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Open-Die Forging

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Open-Die Forging --- Aheadmold

Open-die forging is the simplest forging process. Although most open-die forging generally weighs 15 kg~500 kg, forging as heavy as 300 tons have been made. Sizes may range from very small parts up to shafts some 23 m long (in the case of ship propellers).
The open-die forging process can be depicted by a solid workpiece placed between two flat dies and reduced in height by compressing it (Fig. 4-2). This process is also called upsetting or flat-die forging. The die surfaces in open-die forging may have simple cavities, to produce relatively simple forgings. The deformation of the workpiece under ideal conditions is shown in Fig. 4-2 (b). Because constancy of volume is maintained, any reduction in height increases the diameter of the forged part.
Note that, in Fig. 4-2 (b), the workpiece is deformed uniformly. In actual operations, the part develops a barrel shape (Fig. 4-2 (c)); this deformation is also known as  pancaking. Barreling is caused primarily by frictional forces at the die-workpiece interfaces that oppose the outward flow of the materials at these interfaces. Barreling can be minimized if an effective lubricant is used.
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Fig. 4-2   (a) Solid cylindrical billet upset between two flat dies (b) Uniform deformation of the billet without friction (c) Deformation with friction

Barreling can also occur in upsetting hot workpieces between cold dies. The material at and near the interfaces cools rapidly, while the rest of the workpiece remains relatively hot. Thus, the material at the ends of the workpiece has higher resistance to deformation than the material at its center. Consequently, the central portion of the workpiece expands laterally to a greater extent than do its ends. Barreling from thermal effects can be reduced or eliminated by using heated dies; thermal barriers such as glass cloth at the die-workpiece interfaces are also used.
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Fig. 4-3   Two views of a cogging operation on a rectangular bar. Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil

Cogging, also called drawing out, is basically an open-die forging operation in which the thickness of a bar is reduced by successive forging steps at specific intervals (Fig. 4-3). Because the contact area per stroke is small, a long section of a bar can be reduced in thickness without requiring large forces or machinery. Blacksmiths perform such operations with a hammer and an anvil using hot pieces of metal; iron fences of various designs are often made by this process.

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