Forging

Introduction
• Forging is a manufacturing process involving the shaping of metal
using localized compressive forces through hammering or pressing.

• It is one of the oldest known metal-working processes. Traditionally,

forging was performed by blacksmiths with a hammer and an
anvil. Machinery soon took over the act of forging after the
Industrial Revolution. Today there is a wide variety of forging
machinery that is capable of making parts ranging from a bolt to an
entire plane wing.
Two major classes of equipment are used –

1. forging hammer, which delivers rapid impact

blows to the surface of the metal, while

2. forging press subjects the metal to a slow

speed compressive force.
 Classification of forging processes
Forging processes can be classified based on forging temperature as,
a) Hot, or
b) Cold.
This classification is based on whether forging of a material is
done above(hot forging/hot working) or below(cold forging/cold
working) its recrystallization temperature. Most forging operations
are carried out hot since work hardening effects due to deformation
are negated by the recrystallization process. Cold forging typically
results in work hardening of the workpiece.
 Classification of forging processes
Based on the type of die used, forging processes can be

classified broadly as,

Open die forging – generally carried out between flat dies or dies

of simple shape.

• Generally used when parts are large and number of parts

produced is small.

• It is often used to preform the workpiece for closed die

forging.
Closed die forging – workpiece is deformed
between two die halves which carry impressions
of desired final shape.

• Deformation is done under high pressure in a

closed cavity, hence precision forgings with
close dimensional tolerances can be produced.
 Types of open die forging
a) Upsetting of a cylindrical billet between two flat
dies.

• Here the metal flows laterally between the

advancing die surfaces

• there is less deformation at the die interfaces due

to frictional forces than at mid-height plane.
• Thus the sides of the upset cylinder become
barreled.

• Generally the metal tends to flow most easily

towards the nearest free surface because this
represents the least frictional path.

This effect of friction in restraining metal flow is

used to produce shapes with simple dies.
b) Edging is done to shape the ends of bars and
to gather metal. The metal is confined by the
die from flowing horizontally but it is free to
flow laterally to fill the die.
c) Fullering is done to reduce the cross-section
area of a portion of the stock. The metal flow
is outward and away from the centre of the
fullering die.
 Fuller move fast and moves metal
Fullers come perpendicular to the face
in different
shapes
www.anvilfire.com

Fullers
d) Drawing – The reduction in cross-sectional of
the workpiece with concurrent increase in
length is called drawing down or drawing out.
e) Swagging – In drawing, if concave dies are
used so as to produce a rod of smaller
diameter, the process is called swagging.
Swaging provides a reduced round cross section
suitable for tapping, threading, upsetting or other
subsequent forming and machining operations.
Other commonly used processes of forging are,
• Bending,
• Twisting,
• Extrusion,
• Piercing,
• Punching, and
• Indenting.
 Closed die forging
• Uses carefully machined matching die blocks to produce forgings with
close dimensional tolerances.

• Generally the billet is first fullered and edged to place the metal in the
correct places for subsequent forging.

• The preshaped billet is then placed in the cavity of the blocking die and
forged to close to final shape. Greatest change of shape occurs in this step.
• Then it is transferred to the finishing die where
it is forged to final shape and dimensions.

• Usually blocking cavity and finishing cavity

are machined into the same die block along
with fullering and edging impressions on the
edges of the die block.
 Closed die forging
• It is important to use enough metal in the billet so that die cavity is
completely filled and since it is difficult to put exact amount of metal, a
slight excess of metal is used.

• During finishing, this excess metal comes out of the cavity as a thin ribbon
called flash. In order to prevent very wide flash formation a flash gutter is
used as shown.
• Final step is the removal of the flash by a
process called trimming.

• It is also more appropriately called impression

die forging, because of the flash.

• Flash serves two main purposes, one is that it

acts as a safety valve for excess metal in the
cavity. It also regulates the escape of metal,
thin flash greatly increases flow resistance of
the system so that the pressure builds up to
high values to ensure that the metal fills all
recesses in the cavity.
The trick in designing flash is to minimise the flash needed for the job such
that its not too low to cause breakage and die wear and so that it makes it more
difficult for metal to flow through the flash than within the cavity, so that it can
fill all intricate details within the cavity first. Forging pressure increases with
decreasing flash thickness and increasing flash width.
Metal flow is greatly influenced by the part geometry. Spherical or blocklike
shapes are easiest to forge in impression dies, while shapes with thin and long
sections or projections are most difficult due to the large surface area per unit
volume due to which their friction and temperature effects are enhanced. It is
particularly difficult to produce parts with sharp fillets, wide thin webs and
high ribs as shown. More over forging dies must be tapered to facilitate
removal of the finished piece, which is generally 5 deg for steel forgings.
 Forming machines
There are four basic types of forging machines
 Hammer and press forging processes
There are two basic types of forging
hammers used;
Forging hammers
• Board hammer
• Power hammer

There are two basic types of forging

presses available;
Forging presses
• Mechanical presses
• Hydraulic presses
 Board hammer –forging hammer
•The upper die and ram are raised by friction rolls
gripping the board.
Potential energy = mgh
•After releasing the board, the ram falls under gravity to
produce the blow energy.

•The hammer can strike between 60-150 blows per

minute depending on size and capacity.

•The board hammer is an energy- restricted machine

•.
• The blow energy supplied equal the potential energy
due to the weight and the height of the fall.
Board hammer

This energy will be delivered to the metal workpiece to produce plastic

deformation.
 Forging hammer or drop hammer

• Provide rapid impact blows to the surface of the

metal.

Bel
• Dies are in two halves
t - Lower : fixed to anvil
- Upper : moves up and down with the TUP.
•Energy (from a gravity drop) is adsorbed
onto the metal, in which the maximum
impact is on the metal surface.
•Dies are expensive being accurately
machined from special alloys (susceptible
to thermal shock).
•Drop forging is good for mass production
of complex shapes.
Example: Forging hammer or drop hammer

The energy
supplied by the
blow is equal to the
potential energy
due to the weight
of the ram and the
height of the fall.

Potential energy = mgh

Forging machine
 •Power hammer provides greater capacity,
Power hammer in which the ram is accelerated on the
downstroke by steam or air pressure in
addition to gravity.

•Steam or air pressure is also used to raise

the ram on the upstroke.

•The total energy supplied to the blow

in a power drop hammer is given by

W  1 mv 2  pAH  (mg  pA)H

2
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 downstroke
A = area of ram cylinder
H = height of the ram drop
Power hammer
 Hydraulic press forging

•Using a hydraulic press or a

mechanical press to forge the
Ra
metal, therefore, gives continuous
m forming at a slower rate.

Die •Provide deeper penetration.

•Better properties (more homogeneous).

•Equipment is expensive.
Example: Hydraulic Press forging

Hydraulic presses are

load- restricted
machines in which
hydraulic pressure
moves a piston in a
cylinder.

•The full press load is

available at any point
during the full stroke of
the ram. Therefore,
hydraulic presses are
ideally suited for
extrusion-type forging
operation.
• Due to slow speed, contact time is longer at
the die-metal interface, which causes problems
such as heat lost from workpiece and die
deterioration.

• Also provide close-tolerance forging.

• Hydraulic presses are more expensive than

mechanical presses and hammers.
 Mechanical press forging
• Crank press translates rotary
motion into reciprocating linear
motion of the press slide.

• The ram stroke is shorter than in

a hammer or hydraulic press.

Mechanical press • Presses are rated on the basis of

the force developed at the end of
the stroke.
• The blow press is more like squeeze than like the
impact of the hammer, therefore, dies can be less
massive and die life is longer than with a hammer.

• The total energy supplied during the stroke of a

press is given by

Where I is moment of inertia of the flywheel

 is angular velocity, o-original, f-after deformation, rad.s-1
Typical values of velocity for different forging equipment

Forging machine Velocity range, ms-1

Gravity drop hammer 3.6-4.8

Power drop hammer 3.0-9.0
HERF machine 6.0-24.0
Mechanical press 0.06-1.5
Hydraulic press 0.06-0.30

Remark: HERF – High Energy RateForging

OPEN-DIE FORGING
• Typically deals with large, relatively simple
shapes.

• Formed between simple dies in a large

hydraulic press or power hammer.

• Parts made in open-die forging : ship propeller

shafts, rings, gun tubes, pressure vessels.
• Workpiece is usually larger than the tool. So at any point in
time, deformation is confined to a small portion of the
workpiece.

• Chief mode of deformation is compression, accompanied by

considerable spreading in the lateral directions.

• Simplest open-die forging operation is cogging a billet

between flat tools to reduce the cross-sectional area, without
changing the final shape of the cross-section.
• Tomlinson and Stringer defined a coefficient of
spread S
S = width elongation = ln(w₁/w₀) (1)
thickness contraction ln(h₀/h₁)

• Because of barreling of the bar, it is difficult to

measure the ‘width natural strain’, but increase in
length can be measured accurately.
• S depends chiefly on the bite ratio (b/w₀)
S= (b/w₀)
1+(b/w₀)
• Equation (1) can also be expresses in terms of the “spread
law”.
β = (1/γ)ˢ
where β=spread ratio=w₁/w₀
γ=squeeze ratio=h₁/h₀
• There are certain limiting ranges of these variables which
must be considered.
• Constancy of volume : h₁w₁l₁ = h₀w₀l₀ (or)
ln(h₁/h₀)+ln(w₁/w₀)+ln(l₁/l₀) = 0

• Substituting in equation(1)
1-S = length elongation = ln(l₁/l₀)
thickness contraction ln(h₀/h₁)

• If S=1, all of the deformation would go as spread;

if S=0, all of the deformation would go into elongation.
• Since only that part of the surface under the bite is being
deformed at any one time, there is danger of causing surface
laps at the step separating forged & unforged portions.

• For a given geometry of tooling there will be a critical

deformation which will produce laps.

• Wistreich and Shutt recommend that squeeze ratio (h₀/h₁)

should not exceed 1.3
• Since this is done frequently on large sections, it is important to
ensure that the billet is deformed through to the center

• Bite ratio should not be less than 1/3 to minimize inhomogeneous

deformation.

• Using these criteria, Wistreich and Shutt developed optimization

techniques for selecting the forging schedule from the thousands of
possible combinations which would require the least number of
steps.
• Load required to forge a flat section in open dies may be estimated by

P = σAC

C-constraint factor to allow for inhomogeneous deformation

• Deformation resistance increases with Δ=h/L.

• Hill constructed slipline fields for forging with various conditions of Δ,

and the results can be summarized by the relation

C = 0.8+0.2(h/b) = 0.8+0.2Δ
Closed die Forging
 Complexity
• Several intermediate steps are required, which require
considerable expertise and skill.(Preform making)

• Deep understanding of the following factors needed for

good die-geometry:

1. Flow stress of material

2. Frictional conditions

3. Heating effects
 Difficulties
• Chilling due to contact with metallic die.
This can be rectified by isothermal forging with heated
dies.

• Flash formation and wastage of metal.

• Shape and symmetry needs to be taken into account.

Shape classification(qualitative)
Shape classification(qualitative)
 Parameters needed

1. Workpiece volume and weight

2. Number of preforming steps and their configuration

3. Flash dimensions in preforming and finishing dies

4. Load and energy requirements for forging operations.

 Preform design aspects

• Preform is critical since it assures defect-free flow,

complete die-fill and minimal flash loss.

• Types of metal flow(extrusion, upsetting) occur

simultaneously and need to be considered.
 Preform design aspects
Three rules

1. Area at each cross section should equal area of finished sections+ flash
area.

2. Concave radii on the preform should be larger than radii on forged part.

3. Cross section should be higher and narrower than final c/s (to increase
upsetting and minimise extrusion)
 CAD-CAM in forging
•CAD- computer aided design

•CAM-computer aided manufacturing

•APT- language for describing geometric changes produced in a metal cutting

•N/C machining- Numerical controlled machining

Helps establish neutral surfaces, shape difficulty factor, CS area, flash

geometry, stresses, centre of loading.
 Forging Defects
• Several types of defects are possible in
forging:
– Segregation
– Surface cracking
– Cold shut
– Scale
– Internal cracking
– Fiber structure
 Segregation

• If deformation during forging is limited to the

surface layers
– As when light hammer blows are used

• The dendritic ingot structure will not be

broken down at the interior of the forging.
• Incomplete forging penetration can readily be detected
by macro-etching a cross-section of the forging.

• The examination of a deep etch disk for segregation,

dendritic structure, and cracks is a standard quality-
control procedure with large forgings.

• Incomplete penetration can be minimized by making

forgings of large cross-section on a forging press.
 Surface cracking
• Result of excessive working of the surface at too
low a temperature or as a result of hot shortness.

• High sulfur concentration in the surface

atmosphere can produce hot shortness in steel and
nickel.
Cracking at the flash

Closed-die forging
• Cracking at the flash of closed-die forgings is

another surface defect.

– The crack penetrates into the body of the forging

when the flash is trimmed off.

• More prevalent the thinner the flash in relation

to the original thickness of the metal.

• Can be avoided by

– Increasing the flash thickness.

– Relocating the flash to a less critical region of the

forging.

– Hot trimming or stress relieving the forging prior

to cold trimming of the flash.

 Cold shut

• Also called fold.

• A discontinuity produced when two surfaces of

metal fold against each other without welding
completely.
• Happens when metal flows past part of the die
cavity that has already been filled or that is
only partially filled because the metal failed to
fill in due to a sharp corner, excessive chilling,
or high friction.

• Too small a die radius – common cause.

Cold shut or fold
 Scale
• Loose scale or lubricant residue that
accumulates in deep recesses of the die forms
scale pockets and causes underfill.

• Incomplete descaling of the workpiece results

in forged-in scale on the finished part.
 Internal cracking
• Secondary tensile stresses can develop during
forging.
– May lead to cracking.

• Internal cracks can develop during the

upsetting of a cylinder or a round, as a result of
the circumferential tensile stresses.
Internal cracking due to secondary tensile stresses

Upsetting of cylinder and round

• Proper design of dies can minimize this type of
cracking.
– In order to minimize bulging during upsetting and
the development of circumferential tensile stresses,
it is usual practice to use concave dies.
Internal cracking is less prevalent in closed die
forging because lateral compressive stresses are
developed by the reaction of the work with the
die wall.
 Fiber structure
• The deformation produced by forging results in a
certain degree of directionality to the
microstructure.
– Second phases and inclusions are oriented parallel to
the direction of greatest deformation.

• At low magnification, this appears as flow lines,

or fiber structure.
• Characteristic of all forgings.
– Not to be considered as a forging defect.

• However it results in lower tensile ductility and fatigue

properties in the direction normal to it (transverse
direction).

• To achieve optimum balance between the ductility in the

longitudinal and transverse directions
– Limit the amount of deformation to 50-70 percent reduction in
cross section.
• Several defects may be observed in forged
products.

• Defects can be minimized by

– Using appropriate forging method
– Adjusting work temperature and atmosphere
– Thorough descaling
– Proper design of die and die cavity
– Controlling the amount of deformation
Powder Metallurgy Forging
The use of sintered P/M preforms rather than bar stock offers
the following advantages :

1) Improved material utilization through reduction or

elimination of machining.

2) Forming of final size in one forging stroke.

3) Uniformity of structure and reduced directionality of

properties relative to conventional forging parts.
Since P/M preforms contain a dispersion of
interconnected voids, the deformation of a P/M
preform in much different from a conventional
fully dense workpiece.

• The workpiece decreases in volume during plastic

deformation as the porosity is closed up and
eliminated by plastic deformation.
Presence of these voids causes significant
decrease in local ductility which increases the
probability of fracture during forging. It also
increases the surface area for unfavourable
oxidation and contamination reactions to occur.
The basic plasticity mechanics of a porous P/M preform can be
described by the following relations.

The relationship between plastic deformation and densification is

achieved through relating the plastic Poisson ratio ν to the fraction of
theoretical density ρ/ρt

(1)
This relationship holds for hot and cold working, provided the
preform has been sintered.
For frictionless compression of a cylinder the relative density
change is given by :

(2)
And from the definition of Poisson’s ratio :

(3)

Substituting Eq. (1) in Eq. (3)

Integrating :

Where ρi is the initial density of the cylinder.

The classical theory of plasticity is based on the assumption of

constancy of volume, which leads to further condition that
yielding is unaffected by the hydrostatic component of the stress
state.

A modification of the von Mises’ criterion is needed for dealing

with porous materials which densify with plastic deformation.
Kuhn has shown that a workable criterion is :

The first term is the usual von Mises’ Criterion,

and the second term accounts for the porosity
through Poisson’s ratio and Eq. (1)
Thank you ….