Page 359 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 14.3 Impression-die and Closed-die Forging

Forging Force. The forging force, F, in an open-die forging operation on a solid
cylindrical workpiece can be estimated from the formula
1 + %
2
F = Yfvrrz , (14.1)

where Yf is the flow stress of the material (see Example 14.1), /,L is the coefficient of
friction between the workpiece and the die, and r and /1 are, respectively, the instan-
taneous radius and height of the workpiece. (Derivations of this formula and of
others for various forging processes are given in references listed in the bibliography
at the end of the chapter.)

EXAMPLE l4.| Calculation of Forging Force in Upsetting

A solid cylindrical slug made of 304 stainless steel is the workpiece has undergone at the end of the stroke
150 mm in diameter and 100 mm high. It is reduced in this operation is
in height by 50% at room temperature by open-die 100
forging with flat dies. Assuming that the coefficient of s - ln( 50 > - 0.69.
friction is 0.2, calculate the forging force at the end of
the stroke. We can determine the flow stress by referring to
Eq. (2.8) and noting from Table 2.3 that, for 304 stain-
less steel, K = 1275 MPa and n = 0.45. Thus, for a
Solution: The forging force at the end of the stroke is
true strain of 0.69, the flow stress is calculated to be
calculated using Eq. (14.1), in which the dimensions
1100 MPa. Another method is to refer to Fig. 2.6 and
pertain to the final dimensions of the forging. Thus, note that the flow stress for 304 stainless steel at a true
the final height is lv = 100/2 = 50 mm, and the final
strain of 0.69 is about 1000 MPa. The small difference
radius, r, is determined from volume constancy by between the two values is due to the fact that the data
equating the volumes before and after deformation.
in Table 2.3 and Fig. 2.6 are from different sources.
Hence,
Taking the latter value, the forging force now can be
calculated, noting that in this problem the units in Eq.
(1f)(75)2(100) = (ff)(f)2(50)-
(14.1) must be in N and ni. Thus,
Therefore, r = 106 mm. P = (1o0o>(1o6)(¢f>(o.1o6)2(1> +-----
The quantity Yf in Eq. (14.1) is the flow stress of (2)(0.2)(0.106)
the material, which is the stress required to continue (3)(o.oso)
plastic deformation of the workpiece at a particular
true strain. The absolute value of the true strain that = 4.5 >< 107N = 45 MN.

14.3 Impression-die and Closed-die Forging

In impression-die forging, the workpiece takes the shape of the die cavity while
being forged between two shaped dies (Figs. 14.5a through c). This process usually
is carried out at elevated temperatures to lower the required forces and attain en-
hanced ductility in the workpiece. Note in Fig. 14.5c that, during deformation, some
of the material flows outward and forms a flash. The flash has an important role in
impression-die forging: The high pressure and the resulting high frictional resistance
in the flash presents a severe constraint on any outward flow of the material in the
die. Thus, based on the principle that in plastic deformation the material flows in
the direction of least resistance (because it requires less energy), the material flows
preferentially into the die cavity, ultimately filling it completely.