10.7 Factors Affecting
The following factors
affect the recrystallization behavior. These are discussed below:
(a) Temperature and time - Figure 10.6 shows the
dependence of recrystallization on time and temperature as discussed above. It shows that
the higher is the temperature, the shorter it takes to recrystallize the coldworked
material, and vice versa. Furthermore, the material can be recrystallized at any
temperature provided enough time is allowed for the process to occur. In practice, for
most metals the recrystallization times are prohibitively long for any meaningful
recrystallization to occur at room temperature. The exceptions are lead, tin and indium
which recrystallize at room temperature. Since the annealing of coldworked materials is
done routingly in industry, the recrystallization temperatures quoted in the literature
are for an annealing time of one hour.
(b) Coldwork - The prior coldwork is essential for
recrystallization because an annealed material does not recrystallize even when heated
close to its melting point. It is so because the driving force for recrystallization is
the strain energy from coldwork, a part of which was recovered during the recovery stage.
It is also found that a certain minimum coldwork, called critical coldwork, is
needed for the recrystallization to occur.
The recrystallization temperature varies with percent
coldwork as shown in Table 10.1. The higher the percent coldwork, the lower is the
recrystallization temperature. It is so because the material with higher amount of
coldwork has greater strain energy which is the driving force for the process. Thus, the
contribution of strain energy to the energy needed for recrystallization is greater and so
the external energy needed to heat the material is lower.
(c) Initial Grain Size - The
smaller the grain size of the material before coldworking, the larger will be the grain
boundary surface area after coldworking. The lattice strain from coldwork in this case
will also be larger. Since the number of nuclei formed in recrystallization is directly
proportional to the grain boundary surface area and the strain intensity, a finer grain
size after recrystallization will result in the case of a material with finer initial
(d) Material Purity - With its
presence in the missing plane of atoms in a dislocation, an impurity atom lowers the
strain energy in the dislocation region. The overall result of this is reduced strain
energy which requires higher temperature for recrystallization.