Forging was the first of the indirect compression-type process and it is probably the oldest method of metal forming. It involves the application of a compressive stress, which exceeds the flow stress of the metal. The stress can either be applied quickly or slowly. The process can be carried out hot or cold, choice of temperature being decided by such factors as whether ease and cheapness of deformation, production of certain mechanical properties or surface finish is the overriding factor.
There are two kinds of forging process, impact forging and press forging. In the former, the load is applied by impact, and deformation takes place over a very short time. Press forging, on the other hand, involves the gradual build up of pressure to cause the metal to yield. The time of application is relatively long. Over 90% of forging processes are hot.
Impact forging can be further subdivided into three types:
- Smith forging,
- Drop forging,
- Upset forging.
This is undoubtedly the oldest type of forging, but it is now relatively uncommon. The impact force for deformation is applied manually by the blacksmith by means of a hammer. The piece of metal is heated in a forge and when at the proper temperature is placed on an anvil. This is a heavy mass of steel with a flat top, a horn which is curved for producing different curvatures, and a square hole in the top to accommodate various anvil fittings. While being hammered the metal is held with suitable tongs.
Formers are sometimes used; these have handles and are held onto the work piece by the smith while the other end is struck with a sledgehammer by a helper. The surfaces of the formers have different shapes and are used to impart these shapes to the forgings. One type of former, called fuller, has a well-rounded chisel-shaped edge and is used to draw out or extend the work piece. A fuller concentrates the blow and causes the metal to lengthen much more rapidly than can be done by using a flat hammer surface. Fullers are also made as anvil fittings so that the metal is drawn out using both a top and bottom fuller. Fittings of various shapes can be placed in the square hole in the anvil.
The working chisels are used for cutting the metal, punches and a block having proper-sized holes are used for punching out holes. Welding can be done by shaping the surfaces to be joined, heating the two pieces then adding a flux to the surfaces to remove scale and impurities. The two pieces are then hammered together producing welding.
The easiest metals to forge are the low and medium carbon steels and most smith forgings are made of these metals. The high carbon and alloy steels are more difficult to forge and require great care.
This is the modern equivalent of smith forging where the limited force of the blacksmith has been replaced by the mechanical or steam hammer. The process can be carried out by open forging where the hammer is replaced by a tup and the metal is manipulated manually on an anvil.
The quality of the products depends very much on the skill of the forger. Open forging is used extensively for the cogging process where the work piece is reduced in size by repeated blows as the metal gradually passes under the forge.
The cogging of a prismatic bar can be used to assess the parameters involved and how they are controlled. The objective is to reduce the thickness of the work piece in a stepwise sequence from end to end. Several passes may be required to complete the work and edging is usually carried out to control the width. The reduction in thickness is accompanied by elongation and spreading. The relative amounts of elongation and spread cannot be calculated theoretically but they have been determined experimentally for mild steel.
Die drop forging. Closed-die drop forging is widely used and the tup and anvil are replaced by dies. Matching dies fit into the anvil and the tup. The dies have a series of grooves and depressions cut into them and the work piece is passed in sequence through a shaping series.
These stations have names such as fullering, blocking, edging, bending and cut off. Where several stages are involved, care must be taken to ensure that the metal does not become excessively chilled before the last station is reached. To ensure that the die cavity is completely filled the volume of the starting billet is greater than that of the final forging. The excess metal appears as a "flash" at each stage, this is a thin fin around the perimeter of the forging at the parting line. This flash is cut away in a further press operation generally at a high temperature. The weight of flash may be a small percentage of the total weight for forgings of simple shapes but may exceed the weight of the actual forging for those of complex shape.
Each size and shape of forging will thus require a separate set of forging and trimming dies. The production tolerance for the initial metal must involve excess, e.g. ~10 mm. The over-tolerance metal is accommodated by a gutter around the die cavity which allows the formation of the fin referred to earlier.
This process was developed originally to gather, or upset metal to form heads on bolts. Today the purpose of this machine has been broadened to include a wide variety of forgings.
It is essentially a double-acting press with horizontal motions rather than vertical. The forging machine has two actions. In the first, a movable die travels horizontally towards a similar stationary die. These two dies have semi-circular horizontal grooves, which grip the bars. A bar heated at the end is inserted between the movable and stationary die. While thus held, the end of the bar is upset or pressed into the die cavity by a heading tool mounted on a ram, which moves towards the front of the machine.
If hexagon heads are desired, a heading tool will upset some of the metal into a hexagon-shaped die cavity. For more complex forgings, as many as six different dies and heading tools may be used in turn in a similar manner to the different stations in die drop forging.
Whereas impact forging usually involves a mechanical press, press forging, on the other hand, requires hydraulic power. The largest forgings are invariably produced on large hydraulic presses. These have vertically moving rams, which move down slowly under considerable pressure.
A typical press forge would be capable of loads of the order of 6000 to 10000 tones. Forgings up to 100 tones weight can be handled easily in this forge and the highest-quality products are manufactured by this technique.
Structure and Properties of Forgings
Forgings are invariably produced by the hot-working process and this controls the resultant structure and properties. There are, however, important differences in forgings produced by different techniques.
The fact that the impact forge applies a stress for a very short period compared to the long period for the press forge results in totally different structures in the product. In the case of impact, the mechanical working is concentrated in the surface layers, since rapid removal of the stress after the blow results in metal relaxation before the effect of the blow has penetrated into the center. Impact forging of a large "as cast" piece of metal at high temperature will result in a very inhomogeneous structure, the outside layers showing a typical hot-worked structure whilst the center is still as cast. Any attempt to achieve greater penetration by increasing the impact load usually leads to internal cracking. Impact forging is therefore limited to relatively small work pieces.
Press forging invariably results in total penetration of the effect of the applied stress into the center of the work piece. The process is generally less severe on the metal than impact. The end result is a more homogeneous product having very high quality. Since the process is much slower and the equipment used is much larger, press forged articles are more expensive than impact forged components.
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