10.7

 

10.7 Factors Affecting Recrystallization

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.

% Coldwork

Recrystallization temperature, F

5

968

10

950

50

847

75

770

          Table 10.1 Recrystallization temperatures for iron/steel with varying coldwork.

The material with a higher percent coldwork has a finer grain structure at the end of recrystallization, as shown in Figure 10.7. This has to do with the increased grain boundary surface area that is available for nucleation as a consequence of the greater distortion of grains with higher coldwork. The recrystallization process involves nucleation followed by the growth of these nuclei. With the higher number of nuclei formed and considering the same probability for the survival of nuclei, the number of grains resulting at the end of recrystallization will be higher. This will result in a finer grain structure.

Fig10.7.jpg (24722 bytes)
Figure 10.7 Effect of coldwork on recrystallized grain size and recrystallization temperature.

(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 grain size.

(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.

 

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