Forging is a process in which material is shaped by the application of localized compressive forces exerted manually or with power hammers, presses or special forging machines. The process may be carried out on materials in either hot or cold state. When forging is done cold, processes are given special names. Therefore, the term forging usually implies hot forging carried out at temperatures which are above the recrystallization temperature of the material.
Forging is an effective method of producing many useful shapes. The process is generally used to produce discrete parts. Typical forged parts include rivets, bolts, crane hooks, connecting rods, gears, turbine shafts, hand tools, railroads, and a variety of structural components used to manufacture machinery. The forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications.
A variety of forging processes have been developed that can be used for either producing a single piece or mass – produce hundreds of identical parts. Some common forging processes are:
Open – Die Hummer Forging.
It is the simplest forging process which is quite flexible but not suitable for large scale production. It is a slow process. The resulting size and shape of the forging are dependent on the skill of the operator.
Open die forging does not confine the flow of metal, Fig 2.1. The operator obtains the desired shape of forging by manipulating the work material between blows. Use may be made of some specially shaped tools or a simple shaped die between the work piece and the hammer or anvil to assist in shaping the required sections (round, concave, or convex), making holes, or performing cut – off operations. This process is most often used to make near – final shape of the part so that some further operation done on the job produces the final shape.
Forging Force. In open die forging operation, the forging force F, to be applied on a solid cylindrical component can be determined from the relation.
Where s f is the flow stress of the material, µ is the coefficient of friction, and d and h are the diameter and height of the work piece, respectively.
Example. Using open-die forging operation, a solid cylindrical piece of 304 stainless steel having 100 mm dia x 72 mm height is reduced in the height to 60 mm at room temperature. Assuming the coefficient of friction as 0.22 and the flow stress for this material at the required true strain as 1000 MPa, calculate the forging force at the end of stroke.
Solution . Initial diameter = 100 mm
If final diameter is d, (100)2 x 72 = d2 x 60
Impression – Die Drop Forging (Closed – Die Forging)
The process uses shaped dies to control the flow of metal. The heated metal is positioned in the lower cavity and on it one or more blows are struck by the upper die. This hammering makes the metal to flow and fill the die cavity completely. Excess metal is squeezed out around the periphery of the cavity to form flash. On completion of forging, the flash is trimmed off with the help of a trimming die.
Most impression – die sets contain several cavities. The work material is given final desired shape in stages as it is deformed in successive cavities in the die set. The shape of the cavities cause the metal to flow in desired direction, thereby imparting desired fibre structure to the component.
Auto – Forging:
This is a modified form of impression – die forging, used mainly for non – ferrous metals.
In this a cast preform, as removed from the mold while hot, is finish – forged in a die. The flash formed during die forging is trimmed later in the usual manner. As the four steps of the process – casting, transfer from mold to the forging die, forging, and trimming are in most applications completely mechanized, the process has acquired the name Auto – forging.
It is a closed – die forging process used mainly for minting coins and making of jewelry. In order to produce fine details on the work material the pressures required are as large as five or six times the strength of the material. Lubricants are not employed in this process because they can get entrapped in the die cavities and, being incompressible, prevent the full reproduction of fine details of the die.
Net - shape Forging (Precession Forging)
Modern trend in forging operation is toward economy and greater precision. The metal is deformed in cavity so that no flash is formed and the final dimensions are very close to the desired component dimensions. There is minimum wastage of material and need for subsequent machining operation is almost eliminated.
The process uses special dies having greater accuracies than those in impression – die gorging, and the equipment used is also of higher capacity. The forces required for forging are high. Aluminum and magnesium alloys are more suitable although steel can also be precision – forged. Typical precision – forged components are gears, turbine blades, fuel injection nozzles, and bearing casings.
Because of very high cost of toolings and machines, precision forging is preferred over conventional forging only where volume of production is extremely large.
Forging Force Requirement:
The forging force, F, required to forge material by impression – die forging operation can be determined by the relation
where k is a constant (whose value can be taken from Table 2.1 s f is the flow stress of material at the forging temperature, and A is the projected area of the forging including the flash.
In hot forging of most non – ferrous metals and alloys, the forging pressure is generally in the range of 500 MPa to 1000 MPa.
Press forging, which is mostly used for forging of large sections of metal, uses hydraulic press to obtain slow and squeezing action instead of a series of blows as in drop forging. The continuous action of the hydraulic press helps to obtain uniform deformation throughout the entire depth of the workpiece. Therefore, the impressions obtained in press forging are more clean.
Press forgings generally need smaller draft than drop forgings and have greater dimensional accuracy. Dies are generally heated during press forging to reduce heat loss, promote more uniform metal flow and production of finer details.
Hydraulic presses are available in the capacity range of 5 MN to 500 MN but 10 MN to 100MN capacity presses are more common.
Upset forging involves increasing the cross – section of a material at the expense of its corresponding length. Upset – forging was initially developed for making bolt heads in a continuous manner, but presently it is the most widely used of all forging processes. Parts can be upset – forged from bars or rods upto 200 mm in diameter in both hot and cold condition. Examples of upset forged parts are fasteners, valves, nails, and couplings.
The process uses split dies with one or several cavities in the die. Upon separation of split die, the heated bar is moved from one cavity to the next. The split dies are then forced together to grip the and a heading tool (or ram) advances axially against the bar, upsetting it to completely fill the die cavity. Upon completion of upsetting process the heading tool comes back and the movable split die releases the stock.
Upsetting machines, called upsetters, are generally horizontal acting.
When designing parts for upset – forging, the following three rules must be followed.
This process is used to reduce the thickness of round or flat bar with the corresponding increase in length. Examples of products produced by this process include leaf springs, axles, and levers.
The process is carried out on a rolling mill that has two semi – cylindrical rolls that are slightly eccentric to the axis of rotation. Each roll has a series of shaped grooves on it. When the rolls are in open position, the heated bar stock is placed between the rolls. With the rotation of rolls through half a revolution, the bar is progressively squeezed and shaped. The bar is then inserted between the next set of smaller grooves and the process is repeated till the desired shape and size are achieved.