Q. Explain the grain growth.
Ans. Grain growth: After Recrystallization is complete, the strain free grains will continue to grow if the metal specimen is left at elevated temperature. This phenomenon is called grain growth.
During grain growth temperature gives atoms of crystal sufficiently mobility to arrange themselves in low energy configuration. Atoms are moves across the interface from small to large grains due to enhance thermal vibration created by temperature.
The salient features of grain growth are:
(1) The required energy for grain growth is less than for Recrystallization.
(2) At Recrystallization temperature grain growth are less although grain growth is depend on temperature.
(3) Due to grain growth, grains of metals have tendency to come back in more stable state.
(4) There is maximum size of grain for a particular temperature. After it grain size will not increase even though temperature will rise.
(5) Fine grains are harder and stronger than coarse grains.
(6) The appearance of metal surface depends on grain size.
(7) Grains are abnormally increase if there is no uniform distribution of impurities in metal.
The factors that control the grain growth of grain size are
(a) Degree of pre deformation
(b) Time and temperature of annealing process
(c) Insoluble impurities present
(a) Degree of pre deformation: If degree of pre deformation is increases then it produces effective nucleation sites and reduces the final grain size.
(b) Time and Temperature: Grain growth is the result of increasing temperature above the Recrystallization temperature. Shorter the time of heating to the annealing temperature quick heating finer grain produced. On the other hand slow heating at the annealing process leads to coarse grains.
(c) Insoluble impurities: Finer impurities with uniform distribution will result in finer grains.