In this article we will discuss about:- 1. Introduction to Forging 2. Temperature Range for Forging 3. Advantages and Disadvantages 4. Guidelines 5. Open Die Forging 6. Operation 7. Closed Die Forging Die Design 8. Characteristics 9. Defects 10. Forces 11. Forces.
- Introduction to Forging
- Temperature Range for Forging
- Characteristics of the Forged Parts
- Guidelines for Forging
- Open Die Forging
- Operation Performed by Forging
- Closed Die Forging Die Design
- Defects in Forged Parts
- Forces in Forging
- Forces of Forging
- Advantages and Disadvantages of Forging
1. Introduction to Forging:
Forging can be defined as the controlled plastic deformation of metals at elevated temperatures into a predetermined size or shape using compressive forces exerted through some type of die by a hammer, a press or an upsetting machine. It consists essentially of altering the shape and section of a steel specimen by pressing or hammering it at a temperature of the order of 1000°C, at which the steel is entirely plastic and will flow under pressure.
By definition, forging involves the shaping of metal by the application of impact or pressure but the primary difference between the various forging methods is the rate at which the energy is applied to the work-piece. The forging hammer has a relatively high rate of deformation, while the hydraulic press has a relatively slow rate of deformation. The raw material for forging is usually a bar, billet, or blank.
By forging, the original crystals are deformed and many of the constituents are precipitated at high temperature which again becomes soluble in the solid iron on freezing, thereby increasing the local homogeneity of the material. Forging is generally employed for those components which require high strength and resistance to shock or vibration and uniform properties.
However, by forging, the insoluble segregates and the non-metallic inclusions cannot be dispersed, but their undesirable effects are reduced in relation to the properties parallel to the main direction of flow of the material during forging.
The structure formed by flowing of these segregated phases in the direction of working is called grain fibre, fibre structure of flow lines. With pronounced grain flow the properties like elastic limit, ultimate tensile strength are steadily improved upto a limit in a direction parallel to the principal grain flow.
The properties in transverse direction may increase at first due to preliminary working of the metal but the new directional grains become more and more defined and the properties in transverse direction fall off rapidly and may be lower than even those exhibited by a cast structure.
2. Temperature Range for Forging :
Forging should be carried out at temperature high enough so that steel can be easily deformed by applied loads. It should be above recrystallization temperature to avoid work hardening (and thus depends on carbon content of steel) but should not be too high so as to cause excessive grain growth and burning.
The suitable range is 1150 to 1300°C for steel with carbon content of 0.2% and 1000—1500°C for steel with carbon content of 0.7%.
3. Characteristics of the Forged Parts:
(a) It refines the structure of metal by closing up the cavities and by smashing up large grain formations.
(b) Forged parts have directional properties and hence have good strength.
(c) Mechanical properties such as percentage elongation, percentage reduction of area and resistance to shock and vibration are improved.
(d) Cracks and blow holes are minimised.
4. Guidelines for Forging:
There are certain design rules which should be adhered to for obtaining sound forgings:
i. Parting line for two dies should be so chosen that the part lies entirely in one die half, there being no extrusion of part to other half of die. Parting line should be at half the height of part.
ii. Parting line should lie in different planes but be co-planer. Parting line should be so located that metal flows parallel to the parting line.
iii. Tapers assisting in forging and ejection of part should be provided on part and dies also.
iv. Sharp changes in cross-sections be avoided. Also avoid the cross-sections which project excessively into the die as it would be difficult to eject such parts.
v. Very thin sections should also be avoided.
5. Open Die Forging:
It makes use of relatively simple tooling. Numerous squeezes or blows are applied to portions of the piece. To present new material, the piece is moved in between. The working to shape is gradual; employing a step-wise process.
It is used to change the shape of raw ingot and improve the mechanical properties by closing and welding internal cavities and porosity, and by refinement of coarse and heterogeneous metallurgical structure of ingot.
6. Operation Performed by Forging:
Various operations performed are:
i. Cogging (Blocking, Drawing):
The raw ingot is progressively extended to elongate shape. The tools used may be flat., swage, or vee-set form. Flat tools are rectangular block with founded edges so that the impressions made by tools have a similar contour to prevent surface folding. Swage tools are suitable for working round forgings and give faster forging.
There is a limited range of sizes that can be made by one set of swage tools. Vee-set tools consist of flat top tool and a vee bottom tool and these are used for working round forgings with several changes in size along the length.
It consists to reducing the height of a forging and increasing its diameter. Usually completely overlapping tool is used.
iii. Expanding (Becking, Saddling):
It is the process of increasing the diameter of hollow products. It uses a narrow tool. Forging is supported on an expanding bar which is rotated between squeezes. Thinning is progressively effected round the circumference and the diameter of the stock is proportionately increased.
iv. Hollow Forging:
The raw material that is available for any forging operation is in the form of a bar or billet.
The formation of shape by forging consists of a combination of two or more number of relatively simple operations described below:
It is the operation of spreading or thinning action and is accomplished by striking the work with flat dies. Due to impact of die on metal, its thickness is reduced and length is increased.
(2) Upsetting (Refer Fig. 5.22):
This is just opposite to drawing and involves increasing of the cross-sectional area usually by pressing or hammering in a direction parallel to the original ingot axis. In the process of upsetting, the shaft or rod is generally gripped in dies, and the head or flange upset either by a plain flattened ram or with further dies, shaped to give the desired contour.
(3) Punching (Refer Fig. 5.23):
It is process of producing holes generally cylindrical by using a hot punch over a cylindrical die.
(4) Setting Down:
It is local thinning down operation effected by the set hammer or set. Usually the work is fullered at the place where the setting down commences.
It is one of the most important processes of forging and is very frequently used. Bends may be classified as sharp cornered bends or more gradual bends. The operation is performed by hammering the metal over the edge of the anvil, or over a block of metal held in a vice.
When the metal is bent by hammering, the outer and inner surfaces do not remain same. The inside surface is shortened while the outer surface is stretched which causes bulging of the side at the inner surface and a radius on the outer surface. If a sharp corner is required, an additional metal is required at the place where the bend occurs in order to permit stretching of metal at outer surface.
Metals like wrought iron and steel are welded by pressing together two surfaces, after they have been raised to the correct welding temperature. Normally these metals come in the plastic stage at 1350°C, when the metal is white-hot. The operation of such a type of welding is performed in forge shop and hence is also sometimes called forge welding.
In order to perform a rapid cutting operation by chiseling, the metal is heated in blacksmith fire to a temperature of 850 to 900°C and then hammer blows are directed on the chisel head. If the thickness of metal to be cut is more, then two notches or grooves are made 180° apart.
(8) Flat-Die Forging:
It is the simplest type of forging in which the stock is laid on a flat anvil and the hammer (or press) with its own flat face is hammered upon the work. The work may be turned either by tongs or by a machine called a manipulator which grasps the work and gradually turns it between blows of the hammer.
In this operation much depends upon the skill of the Hammersmith who directs the forging stock on the anvil, and the speed and intensity of the forging blow. This method is used if the work is bulky and simple, and the quantity to be produced in small. The flat dies may be supplemented by dies with fullering or edging devices for spreading or trimming the forging.
By this process the stock may be rounded, reduced and lengthened, or a thick piece may be flattened. Generally the forgings produced by using open dies require machining all over to adopt them to use in mechanical assembly, but it is possible to work with sufficient exactness is some cases to eliminate machining except where parts fit together.
(9) Impression-Die or Closed Die Forging:
This is used in cases where the piece is small and intricate in shape. A piece of heated metal is placed on the lower die block. The metal is forced to take the shape of the die (upper and lower) by blows from a machine hammer. Half the die is mounted on the anvil and half is mounted on the hammer.
Greater accuracy can be achieved and production rate is also higher with closed dies than with flat dies. During forging the cavities of the die are completely filled, and excess metal (provided purposely) is squeezed out between the dies in the form of a thin fin or flash. It may be necessary to perform several operations in steps to obtain the desired deformation, often within a single pair of die blocks.
It is the term applied to the first operation in an impression-die forging process. It is actually same as setting down and consists of reducing cross-section of the work piece or lengthening a preparation of the stock in preparation for subsequent operations.
It is the term applied to the operation of giving an intermediate shape to work. It follows fullering and precedes the final finished impression. Blocking dies are formed without sharp corner and with very generous radii. The number of blocking impressions depends upon the size and intricacy of the required forging.
(12) Closed Die-Forging:
It is a special form of impression-die forging which is usually performed on an up setter. The material is deformed in a cavity in such a way that it has no chance to escape or to form flash.
It is the operation of removing flash from a forging. It is done by a hammer or a press and consists of pushing the work through a trimming blade with a punch.
(14) Coining and Ironing:
There are sizing operations performed in dies by applying pressure to all or part of the forging to obtain closer tolerances, smoother surfaces, or to eliminate draft. Specific coining dies are used for this operation.
Coining is usually done on surfaces parallel to the parting line, while ironing usually means pushing the work piece through a ring to size its outside diameter. Very little metal flow accompanies these processes.
7. Closed Die Forging Die Design:
Die life depends on the shape, size and detailed design of a forging as well as on the forging stock. The life is greatly decreased by sharp fillets and corner radii, thin flanges, deep narrow fins, and unnecessarily close tolerances.
To facilitate removal of forgings from the die, draft of 5 — 7° for outside faces and 7 — 10° for inside face must be provided. (Refer Fig. 5.26 (a)) However where mechanical ejectors are provided, draft angle can be reduced to 2 — 4°.
Fillet and corner radii are necessary to avoid the formation of forming defects and to prolong die life. Die wear is common at the entrance radius to the flash land and this area should not be selected for jig location points.
As far as possible, parting line (the line separating top and bottom dies) should be straight or symmetrically cranked. Asymmetrically cranked parting line should be avoided. (Refer Fig. 5.26(b)).
8. Defects in Forged Parts:
(1) A mismatch occurs in drop-forging when the dies are incorrectly aligned, and results in a lateral displacement between portions of the forging.
(2) Scale pits are shallow surface depressions, caused by not removing scale from the dies. The scale is subsequently worked into the surface of the forging.
(3) An unfilled section is similar to a mis-run in casting and occurs when metal does not completely fill the die cavity. It is usually caused by using insufficient metal or insufficient heating of the metal.
(4) Defects resulting from the melting practice, such as dirt, slag and blow holes.
(5) Ingot defects such as pipes, cracks, scabs and segregation.
(6) Defects resulting from improper heating and cooling of the forging such as burnt metal, decarburised steel and flakes.
(7) Defects resulting from improper forging such as seams, cracks, laps etc.
The above mentioned defects are mostly found in all metals which are heated to plastic stage and then shaped.
(i) Forces in Forging of Strip:
Let us consider forging of strip of length 2l, and thickness t.
In forging, at free ends, i.e. at x = 0, and x = 2l, the forces will be symmetrical about centre of forging (i.e. x = I). Between 0 and I, a sliding between the work piece and the dies will take place to allow required expansion of work piece.
However beyond a certain point, say at x = x’, no sliding will take place between the work piece and the dies, because the increasing frictional stress reaches the maximum value equal to the shear yield stress at x = x’ and remains so from x = x’ to x = I.
Thus shear stress
ss = µp (0 ≤ x ≤ x’) (sliding or non-sticking zone)
and ss = shear yield stress K (x’ ≤ x ≤ l) (sticking zone)
A strip of size 24 x 24 x 200 mm is to be forged four times, keeping length constant. Determine the forging force, taking coefficient of friction between strip and die as 0.25. Average yield stress of strip in tension is 7 N/mm2.
Four times reduction in thickness will mean final thickness is 6 mm and width = 96 mm.
(ii) Forging Forces in Forging of Disc:
Let us consider a disc of radius R and thickness t. Beyond a certain radius r’, a sliding takes place at the interface to allow the radial expansion of the work piece.
(iii) Work Done in Forging:
The work done in forging or any deformation process consists of ideal work of deformation, work to overcome friction at metal tool interface, and in internal shaping process due to non-uniform deformation because of friction.
In case of open die forging between upper and lower die with lateral flow at sides, the variation of pressure (p) between dies, and the variation of horizontal stress (sx) along length of die is shown in Fig. 5.28 for the case of sliding friction, i.e. no constraint for flow of metal at sides.
However with ends constrained, the metal does not slide along the die face (sticking friction). Sliding friction exists near the end of work piece and sticking friction exists at some distance near the centre. Frictional shear stress at die interface should be lower than yield stress of material.
In case of hot forging, due to high friction (0.5 to 0.6), sticking friction extends, over the entire interface.
In case of closed die forging, value of constant is 1.2 to 2.5 for upsetting a cylinder between flat dies, and 3 to 8 for closed die forging of simple shapes with flash, and 8 to 12 for complex shapes.
10. Examples of Forging:
Any job by forging involves at least two of the above mentioned forging operations. Here is given a brief description of simple parts produced by forging.
Making the head of a bolt (Refer Fig. 5.25):
(а) Heat one end of the bar for a sufficient length to make the head by repeated hammering on the anvil.
(b) Flatten the head by hammering against the end of bush through which shank can pass.
(c) Swage head to size.
(d) Forge camber on the head by a cupping tool.
11. Advantages and Disadvantages of Forging:
The forgings have high strength and ductility and offer great resistance to impact and fatigue loads due to extra working during the process and the opportunity of aligning the grain flow. Forging improves the structure of the metal and hence its mechanical properties.
Forging distorts the previously created unidirectional fibre (obtained due to rolling which spreads the segregation of impurities into fibrous structure) in such a very as to strengthen the component. Because of intense working, flaws are seldom found and the work piece has a high reliability.
Forging renders the parts uniform in density as well as dimensions. It also allows the material to be displaced where it is needed, thus effecting a considerable weight and cost reduction.
The initial cost of dies and the cost of their maintenance is high. The rapid oxidation of metal surface at high temperature results in scaling which wears the dies.
Some materials are not readily worked by forging. Forging operation is limited to simple shapes and has limitation for parts having under-cuts, re-entrant surfaces etc.