PRESSES AND PRESSWORK. The method of convert ing ingots of steel into forgings depends upon the size of the completed pieces and the number of these required. Hammer heads are formed in dies under a drop hammer. (See DROP FORG ING.) Locomotive connecting rods would probably be forged under a steam hammer (q.v.). Ingots larger than 3 ft. in diameter, however, would usually be forged under a hydraulic press. The present article will confine itself to the latter type, although many hydraulic presses are used for the manufacture of seamless tubes, shells or flasks, and adapted to many forming operations in trades other than metal working. Machines for forming cold sheet metal and thin plate (see PRESSED METAL) are also called presses, although the dies are opened and closed by cam or eccentric-driven levers.
In a forging press the hot metal rests on a stationary anvil and is squeezed by a pallet forced downward by a hydraulic ram. Four heavy round columns act as tension members with both ends fixed into massive rectangular slabs called entablatures; the lower entablature rests directly on the foundation and supports the anvil; the upper entablature or cap carries the hydraulic cylinder. Fluid pressure in this cylinder forces a closely fitting a great danger to cylindrical objects which must withstand burst ing pressure, because the greatest stress is in a tangential direc tion. Transverse weakness may be minimized by making ingots of cleanest steel, and also doing no more work in forging than is necessary. Hence the modern method of minimizing the size of ingots and degree of forging avoids undue trouble from segrega tion and transverse weakness. Tensile properties taken from representative large forgings are as follows: ram downward; to the bottom of the ram is attached a cross head guided by f our bushings sliding over the columns, and to this cross head is secured the upper working tool or pallet. Auxiliary cylinders are provided for lifting the cross head after the force has been exerted, and to move the ram up to its work. High pressure in the ram cylinder may be built up directly from a pump, from an accumulator (see STEAM ACCUMULATORS) or by a steam intensifier. Frequently more than one pressure is avail able so that work of various sizes can be handled. Reliable valves and packing at all sliding joints are essential. The capacity of a press is rated by the total squeeze it can exert. The smaller sizes, which compete with steam hammers, are designed for rapid operation and economy of power and water ; in the bigger ones these considerations are subordinated to the necessity for complete reliability.
So much improvement has been made in the art of casting and heat treating alloy steels that the large expense of forging is now justified only when the utmost of uniformity, soundness and toughness is essential. Castings and ingots are made up of inter locked fern-like crystals of metal, whose size depends upon the slowness of solidification. Long continued heat treatment may be able to refine this structure into an aggregate of microscopic crys tals, but it is much more amenable to heat treatment if the hot ingot has been squeezed and the coarse crystals more or less broken up mechanically. Accidental internal cavities may also be closed up and welded shut. This kneading of metal, to be effective, should obviously extend to the very centre of the section. Hence large ingots need the slow, deliberate, irresistible squeeze of a hy draulic press. The sharp blow of the biggest hammer is cushioned by the outer layers of metal and fails to reach the centre of the slab. American railroad practice in 1929 calls for a 5 to i reduc tion from the ingot and a 4 to 1 from rolled billets. Recent investi gations have shown, however, that fine grain can be obtained with even less than 3 to 1 reduction if proper precautions are taken. The irregularities caused by segregation (see IRON AND STEEL) can more than counteract the advantages of large reduction under the press. The carbon is lower in the 'bottom and sides of the ingot and higher in the centre and top; to counteract this in big ingots requiring the full capacity of several furnaces, metal con taining less and less carbon is poured in the successive heats.
It is impossible to make absolutely clean steel. Tiny non metallic inclusions will be caught between the crystals throughout the ingot; soluble impurities like phosphorus will concentrate in the metal locked within the crystal branches. Forging tends to arrange these impurities into planes parallel to the pallet faces; the more the reduction the more pronounced would these "flow lines" become. Thus if tensile test pieces are cut from a thick forged slab in three directions (a) parallel to the main extension, (b) crosswise and (c) from front to back, the ultimate strength of all might be fairly close. However, the first or longitudinal piece would have satisfactory contraction of area and impact toughness; the second or tangential test piece would have markedly less toughness; the third, or transverse test piece would be the most brittle. Such "transverse weakness," as it is called, is