Page 370 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 14 Metal-Forging Processes and Equipment
With continuing advances in developing reliable simulation of all types of metal-
working operations, software is available to help predict material flow in forging-die
cavities. The simulation incorporates various conditions, such as workpiece tempera-
ture and heat transfer, to tooling, frictional conditions at die-workpiece contact
surfaces, and forging speed. Such software can be very helpful in die design and in
eliminating future problems with defective forgings (see also Section 38.7).

Preshaping. In a properly preshaped workpiece, the material should not flow eas-
ily into the flash (otherwise die filling will be incomplete), the grain flow pattern
should be favorable for the products’ strength and reliability, and sliding at the
workpiece~die interfaces should be minimized in order to reduce die wear. The selec-
tion of preshapes requires considerable experience and involves calculations of
cross-sectional areas at each location in the forging. Computer modeling and simu-
lation techniques are useful in such calculations.

Die Design Features. The terminology for forging dies is shown in Fig. 14.5 d, and
the significance of various features is described next. Some of these considerations
are similar to those for casting (Section 122). For most forgings, the parting line is
located at the largest cross section of the part. For simple symmetrical shapes, the
parting line is normally a straight line at the center of the forging, but for more com-
plex shapes, the line may not lie in a single plane. The dies are then designed in such
a way that they lock during engagement, in order to avoid side thrust, balance
forces, and maintain die alignment during forging.
After sufficiently constraining lateral flow to ensure proper die filling, the flash
material is allowed to flow into a gutter, so that the extra flash does not increase the
forging load excessively. A general guideline for flash thickness is 3% of the maxi-
mum thickness of the forging. The length of the land is usually two to five times the
flash thickness.
Draft angles are necessary in almost all forging dies in order to facilitate re-
moval of the part from the die. Upon cooling, the forging shrinks both radially and
longitudinally, so internal draft angles (about 7° to 1O°) are made larger than exter-
nal ones (about 3° to 5°).
Selection of the proper radii for corners and fillets is important in ensuring
smooth flow of the metal into the die cavity and improving die life. Small radii
generally are undesirable because of their adverse effect on metal flow and their
tendency to wear rapidly (as a result of stress concentration and thermal cycling).
Small fillet radii also can cause fatigue cracking of the dies. As a general rule, these
radii should be as large as can be permitted by the design of the forging. As with the
patterns used in casting, allowances are provided in forging-die design when
machining the forging is necessary to obtain final desired dimensions and surface
finish. Machining allowance should be provided at flanges, at holes, and at mating
surfaces.

Die Materials. Most forging operations (particularly for large parts) are carried
out at elevated temperatures. General requirements for die materials therefore are

° Strength and toughness at elevated temperatures
° Hardenability and ability to harden uniformly
° Resistance to mechanical and thermal shock
° Wear resistance, particularly resistance to abrasive wear, because of the pres-
ence of scale in hot forging.
Common die materials are tool and die steels containing chromium, nickel, molyb-
denum, and vanadium (see Tables 5.7 and 5.8). Dies are made from die blocks,