Forging
Forging
Forging
CHAPTER 5
METAL WORKING PROCESSES
5.1 INTRODUCTION
5.1.1 DEFINITIONS
Plastic Deformation Processes
Operations that induce shape changes on the workpiece by plastic deformation
under forces applied by various tools and dies.
Sheet-Forming Processes
In sheet metalworking operations, the cross-section of workpiece does not
change—the material is only subjected to shape changes. The ratio cross-section
area/volume is very high.
Sheet metalworking operations are performed on thin (less than 6 mm) sheets,
strips or coils of metal by means of a set of tools called punch and die on machine
tools called stamping presses. They are always performed as cold working
operations.
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5.2 FORGING
Forging is the working of metal into a useful shape by hammering or pressing. It is
the oldest of the metal working processes.
Most forging operations are carried out hot, although certain metals may be cold
forged.
The two broad categories of forging processes are open-die forging and closed-die
forging.
Closed-die forging uses carefully machined matching die blocks to produce forging to
close dimensional tolerances.
h0
hf
Flash Gutter Flash
(çapak haznesi) (çapak) Fig. 5.7 Forging
A0h0 = Afhf
processes
Open die forging Closed die forging
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According to the degree to which the flow of the metal is constrained by the dies
there are three types of forging:
1. Open-die forging
2. Impression-die forging
3. Flashless forging
Fig. 5.8 Three types of forging: (a) open-die forging, (b) impression die forging, and (c) flashless
forging
Open-die forging
Impression-die forging
In impression-die forging, some of the material
flows radially outward to form a flash:
Flashless forging
The work material is completely surrounded by the die cavity during compression and
no flash is formed:
Fig. 13 Flashless forging: (1) just before initial contact with the workpiece, (2) partial compression,
and (3) final push and die closure. Symbol v indicates motion, and F - applied force.
Most important requirement in flashless forging is that the work volume must equal
the space in the die cavity to a very close tolerance.
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Coining
Special application of flashless forging in which fine detail in the die are
impressed into the top and bottom surfaces of the workpiece.
There is a little flow of metal in coining.
Fig.14 Coining operation: (1) start of cycle, (2) compression stroke, and (3) ejection of
finished part
Raising Rollers
PE = m g h = W h
W Upper Die
Lower Die
Mechanical 0.06-1.5
Fig. 5.17 Drop forging hammer, fed by conveyor and heating unit at the right of the scene.
In determination forging load for free upsetting (open die), two assumptions can
be made for simplicity:
o
Stroke
W
Load (P)
P0 = A0 0 and P = A 0
where
P: forging load (press force)
A: area and
o: flow strength
V 0V
Volume = A0h0 = Ah = Const. A = then..P =
h h
h0
h0
The work done
W = Pdh = 0V ln
hf
hf
If there is friction: 0V d
P= (1 + ) Schey Equation
h 3h
P = c1 0 AT
AT : total cross sectional area and
c1 = 1.2-2.5 for open die forging
c1 = 3.0-8.0 for simple shape closed die forging
c1 = 8.0-12.0 for complex shape closed die forging
5.3. ROLLING
Rolling is a metal deformation process where the thickness of the metal is reduced
by successive passes from rolls.
The metal emerges from the rolls traveling at the higher speed than it enters
T-t A o − Af
Reduction ratio = 100 or Reduction in Area = x100
T Ao
Hot rolling
(after recrystallization
new fine grains)
T t
A0 Af
Flat Rolling
Two-high mill
Three-high mill
Planetary roll
(For high reduction)
Cluster mill
(For stronger material)
COLD WORK
Fig. 5.20-b Various configurations of rolling mills
Steps in rolling
The preheated at 1200oC cast ingot (the process is known as soaking) is rolled
into one of the three intermediate shapes called blooms, slabs, or billets.
Shape rolling
The work is deformed by a gradual reduction into a contoured cross section (I-
beams, L-beams, U-channels, rails, round, squire bars and rods, etc.).
Ring rolling
Thick-walled ring of small diameter is rolled into a thin-walled ring of larger
diameter:
Fig. 5.22 Ring rolling used to reduce the wall thickness and increase the diameter of a ring
Thread rolling
Threads are formed on cylindrical parts by rolling them between two thread dies:
Gear rolling
Gear rolling is similar to thread rolling with three gears (tools) that form the gear
profile on the work
B
R
A
V0
Pr F
h0 Vf hf
Tback Tfront
LP
Plain strain conditions are valid for rolling (i.e. no change in width of plate) and
speed of neutral plane (N) is equal to tangential speed of rolls:
b ho vo = b hf vf = b h v b: width
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The angle between the entrance plane and the centerline of the rolls is called angle of
contact or angle of bite. For the workpiece to enter into the gap between the rolls, horizontal
component of the normal force Pr and the frictional force F should be equal or frictional force
should be bigger.
B
R
A
V0
Pr F
h0 Vf hf
Tback Tfront
LP
F Sin
FCos Pr Sin Tan
Pr Cos
F
Tan where F = Pr
Pr
F
= Tan limiting condition for rolling.
Pr
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2 1 Q L p
P= 0 (e − 1)b Rh Q= and L p = R h
3 Q hm
R: radius of rolls,
hm: mean thickness between entry and exit, and
h: reduction in thickness.
Vr
W0
V0
Wf
R
Vf
t0/2
tf /2
t0 w0l0 = t f w f l f Volume
v0 vr v f
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Before and after this point slipping and friction occur between roll and workpiece.
The amount of the slip can be measured by means of the forward slip: v f − vr
S=
vr
T − t t0 − t
The reduction ratio sometimes used as draft r = = d = t0 − t f
T t0
μ : coefficient of friction
R : roll radius
μ ~ 0.1 0.2 0.4
cold warm hot
Length of contact:
L = R Power:
θ: angle of contact (rad.) P = NT on each roll
w : width
Torque:
T = 0.5FL
Example:
A 300x25 mm strip is fed through a rolling mill with two powered rolls each of 250
mm radius. The work thickness is to be reduced to 22 mm in one pass at a roll
speed of 50 rpm. The flow stress of work material is 180 MPa and the coefficient
of friction between roll and work is about 0.12. Determine if the friction is
sufficient to permit rolling operation to be accomplished. If so calculate the roll
force, torque and horsepower.
RCosØ
12.5
11
OR draft d = 25 − 22 = 3mm
d max = 2 R = (0.12)2 250 = 3.6mm
d d max feasible
L = R = 250 6.28 = 27.4mm
180
N
F = 0 wL = 180 2
300mm 27.4mm = 1.4797MN
mm
1m
T = 0.5FL = 0.5 (1.4797 106 N ) 27.4mm 3
= 20.272kNm
10 mm
1rev 1 min 2 1m
P = NFL = 50 (1.4797 10 N ) 27.4mm 3
6
P = 212.287kw
P = 212.287 1.34
P = 284.46hp
5.4 EXTRUSION
Extrusion is a Bulk Deformation Process in which the work is forced to flow through a
die opening to produce a desired cross-sectional shape.
Extrusion is the process by which a block of metal is reduced in cross-section by
forging it to flow through a die under pressure. In general, extrusion is used to
produce cylindrical bars or hollow tubes, but irregular cross-sections may also be
produced. Lead, tin, aluminum alloys can be cold extruded. Horizontal type presses
are used. Speed is depends on temperature and type of material used.
Direct extrusion
Fig. 5.27 Direct extrusion to produce solid cross section. Schematic shows the various
equipment components.
Indirect extrusion
Fig. 5.28 In indirect extrusion (backward, inverse extrusion) the material flows in the
direction opposite to the motion of the ram to produce a solid (left) or a hollow cross
section (right)
Df
ho hf
It can be calculated similar to forging. Here, ram power and ram force are:
hf
W = 0V ln and P = 0 A0 ln
A0
h0 Af
Wire and Bar Drawing is a Bulk Deformation Process in which the cross-section of
a bar, rod or wire is reduced by pulling it through a die opening, as in the next
figure:
A0
P = 0 Af ln
Af Pback Pfront
Drum Drum
The number of dies varies between 4 to12. The maximum possible reduction per
pas is 0.63. In practice, draw reductions per pass are well below the theoretical
limit. Reductions of 0.5 for single-draft bar drawing and 0.3 for multiple-draft wire
drawing seem to be the upper limits in industrial practice.
THE END