Page 357 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 14.2 Open-die Forging

parts have a good surface finish and dimensional accuracy. Hot forging requires lower
forces, but the dimensional accuracy and surface finish of the parts are not as good as
in cold forging.
Forgings generally are subjected to additional finishing operations, such as
heat treating to modify properties and machining to obtain accurate final dimen-
sions and a good surface finish. These finishing operations can be minimized by
precision forging, which is an important example of net-shape or near-net-shape
forming processes. As we shall see throughout this book, components that can be
forged successfully also may be manufactured economically by other methods, such
as casting (Chapter 11), powder metallurgy (Chapter 17), or machining (Part IV).
Each of these will produce a part having different characteristics, particularly with
regard to strength, toughness, dimensional accuracy, surface finish, and the possibil-
ity of internal or external defects.

l4.2 Open-die Forging

Open-die forging is the simplest forging operation (Table 14.1). Although most
open-die forgings generally weigh 15 to 500 kg, forgings as heavy as 275 metric tons
have been made. Part sizes may range from very small (the size of nails, pins, and
bolts) to very large (up to 23 m, long shafts for ship propellers). Open-die forging
can be depicted by a solid workpiece placed between two flat dies and reduced in
height by compressing it (Fig. 14.3a)-a process that is also called upsetting or flat-
die forging. The die surfaces also may have shallow cavities or incorporate features
to produce relatively simple forgings.
The deformation of a workpiece under frictionless conditions is shown in
Fig. 14.3b. Because constancy of volume is maintained, any reduction in height in-
creases the diameter of the forged part. Note that the workpiece is deformed
uniformly. In actual operations, however, there is friction, and the part develops a
barrel shape (Fig. 14.3c)-a deformation mode also known as panciz/Qing.
Barreling is caused primarily by frictional forces that oppose the outward flow
of the workpiece at the die interfaces and thus can be minimized by using an effective

TABLE |4.l
General Characteristics :rf Forging Processes
Process Advantages Limitations
Open die Simple and inexpensive dies; wide range of part Limited to simple shapes; difficult to hold close
sizes; good strength characteristics; generally tolerances; machining to final shape necessary;
for small quantities low production rate; relatively poor utilization
of material; high degree of skill required
Closed die Relatively good utilization of material; generally High die cost, not economical for small
better properties than open-die forgings; good quantities; machining often necessary
dimensional accuracy; high production rates;
good reproducibility
Blocker Low die costs; high production rates Machining to final shape necessary; parts with
thick webs and large fillets
Conventional Requires much less machining than blocker Higher die cost than blocker type
type; high production rates; good utilization of
material
Precision Close dimensional tolerances; very thin webs High forging forces, intricate dies, and
and flanges possible; machining generally provision for removing forging from dies
not necessary; very good material utilization