Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 75
A minimum degree of prior cold work is necessary before a material will
recrystallize and the minimum temperature for recrystallization (T R ) is
dependent on several factors:
(i) It is inversely proportional to the time of anneal and to the degree of
prior cold work.
(ii) It is directly proportional to the initial grain-size and to the temperature
of prior cold work.
The effect of the degree of prior strain and the temperature of anneal upon
the final grain size produced after recrystallization is indicated in Fig. 3.3,
where is it seen that, in general, coarse grain sizes result after small strains
and high annealing temperatures, whereas fine-grained structures form after
high strains and low annealing temperatures.
If the degree of prior strain or the annealing temperature is too low to
cause recrystallization, the material may still undergo softening by a process
known as recovery. Here, no microstructural change is apparent, but some
lattice defects are removed by the thermal treatment.
Thermo-mechanical treatment
When materials are hot-worked, dynamic recrystallization may take place
during the deformation process itself. Depending on the degree of strain and
0.4
Grain size (mm 2 )
0.3
0.2
0.1
0
0 10 20 30 40 50 60 75
Deformation (%)
800
700
600
500
400
Temperature, T(°C)
3.3 Showing the recrystallized grain size as a function of prior
deformation and recrystallisation temperature.