ASM Metals HandBook Vol. 14 - Forming and Forging
Подождите немного. Документ загружается.
Fig. 36 Extrusion-type defect in centrally located rib (a) and die-
design modification (b) used to avoid defect.
Source: Ref 34.
Most of the defects summarized above occur in hot forging, which is most common for impression-die forging.
Therefore, defect formation may also involve entrapment of oxides and lubricant. When this occurs, the metal is
incapable of rewelding under the high forging pressures; the term cold shut is frequently applied in conjunction with laps,
flow-through defects, and so on, to describe the flaws generated.
These defects are all related to the velocity field distribution of the deforming metal. They can be avoided by proper die
design, preform design, and choice of lubrication system. Strictly speaking, these defects are not fundamental to the
workability of the material. However, knowledge of these common forging defects is necessary for a practical
understanding of workability. These are the defects that commonly limit deformation in the forging process.
References cited in this section
31.
T. Okamoto, T. Fukuda, and H. Hagita, Material Fracture in Cold Forging--Systematic Classification o
f
Working Methods and Types of Cracking in Cold Forging, Sumitomo Search, No. 9, May 1973, p 46;
Source Book on Cold Forming, American Society for Metals, 1975, p 216-226
34.
A.M. Sabroff, F.W. Boulger, and H.J. Henning, Forging Materials and Practices, Reinhold, 1968
35.
F.N. Lake and D.J. Moracz, "Comparison of Major Forging Systems," Technical Report AFML-TR-71-
112,
TRW, Inc., May 1971
Workability Tests
George E. Dieter, University of Maryland
References
1. G.E. Dieter, Mechanical Behavior of Materials Under Tension, in Mechanical Testing,
Vol 8, 9th ed.,
Metals Handbook, American Society for Metals, 1985, p 20-27
2.
S.L. Semiatin, G.D. Lahoti, and J.J. Jonas, Application of the Torsion Test to Determine Workability, in
Mechanical Testing, Vol 8, 9th ed., Metals Handbook, American Society for Metals, 1985, p 154-184
3. M.J. Luton, Hot Torsion Testing, in Workability Testing Techniques,
G.E. Dieter, Ed., American Society
for Metals, 1984, p 95-133
4. S. Fulop, K.C. Cadien, M.J. Luton, and H.J. McQueen, J. Test Eval., Vol 5, 1977, p 419
5. J.A. Schey, T.R. Venner, and S.L. Takomana, Shape Changes in the Up-setting of Slender Cylinders,
J.
Eng. Ind. (Trans. ASME), Vol 104, 1982, p 79
6. G. Fitzsimmons, H.A. Kuhn, and R. Venkateshwar, Deformation and
Fracture Testing for Hot Working
Processes, J. Met., May 1981, p 11-17
7.
J.F. Alder and V.A. Phillips, The Effect of Strain Rate and Temperature on the Resistance of Aluminum,
Copper and Steel to Compression, J. Inst. Met., Vol 83, 1954-1955, p 80-86
8.
F.J. Gurney and D.J. Abson, Heated Dies for Forging and Friction Studies on a Modified Hydraulic Forge
Press, Met. Mat., Vol 7, 1973, p 535
9. J.E. Hockett, The Cam Plastometer, in Mechanical Testing, Vol 8, 9th ed., Metals Handbook,
American
Society for Metals, 1985, p 193-196
10.
J.G. Lenard, Development of an Experimental Facility for Single and Multistage Constant Strain Rate
Compression, J. Eng.Mater. Technol. (Trans. ASME), Vol 107, 1985, p 126-131
11.
A.B. Watts and H. Ford, On the Basic Yield Stress Curve for a Metal, Proc. Inst. Mech. Eng.,
Vol 169,
1955, p 1141-1149
12.
J.J. Jonas, R.A. Holt, and C.E. Coleman, Plastic Stability in Tension and Compression, Acta Metall.,
Vol
24, 1976, p 911
13.
S.L. Semiatin and J.J. Jonas, Formability and Wor
kability of Metals: Plastic Instability and Flow
Localization, American Society for Metals, 1984
14.
S.L. Semiatin, Workability in Forging, in Workability Testing,
G.E. Dieter, Ed., American Society for
Metals, 1984, p 197-247
15.
P. Dadras, Stress-Strain Relationships in Bending, in Mechanical Testing, Vol 8, 9th ed., Metals Handbook,
American Society for Metals, 1985, p 118-124
16.
J.A. Bailey, The Plane Strain Forging of Aluminum at Low Strain Rates and Elevated Temperatures,
Int. J.
Mech. Sci., Vol 11, 1969, p 491
17.
O. Pawelski, U. Rudiger, and R. Kaspar, The Hot Deformation Simulator, Stahl Eisen, Vol 98, 1978, p 181-
189
18.
S.M. Woodall and J.A. Schey, Development of New Workability Test Techniques, J. Mech.Work. Technol.,
Vol 2, 1979, p 367-384
19.
S.M. Woodall and J.A. Schey, Determination of Ductility for Bulk Deformation, in Formability Topics--
Metallic Materials, STP 647, American Society for Testing and Materials, 1978, p 191-205
20.
B. Avitzur, Metal Forming: Processes and Analysis, McGraw-Hill, 1968
21.
B. Avitzur and C.J. Van Tyne, Ring Forming: An Upper Bound Approach, J.Eng. Ind. (Trans. ASME),
Vol
104, 1982, p 231-252
22.
G. Garmong, N.E. Paton, J.C. Chesnut, and L.F. Necarez, An Evaluation of the Ring Test for Strain-
Rate
Sensitive Materials, Metall. Trans. A, Vol 8A, 1977, p 2026, 2027
23.
A.T. Male and V. DePierre, The Validity of Mathematical Solutions for Determining Friction From the
Ring Compression Test, J. Lubr. Technol. (Trans. ASME), Vol 92, 1970, p 389-397
24.
V. DePie
rre, F.J. Gurney, and A.T. Male, "Mathematical Calibration of the Ring Test With Bulge
Formation," Technical Report AFML-TR-37, U.S. Air Force Materials Laboratory, March 1972
25.
G. Saul, A.T. Male, and V. DePierre, "A New Method for the Determination of
Material Flow Stress
Values Under Metalworking Conditions," Technical Report AFML-TR-70-
19, U.S. Air Force Materials
Laboratory, Jan 1970
26.
R.E. Bailey, R.R. Shiring, and H.L. Black, Hot Tension Testing, in Workability Testing Techniques,
G.E.
Dieter, Ed., American Society for Metals, 1984
27.
E.F. Nippes, W.F. Savage, B.J. Bastian, H.F. Mason, and R.M. Curran, An Investigation of the Hot
Ductility of High Temperature Alloys, Weld. J., Vol 34, April 1955, p 183-196
28.
R.L. Plaut and C.M. Sellars, Analysis of Hot Tension Test Data to Obtain Stress-
Strain Curves to High
Strains, J. Test Eval., Vol 13, 1985, p 39-45
29.
A.L. Hoffmanner, "Plasticity Theory as Applied to Forging of Titanium Alloys," Paper presented at the
Symposium on the Thermal-Mechanical Treatment of Metals, London, May 1970
30.
R.P. Daykin, Ladish Company, unpublished research, 1951
31.
T. Okamoto, T. Fukuda, and H. Hagita, Material Fracture in Cold Forging--
Systematic Classification of
Working Methods and Types of Cracking in Cold F
orging, Sumitomo Search, No. 9, May 1973, p 46;
Source Book on Cold Forming, American Society for Metals, 1975, p 216-226
32.
S.L. Semiatin and G.D. Lahoti, The Occurrence of Shear Bands in Nonisothermal, Hot Forging of Ti-6Al-
2Sn-4Zr-2Mo-0.1Si, Metall.Trans. A, Vol 14A, 1983, p 105
33.
M.C. Mataya and G. Krauss, A Test to Evaluate Flow Localization During Forging, J. Appl. Metalwork.,
Vol 2, 1981, p 28-37
34.
A.M. Sabroff, F.W. Boulger, and H.J. Henning, Forging Materials and Practices, Reinhold, 1968
35.
F.N. Lake and D.J. Moracz, "Comparison of Major Forging Systems," Technical Report AFML-TR-71-
112, TRW, Inc., May 1971
Workability Tests
George E. Dieter, University of Maryland
Selected References
•
T.G. Byrer, Ed., Forging Handbook, Forging Industry Association and American Society for Metals, 1985
•
G.E. Dieter, Ed., Workability Testing Techniques, American Society for Metals, 1984
Workability Theory and Application in Bulk Forming Processes
Howard A. Kuhn, University of Pittsburgh
Introduction
WORKABILITY, as described in the two previous articles in this Section, is not merely a property of a material but a
characteristic of the material/process system. The role of the material is measured by a simple test or tests and should be
expressed in a quantitative form that is universally applicable. This measure is taken to be a basic property of the material
composition and structure, and it reflects the macroscopic outcome of the micromechanisms of plastic flow perturbed by
such inhomogeneities as voids, inclusions, and grain boundaries (see, for example, Fig. 3 to 5 in the article "Introduction
to Workability" in this Section). Such phenomena are dependent not only on the material structure but also on the process
parameters (strain rate and temperature), which define the role of the process in determining workability.
The micromechanisms of ductile fracture in bulk forming processes are strongly influenced by the stress and strain
environment imposed by the process. It was shown in Fig. 14 in the article "Introduction to Workability" in this Section
that an overall measure of strain to fracture could be related to an overall value of hydrostatic stress in the material during
processing. In the spectrum of processes (extrusion, rolling, forging, and wire drawing), the average hydrostatic stress
becomes increasingly tensile, and the strain to fracture progressively decreases. Within each of these processes, however,
the stress and strain states can be considered on a localized basis in the specific regions in which fractures initiate. These
localized conditions are controlled by the geometry of the workpiece, die design, and friction at the die/workpiece
interface. These three factors, in addition to the strain rate and temperature parameters mentioned above, embody the role
of the process in determining workability.
This article will focus on the effects of mechanical plasticity on workability, that is, process control of localized stress and
strain conditions to enhance workability. First, the nature of local stress and strain states in bulk forming processes will be
described, leading to a classification scheme that facilitates the application of workability concepts. This defines testing
procedures and specific process measurements for an experimental approach to workability evaluation. Theoretical
models and fracture criteria will then be described and compared with experimental results. Finally, the application of
workability concepts to forging, rolling, and extrusion processes will be discussed.
Workability Theory and Application in Bulk Forming Processes
Howard A. Kuhn, University of Pittsburgh
Stress and Strain States
Forging, extrusion, and rolling processes are generally considered to involve the application of compressive force to
material to impart a change in shape and dimensions. Upon close examination, however, it is clear that deformation
resulting from the applied load causes secondary stress and strain states that vary from point to point throughout the
deforming workpiece. These stress and strain states may include tension, so fracture can occur at certain locations in a
material even though the primary (applied) load is compressive.
To explore this possibility further, it is useful to review plasticity theory briefly. In the article "Introduction to
Workability" in this Section, the von Mises yield criterion was given; this yield criterion is the foundation of plasticity
theory for isotropic materials. It gives the relationships between normal and shear stress components at yielding. From
this, relationships between stress and strain components are derived:
(Eq 1)
where ε denotes strain, σ denotes stress, and the subscripts 1, 2, and 3 designate the three directions in an orthogonal
coordinate system. Here, λ is a proportionality factor dependent on the deformation history and flow stress curve of the
material. The resulting strain in a given direction is affected by the stress in all three coordinate directions. In addition,
these relationships satisfy the volume constancy condition, ε
1
+ ε
2
+ ε
3
= 0.
To illustrate these relationships, it is useful to consider a two-dimensional case of plane stress, σ
3
= 0:
(Eq 2)
Referring to Fig. 1 and Eq 2, a reduction in thickness (compressive strain in the 2-direction) can be accomplished either
by a compressive stress, P
2
(or -σ
2
), or by a tensile stress, σ
1
. Although the nature of the deformation is the same in both
cases, accomplishing that deformation in the latter case could risk fracture because it involves a tensile stress.
Fig. 1 Similarity of deformation under horizontal tension (a) and vertical compression (b).
For more general cases, Eq 2 can be rearranged to determine the following stresses:
(Eq 3)
Therefore, the stresses depend on the localized strains ε
1
and ε
2
that result from metal flow. Equation 3 is a more
convenient representation of plasticity relationships for workability study.
For example, the cylindrical surface of a compression test undergoes various combinations of axial and circumferential
strains, depending on the aspect ratio and the friction at the die contact surfaces (Fig. 2). When no friction exists, the ratio
of circumferential strain to axial strain is ε
1
/ε
2
= - . According to the first part of Eq 3, σ
1
= 0 for this case. The
deformation in this case is referred to as homogeneous compression, because the only stress acting is σ
2
and it is uniform
throughout the specimen. Therefore, the homogeneous compression test is suitable for measuring flow stress.
Fig. 2 Localized strains on the bulging cylindrical surface of an up
set test (a) and their variation with aspect
ratio and friction conditions (b). Source: Ref 1.
When friction exists at the die contact surfaces, material at these surfaces is constrained from moving outward, while
material at the midplane is not constrained. As a result, bulging occurs, as shown in Fig. 2(a). Under these conditions, the
circumferential strain ε
1
(>0) increases, and the localized axial compressive strain ε
2
(<0) decreases. From Eq 3 as ε
1
/ε
2
becomes more negative (< - ), σ
1
becomes more positive. Therefore, increasing bulging due to friction during the
compression of a cylinder increases the secondary tensile stress σ
1
and enhances the likelihood of fracture.
Similarly, at the edges of bars during rolling (Fig. 3), the elongation strain
1
is determined by the overall reduction in
area. The localized compressive vertical strain ε
2
, however, depends on the shape of the edge. Greater convexity and
sharpness of the edge decrease the compressive vertical strain for a given reduction, which, from the first part of Eq 3,
increases the secondary tensile stress σ
1
at the edge. Therefore, edge cracking during rolling is also due to secondary
stress states.
Fig. 3 Localized strains at the edges of bars during rolling (a) and their variation with edge profile (b).
Source:
Ref 2.
More complex cases of the same type of secondary tensile stress states occur in forging (Fig. 4). During the forging of a
hub shape, for example, the top surface of the hub is subjected to biaxial tension because of friction during flow around
the die radius (Fig. 4a). This is identical to the conditions present at the nose of a billet that is being extruded or rolled.
Similarly, during forging, the top surface of a rib undergoes tensile strain in the direction of curvature, and essentially no
strain occurs along the length direction of the rib (Fig. 4b). In both cases, localized secondary tensile stresses are
generated that may cause fracture. These stresses can be calculated from measured strain values using Eq 3.
Fig. 4 Strains at the free surfaces of forgings. (a) Axisymmetric hub. (b) Rib-web forging.
Figures 2, 3, and 4 show examples of plane stress, with the stress normal to the free surface being zero. Other regions of
workpieces in bulk deformation processes, however, are subjected to three-dimensional stress states. For example,
material at the die contact surfaces in forging, rolling, and extrusion (Fig. 5) is subjected to strains
1
and
2
in the plane
of the surface, as in Fig. 2, 3, and 4. In Fig. 5, however, this surface is also acted upon by pressure P
3
normal to the plane.
Similarly, at internal locations of the workpieces in such processes as forging or wire drawing (Fig. 6), a material element
of the central longitudinal plane is subjected to strains
1
and
2
and stress normal to the plane.
Fig. 5 An element of material at the
die contact surfaces during forging (a), drawing or extrusion (b), and
rolling (c).
Fig. 6 An element of material at the cen
ter of a forging undergoing double extrusion (a) and at the center of
wire being drawn (b). The element undergoes strains ε
1
and ε
2
, as in Fig. 2, 3, and 4, with stress σ
3
normal to
the 1-2 plane.
In Fig. 5 and 6, the material element can be thought of as the plane-stress elements in Fig. 2, 3, and 4 with σ
3
acting
normal to the plane. If ε
1
and ε
2
are taken to be the strains in this plane, then Eq 3 becomes:
In other words, for the same deformation (that is, the same values of ε
1
and ε
2
as in Fig. 2, 3, 4), the stresses σ
1
and σ
2
in
Eq 3 are biased by σ
3
. The stress normal to the surface, then, increases the hydrostatic stress component of the stress state
by σ
3
. This reflects the basic concept that hydrostatic stress does not affect yielding or plastic deformation. As shown in
Fig. 14 in the article "Introduction to Workability" in this Section, however, the hydrostatic stress has a significant effect
on fracture. In Fig. 5, the compressive stress (die pressure) P
3
would increase the strains at fracture, but in Fig. 6 the
internal stress σ
3
may be tensile and decrease the strains at fracture.
The above discussion of stress and strain states in various regions of workpieces suggests a convenient method of
classifying cracking defects, including those shown in Fig. 32 in the article "Workability Tests" in this Volume. Figures 2,
3, and 4 in this article illustrate the locations at which free surface cracks occur in forging and rolling, as well as the nose
ends of billets in extrusion or rolling. They are represented by cracks in Fig. 32 in the article "Workability Tests."
Figure 5 in this article shows examples of circumstances in which contact surface cracking occurs--for example, the fir
tree defect in extrusion or the longitudinal surface cracks in rolled plates. Other examples in forging include γ, η, and λ
cracks as shown in Fig. 32 in the article "Workability Tests."
References cited in this section
1.
H.A. Kuhn, P.W. Lee, and T. Erturk, A Fracture Criterion for Cold Forging, J. Eng.
Mater. Technol. (Trans.
ASME), Vol 95, 1973, p 213-218
2.
H. Ortiz and H.A. Kuhn, Physical Modeling of Bar Rolling for Workability Study, in
Physical Modeling of
Bulk Metalworking Processes, Conference Proceedings, ASM INTERNATIONAL, 1987
Workability Theory and Application in Bulk Forming Processes
Howard A. Kuhn, University of Pittsburgh
Empirical Criterion of Fracture
The stress and strain environments described in the previous section in this article suggest that a workability test should
be capable of subjecting the material to a variety of surface strain combinations. A capability for testing under
superimposed normal stress would also be desirable.
When considering workability tests, it is important to recognize that fractures initiate in localized regions where
interaction between the stress and strain states and the material structure reaches a critical level. Orientation, shape, and
volume fraction of inclusions and other inhomogeneities have a dominant effect on the fracture process. Therefore, it is
critically important that workability test specimens contain material having the same microstructural features as the
material in the localized, potential fracture regions of the actual process.
Specifically, when evaluating a workpiece for surface fractures, specimen surfaces must contain the as-received surface of
the workpiece under consideration because it may contain laps, seams, a decarburized layer, and so on, which affect
fracture initiation. By the same argument, evaluation of material for internal fractures, such as central burst, must involve
test specimens taken from the middle of the workpiece, where, for example, segregation of second phases may have
occurred. Also, because of possible anisotropy effects, orientation of the critical stresses with respect to any inclusion
alignment must be the same in the test specimens as it is in the actual process and material of interest.
The compression test has become a standard for workability evaluation. As shown in Fig. 2, a range of strain
combinations can be developed at the cylindrical free surface simply by altering friction and geometry conditions. The
influence of friction and consequent bulging on circumferential tensile stress development is clearly shown in Fig. 7.
Compression with friction produces circumferential tension that leads to fracture, while frictionless compression avoids
barreling, tension, and cracking as described in Fig. 2 and Eq 3.