2. В процессах прокатки, обжатия или штамповки с вытяж­кой зёрна становятся вытянутыми в направлении обработки, и та­кая зернистая структура обычно называется «волокнистой».

3. При ковке необходимо избегать нарушения волокон, так как это может уменьшить прочность ковки.

4. Match the words on the left with their definitions on the right

1) blow a) a characteristic of wrought metal

indicating directional propetries

2) cogging b) the impact or other pressure pro-

duced by a moving part of any forging unit

3) fibre c) the oxide film that is formed on a

hot metal by the chemical action of the surface metal with oxygen in the air

4) forging d) the reducing operation in working

an ingot into a billet by the use of a forging hammer or a forging press

5) scale e) the process of shaping heated metal

by the application of sudden blows or steady pressure

Unit 20 PREMINARY FORGING OPERATION

Of the several forging operations which make direct use of the laws of plastic metal flow, the edger is the most im­portant and most widely used operation, if by the term «edger» is meant bom forming and rolling the stock. The edger makes use of two fac­tors; first, that the flow of plastic metal is towards the points of the least

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resistance, and second, that the resistance of the flow is greater at the contacting surface ofthe metal and the dies. The general contour ofthe edger is in the form of an oval, confining the metal on the top and bot­tom sides and the two ends, but with no confinement on two sides. This shape allows the metal to flow in a lateral, or sideway direction, but confines it in the axial or longitudinal direction.

In addition to the regular edgers where two sides are open, the use is made ofthe «rolling edger».

The rolling edger finds its use on some shapes conforming to solids of revolution with the metal to be edged confined partially on all sides. The stock is rolled while being struck, and this action moves the metal from one portion of the stock to another portion where it is required.

The fuller finds as its primary function the reduction of the cross-sectional area of the portion of the forging stock between the ends ofthe stock. The fuller has its action as opposite to that ofthe edger and permits the metal to move outward and away from the centre of the fuller. Here again the bar is turned 90 degrees between each blow so that the stock size is reduced in area at the portion being fullered without any area change in the balance ofthe piece.

The use ofthe fuller and the edger is a common combination for preliminary forging operations. Where stock requires such two opera­tions, it is generally fullered first and then edged.

Drawing is a special case of fullering as the drawing operation reduces the stock size at only one end. Drawing may be performed on a flat portion of the dies, or it may be done on a flat slot that has been machined out for this purpose.

The bender as its name implies, moves the various parts or the stock into a proper relation with the shape of the finishing impression where such an impression is of an unsymmetric nature. This operation generally follows such operations as edging, fullering, or drawing where they are necessary. One blow generally suffices for the bending operation.

The blocking impression, or blocker, is a stream-lined model made of the finishing impression, and is required on some types or for-

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gings for one, or both, of two general reasons. The first and the more important reason is that the finishing impression may contain too many obstructions in the form of depression, holes, bosses, plugs, or abrupt contour or section changes to permit a normal flow of metal to all parts of the impression without further preparation from the preceding op­erations. The blocking impression has a general shape of the finishing impression but with all the corners, holes, and abrupt changes, thor­oughly rounded so that the plastic metal may be moved into a suitable position for the more shaping in the finishing impression. The blocking impression aids in the prevention of forging defects, such as laps and cold shuts. The second reason for the use of the blocking impression is to reduce the wear on the finishing impression either of closely toler-anced forgings, or on long production runs.

There are some types of forgings where the blocking impression is the only preparatory operation, and the dies contain only a blocking and a finishing impression. These are usually forgings of a fairly sym­metrical nature, but with some cross-section changes too abrupt for forging in the finishing impression alone.

The finishing impression in the great majority of drop forgings represents the shape required for the finished forging. The exception to this are those forgings where such subsequent operations as bending, forming, drawing, and the like are necessary as special additional op­erations beyond the routine trimming of a flash. If the preliminary steps in the preparation of the forcing stock for the finishing impres­sion are correct, there is little danger of an unsatisfactory finished forg­ing as far aS general physical dimensions are concerned. Sometimes, too much stock or stock of an improper size will either create an excess of flash which prevents the dies from being close to the size of the forging, or the improper sized stock may tlevelop unfilled sections in the forged part. This condition is usually corrected by using the stock of the correct size. In the main the importance of a competent design in the preliminary forging steps must not be underestimated in supplying the finishing impression with the metal so shaped as to permit the nor­mal flow into the various cavities of the finishing impression.

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Notes to the text

solids of revolution типы теловращения

stream-lined model зд. предварительная модель

on long production runs при длительной эксплуатации

as far as ... are concerned что касается ...

in the main в основном

Exercises