Annealing Lab

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

