Unlike the hammer the press exerts a deep working of the piece which can be seen to flow throughout under the stress rather than in surface only as occurs under the hammer. This is very desirable as the interior, which is known to have much coarser grain than outer parts, particularly needs to be “worked.” In plainer terms the press seems to knead the mass much as the bread-maker kneads dough, while the hammering method simply batters down the outside. At a glance an experienced eye can tell from the appearance of the end of a forging whether it has been pressed or hammered.
A Modern Hydraulic Forging Press
Pressures as high as 8,000 pounds per square inch are used in hydraulic presses, though much lower pressures are more common.
Forging vs. Rolling
Though we have not yet considered the rolling mill or its products, we understand that, in general, only products of regular and uniform cross-section and of considerable length can conveniently be rolled. Where they can be obtained of satisfactory shape and size, steel products formed by the rolling process are highly desirable and are usually cheaper than those which are produced by the forging process. Compared with those made by the rolling process, forged products are usually quite costly in labor and time.
Rolling mills, however, cost immensely more to build and equip than do plants installing even the steam hammering outfit, so the rolling process cannot pay except for such articles as are demanded in great quantities. Articles of irregular and odd shape must, of course, be forged and here, especially for very small articles, the drop-forging process is available and highly satisfactory where enough pieces of one kind and size are wanted to pay for the requisite dies.
Forged articles have another advantage which we should not overlook. The physical properties which are imparted during forging are somewhat superior to those which the rolls bestow. The physical properties shown by the latter are very satisfactory, however.
CHAPTER XVII
THE ROLLING PROCESS
