Multiple-Ram Presses (Forging Equipment)
Hollow, flashless forgings that are suitable for use in the manufacture of valve bodies, hydraulic cylinders, seamless tubes, and a variety of pressure vessels can be produced in a hydraulic press with multiple rams. The rams converge on the workpiece in vertical and horizontal planes, alternately or in combination, and fill the die by displacement of metal outward from a central cavity developed by one or more of the punches. Figure 14 illustrates the multiple-ram principle, with central displacement of metal proceeding from the vertical and horizontal planes.
Fig. 14
Examples of multiple-ram forgings. Displacement of metal can take place from vertical, horizontal, and combined vertical and horizontal planes. Dimensions given in inches.
Piercing holes in a forging at an angle to the normal direction of forging force can result in considerable material savings, as well as savings in the machining time required to generate such holes.
In addition to having the forging versatility provided by multiple rams, these presses can be used for forward or reverseextrusion. Elimination of flash at the parting line is a major factor in decreasing stress-corrosion cracking in forging alloys susceptible to this type of failure, and the multidirectional hot working that is characteristic of processing in these presses ecreases the adverse directional effects on mechanical properties.
(ASM Handbook, Volume 1 Properties and Selection Irons, Steels, and High-Performance Alloys, P 43)
Contour Forging (Forging Processes)
· Enhancement of grain flow at specific locations, when demanded by product application
Open-die contour or form forging requiring the use of dedicated dies has been successfully accomplished for carbon, alloy, and stainless steels as well as for superalloys. Contour forging can be advantageous under such circumstances as the following:
· Enhancement of grain flow at specific locations, when demanded by product application
· Reduction of the quantity of starting material; this is especially critical when using expensive materials
such as stainless steels and superalloys
· Reduction of machining costs; this is critical when machinability or excessive material removal are
factors
Open-die contour forging may be a requirement, as in the case of grain flow, or it may be an option, as in the case of material and machining cost savings. The material and machining cost savings typically outweighs the forging tooling costs.
(ASM Handbook, Volume 1 Properties and Selection Irons, Steels, and High-Performance Alloys, P 132)
Turbine wheels, which are commonly 2.54 m (100 in.) in diameter, are forged by first upsetting a block of steel and then contour forging to provide the thick hub and thin rim sections This is done using a shaped (contoured) bottom die, which supports the entire workpiece, and a shaped partial top (contoured swing) die. Successive strokes are taken with the top die as it is indexed around the vertical centerline of the press. The partial top die minimizes the force required to deform the metal, yet produces the desired forge envelope.
(ASM Handbook, Volume 1 Properties and Selection Irons, Steels, and High-Performance Alloys, P 132)
Hot Swaging (Forging Processes)
Hot swaging is used for metals that are not ductile enough to be swaged at room temperature or for greater reduction per pass than is possible by cold swaging. The tensile strength of most metals decreases with increasing temperature; the amount of decrease varies widely with-different metals and alloys. The tensile strength of carbon steels at 540 °C (1000 °F) is approximately one-half the room-temperature tensile strength; at 760 °C (1400 °F), about one-fourth the room temperature strength; and at 980 °C (1800 °F), about one-tenth the room-temperature strength. In practice, reductions greater than those indicated in Table 1 are sometimes possible by cold swaging without intentionally heating the work metal, because sufficient heat is generated during swaging to cause a substantial decrease in strength and increase in the ductility of the work metal.
The decrease in strength at elevated temperature does not make possible unlimited reductions at high temperatures. Because of the design and capabilities of swaging machines, the work metal must be strong enough to permit feeding of the workpiece into the machine. When the work metal has lost so much of its strength that it bends rather than feeds in a straight line, chopper dies must be used. This type of die limits the reduction in area to 25% regardless of work metal ductility. The temperature to which a work metal is heated for swaging depends on the material being swaged and on the desired reduction per pass.
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