Transcript Hot Working Processes

HOT- WORKING PROCESSES
•Shaping of metal by deformation is a very very old
tradition.
•Processes such as rolling, wire drawing etc. were
common in the Middle Ages. In North America, by
1680 the Saugus Iron Works near Boston had an
operating drop forge, rolling mill, and slitting mill.
•Although basic concepts of many forming processes
have remained largely unchanged throughout
history, details and equipment have evolved
considerably.
INTRODUCTION
•Manual processes were converted into machine
processes during the industrial revolution.
•The machinery then became bigger, faster, and more
powerful.
•Waterwheel power was replaced by steam and then
electricity.
•More recently, computer-controlled, automated
operations have emerged
CLASSIFICATION OF DEFORMATION PROCESSES
• Processes divide into the following :–Primary processes reduce a cast material into
intermediate shapes, such as slabs, plates, or
billets.
– Secondary processes further convert these
shapes into finished or semi-finished products.
CLASSIFICATION OF DEFORMATION PROCESSES
• Bulk deformation processes
–Surface area of work piece changes significantly.
–Thicknesses or cross sections of material are reduced
–Shapes are changed.
–As volume of material remains constant, other
dimensions must change in proportion.
–Enveloping surface area is altered, usually increasing
as product lengthens.
CLASSIFICATION OF DEFORMATION PROCESSES
• Sheet-forming operations
–Involve deformation of material where thickness
and surface area remain relatively constant.
–Coining, for example, begins with sheet material
but alters the thickness in a complex manner that
is essentially bulk deformation.
HOT-WORKING PROCESSES
•Hot-working processes provide means of producing
a desired shape.
•At elevated temperatures, metals weaken and
become more ductile.
•With
continual
re-crystallization,
massive
deformation take place without exhausting material
plasticity.
HOT-WORKING PROCESSES
Some major modern hot-working manufacturing
processes are:
– Rolling
– Forging
– Extrusion
– Hot drawing
– Pipe welding
– Piercing
ROLLING
• Rolling is usually first process used to convert
material into a finished wrought product.
• Stock can be rolled into blooms, billets, slabs, or
these shapes can be obtained directly from
continuous casting.
– A bloom has a square or rectangular cross
section, with a thickness greater than 6 inches
and a width no greater than twice the thickness.
ROLLING
– A billet is usually smaller than a bloom and has a
square or circular cross section. Billets are usually
produced by some form of deformation process,
such as rolling or extrusion.
– A slab is a rectangular solid where the width is greater
than twice the thickness. Slabs can be further rolled to
produce plate, sheet, and strip
ROLLING
• These hot-worked products use for subsequent
processing techniques such as cold forming or for
machining.
– Sheet and strip fabricated into products or cold
rolled into thinner, stronger material even into
foil.
– Blooms and billets rolled into finished products,
such as structural shapes or railroad rail, or
processed into semi-finished shapes, such as bar,
rod, tube, or pipe
ROLLING
• Hot Rolling :– is prominent among all manufacturing processes
– equipment and practices are sufficiently advanced
– is standardized
– produce uniform-quality products at relatively low
cost
– products are normally obtained in standard shapes
and sizes
Basic Rolling Process
• Heated metal is passed between two rolls that
rotate in opposite directions
• Gap between rolls is less than thickness of entering
metal.
• Rolls rotate with surface velocity that exceeds
speed of incoming metal, friction along the contact
interface acts to propel the metal forward.
Basic Rolling Process
• Metal is squeezed and elongates result in decrease of the
cross-sectional area.
• Amount of deformation in a single pass depends on the
friction conditions along the interface.
• If too much material flow is demanded, rolls cannot
advance the material and simply skid over its surface.
• Too little deformation per pass results in excessive
production cost ??
Rolling Temperatures
• Temperature control is crucial to the success of the
hot rolling process.
• If the temperature of the billet is not uniform, the
subsequent deformation will not be uniform.
Rolling Temperatures
• For example
If a part cools prior to working, the cooler
surfaces will tend to resist deformation.
Cracking and tearing of surface may result as
hotter, weaker interior tries to deform.
Rolling Temperatures
• Cooling from solidification is controlled to enable
direct insertion into a hot-rolling operation without
additional handling or reheating.
• Brought to rolling temperature, usually gas- or oilfired soaking pits or furnaces are normally used.
• Plain-carbon and low-alloy steels soaking
temperature is approximately 22000F (l2000C)
Rolling Temperatures
• For smaller cross sections, induction coils may be
used to heat material for rolling
• Hot rolling is usually terminated when temperature
falls to about 100 to 2000F (50 to 100°C) above the
recrystallization temperature of material
• Finishing temperature assures the production of a
uniform fine grain size and prevents possibility of
unwanted hardening
• Before additional deformation, a period of
reheating is required to reestablish desirable hotworking conditions
Rolling Mill Configurations
• Rolling mill stands are available in a variety of roll
configurations.
• Early reductions (often called primary roughing or
breakdown passes), employ two- or three-high
configuration with 24- to 55-in. (600- to 1400-mm)
diameter rolls
• Two-high non-reversing mill simplest design from
which material can only pass in one direction
• Two-high reversing mill permits back-and-forth
rolling, rolls may stop, reversed, and brought back
to rolling speed between each pass
Rolling Mill Configurations
• The three-high mill eliminates need for roll reversal
but requires some form of elevator on each side of
mill to raise or lower material and mechanical
manipulators to turn or shift product between
passes
– smaller-diameter rolls produce less length of
contact for a given reduction and therefore
require lower force and less energy to produce a
given change in shape
– smaller cross section, however, provides reduced
stiffness and pressed apart by the metal passing
through the middle
Rolling Mill Configurations
• Four-high and cluster arrangements use
rolls to support the smaller work rolls
backup
– used in hot rolling of wide plate and sheets, and
in cold rolling, where small negligence would
result in an unacceptable variation in product
thickness
– Foil is rolled on cluster mills since small thickness
requires small-diameter rolls
– In a cluster mill, the roll in contact with the work
can be as small as 1/4 in. in diameter
Rolling Mill Configurations
– Pack Rolling, a process where two or more layers
of metal are rolled simultaneously as a means of
providing a thicker input material
– Household aluminum foil is usually pack rolled,
as evidenced by the one shiny side (in contact
with the roll) and one dull side (in contact with
the other piece of foil)
– In rolling of non-flat or shaped products, such as
structural shapes and railroad rail, the sets of
rolls contain contoured grooves that sequentially
form desired shape, cross section and control
metal flow
Continuous Rolling Mills
• When the volume of a product justifies investment,
it may be rolled on a continuous rolling mill.
– Billets, blooms, or slabs are heated and fed
through an integrated series of non-reversing
stands
– Continuous mills for the hot rolling of steel strip,
for example, often consist of a roughing train of
approximately four four-high mill stands and a
finishing train of six or seven additional four-high
stands.
Continuous Rolling Mills
– In a continuous structural mill, the rolls in each
stand contain only one set of shaped grooves, in
contrast to the multi-grooved rolls used when
the product is produced by back-and-forth
passes through a single stand.
– In a continuous rolling mill, same amount of
material must pass through each stand in a given
period of time.
– If cross section is reduced, speed must be
increased proportionately.
Continuous Rolling Mills
– Thus rolls of each successive stand must turn
faster than those of preceding one by an amount
equal to change in cross-sectional area
– If this synchronization is not maintained, material
may accumulate between stands, or demand for
incoming material may place material under
excessive tension, and cause a tearing or rupture
Continuous Rolling Mills
– Synchronization of six or seven mill stands is not
an easy task, especially when key variables such as
temperature and lubrication may change during a
single run and product may be exiting final stand
at speeds in excess of 70 miles per hour (110
kilometers per hour).
– Computer control is important to successful
rolling, and modern mills are equipped with
numerous sensors to provide the needed
information.
Ring Rolling
• In ring rolling process, one roll is placed through the
hole of a thick-walled ring, and a second roll presses
in from outside.
• As the rolls squeeze and rotate, wall thickness is
reduced and diameter of ring increases.
• Shaped rolls can be used to produce a wide variety
of cross-section profiles.
• Resulting seamless rings find application in products
such as rockets, turbines, airplanes, pipelines, and
pressure vessels.
Characteristics of Hot-Rolled Products
• Because they are rolled and finished above
recrystallization temperature, hot-rolled products
have little directionality in their properties and are
relatively free of deformation - induced residual
stresses.
• These characteristics may vary, depending on
thickness of product and presence of complex
sections.
Characteristics of Hot-Rolled Products
• Substantial residual stresses can be induced during
hot working.
Characteristics of Hot-Rolled Products
• Thin sheets often show some definite directional
characteristics, whereas thicker plate (such as that
above 0.8 in. or 20 mm) will usually have very little.
• Because of the high residual stresses in rapidly
cooled edges, a complex shape, such as an T- or Hbeam, may warp noticeably if a portion of one
flange is cut away.
Characteristics of Hot-Rolled Products
• As result of hot deformation and good control hotrolled products are normally of uniform and
dependable quality and reliability.
• It is quite unusual to find any voids, seams, or
laminations when these products are produced by
reliable manufacturers.
• Surfaces of hot-rolled products are usually a bit
rough and are originally covered with a tenacious
high-temperature oxide, known as mill scale.
Characteristics of Hot-Rolled Products
• Removed by an acid pickling operation, resulting in
a surprisingly smooth surface finish.
• Dimensional tolerances of hot-rolled products vary
with kind of metal and size of the product.
• For most products produced in reasonably large
tonnages, tolerances are within 2 to 5% of specified
dimension (either height or width).
Flatness Control and Rolling Defects
• Rolling of flat material with uniform thickness
requires uniform gap between rolls attaining such
an objective may be difficult.
• Consider upper roll in a set that is rolling sheet or
plate material presses upward in the middle of roll
supported in mill frame.
Flatness Control and Rolling Defects
• Roll is loaded in three-point bending and tends to
flex in a manner that produces a thicker center and
thinner edge.
• If roll is always used to roll same material at same
temperature, forces and deflections can be
predicted, and roll can be designed to have a
specified amount of crowning.
• When roll is subjected to a specified load, it will
“deflect into flatness”.
Flatness Control and Rolling Defects
• If applied load is not of designed magnitude, profile
will not be flat and defects may result.
The thinner material will try to become longer but
must remain attached to the thicker. Result may be
wavy edges or fractures in center. If correction is
excessive, center becomes thinner and longer, and
result can be a wavy center or cracking of the
edges.
Thermo-mechanical Processing and Controlled Rolling
• A rolling process is generally used as being a means
of changing shape of material.
• Heat may be used to reduce forces and promote
plasticity while mechanical properties (heat
treatments) are usually performed as subsequent
operations.
• Thermo-mechanical processing consists of both
deformation and controlled thermal processing to
produce desired levels of strength and toughness in
the working product.
Thermo-mechanical Processing and Controlled Rolling
• Possible goals of thermo mechanical includes :– Production of uniform fine grain size
– Controlling nature
– Size and distribution of various transformation
products (such as ferrite, pearlite, bainite, and
martensite in steels)
– Controlling the reactions that produce solid
solution strengthening or precipitation hardening
– Producing a desired level of toughness.
Thermo-mechanical Processing and Controlled Rolling
• Following must all be specified and controlled:– Starting structure (controlled by composition and
prior thermal treatments),
– deformation details,
– temperature during the various stages of
deformation,
– the cool down from the working temperature.
Thermo-mechanical Processing and Controlled Rolling
Computer-controlled rolling facilities are almost a
necessity if thermo-mechanical processing is to be
performed successfully.
Possible benefits of thermo-mechanical processing include
– improved product properties;
– substantial energy savings (by eliminating subsequent
heat treatment);
– Possible substitution of a cheaper, less-alloyed metal for
a highly alloyed one that responds to heat treatment.
FORGING
• Forging is term applied to a family of processes
where deformation is induced by localized
compressive forces.
• The equipment can be manual or power hammers,
presses, or special forging machines. The term
forging usually implies hot forging done above the
recrystaIlization temperature.
FORGING
• The forging material may be
– Drawn out to increase its length and decrease its
cross section,
– Upset to decrease the length and increase the
cross section,
– Squeezed in closed impression dies to produce
multidirectional flow.
FORGING
•Common forging processes include:
– Open-die drop-hammer forging
– Impression-die drop forging
– Press forging
– Upset forging
– Automatic hot forging
– Roll forging
– Swaging
Open-Die Drop-Hammer Forging
• Open-die hammer forging is the same type of
forging done by blacksmith. Metal is first heated
to proper temperature by gas, oil, or electric
furnaces.
• Impact delivered by some type of mechanical
hammer like gravity drop or board hammer.
Operation on a Rectangular Bar
Blacksmiths use this process to reduce the thickness of bars by hammering the part on an
anvil. Reduction in thickness is accompanied by barreling, as in Fig. 14.3c. (b) Reducing the
diameter of a bar by open-die forging; note the movements of the dies and the workpiece. (c)
The thickness of a ring being reduced by open-die forging.
Open-Die Drop-Hammer Forging
• Steam or air hammers use pressure to :
– give higher striking velocities,
– more control of striking force,
– easier automation,
– the ability to shape pieces up to several tons.
Open-Die Drop-Hammer Forging
• Computer controlled hammers
– greatly increase the efficiency of the process
– minimize amount of noise and vibration
– Operator obtain desired shape by orienting and
positioning work piece between blows.
Open-Die Drop-Hammer Forging
•.
• Open-die forging is usually employed to pre-shape
metal for further manufacturing operations for
example consider such massive parts turbine rotors
and generator shafts which may be 70 ft in length
and up to 3 ft in diameter.
• Open-die forging is used to minimize the amount of
subsequent machining.
Impression-Die Drop-Hammer Forging
• Open-die hammer forging (or smith forging) is:– simple and flexible process,
– not practical for large-scale production.
– It is slow operation
– size and shape of resulting workpiece are
dependent on skill of operator.
• Impression-die or closed-die forging overcomes
these difficulties by using shaped dies to control the
flow of metal.
• Consist of set of dies, one half of which attaches to
hammer and other half to anvil.
Impression-Die Drop-Hammer Forging
• Heated metal is positioned in lower cavity and
struck one or more blows by upper die.
• Hammering causes the metal to flow to completely
fill die cavity.
• Excess metal is squeezed out around the periphery
of the cavity to form a flash.
• When final forging is completed, flash is trimmed
off by trimming die.
Impression-Die Drop-Hammer Forging
• Accurate workpiece
sizing
is
required
since
complete filling of cavity must be assured with no
excess material.
• Major advantage is elimination of scrap generated
during flash formation.
Impression-Die Forging
(a) through (c) Stages in impression-die forging of a solid round billet. Note the formation
of flash, which is excess metal that is subsequently trimmed off (d) Standard terminology
for various features of a forging die.
Impression-Die Drop-Hammer Forging
• Final shape and size are set by additional forging in
the final or finisher impression
• The shape of the various cavities forces the metal to
flow in the desired direction.
Impression-Die Drop-Hammer Forging
Board hammers, steam hammers, and air
hammers are all used in impression die
forging.
Impression-Die Drop-Hammer Forging
–After forging, the flash is trimmed
–The part is
temperature.
quenched
to
room
Trimming Flash After Forging
Trimming flash from a forged part. Note that the thin
material at the center is removed by punching.
Design of Impression-Die Forgings
• Forging dies are made of high-alloy or tool steel
– Are costly to design and construct
– Ability to withstand cycles of rapid heating and
cooling
– Care is required to produce and maintain a
smooth and accurate cavity.
Design of Impression-Die Forgings
•
Better and economical results are obtained if
following are observed:
1. Dies should be divided in a flat plane if possible.
2. Parting surface should be a plane through
center of forging
3. Adequate allowance should be provided-at
least 3° for aluminum and 5 to 7° for steel.
4. Generous fillets and radii should be provided.
5. Ribs should be low and wide.
Design of Impression-Die Forgings
6. Various sections should be balanced to avoid
extreme temperature differences in metal flow.
7. Full advantage should be taken of material flow
lines.
8. Dimensional tolerances should not be closer
than necessary.
Design of Impression-Die Forgings
• Computer-aided design has made notable advances and
development, expanded-memory computers has enabled
accurate modeling of complex shapes.
• Good dimensional accuracy is one motivation for using
impression-die forging. With care, these dimensions (for
steel products) can be maintained within the tolerances.
•
Design of Impression-Die Forgings
Mass of Forging
Lb
I
2
5
10
20
50
100
Minus
kg
0.45
0.91
2.27
4.54
9.07
22.68
45.36
in.
0.006
0.008
0.010
0.011
0.013
0.019
0.07.9
mm
0.15
0.20
0.25
0.28
0.33
0.48
0.74
Plus
in.
0.018
0.024
0.03
0.033
0.039
0.057
0.087
mm
0.48
0.61
0 0.76
0.84
0.99
1.45
2.21
Selection of a lubricant is also critical to successful forging. The lubricant
not only affects the friction and wear and associated metal flow, but act
as a thermal barrier (restricting heat flow from the workpiece to dies)
and a parting compound (preventing part from sticking in cavities).
Press Forging
• Required when larger pieces or thicker products
must be formed. Deformation is analyzed in terms
of forces or pressures. Produce a more uniform
deformation and flow.
• Problems can arise because of longer time of
contact between the dies and work-piece.
Press Forging
• Heated dies are generally used to:–Reduce heat loss
–Promote surface flow
–Enable production of finer details and closer
tolerances
Press Forging
• Forging presses are of two basic types:–Mechanical presses
•Use means such as cams, cranks etc
•Because of their mechanical drives, production
presses are capable of up to 50 strokes per
minute
Press Forging
Hydraulic presses are :–Slower
–More massive
–More costly to operate.
–usually more flexible
–Have greater capacity.
–Can be programmed to have different strokes for
different operations and even different speeds
within a stroke.
Press Forging
• Hydraulic presses with capacities up to 50,000 tons
(445 MN) are in operation in the United States.
• Press forgings have higher dimensional accuracy
and can often be completed in a single closing of
dies and the process can be readily automated.
Cost-per-piece in Forging
Figure 14.18 Typical (cost-per-piece) in forging; note how the setup and the tooling costsper-piece decrease as the number of pieces forged increases if all pieces use the same die.
Upset Forging
• Upset-forging involves increasing diameter of
material by compressing its length
• It is the most widely used of all forging processes.
Parts can be upset forged both hot and cold on
special high- speed machines
• Work-piece is rapidly moved from station to station.
Automatic Hot Forging
• Highly automated upset equipment in which milllength steel bars (typically, 24 ft long) are fed into
one end at room temperature and hot-forged
products emerge from the other end at rates of up
to 180 parts per minute( i.e. 86,400 parts per 8hour shift).
• These parts can be solid or hollow, round or symmetrical,
up to 12 Ib (6 kg) in weight, and up to 7 in. (180 mm) in
diameter.
Automatic Hot Forging
– Small parts can be produced at up to 180 parts per
minute, with rates for larger pieces on the order of 90
parts per minute
Automatic Hot Forging
• The process has a number of attractive features.
– Low-cost input material
– High production speeds
– Minimum labor is required, and since no flash is
produced,
– Material savings can be as much as 20 to 30%
over conventional forging.
Automatic Hot Forging
• The benefits of the combined operations include
– high-volume production at low cost,
– Precision,
– Surface finish,
– Characteristic of a cold finished material.
Roll Forging
• In roll forging, round or flat bar stock is reduced in
thickness and increased in length to produce such
products as axles, tapered levers, and leaf springs.
• Roll forging is performed on machines that have
two cylindrical or semi-cylindrical rolls, each
containing one or more shaped grooves.
Roll Forging
• When the bar encounters a stop, the rolls rotate,
and the bar is progressively shaped as it is rolled
out.
• The piece can be reinserted between the next set of
grooves and the process repeated to produce the
desired size and shape.
Swaging
• Swaging generally involves the hammering of a rod
or tube to reduce its diameter where the die itself
acts as the hammer
• Term swaging is also applied to processes where
material is forced into a confining die to reduce its
diameter.
Swaging
Schematic illustration of the rotary-swaging process. (b) Forming internal profiles on a
tubular workpiece by swaging. (c) A die-closing swaging machine showing forming of a
stepped shaft. (d) Typical parts made by swaging. Source: Courtesy of J. Richard
Industries.
Swaging with and without a
Mandrel
(a) Swaging of tubes without a mandrel; note the increase in wall thickness in
the die gap. (b) Swaging with a mandrel; note that the final wall thickness of the
tube depends on the mandrel diameter. (c) Examples of cross-sections of tubes
produced by swaging on shaped mandrels. Rifling (internal spiral grooves) in
small gun barrels can be made by this process.
EXTRUSION
• In the extrusion process, metal is compressed and
forced to flow through a suitably shaped die to form
a product with reduced but constant cross section.
• Extrusion may be performed either hot or cold, hot
extrusion is commonly employed for many metals
to reduce the forces required.
EXTRUSION
• Extrusion process is like squeezing toothpaste out
of a tube. In the case of metals, a common
arrangement is to have a heated billet placed inside
a confining chamber. As ram continues to advance,
pressure builds until material flows plastically
through the die.
EXTRUSION
• Lead, copper, aluminum, magnesium, and alloys of
these metals are commonly extruded, because of
relatively low yield strengths and low hot-working
temperatures.
• Steels, stainless steels, and nickel-based alloys are
far more difficult to extrude.
EXTRUSION
• Almost any cross-sectional shape can be extruded
from nonferrous metals..
Extrusions and Products Made
from Extrusions
Extrusions and examples of
products made by sectioning off
extrusions. Source: Courtesy of
Kaiser Aluminum.
EXTRUSION
• Extrusion has a number of attractive features.
• Extrusion dies can be relatively inexpensive, and one die only be
required to produce a product. Conversion from one product to
another requires only a single die change, so small quantities of a
desired shape can be produced economically.
• Major limitation of process is a requirement that cross section be
uniform for entire length of product.
• Extruded products have good dimensional precision.
EXTRUSION
• For most shapes, tolerances with a minimum of +
0.003in. are easily attainable.
• Grain structure is typical of other hot-worked
metals, but strong directional properties
(longitudinal versus transverse) are usually
observed.
• Standard product lengths are about 20 to 24 ft, but
lengths in excess of 40 feet have been produced.
Extrusion Methods
• Extrusions is produced by various techniques and
equipment configurations. Hot extrusion is usually
done by either the direct or indirect method
• Direct extrusion, a solid ram drives entire billet to
and through a stationary die, must provide
additional power to overcome frictional resistance
between surface of moving billet and confining
chamber.
Extrusion Methods
• Indirect extrusion, a hollow ram drives die back
through a stationary, confined billet.
Direct-Extrusion
Schematic illustration of the direct-extrusion process.
Types of Extrusion
Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral;
Extrusion Methods
• Lubrication is another primary concern.
• Lubricant applied to billet must thin considerably as
material passes through die.
• At all stages of process, it will be expected to
function both as a lubricant and a barrier to heat
transfer.
Extrusion of Hollow Shapes
• Hollow shapes, and shapes with more than one
longitudinal cavity, can be extruded by several
methods. For tubular products, the stationary or
moving mandrel processes of are quite common.
• For products with multiple or more-complex
cavities, a spider-mandrel die (also known as a
porthole, bridge, or torpedo die) may be required.
•
Extrusion of Hollow Shapes
• Process is limited to materials that can be extruded
without lubrication and that can easily be pressure
welded.
• Hollow extrusions will obviously cost more than
solid ones as additional tooling is required
• However, a wide variety of shapes can be produced
that cannot be made economically by any other
process.
Metal Flow in Extrusion
• The flow of metal during extrusion is often quite complex
and some care must be exercised to prevent surface
cracks, interior cracks, and other flow-related defects.
• Metal near center of chamber can often pass through die
with little distortion, while metal near surface undergoes
considerable shearing.
• In direct extrusion, friction between forward- moving
billet and stationary chamber and die serves to further
impede surface flow.
Metal Flow in Extrusion
• If surface regions of billet undergo excessive
cooling, the deformation is further impeded and
cracks tend to form on the product surface.
• If quality is to be maintained, process control must
be exercised in the areas of design, lubrication,
extrusion speed, and temperature.
Types of Metal Flow in Extrusion
with Square Dies
Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction or
in indirect extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces.
(c) Pattern obtained at high friction or with coiling of the outer regions of the billet in the
chamber. This type of pattern, observed in metals whose strength increases rapidly with
decreasing temperature, leads to a defect known as pipe (or extrusion) defect.
HOT DRAWING OF SHEET AND PLATE
• Drawing is a plastic deformation process in which a
flat sheet or plate is formed into a recessed, threedimensional part with a depth more than several
times the thickness of the metal.
• As a punch descends into a mating die (or the die
moves upward over a mating punch), the metal
assumes the desired configuration.
• Hot drawing is used for forming relatively thickwalled parts of simple geometries, usually
cylindrical.
• Because the material is hot, there is often
considerable thinning as it passes through the dies.
HOT DRAWING OF SHEET AND PLATE
• In contrast, cold drawing uses relatively thin metal, changes
thickness very little or not at all, and produces parts in a
wide variety of shapes.
• A punch then descends, pushing metal through die,
converting circular blank to a cylindrical cup.
• Height of cup walls is determined by difference between
the diameter of original blank and diameter of punch.
• This dimension is limited by formation of several defects.
HOT DRAWING OF SHEET AND PLATE
• Wrinkles can appear in cup walls as circumference is
reduced, or punch can act as a piercing tool,
tearing blank around punch perimeter.
• There are several means of producing parts with
taller walls.
• If gap between punch and die is less than thickness
of incoming material, cup wall is thinned and
elongated simultaneously (a process often called
ironing or wall ironing).
HOT DRAWING OF SHEET AND PLATE
• If this thinning is objectionable, an intermediate
shape can be produced, with the further reduction
in diameter (and concurrent increase in wall height)
being taken in a subsequent redrawing with a
smaller punch and die,
• Some drawn products are designed to utilize part of
original disk as a flange around the lip of the cup.
• In this case the punch does not push the material
completely through the die, but descends to a
predetermined depth and then retracts.
• The partially drawn product is then ejected upward, and
the perimeter of the remaining flange is trimmed to the
desired size and shape
PIPE WELDING
• Large quantities of steel pipe are made by two processes
that use hot forming of steel strip coupled with
deformation-induced welding of its free edges.
• Both of these processes, utilize steel in the form of skelplong strips with specified width, thickness and edge
configuration.
• Because the skelp has been hot rolled previously and
welding process produces further compressive working and
recrystallization, pipe welded by these processes tends to
be very uniform in quality.
PIPE WELDING
Butt-Welded Pipe
• In the butt-welding process for making pipe, steel
skelp is heated to a specified hot-working
temperature by passing it through a furnace.
• Upon exiting the furnace, it is pulled through
forming rolls that shape it into a cylinder and bring
the free ends into contact.
• The pressure exerted between the edges of the
skelp is sufficient to upset the metal and produce a
welded seam.
• Additional sets of rollers then size and shape the pipe and it
is cut to standard, preset lengths.
• Product diameters range from 1/8 in. (3 mm) to 3 in. (75
mm), and speeds can approach 500 ft/min.
PIPEWELDING
Lap-Welded Pipe
• Lap-welding process for making pipe differs from buttwelding technique in that skelp now has beveled edges and
the rolls form the weld by forcing lapped edges down
against a supported mandrel.
• This process is used primarily for larger sizes of pipe, from
about 2 in. (50 mm) to 14 in. (400 mm) in diameter.
• Because product is driven over a supported mandrel,
product length is limited to about 20 to 25 feet.
PIERCING
• Thick-walled seamless tubing can be made by rotary
piercing,
• A heated billet is fed longitudinally into the gap
between two large, convex-tapered rolls.
• These rolls are powered to rotate in the same
direction, but axes of rolls are offset from axis of
billet by about 6°, one to right and the other to left.
• Clearance between rolls is preset at a value less
than diameter of billet.
PIERCING
• As billet is caught by rolls, it is simultaneously
rotated and driven forward.
• The reduced clearance between rolls forces billet to
deform into a rotating ellipse.
• Rotation of the elliptical section causes the metal to
shear about the major axis.
• A crack tends to form down the center axis of billet,
and cracked billet is then forced over a pointed
mandrel that enlarges and shapes the opening to
form a seamless tube.
PIERCING
• The result is a short length of thick-walled seamless
tubing, which can then be passed through a reeler
and sizing rolls to straighten it and reduce the
diameter and/or wall thickness.
• Seamless tubes can also be expanded in diameter
by passing them over a larger mandrel.
• As the diameter and circumference increase, the
walls correspondingly thin.
PIERCING
• Mannesmann mills used in hot piercing can
produce tubing upto 12 in. (300 mm) in diameter.
• Larger-diameter tubes can be produced on Stiefel
mills, which use same principle but replace convex
rolls of Mannesmann mill with larger-diameter
conical disks.