FORGE, FORGING and FORGING MACHINES. A forge is a furnace or open fireplace, having a forced draft, for heating metals, especially iron and steel, to be shaped by hammering or pressure. Previous to the 19th century, most iron work was forged by smith on anvils. The coal used now is usually soft, and is sometimes termed blacksmith's coal. Portable forges are made for light work, and a common form of these has four legs made of iron tubing, a shallow bowl to hold the coal and contain the fire, and a hood above. At the rear is a small rotary fan, operable by a wheel-crank. As forges increase in size and capacity, they develop into what are more properly called furnaces, such as the immense heating furnace of a steel works. In forging iron by hand the smith thrusts the metal into the centre of his fire, and its color tells him when it is ready to be worked. As it heats, it becomes black red, then a low red, then bright red, then a white heat, and if the heating continues it next begins to burn. If the smith desires the iron to be very soft, that it may yield quickly to his blows, he lets it come to a white heat, then takes it with his tongs on to his anvil, and begins to hammer it rapidly into the desired shape. Often a helper stands by to strike blows alternately with the smith, in order that as much hammer ing as possible may be done before the iron cools to a point where it requires reheating. Long practice makes very expert smiths, and many of them will shape ,a bar of iron into intricate form in a surprisingly short space of time. When the smith desires to weld together two pieCes of iron, be must heat both pieces at the same time to a uniform white heat. As the iron approaches a white heat he throws sand on it, which, if done at the proper tem perature, adheres and melts, forming a liquid glass-like fluid that flows over the surface of the iron and keeps the air from it, thus pre venting its burning. If the ends of two bars are to be welded, each is first brought to a flat point, so that there will be considerable surface to weld together. After the surfaces adhere, the weld may be reduced by reheating and hammering down to the size of the other portions of the bars. When pieces are joined end to end, this is termed a butt-weld, which is not apt to be as strong as a lap-weld. Where forgings are so large that a smith and helper cannot work them satisfactorily, because there is too much and too heavy, hammering to be accomplished in the short time the metal re tains its great heat, the machine hammer is resorted to. This was made in various forms, and was an important tool during the early part of the 19th century, finding its culmination in the great steam-hammer of Neysmith, which was devised especially for such large forgings as the shafts of paddle-wheel steamships, then coming into use. The trip-hammer and the steam-hammer were the chief tools used for large forgings until about 1890, when the hydraulic press began to supersede the hammer for large work. 'About 1884, when the United States government began to seriously take up the rebuilding of the navy, and the construction di steel battleships and cruisers, there came a demand to American for very large forgings, which had not previously been produced here. The forges of that date were entripped with power bammars,, of a capacity suited to welding wrought iron into forgings of moderate size. The introduction of steel rails taught American iron manufacturers that steel forgings were desirable in all classes of machinery as greatly reducing the size of the parts, because of the increased strength.
The hydraulic compression system of forg ing, which was introduced in America when our steel plants were called upon to provide the large forgings for the navy, has met with general approval, and is steadily increasing in use. It has been demonstrated that forgings made by hammering were subject to all sorts of internal strains due to differences in com pression and differences in cooling. When such a forging is cut, as for making a keyseat, a distortion of form follows as the result of certain strains that have been relieved. A large forging requires a pressure great enough to penetrate to every part of the mass of metal, so that the flow of metal resulting shall take place uniformly throughout. Such a flow re quires time and a pressure that cannot be ob tained by a quick hammer blow, and a series of blows does not produce the same effect as does the persistent pressure of the hydraulic forging machine. Fluid steel begins to crystal lize at the point where the degree of heat is reduced so that it solidifies (about 2,600° F.), and the more slowly it is cooled from this point downward, the larger will be the crystals of the ingot. Forging during the cooling tends to check crystallization, just as stirring water at a temperature below the freezing point would check the formation of ice crystals. The more irregular the method of forging is, as in the case of hammer blows, the more it interferes with an even crystallization. The pressure system not only interferes much less than hammering, but forgings are farther im proved in modern practice by reheating above the recalescent point, which is between 1,200 and 1,300° F., depending on the percentage of carbon and annealing to secure a new crystal lization, and oil tempering.
An examination of the grain or fibre of hydraulic forgings shows that the steel tends to arrange itself in layers following the outer con tour of the piece of metal, thus securing the most strength. A sharp angle in the pattern will tend to cut off the flow of metal, and if such is required, the method is to round out the place in the forging, to secure the best flow of metal, and then machine off the surplus later.
To avoid internal strains as much as •os sible such large forgings as steamship shafts are preferably made hollow. That is to say, the cylindrical ingot from the furnace after cooling is bored through the centre, mounted on a steel mandrel, heated and forged by com pression. Ingots produced with fluid compres sion are well adapted to being bored and hol low-forged, and this method produces the most perfect forgings. These large forgings are sub jected to a hydraulic pressure of about 7,000 pounds to the square inch, which pressure is maintained while the mass slowly cools. Shafts of more than a foot diameter are best bored, as the bore hole not only assists uniform heat in*. but gives opportunity •for..inspection.
As the art of forging advanced, small forg ings of iron, steel and other metals were com monly produced by drop-forging. A press containing a top and bottom pattern (die and counter) received the roughly shaped hot metal, and at one or more blows it was brought to the desired shape. This method has been im proved upon by the use of pressure, as de veloped in forging machines which accomplish, on a smaller scale, work similar to hydraulic forgingpresses. See MACHINE Bo. LOWS; FURNACE; HAMMER; IRON; Puns; STEEL ; WELDING.