Forging, one of the oldest known metal-forming operations, has come a long way since the days of banging hot steel by hand. However, the fundamental process has remained the same for thousands of years. In every situation, forging involves the application of a compressive stress, which exceeds the flow stress of the metal.
The forging process typically begins with a standard billet, with the objective change its dimensions. In its more sophisticated form it produces complex shapes to a very high degree of accuracy. Forging can either be applied quickly or slowly. The process can be carried out hot or cold. Depending on whether the operation is carried out hot, or cold, forging affects the structure and properties of the forged component to varying degrees. 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, involves the gradual build up of pressure to cause the metal to yield.
Impact forging can be further subdivided into three types:
-Smith Forging
-Drop Forging
-Upset Forging
Smith forging is the oldest type of forging, but it is now relatively unused and considered outdated for most applications. The impact force for deformation of steel is applied manually by the blacksmith by means of a hammer. While being hammered the metal is held with suitable tongs to protect the blacksmith from the excessive heat required to forge the steel.
Drop forging is the modern equivalent of smith forging. limited force of the blacksmith has been replaced by the mechanical hammer. With this system, the quality of products depends on the skill of the worker.
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 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.
Closed-die drop forging is widely used in modern applications. It requires the dies to have a series of grooves and depressions cut into them, allowing the work piece to pass in sequence through a shaping series. 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", 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. Each size and shape of forging will thus require a separate set of forging and trimming dies.
Upset forging: This process was developed originally to 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.
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. However, only press forging should be considered for critical steel components.
Modern forging shops generally utilize hyrdraluic, steam or electric powered presses to generate up to thousands of tons of force to quickly and accurately manipulate a custom shape in hot steel. The automotive, railway, construction and aerospace industries require millions of tons of high quality forgings annually. However, there is a limited number of forging facilities capable of producing forgings under the modern processes accepted by very regulated companies.
An example of a large, well-equipped modern forging facility that performs open and closed die hot forging is Yapi-Tek Celik Sanayi in Turkey. This facility provides one of the best examples of a large-scal operation capable of producing high-quality, custom steel forgings. For more information visit www.celiksanayi.com
