Annealing and Recrystallization
Materials Science 2- ENME 2421
January 31, 2014
Instructor: Maciej S. Kumosa
Author: Alisha Alomia
Team Members: Gregg Gugegenheim, Matt Minuti, Luke Skelly
Introduction
Deformations of certain materials are used to obtain desired dimensions
and/or properties by applying stresses or heat treatment called annealing.
Annealing is used to change the dislocation energy and the properties of the
material. The annealing process consists of four different stages to change the
microstructure of the material. The four stages are, cold work, recovery,
recrystallization and grain growth. Cold work is when there has been not been any
type of deformation performed on the material. Recovery is the softening of the
material through the removal of primary dislocations and internal stresses. The
recovery process occurs at lower temperatures and there is no appearance of strain
free grains. Recrystallization is when strain free grains appear and begin to replace
those that are deformed from internal stresses. Lastly, grain growth is when the
microstructure begins to coarsen and the material may loose a substantial amount
of its original strength. If the original strength is altered it can be recovered through
hardening.
The ductility, strength and hardness of the material change respectively with
each stage in the annealing process. The most change is seen during
recrystallization. The annealing process is typically done with varying temperatures,
typically starting at 260 degrees Celsius and ending at 760 degrees Celsius. During
the recrystallization process, a large decrease in the materials strength is seen while
the ductility of the material increases.
Procedure
Four different samples of 70/30 brasses were used to compare crystal
structures. The annealing temperatures of the samples of brass that were given
were, not annealed, 380, 525 and 725 degrees Celsius respectively. The samples
were first polished with #240, #400 and #600 sand paper, making sure the sand
paper stayed damp while polishing. The specimen could not be rotated and an even
pressure had to be applied constantly during the polishing process to ensure even
polishing on the surface. When starting with a finer grit sandpaper, the brass had to
be positioned so that the new scratches would be perpendicular to the previous
ones.
The final step in the polishing process was polishing the brass with metcloth.
Beuhler alpha polishing Alumina No.1 was poured onto the metcloth while
polishing. The brass was then inserted into an ultrasonic cleaner for 3 minutes and
followed by polishing again with the metcloth and gamma micropoish Alumina #3,
following the same process done with the Alumina No. 1. While polishing with the
metcloth, the scratches still had to be perpendicular to the previous ones.
After polishing the brass the grains were exposed with an etching process
using 45% Nitric Acid. The nitric acid allows the boundaries of the grains to corrode
quickly while highlighting the edges. Only 80-95% nitric acid was available. The
nitric acid was diluted using 4.8 mL of Nitric Acid and 10.67 mL of water. A high
power optical microscope was used to capture the grains of the different specimens.
A Rockwell hardening test was taken of each of the brass samples. Hardness
was measured as a function of the ratio of the depth of penetration of the indenter
due to the test load and the depth of penetration due to a minor pre-load. The B and C
scales were used. Multiple were readings were taken to ensure accurate data.
Results
The results of the average hardness at each annealing temperature are listed
below. The average was found by taking the average of the four trials. After looking
at the microscopic pictures (Appendix: Figure 4, Figure 5, Figure 6, Figure 7) the
average grain size was determined. The scale used in the pictures is 100
micrometers and 500 micrometers.
Table 1:
Annealing
Temperature
Hardness Testing (F-scale)
Trial 1 Trial 2 Trial 3 Trial 4 Average
Average
grain size
(micro
meters)
20⁰C 67 67.5 67 67 67.125 25
380⁰C 55 57.5 57.5 57.5 56.875 50
525⁰C 33 35 36 35 34.75 30
725⁰C 21 24 24.5 24 23.375 250
0 100 200 300 400 500 600 700 8000
10
20
30
40
50
60
70
80
Average Hardness at Annealing Tem-perature
Series2
Annealing Temperature (C)
Har
dn
ess
Figure 1: Average Hardness at Annealing Temperatures
20 C⁰ 380 C⁰ 525 C⁰ 725 C⁰0
50
100
150
200
250
300
Average Grain Size due to Annealing Temperature
Series1
Annealing Temperature (C)
Ave
rage
Gra
in S
ize
(mic
ro m
eter
s)
Figure 2: Average Grain Size Due To Annealing Temperature
From the two plots above it can be seen that the recrystallization
temperature is approximately 380 degrees Celsius. This can be determined because
the hardness significantly changes at 380 degrees Celsius. The plateau before 380
degrees Celsius show that the material entered recrystallization after. It is seen in
figure two as well because there is a slight peak at 380 degrees Celsius.
Discussion
The results of the experiment are shown above in the two figures and table.
Figure 1 shows a graphical relationship of the annealing temperature against the
hardness of the material. Figure 2 shows a graphical relationship of the annealing
temperature against the average grain size. Both tables show that the
recrystallization stage begins at 380 degrees Celsius. When observing Figure one it
can be seen that the recrystallization phase occurred starting around 380 degrees
and ending around 550 degrees Celsius. The new grains started to form only if the
material was annealed at 380 degrees Celsius or above. Table 1 shows that while the
hardness of the brass decreased, grain size increases. Both of the factors were
measure by the use of hardness testing and the polishing and etching process to see
the grains.
Although figure one shows that grain growth begins around 550 and 600
degrees, it does not show when the annealing process ends. Further research would
have to be done to see when there is no longer a change in the grain size. At that
temperature is when the annealing process would be completely done.
During this lab there were three main areas where sources of area could
have arisen. The first source of error could have been that the samples of brass were
not annealed correctly. There is no way to test that they were done correctly so
assumptions had to be made that there was no error in the annealing process.
Another source of error was not polishing and etching the samples correctly. Even
with supervision of teaching assistants mistakes were still made. If pressed too hard
the sample would have an uneven polish, causing there to not be a large enough
area to see the grain sizes under the microscope. The last source of error could have
been the hardness test. Although it was calibrated using samples with known errors,
there were still variations in the hardness for each sample. Taking the average
hardness of the samples did not give precise results unless many more samples
were taken and the results were averaged.
Conclusion
In the experiment specific specimens of 70/30 brass were annealed at
different temperatures that met the four stages of the annealing process. Hardness
samples were taken from each of the samples to compare to the grain size of the
samples. The grains size was found through the polishing and etching process.
Tables and plots were made to show how the grain size, temperature and hardness
were relative to each other. It can be seen that as the temperature increases, the
hardness decreases and the grains size increases showing that the material becomes
more ductile.
Executive Summary
Four samples of brass were annealed at different temperatures, then
polished and etched to observe the grain size. The samples were tested for
hardness. The results showed the relationship between the annealing temperature,
grain size, and the hardness of the material. It can be seen that as the hardness
decreases, the temperature and grain size increase.
References
"Annealing Review Heat Treatment - Engineers Edge." Annealing Review Heat
Treatment - Engineers Edge. N.p., n.d. Web. 31 Jan. 2014.
ENME 2421 Laboratory Manual Winter 2011
Appendix
Calculations: Need 45% nitric acid from 80-95% source
Assume 80% 6ml*0.8 = 4.8 ml nitric acid
4.8/.45 = 10.667 ml total
10.667 ml - 6 ml = 4.667 ml to add
Figure 3: Nitric Acid Calculations
Figure 4: Microscopic Pictures of Brass As Received
Figure 5: Microscopic Pictures of Brass Annealed at 380 Degrees Celsius
Figure 6: Microscopic Pictures of Brass Annealed at 525 Degrees Celsius
Figure 7: Microscopic Pictures of Brass Annealed at 725 Degrees Celsius
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