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TitleCold and Hot Forging Fundamentals and Applications
TagsDeformation (Engineering) Stress (Mechanics) Plasticity (Physics) Yield (Engineering) Forging
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First printing, February 2005

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Library of Congress Cataloging-in-Publication Data

Cold and hot forging : fundamentals and applications / edited by Taylan Altan, Gracious
Ngaile, Gangshu Shen.

p. cm.
Includes bibliographical references and index.
ISBN: 0-87170-805-1
1. Forging. I. Altan, Taylan. II. Ngaile, Gracious. III. Shen, Gangshu.

TS225.C63 2004
671.3�32—dc22 2004055439

SAN: 204-7586

ASM International�
Materials Park, OH 44073-0002

Printed in the United States of America

© 2005 ASM International. All Rights Reserved.
Cold and Hot Forgings: Fundamentals and Applications (#05104G)

Page 166

Process Design in Impression-Die Forging / 167

Fig. 14.10 Metal flow and the corresponding load-stroke
curve. (a) Upsetting. (b) Filling. (c) End. (d) Load-

stroke curve [Altan et al., 1983]
Fig. 14.9 Typical load-stroke curve for closed-die forging

[Altan et al., 1983]

The geometry of the finisher die is essentially
that of the finish forging augmented by flash
configuration. In designing finisher dies, the di-
mensions of the flash should be optimized. The
designer must make a compromise: on the one
hand, to fill the die cavity it is desirable to in-
crease the die stresses by restricting the flash di-
mensions (thinner and wider flash on the dies);
but, on the other hand, the designer should not
allow the forging pressure to reach a high value,
which may cause die breakage due to mechani-
cal fatigue. To analyze stresses, “slab method of
analysis” or process simulation using finite-ele-
ment method (FEM)-based computer codes is
generally used. The FEM approach is discussed

By modifying the flash dimensions, the die
and material temperatures, the press speed, and
the friction factor, the die designer is able to
evaluate the influence of these factors on the
forging stresses and loads. Thus, conditions that
appear most favorable can be selected. In addi-
tion, the calculated forging stress distribution
can be utilized for estimating the local die
stresses in the dies by means of elastic FEM
analysis. After these forging stresses and loads
are estimated, it is possible to determine the cen-
ter of loading for the forging in order to locate
the die cavities in the press, such that off-center
loading is reduced.

14.4.1 Flash Design and Forging Load

The flash dimensions and the billet dimen-
sions influence the flash allowance, forging load,
forging energy, and the die life. The selection of
these variables influences the quality of the
forged part and the magnitude of flash allow-
ance, forging load, and the die life. The influence
of flash thickness and flash-land width on the
forging pressure is reasonably well understood
from a qualitative point of view. The forging
pressure increases with:

● Decreasing flash thickness
● Increasing flash-land width because of the

combinations of increasing restriction, in-
creasing frictional forces, and decreasing
metal temperatures at the flash gap

A typical load-versus-stroke curve from an
impression-die forging operation is shown in
Fig. 14.9. Loads are relatively low until the more
difficult details are partly filled and the metal
reached the flash opening (Fig. 14.10). This
stage corresponds to point P1 in Fig. 14.9. For
successful forging, two conditions must be ful-
filled when this point is reached: A sufficient
volume of metal must be trapped within the con-
fines of the die to fill the remaining cavities, and
extrusion of metal through the narrowing gap of
the flash opening must be more difficult than
filling of the more intricate detail in the die.

As the dies continue to close, the load in-
creases sharply to point P2, the stage at which
the die cavity is filled completely. Ideally, at this
point the cavity pressure provided by the flash
geometry should be just sufficient to fill the en-

Page 167

168 / Cold and Hot Forging: Fundamentals and Applications

Fig. 14.12 Relationships among flash width/thickness ratio,
excess stock material, forging load, and energy

for a constant flash thickness, t, of 0.04 in. (1.0 mm) (same forging
as that shown in Fig. 14.11) [Vieregge, 1968]

Fig. 14.11 Relationships among excess stock material, flash
thickness, flash width/thickness ratio, and forg-

ing load for mechanical press forging of a round part approxi-
mately 3 in. (7.6 cm) in diameter by 3.5 in. (8.9 cm) high [Vi-
eregge, 1968]

tire cavity, and the forging should be completed.
However, P3 represents the final load reached in
normal practice for ensuring that the cavity is
completely filled and that the forging has the
proper dimensions. During the stroke from P2 to
P3, all the metal flow occurs near or in the flash
gap, which in turn becomes more restrictive as
the dies close. In that respect, the detail most
difficult to fill determines the minimum load for
producing a fully filled forging. Thus, the di-
mensions of the flash determine the final load
required for closing the dies. Formation of the
flash, however, is greatly influenced by the
amount of excess material available in the cav-
ity, because that amount determines the instan-
taneous height of the extruded flash and, there-
fore, the die stresses.

The effect of excess metal volume in flash
formation was studied extensively [Vieregge,
1968]. It was found that a cavity can be filled
with various flash geometries provided that there
is always a sufficient supply of material in the
die. Thus, it is possible to fill the same cavity by
using a less restrictive, i.e., thicker, flash and to
do this at a lower total forging load if the nec-
essary excess material is available (in this case,
the advantages of lower forging load and lower
cavity stress are offset by increased scrap loss)
or if the workpiece is properly preformed (in

which case low stresses and material losses are
obtained by extra preforming). These relation-
ships are illustrated in Fig. 14.11 and 14.12.

14.4.2 Empirical Methods for
Flash Design

The “shape classification” (Fig. 14.8) has
been utilized in systematic evaluation of flash
dimensions in steel forgings. For this purpose,
1500 forgings from eight different forging com-
panies were classified into shape groups, as
shown in Fig. 14.8. By evaluating the flash de-
signs suggested for these forgings, an attempt
was made to establish a relationship between
forging weight and flash dimensions. The results
for group 224 are presented in Fig. 14.13 as an
example [Altan et al., 1973]. This figure can be
used for selecting the flash thickness based on
the forging weight, Q, of the forging. This graph
also shows the relationship between the flash
width/thickness (w/t) ratio and the forging
weight. Thus, knowing the weight of the part to
be forged, it is possible to find the corresponding
flash thickness and w/t ratio. Thus, the user can
obtain the flash dimensions based on the weight
of the forging.

There is no unique choice of the flash dimen-
sions for a forging operation. The choice is vari-
able within a range of values where the flash
allowance and the forging load are not too high.
There has to be a compromise between the two.
In general, the flash thickness is shown to in-
crease with increasing forging weight, while the
ratio of flash width to flash thickness (w/t) de-
creases to a limiting value. In order to investi-

Page 332

Index / 341

Temperature, and metal forming 59–66(F, T),

Tensile testing, and flow stress 25(F), 26(F), 27–29
Titanium and titanium alloys

aircraft component, process modeling for 203(F),

alpha and beta stabilizers for 262(T)
beta transus and forging temperatures for 262(T)
flow stress-strain 46(T)
hot forging temperatures 163(T)
isothermal and hot-die forging of 259–262(F, T),

263(F), 264(F), 265(F)
lubrication 73(T), 75(T)
preform dies 174, 175(T)
ring compression testing 65(T)

and die design 164
and manufacturing process 2–3(F), 4(F)
in precision forging 319–323(F)

Tool steels
AISI classification and composition 278(T)
for cold forging 286(T), 287(T)

advances in 323–326(F)
automotive, failure investigation using FEM 202(F),

for cold forging 225–228(F), 230–233(F), 286–289(T)
and friction/lubrication 69–70
for isothermal and hot-die forging 263–264
materials for 277–289(F, T)
as process variable 8(F), 9, 196, 237–238

Torsion testing, and flow stress 36
Toughness, of die steels 281–283(F)
Training 332–334
Transverse rolling 142, 144(F), 145(F)
Trapped-die forging. See Enclosed-die forging
Tresca yield criterion 52–53(F), 54–55(F)
True strain 23(F)
Tungsten and tungsten alloys

hot forging temperatures 163(T)
lubrication 75(T)

Tungsten carbide, for tooling 287, 288(T)


Upper-bound analysis 92(T), 97–98
Upset forging. See also Electro-upsetting

closed-die 216–217(F)
cold 215–217(F, T)
and finite element analysis 101–102, 103(F), 312–314(F,

and friction 67(F)
and horizontal machines 144–145, 147(F)
load vs. displacement 109(F)
overview 13–14(F)
and slab analysis 93–97(F), 216–217, 218(F)
and upper-bound analysis 97–98

Uranium and uranium alloys, flow stress-strain 47(T)


Velocity field
and local deformation 20
and slab analysis 93–94(F), 95–96
and strain rate 21

Visioplasticity analysis 92
Von Mises yield criterion 53–55(F), 56(F)


Warm forging. See Hot forging
Wear mechanisms, in die failure 296–297(F), 298(F, T)
Wear resistance, of die steels 279–280(F)


Yield criteria, and plastic deformation 52–55(F), 56(F)


Zinc phosphate, for lubrication 71–73(F, T), 214(T)
Zirconium and zirconium alloys, flow stress-strain


© 2005 ASM International. All Rights Reserved.
Cold and Hot Forging: Fundamentals and Applications (#05104G)

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