. To minimize the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06 %. However when welding a highly restrained joint using high strength teels, a combined level below 0.03 might be needed.