07-12-2022

Forging

Introduction: Forging
 Forging is a deformation process in which the work is compressed
between two dies, using either impact or gradual pressure to form the
part.
 Forging is said to be a process of shaping and forming metals through
by performing some operations such as hammering, rolling or pressing.
 The process usually starts with a stock present initially known as starting
stock, generally a cast ingot (or a “cogged” billet which already has
been forged out from a cast ingot), which then undergoes heating till its
plastic deformation temperature, then “kneaded” or upset between
dies to the required size and shape.
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Uses and Application of Metal

Forgings
 Metal forgings can be a small automobile part or a huge part for the construction of
a machine. The application of forging can be found across the engineering
industries:
 Aircraft manufacturing-Jet engine shafts, turbine items, landing gear, etc.
 Automobile industry-Crankshafts, connecting rods, gears, levers, valve stems, cam,
axles, etc.
 Hand tools-Open end spanners, ring spanners, Allen keys, hook spanners, wrenches,
pliers, etc.
 Hardware items-Screws, bolts, rivets, nails, crane hooks, etc.
 Kitchenware like knives, tongs, etc.
 Military-Barrel of the guns are cold-forged
 Jewelry industry-Gold, silver, and platinum jewelry.

Forging
 Forging refines the microstructure of the metal, eliminates the
hidden defects such as hair cracks and voids, and rearranges
the fibrous macrostructure to conform with the metal flow. By
successful design of the dies, the metal flow during the process
can be employed to promote the alignment of the fibers with
the anticipated direction of maximum stress.
 The process begins with starting stock, usually a cast ingot,
which is heated to its plastic deformation or above
recrystalisation temperature, then upset or "kneaded" between
dies to the desired shape and size.
 During this hot forging process, the cast, coarse grain structure is
broken up and replaced by finer grains. Low-density areas, gas
porosity and microshrinkage inherent in the cast metal are
consolidated through the reduction of the ingot, achieving
structural integrity.
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Forging
 Mechanical properties are therefore improved through the
elimination of the cast structure, enhanced density, and
improved homogeneity.
 Forging also provides means for aligning the grain flow to
best obtain desired directional strengths

Grain Flow Comparison Forged Bar: Forging: advantages

 Forged parts have a better grain structure and strength compared to
 Forged Bar: casting or machining.
Directional alignment through the forging process has been  Forging improves the structural integrity of the part, by taking care of the
deliberately oriented in a direction requiring maximum defects in the cast workpiece viz. porosity, voids, cracks, shrinkage,
inclusions, etc.
strength. This yields to increase resistance to impact and
fatigue.  Forging forms the workpiece metal to the nearest geometric shape of the
part, and this results in the reduction of material cost and machining cost.
 Machined Bar:
 Forged parts have better tolerance and higher reliability compared to cast
Unidirectional grain flow has been cut when changing or machined parts.
contour. This renders the material more liable to fatigue and  The forging process produces parts with consistent metallurgical quality and
more sensitive to stress corrosion cracking. with good strength and fatigue properties.
 Cast Bar:  For a part like a crane hook, forging is the only process that gives the
required strength due to its grain flow direction.
No grain flow or directional strength is achieved through the
casting process.  Forging modifies the microstructure of the workpiece metal and the forged
part has a finer grain structure with the grain flow in the shape of the part.
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Limitations of Forging

 High initial cost towards forging machinery, forging setup, and forging
dies. The forging process requires trained personnel, and the
workshop building should be able to absorb the vibration and shock
generated by the forging operations.
 The process is hazardous since the person has to deal with hot metal
and the sound of the presses and hammers.
 Complex shapes cannot be forged.

Open die forging Closed die forging Cold forging

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Open Die Forging or Smith Forging Closed Die Forging

 Smith forging is an open die forging process. When a workpiece
metal is heated and forged by applying repeated hammer blows,
the metal in the plastic condition deforms and flows. The flow and
deformation can be restricted by using a die with a cavity or can  There are mainly three different types of closed die forging
be manipulated with a flat die. When a die with a cavity is used, processes. They are:
the metal in plastic condition takes the shape of the die cavity.  Drop forging
 When the die is flat (no cavity), the smith/worker manipulates the  Press forging
hot metal and shapes it to the desired shape by repeated blows of
the hammer, and since the flat die is used this process is known as  Machine forging
open die forging. The name “open die” indicates that there is no
cavity in the die to restrict the plastic flow of the metal.
 When open die forging is done manually using an anvil and a
handheld hammer it is called hand forging and when it is done
using a power hammer it is called power forging. In both cases, the
smith manipulates the workpiece manually.

Drop Forging
 Drop forging needs a set of die (top die and bottom die).
The bottom die is fixed to the bottom bolster or drop
hammer platform. The top die is fixed to the movable ram.
 Workpiece heated to the recrystallization temperature is
placed on the fixed die and repeated impact force of the
hammer is applied 3 to 4 times.
 More localized deformation takes place at the upper and
lower layer of the workpiece. The surface will be harder
than the workpiece metal. The deformation is not uniform
through the thickness of the metal.
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Press Forging Hot Forging

 Press forging also uses a closed die (set of upper and lower die). However,
unlike in drop forging (where impact force is applied repeatedly), press
forging uses a continuous increase in pressure to perform the squeezing  The other reasons that favours the application of hot-forging are:
operation. The metal is continuously squeezed between the upper and
lower die till the metal flows and fills the cavity. The workpiece metal takes 1. Strength coefficient is substantially less than at room temperature.
the shape of the cavity when the die closes. There may be a flash formation.
2. Strain hardening exponent is zero (theoretically).
 As in drop forging, in press forging also the bottom die is fixed and the top
die is clamped to the movable bolster. The press used can be mechanical 3. Ductility is significantly increased.
or hydraulic, however, the hydraulic press offers more squeezing pressure.
 Due to the high pressure, homogeneous and uniform flow of metal takes  The other reasons that favours the application of hot-forging are:
place and the uniform deformation over the thickness is ensured. The
surface of the forged part is smoother than that obtained in drop forging. 1. Strength coefficient is substantially less than at room temperature.
 The force used in press forging is sufficiently high and can be more than
twice of drop forging. Due to the high pressure required, press forging is used 2. Strain hardening exponent is zero (theoretically).
for comparatively smaller size parts.
3. Ductility is significantly increased.
 Press forging can be cold or hot forging.

Machine Forging Hot Forging

 The machine forging is also known by the name upset forging. Even
though, you use a machine for drop forging as well as press forging, in
 Advantages of Hot Forging :
the forging industry “upsetting forging” is known by the name “machine 1. Lower forces and power requirement than cold working.
2. More intricate work geometries are possible to process.
forging”.
3. Need for annealing may be reduced or eliminated.
 Disadvantagesof Hot Forging :
1. Lower dimensional accuracy.
 Machine forging is used for the upsetting forging process, normally for 2. Higher total energy required (due to the thermal energy to heat
forming a bolt head, rivet head, and can also be used for automotive the workpiece).
3. Work surface oxidation (scale), resulting in a poorer surface finish.
spindle and axles that need upsetting. 4. Shorter tool life.
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Cold Forging

 The cold-forging is the operation of forging product or components

at the room temperature or above the room temperature, but far
below than the recrystallization temperature of the product.
 Advantages of Cold Forging :
1. Higher dimensional accuracy
2. Lower total energy required.
3. There is no work surface oxidation or scale, resulting in high
surface finish
4. There is a longer tool life.
 Disadvantages of Cold Forging :
1. It requires larger forces and power than hot forging.
2. The cold forging cannot be used for complex and intricate shape
work parts.
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Types of Forging Machines

 Hammers : They are the most common types of machine used. They are
often preferred for small to medium batches because of quicker tool
setups and lower overheads.
 They are also used for elongated and branch-type forgings because die
areas can be provided for the larger number of preform dies required for
such shapes.
 Screw Presses : In screw presses, the upper ram and die are connected to
a large vertical screw that can be rotated by a flywheel, so that the ram
can move up and down relative to the fixed die in the bed of the
machine.
 Hydraulic Presses : Hydraulic presses are available in a wide range of sizes
up to the largest at 50,000 tons or more capacity. The moving die is
attached to a ram actuated by a large hydraulic cylinder. Various strokes,
forces, and closing speeds can be obtained on hydraulic presses.

FORGING EQUIPMENT
 Equipment used in forging consists of forging machines, classified as Forging hammers-Board hammer
hammers or presses, and forging dies.
 The force is supplied by a falling weight
of ram.
 In addition, auxiliary equipment is needed, such as furnaces to heat  Deformation of work piece is due to
the work, mechanical devices to load and unload the work, and the application of the kinetic energy of
trimming stations to cut away the flash in impression-die forging. the ram.
 Board hammer
 Forging equipment may be classified with respect to the principle of  It is a stroke restricted machine.
operation.
 Repeatedly the board (weight) is
raised by rolls and is dropped on the
 Forging hammers die.
 Board hammer  Rating is in terms of weight of the ram
 Power hammer
and energy delivered.
 Forging Presses  strike between 60 and 150 blows per
minute depending on size and
 Mechanical presses capacity
 Hydraulic presses
 Screw presses  Potential energy = mgh
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Forging hammers-Power hammer or Steam hammer

Forging Presses
 Range: 5 kN to 200 kN  Presses apply gradual pressure, rather than sudden impact, to
accomplish the forging operation.
 It uses steam in a piston and cylinder
arrangement  Forging presses are of either mechanical or hydraulic design.
 It has greater forging capacity. the Presses are rated on the basis of the force developed at the
ram is accelerated on the down end of the stroke.
stroke by steam or air pressure in
addition to gravity. Forging presses include
 It can produce forgings ranging from  Mechanical presses,
a few kgs to several tonnes
 Preferred in closed die forging  Hydraulic presses, and
 Screw presses.
 The total energy supplied to the blow
in a power drop hammer is given by

Where m = mass, v = velocity of ram at start of deformation, g =

acceleration of gravity
p = air or steam pressure acting on ram cylinder on down stroke, A = area
of ram cylinder

COMPARISSION BETWEEN BOARD AND STEAM HAMMER: Forging Presses- Mechanical

presses  eccentrics, cranks, or knuckle joints-convert
the rotating motion of a drive motor into
the translation motion of the ram
 convert the rotating motion of a drive
motor into the translation motion of the
ram.
 It has more of squeezing action than
hammering action.
 Hence dies can be smaller and have longer
life than with a hammer.
 load ratings from 300 to 12,000 tonnes
 The total energy supplied during the stroke
of a press is given
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Hydraulic presses
 Hydraulic presses use a
hydraulically driven piston to
actuate the ram.
 Hydraulic presses are load-
restricted machines in which
hydraulic pressure moves a
piston in a cylinder.
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Hydraulic presses Slab Analysis of Forging Operation /CALCULATION

Features of Hydraulic Press FOR FORGING LOAD IN OPEN DIE FORGING/ Plain
1. Full press load is available during the full stroke of the ram. strain in Forging
2. Ram velocity can be controlled and varied during the stroke.
3. It is a slow speed machine and hence has longer contact
time and hence higher die temperatures. The slow squeezing
action gives close tolerance on forgings.
4. Initial cost is higher compared to hammers.
5. Hydraulic presses use a hydraulically driven piston to actuate
the ram.

Comparison between Hydraulic Press and Mechanical Press :

Slab Analysis of Forging Operation /CALCULATION
FOR FORGING LOAD IN OPEN DIE FORGING/ Plain
strain in Forging
Consider the effect of friction on an upset forging operation in plane strain
condition. In plane strain condition, as the work piece of width ‘2a’,
thickness (height) ‘h’ and length ‘L’ as shown in Fig, is reduced in height, it
expands laterally and all deformation is confined in the x-y plane.
Lateral flow of metal perpendicular to the ram travel leads to frictional shear
stress at the die contact surfaces. This surface shear is directed towards the
center line, opposing the metal flow.
Consider the force acting on a vertical element of unit length and width dx. The element is
at some distance x from the central point.
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Consider the force acting on a

Slab Analysis of Forging Operation /CALCULATION FOR vertical element of unit length and
FORGING LOAD IN OPEN DIE FORGING/ Plain strain in width dx. The element is at some
Forging distance x from the central point.

The presence of friction causes an imbalance of the force on the element in

x-direction. The force acting on the left side will be σxh and from the right the
force (σx+ dσx)h. The horizontal compressive stress σx varies from a maximum
at the center of the work piece to zero at the edge and changes by dσx
across the element width dx.
To calculate the total forming load : determine the local stresses needed to
deform each element of a work piece of height h and width 2a
It is assuming that there is no redundant work and the material exhibits rigid-
plastic behavior, and all stress on the body is compressive.

Plain strain in Forging Consider the effect of friction on Balancing the horizontal forces acting on
an upset forging operation in the element
plane strain condition (rigid-plastic  x h  ( x   d x ) h  2 xy dx  0
behavior, see Fig).
In plane strain condition, as the d x 2
  xy (1)
work piece is reduced in height, it dx h
expands laterally and all The von Mises yield criterion for a condition of plane strain is given by
deformation is confined in the x-y 2
plane. 1   3  0  0 '
3
Lateral flow of metal perpendicular to the ram travel leads to frictional shear If we define p and σx as positive compressive principle stresses the p= σz and
stress at the die contact surfaces. This surface shear is directed towards the
1   3   0 '  p   x
center line, opposing the metal flow.
Since σ0’ does not change with ‘x’ (dP/dx)=(dσx/dx) and on substituting in
Consider the force acting on a vertical element of unit length and width dx. Eqn (1) the differential equation of equilibrium becomes
The element is at some distance x from the central point.
dp 2
  xy (2)
dx h
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Plain strain in Forging: Case(i) Sliding Columbic friction

Plain strain in Forging: Case(i) Sliding Columbic friction
If the shearing stress is related to the normal pressure by Coulomb’s law of sliding friction,
(5)
dp 2 (3)
 dx
p h
2 x
ln p    ln C where C is a constant of integration
(4)
h
We can evaluate C by looking at the boundary conditions. At the edge of the workpiece where x=a, Both forging pressure P and longitudinal (σx) build up
σx =0 and p= σ0’ .Therefore to a maximum value at the center of the plate (work),
2 a and reduce to a minimum value at the edge (end) of
ln C  ln  0 ' the plate. When this variation in P and σx is plotted
h over the entire length, L= 2a, a peak exists at the
2 x 2 a center, resembling a “hill”. This plot is called as friction
ln p    ln  0 ' hill. Fig. Friction hill in plane strain upsetting under
h h sliding friction
2
ln p  ln  0 ' (a  x )
h

Plain strain in Forging: Case(i) Sliding Columbic friction

 2 
Forging Pressure p   0 'exp  (a  x)  or (5)
 h 
x 2 x3
The above equation (5) can be simplified if we make use the expansion exp x  1  x    ..... (8)
2! 3!

Since µ usually is a small number. We can approximate exp x as (1+x) for small x thus Eqn (5) becomes
 2 ( a  x) 
p   0 ' 1  
 h (6)

(5)
)
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Plain strain in Forging: Case (ii) High friction condition (Sticky friction
condition)
In the case of sticky friction. If we replace the shear stress is with the friction factor ‘m’, where
τxy=mk. Substituting τxy=mk into Eq. (3) gives

2mk 2 dx dx
dp   dx   0 m   0 ' m
(9)
h 3 h h (11)

x
Integrating p   0 ' m  C Since p= σ0 at x=a
h
ma
C   0 '  0 '
h
x ma
p   0 ' m   0 '  0 '
h h
m
p  0 ' (a  x)   0 ' (12)
h

Plain strain in Forging: Case (ii) High friction condition (Sticky friction
condition)
For condition of sticking friction m=1 eq.(10) becomes

 (a  x) 
p 0 '  1
 h  (13)

(10)

Equation ( 10) gives the mean or average forging pressure. Now, substituting equation (10) in( 8)
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Plain strain in Forging: Case (ii) High friction condition (Sticky friction
condition)
CONCEPT OF FRICTION HILL:

(14)
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Boundary between slipping and sticking friction

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KN