Unit 11

(Total Score – 17 points)

1.Прочитайте текст, намагайтесь зрозуміти його зміст і перекажіть його. (Score – 9 points)

1) Для чого поковки із сталі підвержені тепловій обробці;

2) Що відбувається з поковкой під час охолодження;

3) Назвіть методи охолодження поковок. Cooling Forgings

Steel forgings are heat treated for the following purposes: (1) to remove stresses arising in the steel during (1) forging and cooling (2) to homogenise the structure of the metal of the forging (3) to give the steel that degree of hardness which makes it most easy to machine and (4) to improve the mechanical properties of the steel.

As has already been said, the forging of steel is completed at a fairly high temperature (800-900°C). When forgings cool, deformations (stresses) arise in the steel and in the process of forging the magnitude of these stresses may exceed the strength of the steel itself. As a result, what are called microcracks may occur in the steel, which cannot be seen with the naked eye, though sometimes very large cracks also occur. This leads to the rejection of forging.

On cooling, the difference in temperature between the surface and the core of the forging causes internal stresses to arise. As known, a forging begins to cool on the surface. Therefore the temperature at its surface will always be lower than at its core, and the temperature of the metal will increase towards the centre of the forging. The surface of a forging cools and contracts considerably more rapidly than the hotter, central, layers. As a result, the surface layers will tend to compress the internal layers of the forging, and the internal layers will tend to distend the surface layers.

As the layers of metal continue to grow cooler (i.e., when the surface layers of the forging are almost cold), the contraction of the surface layers practically ceases but, as the outside layers of the metal are cold, the metal itself loses a considerable amount of its plasticity. At the same time, the temperature of the inner layers of the metal of the forging will still be high, and these layers will continue to contract. As a result, the direction of the stresses changes: compressive stresses are now present in the outer layers, and tensile stresses in the inner layers.

As has already been mentioned, cracks occur in the forging when the stresses due to the cooling of the steel exceed its tensile strength. The occurrence of cooling cracks depends on the quality of the steel. Heterogeneity of the steel, the presence of bubbles, pipe cavities, non-metallic inclusions, facilitate the development both of internal and surface cracks. Thus the conclusion can be drawn that forgings must be cooled in such a way as to promote the complete or partial absence of stresses in the steel as it cools.

If a forging is cooled slowly from 800-900°C, its temperature throughout its entire cross-section will gradually become equalised. This will result in the disappearance of all residual deforming stresses set up in the steel during its forging the stresses arising in the steel resulting from the difference between the temperatures of the surface and internal layers of the forging as it cools will be reduced.

A forging can be cooled by one of the following methods: (1) cooling in air; (2) cooling in unheated pits (3) cooling in heated pits or in the furnace. The cooling method selected depends on the grade of the steel and the shape and cross-sectional area of the forging, The method of cooling, and the cooling rate, must be indicated in the process chart for a given forging.

The quickest cooling method is that of cooling in air for air cooling, forgings can be packed in three different ways: (1) in a single row, with out clearances: (2) in a single row, with clearances, and (3) in stacks.

The highest cooling rate is attained by placing the forgings in a single row, with clearances between the forgings: they will cool rather more slowly when placed in one row without any clearances between them while the rate of cooling will be slowest when they are tacked one on top of another.

When cooling forgings in air, they must always be placed on dry soil. If placed on damp soil, or on a metal floor, or on cold forgings the cooling will not be uniform. The locality for air cooling must be protected from draughts.

Cooling in pits (or wells) is a slower process than air , cooling. The rate of cooling in pits or wells can be controlled by opening and closing the pit covers, by covering the forgings with layers of low-heat-conduction materials, such as dry sand, slag wool, coal ashes, etc., of varying thickness. Forgings should never be charged into cooling pits at temperatures below 500-550°C. They should be stacked in the cooling pits not touching the cold walls. If the forgings in the pits are to be covered with sand, slag wool, etc., care should be taken to leave a clearance of 25-50mm between the forgings; this clearance must also be packed with a dry, low-heat-conduction material. The forgings must then be covered with an 80-100mm layer of the same material.

Cooling in hot furnaces is employed for more important forgings, whose cooling must be carefully controlled in accordance with a definite predetermined rate. Before charging the forgings into the furnace, the latter must be heated to 500-700°C, but never less than 500-600°C. Forgings are generally cooled in car-bottom hearth furnaces.

If the forging is to be heat-treated, for instance, annealed, the cooling process should be combined with the annealing process. For this purpose, the forgings are charged into the furnace at temperatures not below 500°C, and then heated to the required annealing temperature, after which they are cooled in the furnace according to specified conditions. This method will reduce the cooling time and the fuel consumption, since it eliminates one heat and one cooling.

П. Складіть 4 питання до тексту так, щоб відповіді на них були б коротким переказом змісту. (Score – 8 points)

(Total Score – 17 points)

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