This set of Manufacturing Processes Objective Questions & Answers focuses on “Annealing – 8”.
1. The ratio of ______ is frequently used in the interpretation of recrystallization data.
a) yield strength of test specimen and standard specimen
b) temperature and time
c) rate of nucleation and rate of growth
d) rate of recrystallization and polygonization
Explanation: The ratio of the rate of nucleation to the rate of growth, N to G, is frequently used in the interpretation of recrystallization data. If it is assumed that both N and G are constant or are average values for an isothermal recrystallization process, then the recrystallized grain size can be deduced from this ratio.
2. Many nuclei will form before the recrystallization process is completed if the ratio of the rate of nucleation to the rate of growth is _____
c) equal to 1
d) higher than rate of grain growth
Explanation: If the ratio is high, many nuclei will form before the recrystallization process is completed, and a fine-grain size will result. On the other hand, a low ratio corresponds to a slow rate of nucleation relative to the rate of growth, and to a coarse crystal size in a recrystallized specimen.
3. The ratio N to G decreases in magnitude with decreasing _____
d) grain size
Explanation: As the deformation before annealing is reduced to smaller and smaller values, the rate of nucleation falls much faster than the rate of growth. As a consequence, the ratio N to G decreases in magnitude with decreasing strain and is effectively zero at several percent elongation.
4. The rate of recrystallization is dependent upon how many variables?
d) It is an independent value
Explanation: The rate of recrystallization is dependent upon the following variables:
• temperature of annealing and
• amount of deformation
• purity of the metal
• initial grain size
The recrystallized grain size is independent of the annealing temperature but sensitive to the amount of strain.
5. Which of the following materials has rapid rates of recrystallization?
a) Pure metals
c) Ductile metals
d) Coinage metal
Explanation: Extremely pure metals have very rapid rates of recrystallization. As little as 0.01% of a foreign atom in solid solution can raise the recrystallization temperature by several hundred degrees.
6. The increase in the recrystallization temperature caused due to foreign atoms depends on _____
a) nature of foreign atoms
b) temperature of foreign atoms
c) size of foreign atoms
d) electronegativity of foreign atoms
Explanation: The increase in the recrystallization temperature caused by the presence of foreign atoms depends markedly upon the nature of the solute atoms. The fact that very small numbers of solute atoms have such a pronounced effect on recrystallization rates is believed to indicate that the solute atoms interact with grain boundaries.
7. Decreasing the grain size increases the _____
a) recrystallization time
b) nucleation rate
c) grain boundary area
d) recrystallization temperature
Explanation: Decreasing the grain size increases the grain-boundary area and, as a consequence, the volume and uniformity of distorted metal (that adjacent to the boundaries). This effect increases the number of possible sites of nucleation and, therefore, the smaller the grains of the metal before cold work, the greater will be the rate of nucleation and the smaller the recrystallized grain size for a given degree of deformation.
8. _____ is the driving force for grain growth.
b) Surface energy
d) Surface hardness
Explanation: The driving force for grain growth lies in the surface energy of the grain boundaries. As the grain grow in size and their numbers decrease, the grain boundary area diminishes and the total surface energy is lowered accordingly.
9. Annealing of a slightly deformed metal specimen can lead to a localized, very coarse-grain growth.
Explanation: If only a portion of a metallic object is deformed cold, a region containing a critical amount of cold work must exist between the worked and unworked areas. Annealing in this case can easily lead to a localized, very coarse-grain growth.
10. Foreign atoms lowers the recrystallization rates.
Explanation: When a foreign atom migrates to a grain boundary, both its elastic field, as well as that of the boundary, are lowered. In recrystallization, grain-boundary motion occurs as the nuclei form and grow. The presence of foreign atoms in atmospheres associated with these boundaries strongly retards their motion and therefore lowers the recrystallization rates.
Sanfoundry Global Education & Learning Series – Manufacturing Processes.
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