Page 73 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 1 The Structure of Metals
Recrystallization is also a function of time, because it involves diffusion-the
movement and exchange of atoms across grain boundaries.
The effects on recrystallization of temperature, time, and plastic defor-
mation by cold working are as follows:
a. For a constant amount of deformation by cold working, the time required
for recrystallization decreases with increasing temperature;
b. The more the prior cold work, the lower the temperature required for
recrystallization;
c. The higher the amount of deformation, the smaller the grain size becomes
during recrystallization; this effect is a commonly used method of con-
verting a coarse-grained structure to one having a finer grain, and thus
one with improved properties;
d. Some anisotropy due to preferred orientation usually persists after recrys-
tallization; to restore isotropy, a temperature higher than that required for
recrystallization may be necessary.
3. Grain growth. If the temperature of the metal is raised further, the grains
begin to grow, and their size may eventually exceed the original grain size;
called grain growth, this phenomenon adversely affects mechanical properties
(Fig. 1.13). Large grains also produce a rough surface appearance on sheet
metals, called orange peel, when they are stretched to form a part, or on the
surfaces of a piece of metal when subjected to bulk deformation, such as com-
pression in forging (Chapter 14).

l.8 Cold, Warm, and Hot Working

Cold working refers to plastic deformation that is usually, but not necessarily, carried
out at room temperature. When deformation occurs above the recrystallization tem-
perature, it is called hot working. “Cold” and “hot” are relative terms, as can be seen
from the fact that deforming lead at room temperature is a hot-working process,
because the recrystallization temperature of lead is about room temperature. As the
name implies, warm working is carried out at intermediate temperatures; thus, warm
working is a compromise between cold and hot working. The important technologi-
cal differences in products that are processed by cold, warm, and hot working are
described in Part III.
The temperature ranges for these three categories of plastic deformation are
given in Table 1.2 in terms of a ratio, T/Tm, where T is the working temperature
and Tm is the melting point of the metal, both on the absolute scale. Although it
is a dimensionless quantity, this ratio is known as the homologous temperature.

TABLE l.2
Homulogous Temperature Ranges for Various
Processes
Process T/ Tm
Cold working <O.3
Warm working 0.3 to 0.5
Hot working >0.6