This set of Manufacturing Processes Multiple Choice Questions & Answers (MCQs) focuses on “Annealing – 10”.
1. Temperature dependence of the grain-growth can be explained by ______
a) recrystallization theory
b) planks theory
c) assuming that grain boundaries brake due to thermal vibrations
d) strain energy theory
Explanation: The temperature dependence of the grain-growth exponent can be explained by assuming that the grain-boundary solute atmospheres are broken up by thermal vibrations at high temperatures. Those elements that distort the lattice structure the most have the largest effect on the rate of grain growth.
2. The presence of foreign particles _____
a) increases recrystallization temperature
b) improves surface hardness of the specimen
c) retards the grain boundary motion
d) softens the specimen
Explanation: Solute atoms in solid solution can form grain boundary atmospheres, the presence of which retards the normal surface-tension induced boundary motion. In order for the boundary to move, it must carry its atmosphere along with it.
3. _____ lowers the retarding effect of the solute/foreign atoms.
a) Increase in temperature
b) Decrease in temperature
c) Increase in amount of solute atoms
d) Decrease in size of solute atoms
Explanation: An increase in temperature lowers the retarding effect of the solute atoms and grain growth occurs under conditions more closely resembling the growth of soap cells.
4. At the high temperatures normally associated with annealing, _____
a) grooves may form on the surface
b) irregularity in surface roughness may occur
c) unwanted bump may form
d) decrease in yield strength may occur
Explanation: At the high temperatures normally associated with annealing, grooves may form on the surfaces where grain boundaries intersect the specimen surface.
5. Grain-boundary grooves are important in grain growth because of _____
a) they can counter the unwanted surface irregularities
b) they improve surface finish
c) they tend to fix the ends of grain boundaries
d) they are the result of perfect recrystallization process
Explanation: Grain-boundary grooves are important in grain growth because they tend to anchor the ends of the grain boundaries (where they meet the surface), especially if the boundaries are nearly normal to the surface.
6. Thermal grooving has little effect on the overall rate of growth when ______
a) average grain size of the specimen is very small as compared to the size of the specimen
b) average grain size of the specimen is larger as compared to the size of the specimen
c) temperature of the specimen is lower than recrystallization temperature
d) specimen has a high thermal conductivity
Explanation: When the average grain size of a metal specimen is very small compared to the dimensions of the specimen, thermal grooving, or the lack of curvature in the surface grains, has little effect on the overall rate of growth. However, when the grain size approaches the dimensions of the thickness of the specimen, it can be expected that grain-growth rates will be decreased.
7. During normal grain growth, the moving boundaries will be attached to the particles, so that _____
a) the particles exert a pulling force on the boundary
b) the particles exert a pushing force on the boundary
c) grain growth occurs at higher rate
d) surface structure does not get disturbed
Explanation: The moving boundaries will be attached to the particles so that the particles exert a pulling force on the boundary restricting its motion. The boundary will be attached to the particle along the length. If the boundary intersects the particle surface at 90 degrees, the particle will feel a pull of force.
8. The stabilization of fine grain size during heating at high temperatures requires _____
a) a large volume fraction of very small particles
b) a large volume fraction of bigger particles
c) planner grains
d) small grain boundaries
Explanation: The stabilization of fine grain size during heating at high temperatures requires a large volume fraction of very small particles. If the temperature is too high, the particles tend to coarsen or dissolve. When this occurs, some boundaries can break away before the others and abnormal grain growth occurs, transforming the fine-grain array into a very coarse-grain structure.
9. Metal grain boundaries differ from soap films.
Explanation: Metal grain boundaries differ from soap films for they possess only a single surface, whereas the latter has 2 surfaces. The driving force per unit area for grain boundary movement (∆pin the analogous soap bubble case) can be written in the following manner:
f = (2ϒ)/R
Where f is the force per unit area, ϒ the surface tension of the grain boundary, and R the net radius of curvature of the boundary.
10. Specimen geometry has no part in controlling the rate of grain growth.
Explanation: Specimen geometry may play a part in controlling the rate of grain growth. Grain boundaries near any free surfaces of a metal specimen tend to lie perpendicular to the specimen surface. This means that the curvature becomes cylindrical rather than spherical and, in general, cylindrical surfaces move at a slower rate than spherical surfaces with the same radius of curvature.
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