Uniт 9 (Total Score – 15 points)

I. Прочитайте і перекладіть наступний текст зі словником. (Score – 6 points)

The Drop-Forging Technology

The technological process of drop-forging includes the following ореrations (1) cutting the stock to size_} (2) heat­ing the stock, (3) stamping, (4) trimming (cutting off flash), and (5) straightening. This sequence must be observed both when forging in multi-impression dies and by the flow method. If the drop-forging process is conducted by the combined method, the stock must be forged in a smith hammer after the second operation (beating), reheated and only then drop-forged.

Standard rolled sections and special rolled sections can be used as stock for drop-forgings. The use or special, sections of definite shapes and dimensions is particularly advantageous in large-scale and mass production. These sections are rolled to correspond to the shape of the finished forging, thus obviat­ing the necessity for intermediate operations preceding the finished forging operation in the finished impressions their use also results in a considerable saving of metal.

Sometimes parts of grade “Ст 3” and “Ст 4” steel, which are not designed for heavy duty, are stamped from cast steel blanks. Usually these blanks are cast in such shapes that they can be finish forged directly in the finish impressions of the dies. The use of cast steel blanks for drop-forgings greatly simplifies the production of intricately shaped forgings with­out resorting to the use of expensive multi-impression dies. When drop-forging cast steel forging blanks, casting defects, such as bubbles, cavities, etc., are usually squeezed out. Nevertheless, it is cheaper to use rolled stock for drop-forgings rolled stock of standard and special section is delivered as bars from 3 to 9 meters long, most commonly, in 6 meter lengths.

The first operation in the drop-forging shop is cutting the bars into stock of the requisite length” In mass and large-scale drop-forging shops, stock is cut to length in special bar-and billet-shearing departments^ individual and small-scale drop-forging shops, however, are usually not equipped with such departments, and the stock is cut to length on the main equipment of the forge shop.

Practical experience of the most advanced drop-forging shops shows that every forge shop, whether engaged in indivi­dual or large-scale production, should be equipped with a bar and billet shearing department, as this is particularly import­ant from the point of view of organizing production and econo­mising metal. Such departments must be equipped with all the necessary machines, and all operations must be mechanized. The bar and fillet departments must be designed, not only for cut­ting rolled stock to standard lengths, but also for: reducing the surplus lengths to smaller sections in hammers or in presses; preparing, whenever possible, drop-forging or hammer-forging croppings for further utilization as stock for other forgings; sorting forging croppings into their respective grades, and for other miscellaneous work.

The various means of cutting stock to length are: (a) cutting with shears (b) breaking in cold breaking fixtures; (c) cold saw cutting; electric saw cutting, and (d) gas cutting.

Shear Cutting. The most general method of cutting stock employed in mass and large-scale production forge shops is cutting it with shears or breaking it in cold metal breaking fixtures.

Shears are classified according to design into disc, lever and guillotine. The latter are employed for cutting sheet and plate metal. Shear presses are most widely used in forge shops. These presses operate on the principle of the crank press.

Shears are used for cutting both hot and cold metal. At present, shear presses are manufactured which can cut cold steel up to 300 x 300mm in cross-section.

Shear presses are a highly productive form of equipment, as they make one cut with every stroke, and are capable of making from 3 to 32 strokes per minute however, the greater the cross-section of the metal to be cut, the slower the speed of the shears will be. They can be used for shearing stock from shaped rolled sections, but for this purpose they must be equipped with special fixtures.

Shearing cold metal is accompanied by considerable strains which are liable to develop cracks at the cut. For this reason, high-carbon and ordinary alloy steels must be heated to temperatures ranging from 330 to 700°C before shearing the higher the temperature, the less force will be necessary to cut the steel.

Mild and medium carbon steels up to 200 x 200 mm in cross-section are usually cut cold. After being cut in shears, the ends of a piece of work will be rough; the greater the cross-section of the work and the higher the temperature of the metal, the greater the roughness of the ends will be. Because of this, temperature to which the metal is heated before cutting will depend on the grade of steel and on the method of subsequent forging (stamping).

For subsequent upsetting the cut should be as straight and even as possible. This is not always attainable, especially when shearing high-alloy and high-carbon steels, since, in order to avoid the development of cracks at the shearing section, the steel has to be heated to higher temperatures (from 700 to 800 and even to 1,000°C) and at such high temperatures straight edges cannot be obtained by shearing. For this reason other shearing methods must be employed, such as shearing on cold saws or electric arc cutting, etc.

Stock is also sheared in crank-type and hydraulic presses with the aid of special dies and fixtures. Smith hammers and drop-forging hammers must not be used for cold shearing stock, as this operation is by no means safe.

П. Складіть реферат. (Score – 9 points)