Page 368 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

8 Chapter 14 Metal-Forging Processes and Equipment

|4.5 Forgeability of Metals; Forging Defects

Porgeability is generally defined as the capability of a material to undergo deforma-
tion without cracking. Various tests have been developed to quantify forgeability;
however, because of their complex nature, only two simple tests have had general
acceptance: upsetting and hot twist.
In the upsetting test, a solid, cylindrical specimen is upset between flat dies,
and the reduction in height at which cracking on the barreled surfaces begins is
noted (see also Fig. 2.20d). The greater the deformation prior to cracking, the
greater the forgeability of the metal. The second method is the hot-twist test, in
which a round specimen is twisted continuously in the same direction until it fails.
This test is performed on a number of specimens and at different temperatures, and
the number of complete turns that each specimen undergoes before failure at each
temperature is plotted. The temperature at which the maximum number of turns oc-
curs then becomes the forging temperature for maximum forgeability. The hot-twist
test has been found to be useful particularly for steels.
The forgeability of various metals and alloys is given in Table 14.3 in decreas-
ing order. Forgeability is based on considerations such as ductility and strength of
the material, forging temperature required, frictional behavior, and the quality of
the forgings produced. These ratings should be regarded only as general guidelines.
Typical hot-forging temperature ranges for various metals and alloys are included in
Table 14.3. Note that higher forging temperature does not necessarily indicate
greater difficulty in forging that material. For warm forging, temperatures range
from 200° to 300°C for aluminum alloys and from 550° to 750°C for steels.

Forging Defects. In addition to surface cracking, other defects can develop dur-
ing forging as a result of the material flow pattern in the die, as described next in
Section 14.6 regarding die design. For example, if there is an insufficient volume of
material to fill the die cavity completely, the web may buckle during forging and
develop laps (Fig. 14.16a). On the other hand, if the web is too thick, the excess
material flows past the already formed portions of the forging and develops inter-
nal cracks (Fig. 14.16b).

TABLE l4.3
forgeability uf Metals, in Decreasing Order
Approximate range of
Metal or alloy hot-forging temperatures ( C)
Aluminum alloys 400-550
Magnesium alloys 250-350
Copper alloys 600-900
Carbon- and low-alloy steels 850-1150
Martensitic stainless steels 1100-1250
Austenitic stainless steels 1100-1250
Titanium alloys 700-950
Iron-based superalloys 1050-1180
Cobalt-based superalloys 1 180-1250
Tantalum alloys 1050-1350
Molybdenum alloys 1 150-1350
Nickel-based superalloys 1050-1200
Tungsten alloys 1200-1300