Forging--metal
shaping by plastic deformation--spans a myriad of equipment and
techniques. Knowing the various forging operations and the
characteristic metal flow each produces is key to understanding forging
design.
Fig. 11. Compression between narrow dies.
Hammer and Press Forging
Generally, forged
components are shaped either by a hammer or press. Forging on the
hammer is carried out in a succession of die impressions using repeated
blows. The quality of the forging, and the economy and productivity of
the hammer process depend upon the tooling and the skill of the
operator. The advent of programmable hammers has resulted on less
operator dependency and improved process consistency. In a press, the
stock is usually hit only once in each die impression, and the design
of each impression becomes more important while operator skill is less
critical.
The Processes
Open Die Forging
Open die forging with hammers and presses is a modern-day extension of
the pre-industrial metalsmith working with a hammer at his anvil.
Fig. 12. Roll forging.
In open die forging,
the workpiece is not completely confined as it is being shaped by the
dies. The open die process is commonly associated with large parts such
as shafts, sleeves and disks, but part weights can range from 5 to
500,000 lb.
Most open die
forgings are produced on flat dies. Round swaging dies and V dies also
are used in pairs or with a flat die. Operations performed on open die
presses include:
Drawing out or reducing the cross-section of an ingot or billet to lengthen it.
Upsetting or reducing the length of an ingot or billet to a larger diameter.
Upsetting, drawing out, and piercing--processes sometimes combined with
forging over a mandrel for forging rough-contoured rings.
Fig. 13. Roll forging using speciality shaped rolls.
As the forging
workpiece is hammered or pressed, it is repeatedly manipulated between
the dies until it reaches final forged dimensions. Because the process
is inexact and requires considerable skill of the forging master,
substantial workpiece stock allowances are retained to accommodate
forging irregularities. The forged part is rough machined and then
finish machined to final dimensions. The increasing use of press and
hammer controls is making open die forging, and all forging processes
for that matter, more automated.
In open die forging,
metals are worked above their recrystallization temperatures. Because
the process requires repeated changes in workpiece positioning, the
workpiece cools during open die forging below its hot-working or
recrystallization temperature. It then must be reheated before forging
can continue. For example, a steel shaft 2 ft in diameter and 24 ft
long may require four to six heats before final forged dimensions are
reached.
In open die forging of steel, a rule of thumb says that 50 lb of
falling weight is required for each square inch of stock cross-section.
Compression between
flat dies, or upsetting, is an open die forging process whereby an
oblong workpiece is placed on end on a lower die and its height reduced
by the downward movement of the top die. Friction between end faces of
the workpiece and dies prevents the free lateral spread of the metal,
resulting in a typical barrel shape. Contact with the cool die surface
chills the end faces of the metal, increasing its resistance to
deformation and enhancing barreling.
Upsetting between
parallel flat dies is limited to deformation symmetrical around a
vertical axis. If preferential elongation is desired, compression
between narrow dies (Fig. 1) is ideal. Frictional forces in the ax ial
direction of the bar are smaller than in the perpendicular direction,
and material flow is mostly axial.
A narrower die
elongates better, but a too-narrow die will cut metal instead of
elongate. The direction of material flow can also be influenced by
using dies with specially shaped surfaces.
Compression between
narrow dies is discontinuous since many strokes must be executed while
the workpiece is moved in an axial direction. This task can be made
continuous by roll forging (Fig. 2). Note the resemblance between Fig.
11 and Fig. 12. The width of the die is now represented by the length
of the arc of contact. The elongation achieved depends on the length of
this contact arc.
Larger rolls cause
greater lateral spread and less elongation because of the greater
frictional difference in the arc of contact, whereas smaller rolls
elongate more. Lateral spread can be reduced and elongation promoted by
using specially shaped rolls (Fig. 13).
The properties of
roll-forged components are very satisfactory. In most cases, there is
no flash and the fiber structure is very favorable and continuous in
all sections. The rolls perform a certain amount of descaling, making
the surface of the product smooth and free of scale pockets.
Impression Die Forging
Fig. 14. Impression die forging
In the most basic
example of impression die forging, which accounts for the majority of
forging production, two dies are brought together and the workpiece
undergoes plastic deformation until its enlarged sides touch the die
side walls
(Fig. 14). Then, some material begins to flow outside the die
impression, forming flash. The flash cools rapidly and presents
increased resistance to deformation, effectively becoming a part of the
tool. This builds pressure inside the bulk of the workpiece, aiding
material flow into unfilled impressions.
Impression die
forgings may be produced on a horizontal forging machine (upsetter) in
a process referred to as upsetting. In upsetting, stock is held between
a fixed and moving die while a horizontal ram provides the pressure to
forge the stock (Fig. 15). After each ramstroke, the
multiple-impression dies can open to permit transfer of stock from one
cavity to another.
A form of impression die forging, closed die forging does not depend on
flash formation to achieve complete filling of the die. Material is
deformed in a cavity that allows little or no escape of excess
material, thus placing greater demands on die design.
Fig. 15. Upsetting.
For impression die
forging, forging dies become more important, and operator skill level
is less critical in press forging operations. The press forging
sequence is usually block and finish, sometimes with a preform, pierce,
or trim operation. The piece is usually hit only once in each die
cavity.
The Precision Forging Advantage
Precision forging
normally means close-to-final form or close-tolerance forging. It is
not a special technology, but a refinement of existing techniques to a
point where the forged part can be used with little or no subsequent
machining. Improvements cover not only the forging method itself but
also preheating, descaling, lubrication, and temperature control
practices.
FIG 15b
The decision to
apply precision forging techniques depends on the relative economics of
additional operations and tooling vs. elimination of machining. Because
of higher tooling and development costs, precision forging is usually
limited to extremely high-quality applications.
Stages in the Ring Rolling Process
FIG 15c
Ring Rolling
Ring rolling has evolved from an art into a strictly controlled
engineering process. Seamless rolled rings are produced on a variety of
equipment. All give the same product--a seamless section with
circumferential grain orientation. These rings generally have
tangential strength and ductility, and often are less expensive to
manufacture than similar closed die forgings. In sum, the ring rolling
process offers homogeneous circumferential grain flow, ease of
manufacture, and versatility in material, size, mass, and geometry.
In the ring rolling
process, a preform is heated to forging temperature and placed over the
idler (internal) roll of the rolling machine. Pressure is applied to
the wall by the main (external) roll as the ring rotates. The
cross-sectional area is reduced as the inner and outer diameters are
expanded. Equipment can be fully automated from billet heating through
post-forge handling. Advanced ring rolling equipment can roll contours
in both the inner and outer diameter of the ring, allowing for
excellent weight reductions, material savings, and reduced machining
cost.
There is an infinite variety of sizes into which rings can be rolled,
ranging from rollerbearing sleeves to rings of 25 ft in diameter with
face heights of more than 80 in. Various profiles may be rolled by
suitably shaping the drive and idling rolls.
Extrusion
In extrusion (Fig. 16), the workpiece is placed in a container and
compressed until pressure inside the metal reaches flowstress levels.
The workpiece completely fills the container and additional pressure
causes it to travel through an orifice and form the extruded product.
Fig. 16. a-Foward extrusion; b-backward extrusion; c-tube extrusion; d-container extrusion.
Extrusion can be
forward (direct) or backward (reverse), depending on the direction of
motion between ram and extruded product. Extruded product can be solid
or hollow. Tube extrusion is typical of forward extrusion of hollow
shapes, and backward extrusion is used for mass production of
containers.
Piercing is closely related to reverse extrusion but distinguished by
greater movement of the punch relative to movement of the workpiece
material.
Secondary Processes
Besides the primary forging processes, secondary operations often are
employed. Drawing through a die is a convenient way to eliminate forged
draft (Fig. 17a). The mode of deformation is tangential compression.
The diameter of the drawing ring can be slightly smaller than the outer
diameter of the preforged shell to control or reduce wall thickness and
increase the height of the shell in a drawing or ironing operation
(Fig. 17b).
Fig. 17.a-drawing;
b-ironing
Bending can be performed on the finished forging or at any stage during its production.
Because forging stock may assume complex shapes, it is rare that only a
single die impression is needed. Preforming the forging stock--by
bending or rolling it, or by working it in a preliminary die--may be
more desirable. Gains in productivity, die life, and forging quality
often outweigh the fact that preforming adds an operation and attendant
costs. Forging in one final die impression may be practical for
extremely small part runs.
Since bending of
larger parts requires a machine of long stroke, special mechanical or
hydraulic presses are often necessary. Simple shapes can be bent in one
operation, but more complex contours take successive steps. If complex
shapes are to be formed in a single operation, the tool must contain
moving elements.
Special Techniques
After deformation, forged parts may undergo further metalworking. Flash
is removed, punched holes may be needed, and improved surface finish or
closer dimensional accuracy may be desired.
Trimming--Flash is trimmed before the forging is ready for shipping.
Occasionally, especially with crack-sensitive alloys, this may be done
by grinding, milling, sawing, or flame cutting.
Coining--Coining and ironing are essentially sizing operations with
pressure applied to critical surfaces to improve tolerances, smoothen
surfaces, or eliminate draft.
Coining is usually done on surfaces parallel to the parting line, while
ironing is typified by the forcing of a cup-shaped component through a
ring to size on outer diameter. Little metal flow is involved in either
operation and flash is not formed.
Swaging--This operation is related to the open die forging process
whereby the stock is drawn out between flat, narrow dies. But instead
of the stock, the hammer is rotated to produce multiple blows,
sometimes as high as 2,000 per minute. It is a useful method of primary
working, although in industrial production its role is normally that of
finishing. Swaging can be stopped at any point in the length of stock
and is often used for pointing tube and bar ends and for producing
stepped columns and shafts of declining diameter.
Fig. 18. Hot extrusion of a valve body.
[size=21]Hot
Extrusion--Extrusion is most suitable for forming parts of drastically
shaping by plastic deformation--spans a myriad of equipment and
techniques. Knowing the various forging operations and the
characteristic metal flow each produces is key to understanding forging
design.
Fig. 11. Compression between narrow dies.
Hammer and Press Forging
Generally, forged
components are shaped either by a hammer or press. Forging on the
hammer is carried out in a succession of die impressions using repeated
blows. The quality of the forging, and the economy and productivity of
the hammer process depend upon the tooling and the skill of the
operator. The advent of programmable hammers has resulted on less
operator dependency and improved process consistency. In a press, the
stock is usually hit only once in each die impression, and the design
of each impression becomes more important while operator skill is less
critical.
The Processes
Open Die Forging
Open die forging with hammers and presses is a modern-day extension of
the pre-industrial metalsmith working with a hammer at his anvil.
Fig. 12. Roll forging.
In open die forging,
the workpiece is not completely confined as it is being shaped by the
dies. The open die process is commonly associated with large parts such
as shafts, sleeves and disks, but part weights can range from 5 to
500,000 lb.
Most open die
forgings are produced on flat dies. Round swaging dies and V dies also
are used in pairs or with a flat die. Operations performed on open die
presses include:
Drawing out or reducing the cross-section of an ingot or billet to lengthen it.
Upsetting or reducing the length of an ingot or billet to a larger diameter.
Upsetting, drawing out, and piercing--processes sometimes combined with
forging over a mandrel for forging rough-contoured rings.
Fig. 13. Roll forging using speciality shaped rolls.
As the forging
workpiece is hammered or pressed, it is repeatedly manipulated between
the dies until it reaches final forged dimensions. Because the process
is inexact and requires considerable skill of the forging master,
substantial workpiece stock allowances are retained to accommodate
forging irregularities. The forged part is rough machined and then
finish machined to final dimensions. The increasing use of press and
hammer controls is making open die forging, and all forging processes
for that matter, more automated.
In open die forging,
metals are worked above their recrystallization temperatures. Because
the process requires repeated changes in workpiece positioning, the
workpiece cools during open die forging below its hot-working or
recrystallization temperature. It then must be reheated before forging
can continue. For example, a steel shaft 2 ft in diameter and 24 ft
long may require four to six heats before final forged dimensions are
reached.
In open die forging of steel, a rule of thumb says that 50 lb of
falling weight is required for each square inch of stock cross-section.
Compression between
flat dies, or upsetting, is an open die forging process whereby an
oblong workpiece is placed on end on a lower die and its height reduced
by the downward movement of the top die. Friction between end faces of
the workpiece and dies prevents the free lateral spread of the metal,
resulting in a typical barrel shape. Contact with the cool die surface
chills the end faces of the metal, increasing its resistance to
deformation and enhancing barreling.
Upsetting between
parallel flat dies is limited to deformation symmetrical around a
vertical axis. If preferential elongation is desired, compression
between narrow dies (Fig. 1) is ideal. Frictional forces in the ax ial
direction of the bar are smaller than in the perpendicular direction,
and material flow is mostly axial.
A narrower die
elongates better, but a too-narrow die will cut metal instead of
elongate. The direction of material flow can also be influenced by
using dies with specially shaped surfaces.
Compression between
narrow dies is discontinuous since many strokes must be executed while
the workpiece is moved in an axial direction. This task can be made
continuous by roll forging (Fig. 2). Note the resemblance between Fig.
11 and Fig. 12. The width of the die is now represented by the length
of the arc of contact. The elongation achieved depends on the length of
this contact arc.
Larger rolls cause
greater lateral spread and less elongation because of the greater
frictional difference in the arc of contact, whereas smaller rolls
elongate more. Lateral spread can be reduced and elongation promoted by
using specially shaped rolls (Fig. 13).
The properties of
roll-forged components are very satisfactory. In most cases, there is
no flash and the fiber structure is very favorable and continuous in
all sections. The rolls perform a certain amount of descaling, making
the surface of the product smooth and free of scale pockets.
Impression Die Forging
Fig. 14. Impression die forging
In the most basic
example of impression die forging, which accounts for the majority of
forging production, two dies are brought together and the workpiece
undergoes plastic deformation until its enlarged sides touch the die
side walls
(Fig. 14). Then, some material begins to flow outside the die
impression, forming flash. The flash cools rapidly and presents
increased resistance to deformation, effectively becoming a part of the
tool. This builds pressure inside the bulk of the workpiece, aiding
material flow into unfilled impressions.
Impression die
forgings may be produced on a horizontal forging machine (upsetter) in
a process referred to as upsetting. In upsetting, stock is held between
a fixed and moving die while a horizontal ram provides the pressure to
forge the stock (Fig. 15). After each ramstroke, the
multiple-impression dies can open to permit transfer of stock from one
cavity to another.
A form of impression die forging, closed die forging does not depend on
flash formation to achieve complete filling of the die. Material is
deformed in a cavity that allows little or no escape of excess
material, thus placing greater demands on die design.
Fig. 15. Upsetting.
For impression die
forging, forging dies become more important, and operator skill level
is less critical in press forging operations. The press forging
sequence is usually block and finish, sometimes with a preform, pierce,
or trim operation. The piece is usually hit only once in each die
cavity.
The Precision Forging Advantage
Precision forging
normally means close-to-final form or close-tolerance forging. It is
not a special technology, but a refinement of existing techniques to a
point where the forged part can be used with little or no subsequent
machining. Improvements cover not only the forging method itself but
also preheating, descaling, lubrication, and temperature control
practices.
FIG 15b
The decision to
apply precision forging techniques depends on the relative economics of
additional operations and tooling vs. elimination of machining. Because
of higher tooling and development costs, precision forging is usually
limited to extremely high-quality applications.
Stages in the Ring Rolling Process
FIG 15c
Ring Rolling
Ring rolling has evolved from an art into a strictly controlled
engineering process. Seamless rolled rings are produced on a variety of
equipment. All give the same product--a seamless section with
circumferential grain orientation. These rings generally have
tangential strength and ductility, and often are less expensive to
manufacture than similar closed die forgings. In sum, the ring rolling
process offers homogeneous circumferential grain flow, ease of
manufacture, and versatility in material, size, mass, and geometry.
In the ring rolling
process, a preform is heated to forging temperature and placed over the
idler (internal) roll of the rolling machine. Pressure is applied to
the wall by the main (external) roll as the ring rotates. The
cross-sectional area is reduced as the inner and outer diameters are
expanded. Equipment can be fully automated from billet heating through
post-forge handling. Advanced ring rolling equipment can roll contours
in both the inner and outer diameter of the ring, allowing for
excellent weight reductions, material savings, and reduced machining
cost.
There is an infinite variety of sizes into which rings can be rolled,
ranging from rollerbearing sleeves to rings of 25 ft in diameter with
face heights of more than 80 in. Various profiles may be rolled by
suitably shaping the drive and idling rolls.
Extrusion
In extrusion (Fig. 16), the workpiece is placed in a container and
compressed until pressure inside the metal reaches flowstress levels.
The workpiece completely fills the container and additional pressure
causes it to travel through an orifice and form the extruded product.
Fig. 16. a-Foward extrusion; b-backward extrusion; c-tube extrusion; d-container extrusion.
Extrusion can be
forward (direct) or backward (reverse), depending on the direction of
motion between ram and extruded product. Extruded product can be solid
or hollow. Tube extrusion is typical of forward extrusion of hollow
shapes, and backward extrusion is used for mass production of
containers.
Piercing is closely related to reverse extrusion but distinguished by
greater movement of the punch relative to movement of the workpiece
material.
Secondary Processes
Besides the primary forging processes, secondary operations often are
employed. Drawing through a die is a convenient way to eliminate forged
draft (Fig. 17a). The mode of deformation is tangential compression.
The diameter of the drawing ring can be slightly smaller than the outer
diameter of the preforged shell to control or reduce wall thickness and
increase the height of the shell in a drawing or ironing operation
(Fig. 17b).
Fig. 17.a-drawing;
b-ironing
Bending can be performed on the finished forging or at any stage during its production.
Because forging stock may assume complex shapes, it is rare that only a
single die impression is needed. Preforming the forging stock--by
bending or rolling it, or by working it in a preliminary die--may be
more desirable. Gains in productivity, die life, and forging quality
often outweigh the fact that preforming adds an operation and attendant
costs. Forging in one final die impression may be practical for
extremely small part runs.
Since bending of
larger parts requires a machine of long stroke, special mechanical or
hydraulic presses are often necessary. Simple shapes can be bent in one
operation, but more complex contours take successive steps. If complex
shapes are to be formed in a single operation, the tool must contain
moving elements.
Special Techniques
After deformation, forged parts may undergo further metalworking. Flash
is removed, punched holes may be needed, and improved surface finish or
closer dimensional accuracy may be desired.
Trimming--Flash is trimmed before the forging is ready for shipping.
Occasionally, especially with crack-sensitive alloys, this may be done
by grinding, milling, sawing, or flame cutting.
Coining--Coining and ironing are essentially sizing operations with
pressure applied to critical surfaces to improve tolerances, smoothen
surfaces, or eliminate draft.
Coining is usually done on surfaces parallel to the parting line, while
ironing is typified by the forcing of a cup-shaped component through a
ring to size on outer diameter. Little metal flow is involved in either
operation and flash is not formed.
Swaging--This operation is related to the open die forging process
whereby the stock is drawn out between flat, narrow dies. But instead
of the stock, the hammer is rotated to produce multiple blows,
sometimes as high as 2,000 per minute. It is a useful method of primary
working, although in industrial production its role is normally that of
finishing. Swaging can be stopped at any point in the length of stock
and is often used for pointing tube and bar ends and for producing
stepped columns and shafts of declining diameter.
Fig. 18. Hot extrusion of a valve body.
[size=21]Hot
Extrusion--Extrusion is most suitable for forming parts of drastically