The process of forging may be carried out under hammers or presses. The former range in size from the hand-sledge of the blacksmith to the largest steam-hammers de livering blows of many foot tons. For much of the heaviest forg ing great hydraulic presses are used in which the heated metal is quietly pressed or kneaded into the desired shape. Forging, par ticularly on the large scale, is mainly applied to steel and in that material very large pieces are sometimes handled. The tubes of heavy naval guns are probably the largest of these, but parts of large ships are also very heavy forgings. These very large forgings are always made from the ingot itself, but for much of the smaller work the steel is first brought into the form of bars by rolling and is only then forged. Work of this kind merges into what is known as "drop forging" in which a carefully proportioned piece of steel bar is forged between heavy dies to assume a designed shape. Stamping and pressing may also be regarded as allied to forging, but while true forging is always applied to highly heated metal, stamping and pressing may be done either hot or cold. Here the metal is used in the form of bar or sheet and is forced into the desired shape, usually without much change of section or real flow of metal, but rather by bending accompanied by a certain amount of "drawing." Metal to be used in this way must, of course, possess an ample degree of ductility. The rolling process is used to convert ingots into forms which are small in cross-section in comparison with their length. Bars and sheets, rods and wire, rails and girders and all kinds of structural sections are produced in this way. Rolling is carried out in successive stages. In the early stage the purpose is, while gradually bringing the metal towards the desired shape, to improve its quality--especially its ductility— so that it can better stand the more severe subsequent treatment. Yet for this very purpose it is, in many cases, found necessary to make the first deformations of the metal very drastic. It is found that at almost any stage of mechanical working, if the change of shape or size forced on the material at any one stage is not suffi cient to penetrate and affect the entire thickness, there is risk of damage. Accordingly, where an ingot of large size has to be `'broken down" very large forces and correspondingly heavy de formations are applied. This occurs mainly in the rolling of steel and is carried out in very powerful machinery. The ingots are so large and heavy that they are handled by automatic appliances which take them from the furnace, feed them backwards and f or wards through the great rolls and finally eject the finished billet from the rolling train. Some recently-erected rolling mills of this kind, which are employed for dealing with nickel ingots as well as for steel, are operated electrically and are controlled by two or three men from a control platform—there is no heavy manual labour. For smaller work, however, and in most of the non-ferrous industries, the metal is put through the rolls by hand.
While steel is always "broken down" hot, some of the softer non ferrous metals and alloys can be safely treated cold. The essential nature of the process does not differ as between hot and cold rolling. The effect of cold rolling, however, is always to produce hardening so that, after a time, the metal has to be "annealed" in order to soften it prior to further cold-rolling. Such annealing usually consists in placing the metal in a suitable furnace and heat ing it to a temperature correctly chosen for each metal.
The details of rolling practice vary widely according to the metal employed and the purpose for which the product is required. Heavy plates and extremely thin sheets naturally demand entirely different treatment. Strip metal is sometimes rolled in so-called "continuous" mills and these are especially applied to the produc tion of wire. In these mills, which are mainly used for steel, the hot metal is fed into the first set of rolls rotating at a moderate speed. The material emerging from the first rolls is fed directly into a second set which—since the length is increased by the pas sage through the first rolls—must run at a considerably higher speed. A number of sets of rolls are used in series in this way, the finished product, such as wire, leaving the last rolls at a very high speed. Other types of rolling result in the production of "sections" —i.e., of material having the form of H or I beams, channels, angles, railway rails, etc. These are produced by the action of suitably shaped grooves cut in the rolls, but the differences in the rate of movement at the bottom of such grooves and near the top require careful consideration. Successive passes in such rolls must provide for the easy flow of the metal from one shape to the next. Extrusion.—Another method of manipulating metals, more especially non-ferrous alloys, for the purpose of bringing them quickly and easily into the form of bars or rods having any desired section or into the form of tubes, is that of extrusion. Here the heated billet of metal, previously produced of the correct size and shape, is placed in a heavy steel cylinder and a steel ram is forced down into the cylinder from one end. At the other end there is an aperture or "die" through which the metal is forced to escape in the desired form. Rods or bars produced by this process are sometimes found to possess an interior defect. This is believed to be due to the manner in which the flow of the metal in entering the die from the chamber causes the outer portions of the billet, in cluding the skin, to flow into the centre of the extruded bar. A means of avoiding this difficulty has been devised recently, by a modification of the extrusion press. It is possible to extrude many alloys which are not sufficiently ductile, in the cast state, to stand rolling. In other cases, rolling is still possible if the internal struc ture of the metal has first been modified either by previous extru sion or by preliminary forging.
the great majority of metallic prod ucts are put into use in the condition—so far as their inner structure and strength are concerned—in which the final stages of production have left them, an increasing number of articles (more particularly those intended for important engineering uses) are now subjected to some form of heat-treatment for the purpose of improving their internal structure and mechanical properties. In the case of steel castings, for example, annealing is almost univer sally applied for the purpose of removing some of the impurities from the inter-crystalline boundaries, where they tend to form em brittling cell-walls, and bringing them into the shape of small scat tered globules which are comparatively harmless. The treatment breaks up the original coarse casting structure and a further im provement is effected by either "normalizing" or "quenching and tempering" the steel.
Simple annealing is also used for softening metal hardened by cold working, as in rolling, stamping, pressing, wire-drawing, etc.