Fibre direction

It is common knowledge that metals are made up of crystal line structure which is ordinarily termed «grain». In the cast metal such grains are distributed throughout the piece of a metal -in their natural form without any general direction and the strength of

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the casting is usually uniform in any direction. However, when the cast jteel or metal ingot is worked into the forging bar or billet, the process of changing this ingot into the bar of billet elongates these grains in the (Erection of work so that the rolling, cogging, or extruding operation imparts these elongated grains to the structure of the metal, and such a grain structure is commonly termed «fibre». This fibre may be likened to the fibre structure of wood, with the greatest strength of the metal in fte direction of the fibre. Broadly considered, the difference in the strength of steel along the fibre structure and across the same structure is not very large, when tested by the standard tensile test. The tensile tester usually indicates the strength along the fibre structure as being fiom about 2 to 5 per cent stronger than that across the same structure. In recent years, however, it has been demonstrated that steel is much stronger along the grain direction than across the fibre structure when subjected to impact, shock vibration, or wear, and this knowledge is of much interest in the fibre direction of forged parts.

In many forgings the fibre direction is not of prime importance and in some of them the designation of the part to be forged naturally permits the metal to be forged with the fibres directioned advanta­geously. There are, however, many drop forgings in which the grain fi­bres may be placed in one of two or more positions with relation to the forging, and on some of these it is advantageous to specify definite di­rections for the fibre structure. It must be remembered that in absence of specific information, the forge shop will ordinarily consider a drop forging only from the view point of the most economical method of production. The importance of fibre direction is becoming more and more realized among the users of metal products particularly in for­gings, and fibre direction attracts considerable attention in the specifi­cations when applied to forgings.

The direction or the grain fibre has considerable effect upon the operations which follow the forging operation such as warpage in heat-treatment. Where the fibre structure in the forging contains unequal stresses in the forging, a subsequent heat-treatment will balance these stresses which may change the physical shape of the forging. This is generally known as warpage.

Recent studies on this subject have also indicated the importance <« end fibres. End fibres are the fibres as seen at the end of the bar,

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similar to the end grains in the wood. It is important that these cut grain ends are placed in the forging so as to have a minimum effect on the strength. If the grain fibres are cut or torn in some section where the stresses are severe, the part is more likely to fail in service sooner than a part that has these fibres intact, and some of the failures of for-gings in service have been traced in this case. The strength of the forg­ing when subjected to impact or wear is undoubtedly due to the tough fibrous structure. In forging practice it is essential that the various steps in the drop forging dies be so placed and allocated that the grain fibres are not broken or cut at any point where they may reduce may strength of the forging.

Exercises

I. Answer the following questions

1. What are metals made up of?

2. How are grains distributed in the cast metal?

3. What is commonly termed «fibre»?

4. What operation has the direction of the grain fibre consider­able effect upon?

2. Find the English equivalents for the following Russian word combinations

кристалическая структура операция вытяжки вдоль волокна поперек волокна при испытании горячая штамповка оказывать влияние подвергаясь износу

3. Translate at sight

1. Все металлы имеют кристаллическую структуру, которую обычно называют «зернистой».

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