UNIT III BULK DEFORMATION

PROCESSES
Hot working and cold working of metals – Forging processes
Open, impression and closed die forging – Characteristics
the process – Types of Forging Machines – Typical forgin
operations – Rolling of metals – Types of Rolling mills - Fla
strip rolling – Shape rolling operations – Defects in rolle
parts - Principle of rod and wire drawing -Tube drawing
Principles of Extrusion – Types of Extrusion – Hot and Co
extrusion –– Equipments used
COLD WORKING
Plastic deformation which is carried out within the recrystallization
temperature and over a time interval such that the strain hardening i
not relieved is called cold working.

Some Cold Working Processes:

v Cold rolling
v Cold forging
v Cold extrusion
v Bending
v Drawing
v Shearing
Reason for Cold Working:

Provides better surface finish and dimensional precision.

The advantages of cold working are

1. Handling of material is easy.

2. Good surface finish and better dimensional accuracy.

3. Energy saving since heating is not required.

4. Strength, fatigue and wear properties are improved.

5. Minimum contamination because of low working temperature.

6. No possibility of decarburisation of the surface.

 advantages :

Ductility of metal is reduced.

Deformation energy required is high, so rugged and more powerful equipme

s required, thus equipment cost is high.

Severe stresses are set up, this requires stress relieving, which increases the
cost

Owing to limited ductility at room temperature, the complexity of shapes

that can be readily produced is limited.

Cold working, for large deformation, requires several stages with interstage
annealing which increases the production cost.
1) HOT WORKING

ot working refers to the process

where metals are defromed above
heir recrystallizatıon tempereture
nd strain hardening does not
ccur.

ome Hot Working Processes:

Rolling
Forging
Extrusion Reason for Hot Working:
Hot drawing
At elevated temperatures, metals weake
Pipe welding
and become more ductile.
Piercing
vantages of hot working :
High production rate (since the process is faster).
Very high reduction is possible without fear of fracture.
Metal is made tougher because pores get closed and impurities are
segregated.
Deformation energy required is low, hence, less powerful equipments
are required.
Structure can be altered to improve the final properties.
The process does not change hardness or ductility of the metal since
distorted grains soon change into new undeformed grains.
sadvantages :
Handling of material is not so easy.
Heat resistant tools are required which are expensive.
High temperature may promote undesirable reactions.
Close tolerances cannot be held because of non-
non uniform cooling and
thermal contraction.
Surface finish is poor because of scale formation.
Metallurgical structure may be non-uniform
non because of cooling histor
after deformation.
Cold Rolling Hot Rolling
Bulk deformation processes

Forging
Rolling
Traditionally Hot

Extrusion
Drawing
Four Basic Bulk Deformation Processes
1. Rolling – slab or plate is squeezed between opposing rolls
2. Forging – work is squeezed and shaped between opposing
dies
3. Extrusion – work is squeezed through a die opening, thereby
taking the shape of the opening
4. Wire and bar drawing – diameter of wire or bar is reduced
by pulling it through a die opening
Forging
Deformation process in which work is compressed between
two dies
• Oldest of the metal forming operations, dating from
about 5000 B C
• Components: engine crankshafts, connecting rods, gears,
aircraft structural components, jet engine turbine parts
• Also, basic metals industries use forging to establish basic
form of large parts that are subsequently machined to
final shape and size
vantages of forging
Forged parts possess high ductility and offers great resistance
mpact and fatigue loads.
Forging refines the structure of the metal.
It results in considerable saving in time, labour and material
compared to the production of similar item by cutting from a so
stock and then shaping it.
Forging distorts the previously created unidirectional fiber as creat
by rolling and increases the strength by setting the direction of grains
Because of intense working, flaws are rarely found, so have go
reliability.
The reasonable degree of accuracy may be obtained in forgi
operation.
sadvantages of forging
Rapid oxidation in forging of metal surface at high temperature resu
n scaling which wears the dies.
The close tolerances in forging operations are difficult to maintain.
Forging is limited to simple shapes and has limitation for parts havi
undercuts etc.
Some materials are not readily worked by forging.
The initial cost of forging dies and the cost of their maintenance
high.
The metals gets cracked or distorted if worked below a specifi
temperature limit.
The maintenance cost of forging dies is also very high.
Forging Hammers (Drop Hammers)
Apply impact load against workpart
• Two types:
• Gravity drop hammers - impact energy from falling
weight of a heavy ram
• Power drop hammers - accelerate the ram by pressurized
air or steam
• Disadvantage: impact energy transmitted through anvil into
floor of building
• Commonly used for impression-die
impression forging
wer Forging

Hand hammer blows impact will not be always sufficient enough to affect the proper plastic
low in a medium sized or heavy forging.. It also causes fatigue to the hammer man.

To have heavy impact or blow for more plastic deformation, power hammer are generally
employed.

These hammers are operated by compressed air, steam, oil pressure, spring and gravity.

They are generally classified as spring hammer and drop hammers.

Spring hammers

Spring hammers may be made available in various capacities having the tup weights from 30
to 250 kg.

Those having top weights 50 to 100 kg and speed of blows up to 300 per minute are in
generally used in forging shop.

These hammers have a common drawback in their springs getting broken very frequently du
to severe vibrations during forging of the jobs in the forging shop.
rop Hammers

Drop hammers are operated hydraulically and are widely used for shaping parts by drop
hammering a heated bar or billet into a die cavity.

A drop forging raises a massive weight and allows it to fall under gravity on close dies in
which forge component is allowed to be compressed.

Drop hammers are commonly used for forging copper alloys and steel.
Drop forging hammer, fed by conveyor and heating units at the right
the
Forging Presses
• Apply gradual pressure to accomplish compression operation
• Types:
• Mechanical press - converts rotation of drive motor into
linear motion of ram
• Hydraulic press - hydraulic piston actuates ram
• Screw press - screw mechanism drives ram
Upsetting and Heading
Forging process used to form heads on nails, bolts, and similar
hardware products
• More parts produced by upsetting than any other forging
operation
• Performed cold, warm, or hot on machines called headers or
formers
• Wire or bar stock is fed into machine, end is headed, then
piece is cut to length
• For bolts and screws, thread rolling is then used to form
threads
 Upset Forging

Figure 19.22 An upset forging operation to form a head on a bolt or similar

hardware item The cycle consists of: (1) wire stock is fed to the stop,
(2) gripping dies close on the stock and the stop is retracted, (3) punch
moves forward, (4) bottoms to form the head.
 Heading (Upset Forging)

Figure 19.23 Examples of heading (upset forging) operations: (a) heading a nail
using open dies, (b) round head formed by punch, (c) and (d) two common
head styles for screws formed by die, (e) carriage bolt head formed by punch
and die.
 Trimming After Impression-Die
Impression Forging

Figure 19.29 Trimming operation (shearing process) to remove the flash

after impression-die forging.
Classification of Forging Operations
• Cold vs. hot forging:
• Hot or warm forging – most common, due to the
significant deformation and the need to reduce
strength and increase ductility of work metal
• Cold forging – advantage: increased strength that
results from strain hardening
• Impact vs. press forging:
• Forge hammer - applies an impact load
• Forge press - applies gradual pressure
Types of Forging Dies
• Open-die forging - work is compressed between
two flat dies, allowing metal to flow laterally with
minimum constraint
• Impression-die forging - die contains cavity or
impression that is imparted to workpart
• Metal flow is constrained so that flash is created
• Flashless forging - workpart is completely
constrained in die
• No excess flash is created
 Open-Die
Die Forging

Figure (a) open-die forging.

pen- Die Forging
Compression of workpart between two flat dies
Similar to compression test when workpart has cylindrical cro
section and is compressed along its axis
• Deformation operation reduces height and increases diameter
work
• Common names include upsetting or upset forging
• open-die forgings generally weigh 15 to 500 kg
Part sizes may range from very small (the size of nails, pins, and bolt
to very large (up to 23 m, long shafts for ship propellers)
 Impression-Die
Die Forging
In impression-die forging, the die surfaces contain a shape
impression that is imparted to the work during compression, th
constraining metal flow to a significant degree.
In this type of operation, a portion of the work metal flows beyond t
die impression to form flash, as shown in the figure.
Flash is excess metal that must be trimmed off later.
 Impression-Die Forging
Impression

Figure (b) impression-die forging.

 Impression-Die
Die Forging

Figure 19.14 Sequence in impression-die

impression forging: (1) just prior to initial
contact with raw workpiece, (2) partial compression, and (3) final die
closure, causing flash to form in gap between die plates.
 Advantages and Limitations
§ Advantages of impression-diedie forging compared to
machining from solid stock:
§ Higher production rates
§ Less waste of metal
§ Greater strength
§ Favorable grain orientation in the metal
§ Limitations:
§ Not capable of close tolerances
§ Machining often required to achieve accuracies and
features needed
 Flashless Forging (Precision Forging)
Compression of work in punch and die tooling whose cavity does
not allow for flash
• Starting workpart volume must equal die cavity volume within
very close tolerance
• Process control more demanding than impression die forging
• Best suited to part geometries that are simple and
symmetrical
• Often classified as a precision forging process
 Flashless Forging

Figure 19.9 Three types of forging (c) flashless forging.

 Flashless Forging

Figure 19.17 Flashless forging: (1) just before initial contact with workpiece,
(2) partial compression, and (3) final punch and die closure.
Extrusion
usion

ompression forming process in which work metal is forced to

flow through a die opening to produce a desired
cross-sectional shape
Process is similar to squeezing toothpaste out of a toothpaste
tube
In general, extrusion is used to produce long parts of
uniform cross sections
Latin extrudere, meaning “to force out”)
Two basic types:
• Direct extrusion
• Indirect extrusion
irect extrusion
mmonly extruded materials are aluminum, copper, steel, magnesium, and lead; other metal
d alloys also can be extruded, with various levels of difficulty.

pending on the ductility of the material, extrusion is carried out at room or elevated
mperatures. Extrusion at room temperature often is combined with forging operations, in
hich case it generally is known as cold extrusion
Direct Extrusion

Direct extrusion.
Comments on Direct Extrusion
• Also called forward extrusion
• As ram approaches die opening, a small portion of billet
remains that cannot be forced through die opening
• This extra portion, called the butt, must be separated from
extrudate by cutting it just beyond the die exit
• Starting billet cross section usually round
• Final shape of extrudate is determined by die opening
 Hollow and Semi-Hollow
Hollow Shapes

(a) Direct extrusion to produce a hollow or semi-hollow

semi cross sections; (b) hollow
and (c) semi-hollow
hollow cross sections.
 Indirect Extrusion

Indirect extrusion to produce (a) a solid cross section and (b) a

hollow cross section.
Comments on Indirect Extrusion
• Also called backward extrusion and reverse extrusion
• Limitations of indirect extrusion are imposed by
• Lower rigidity of hollow ram
• Difficulty in supporting extruded product as it exits die
Advantages of Extrusion
• Variety of shapes possible, especially in hot extrusion
• Limitation: part cross section must be uniform
throughout length
• Grain structure and strength enhanced in cold and
warm extrusion
• Close tolerances possible, especially in cold extrusion
• In some operations, little or no waste of material
Wire and Bar Drawing
Cross-section
section of a bar, rod, or wire is reduced by pulling it
through a die opening
• Similar to extrusion except work is pulled through die in
drawing (it is pushed through in extrusion)
• Although drawing applies tensile stress, compression also
plays a significant role since metal is squeezed as it passes
through die opening
Wire and Bar Drawing
Wire Drawing vs. Bar Drawing
• Difference between bar drawing and wire drawing is stock
size
• Bar drawing - large diameter bar and rod stock
• Wire drawing - small diameter stock - wire sizes down to
0.03 mm (0.001 in.) are possible
• Although the mechanics are the same, the methods,
equipment, and even terminology are different
Bar Drawing
• Accomplished as a single-draft
draft operation - the stock is
pulled through one die opening
• Beginning stock has large diameter and is a straight cylinder
• Requires a batch type operation
 Bar Drawing Bench

Hydraulically operated draw bench for drawing metal bars.

Wire Drawing
• Continuous drawing machines consisting of multiple draw
dies (typically 4 to 12) separated by accumulating drums
• Each drum (capstan) provides proper force to draw wire
stock through upstream die
• Each die provides a small reduction, so desired total
reduction is achieved by the series
• Annealing sometimes required between dies to relieve
work hardening
Continuous Wire Drawing

Continuous drawing of wire.

Features of a Draw Die
• Entry region - funnels lubricant into the die to prevent
scoring of work and die
• Approach - cone-shaped
shaped region where drawing occurs
• Bearing surface - determines final stock size
• Back relief - exit zone - provided with a back relief angle
(half-angle) of about 30°
• Die materials: tool steels or cemented carbides
Draw Die Details

Figure 19.43 Draw die for drawing of round rod or wire.

 Rolling
Deformation process in which work thickness is reduced
by compressive forces exerted by two opposing rolls

The rolling process (specifically, flat rolling).

The Rolls
Rotating rolls perform two main functions:

• Pull the work into the gap between them by friction

between workpart and rolls

• Simultaneously squeeze the work to reduce its cross section

Types of Rolling

• Based on workpiece geometry :

• Flat rolling - used to reduce thickness of a
rectangular cross section
• Shape rolling - square cross section is formed
into a shape such as an I-beam
I
• Based on work temperature :
• Hot Rolling – most common due to the large
amount of deformation required
• Cold rolling – produces finished sheet and plate
stock
Rolled Products Made of Steel

Some of the steel products made in a rolling mill.

 Diagram of Flat Rolling

Side view of flat rolling, indicating before and after thicknesses, work
velocities, angle of contact with rolls, and other features.
Shape Rolling
Work is deformed into a contoured cross section rather than
flat (rectangular)
• Accomplished by passing work through rolls that have the
reverse of desired shape
• Products include:
• Construction shapes such as I-beams,
I L-beams, and
U-channels
• Rails for railroad tracks
• Round and square bars and rods
A rolling mill for hot flat
rolling. The steel plate
is seen as the glowing
strip in lower left
corner (photo
courtesy of Bethlehem
Steel).
Rolling Mills
• Equipment is massive and expensive
• Rolling mill configurations:
• Two-high – two opposing rolls
• Three-high – work passes through rolls in both directions
• Four-high – backing rolls support smaller work rolls
• Cluster mill – multiple backing rolls on smaller rolls
• Tandem rolling mill – sequence of two-high
two mills
 Two-High
High Rolling Mill

Figure 19.5 Various configurations of rolling mills: (a) 2-high

2 rolling
mill.
 Three-High
High Rolling Mill

Figure 19.5 Various configurations of rolling mills: (b) 3-high

3 rolling mill.
 Four-High Rolling Mill

Figure 19.5 Various configurations of rolling mills: (c) four-high

four rolling mill.
 Cluster Mill
Multiple backing rolls allow even smaller roll diameters

Figure 19.5 Various configurations of rolling mills: (d) cluster mill

 Tandem Rolling Mill

A series of rolling stands in sequence

Figure 19.5 Various configurations of rolling mills: (e) tandem rolling mill.
Thread Rolling
Bulk deformation process used to form threads on
cylindrical parts by rolling them between two dies
• Important commercial process for mass producing
bolts and screws
• Performed by cold working in thread rolling
machines
• Advantages over thread cutting (machining):
• Higher production rates
• Better material utilization
• Stronger threads and better fatigue resistance due
to work hardening
 Thread Rolling

(1) start of cycle (2) end of cycle

Figure 19.6 Thread rolling with flat dies: