The mechanical treatment of steel, through its influence on the structural units of the metal, is as important as any phase in the production of a suitable finished product. It has the most profound result in perfecting the physical internal structure of the metal, as well as in shaping it for the use desired. The four great methods of shaping metal are: (1) pounding or hammering; (2) rolling; (3) squeezing in hydraulic process; and (4) drawing through dies, as in the making of thin bars and wires. Each of these methods of shaping is especially efficient in making products of certain shapes and will be favored on that account.
Hammering is the most ancient method, and is used largely still, in connection with the crucible process. Hammers have been built to very large size, but are subject to certain mechanical defects and cannot compete with other methods of forming in the shaping of most objects. Hammering gives an especially good working of the surface layers of any object, but, as the impact is so transient, the core of the metal is less worked by this than by any of the other three methods.
Rolling of the metal is the most rapid of all the processes for shaping. If the metal is used at a rather high temperature, it will offer little resistance, to shaping and can be passed through the rolls at an extremely rapid rate. Care must be taken always that rolling speeds are not too great nor the exterior layers drawn by excessive differential motions. The effect of squeezing in rolls is more prolonged than by hammering, and rolled material may have a well worked core. The metallurgist is especially interested in this mechanical kneading of the metal as it passes through the rolls, but also must be thoroughly familiar with the mechanical side of the treatment.
Rolling mills are built in conjunction with nearly all steel plants, and shape up the material into billets, slabs, bars, plates, rails, structural forms, and the other simple shapes used in commerce. Of the divers sorts of mills for working everything from ingots to finished shapes we can show only one - a blooming mill, or that for the 'first passes of large ingots. The self-acting rollers of the tables at both ends of the rolls, Fig. 27, hurry the heavy lengths of metal into the gap between the two horizontal rolls which revolve now in one direction, now in the other, according to the pass. Each time the rolls will be closed a little between passes so that the ingot soon is reduced much in cross-section and is made longer. The driving mechanism is in the room at the right; the gears to make both rolls turn together are in the box at the right end of the two spindles which are coupled to the actual rolls. Above the roll frame is the mechanism to raise and lower the rolls as the pass demands. The motors to drive the live rolls of the tables are in the center foreground hydraulic press about to reduce the size of an ingot. With many turnings and squeeze after squeeze such a broad thick ingot gradually will be drawn out into gun tubes or shapes for other objects.
While the parts of a press such as can be seen in Fig. 28 are essential and comprise as their principal features the upper and lower forging bitts properly connected the main columns supporting the? hydraulic compression cylinder and the two side lifting cylinders, the movable head, the attachments for turning and moving the ingot, etc., yet the mechanical means for supplying the power to the press and causing quick and repeated actions are equally essential and have made a great success for the machine.
Wire drawing gives good internal working to a metal but of course is limited to the peculiar forms which can be made by such a process. As a matter of fact, the strongest metal ever produced has been made by a suitable combination of annealings and drawings.