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Forging Forging is a basic process in which the work piece is shaped by application of compressive forces with the help of tools and dies. Some commonly used forged parts are; large turbine rotors, gears, bolts, rivets etc. Rolling operation produces continuous parts, where as Forging operation produces discrete parts.

Metal parts which are forged have good strength and toughness characteristics, and are highly reliable in high stress and critical operations

Depending upon the Homologous temperature, forging can be termed as Hot or Cold forging

Cold forging is carried out at room temperature and require higher energy input. The work piece under cold forging process must possess adequate amount of ductility at room temperature so as to avoid any sort of cracking while undergoing deformation. The parts produced have good surface finish and dimensional accuracy. Hot forging needs comparatively lower energy input, but has lower dimensional accuracy and surface finish. Open Die Forging In this process, a solid work piece is placed between two flat dies which the height of the work piece is reduced by application of compressive forces. This process is also called up-setting. The figure below illustrates the Open Die forging process of a solid cylindrical billet; without friction and with friction (barrelling).

Due to friction, actual processes, the part develops a barrel shape, it is also called pancaking.

Barrelling can be avoided by using an effective lubricant. This condition can also develop in hot workpieces between cold dies. This can be

avoided by using heated dies. Cogging (Drawing out) is also an open die operation in which the bar thickness is reduced by successive forging steps at specific intervals. The figure below illustrates the cogging operation for a rectangular bar.

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For an open die forging process, the forging force F on a solid cylindrical piece is calculated by the formula

Impression Die Forging In this process, the work piece takes the shape of the die cavity when forged between two shaped dies. For lower energy requirement and enhanced ductility of the work piece, this process is performed at elevated temperatures. During deformation, some of the metal flows outward and forms a flash, and this plays an important role of making the metal flow in the direction of least resistance, thereby completely filling the cavity. The figure below illustrates the process:

Refer to the figure given below, for standard terminology used in a forging die

The blank is prepared by:

a) Cropping from an extruded bar stock b) Performing from operations like powder metallurgy c) Casting

The blank is placed on the lower die. As the upper die begins to descend, the blank

changes shape Performing operations are done to distribute the material properly into various

regions In fullering material is distributed away from an area In edging the material is gathered into a localized area

Forging force for an impression die forging process can be calculated from the formula;

Where k = multiplying factor with typical values ranging from 3-5 for simple shapes without flash to 5-8 for simple shapes with flash. It ranges from 8-12 for complex shapes with flash. A is the projected area of the forging.

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Typical Forging Process 1. Preparing a slug, billet or a preform by processes like cropping (shearing), sawing etc.

Post preparation, the preform is cleaned at its surface by shot blasting. 2. For process of Hot Forging, the work piece is heated in a furnace, and then it is descaled

with a wire brush (if necessary). 3. For Hot forging, preheating and lubrication of dies is performed. Blank is also lubricated. 4. After forging the billet, remove any excess material (flash) by trimming, machining or

grinding. 5. Clean the forging, check for dimensions and machine to final shape (if necessary). 6. Perform any heat treatment operations, if required. 7. Inspect for any defects (internal or external).

Generally, dimensional tolerance ranges between 0.5 and 1% of the

dimensions of the forging. Factors contributing to dimensional inaccuracies:

1. Draft angles 2. Die wear 3. Die closure 4. Radii 5. Fillets

Surface finish of the forging depends upon the following factors: 1. Blank preparation 2. Die surface finish 3. Die wear 4. Effectiveness of lubricant

Various Forging Operations Coining: It is a closed die forging process, and is used to mint coins, medallions and jewellery. To produce the required detail, the pressure applied is almost 5 to 6 times the strength of the material.

Lubricants cannot be used in coining, because they get entrapped in the cavity and prevent full reproduction of shape details.

Heading: It is also called upset forging. It is an upsetting operation, usually performed on the end of a round rod or wire in order to increase the cross section. Some of the products prepared from this operation are nails, bolt heads, screws, rivets and various other fasteners. This process can be carried out cold, warm or hot.

There is a tendency of buckling during the heading process, if its unsupported length-to-diameter ratio is too high.

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Heading operations are carried out on machines called headers. The figure below illustrates the heading operation to form heads on fasteners, such as nails and rivets.

Piercing: This is a process of indenting (not breaking through) the surface of a work piece with the help of a punch to create a cavity or an impression. The work piece may be constrained or unconstrained. The deformation of the work piece will depend on how much it is constrained from flowing freely as the punch descends. An example of this operation is the creation of a hexagonal cavity in the bolt heads.

The piercing force depends upon the following factors: Cross sectional area and tip geometry of the punch Strength of the material Magnitude of friction at the sliding interfaces

Isothermal Forging: It is also known as hot die forging. In this process, the die and the work piece are at the same temperature. Due to hot work piece, flow strength and high ductility is maintained. This results in low forging load with an enhanced material flow in the cavity. The process has high dimensional accuracy. The die for this process are usually made from Nickel or Molybdenum alloys. It is an expensive process and is generally employed to make intricate parts of Aluminium, Titanium and superalloys. Swaging: This process is also called rotary forging, radial forging or rotary swaging. The figure below shows the process in diagrammatically;

The work piece is stationary and the work piece rotates, with the help of rollers in a cage, somewhat similar to roller bearing. Like this a solid rod or tube is subjected to radial impact forces by a set of reciprocating dies. Typical products from this process are; screwdriver blades,

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solder iron tips. The process is also used for operations like pointing (tapering the tip of a cylindrical part) and sizing (finalizing the dimensions of a part).

The swaging process is limited to a maximum work piece diameter of 150 mm Swaging is a versatile process, limited only by the length of the bar supported by

the mandrel, as shown in the figure below;

Tube Swaging: In this process, the internal diameter or thickness of the tube is reduced with or without using internal mandrels. The figure below, illustrates this process with a diagram;

For rifling a gun barrel, swaging is performed on a tube over a mandrel with spiral grooves

Forgeability Forgeability is defined as the capability of a material to undergo deformation without cracking. Two tests which quantify this characteristic of a material are;

1. Upsetting: In this, a solid cylindrical specimen is upset between flat dies, and the reduction in height at which cracking on the barrelled surface begins, is noted. The greater the deformation prior to cracking, greater is the forgeability.

2. Hot twist: In this method, a round specimen is continuously twisted in the same direction until it fails. This test is performed on a number of specimens at different temperatures, and the number of turns before failure for each material is plotted. The temperature at which the maximum number of turns occur, is termed as the temperature for maximum forgeability. This test is done usually for steeld.

Forgeability of a material depends upon: Ductility of the material Strength of the material Forging temperature required Friction behaviour Quality required for a forging

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Die Materials As most of the forging operation is carried out at elevated temperatures, general requirements for a die material are as follows:

Strength and toughness at elevated temperatures Hardenability and the ability to harden uniformly Resistance to mechanical and thermal shocks Wear resistance (scale formation during hot forging)

Some typical die materials are as follows: CHROMIUM, NICKEL, MOLYBDENUM, VANADIUM.