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Cold working and annealing lab 2 che 333
- 2. Introduction:
The reason for conducting this experiment was to discover the different effects cold
working and annealing has on the hardness of the 7030 brass. Cold working is the plastic
deformation of metals below the recrystallization temperature and is very important and
useful. Cold working means you are rolling, drawing, bending, squeezing or shearing a
metal usually at room temperature. Some heat may be applied as long as it stays below
the recrystallization temperature which in this case is 370 C. Cold working a metal
means to mechanical stress to cause a permanent change to the metal's crystalline
structure. Brass can be cold worked to up to 70% before cracking begins. This process is
typically applied to steel, aluminum and copper. The benefits of cold working is that it
increases the tensile strength and hardness. It also improves the finish. The disadvantages
of cold working is that the metal becomes less ductile. In this experiment we will test the
hardness of the 7030 brass which will be rolled out to a several different thicknesses and
the amount of cold working taking place in the brass samples will be calculated. Then we
will anneal the brass and see how the hardness values change over a time period of fifty
minutes. Annealing is a heat treatment process that alters the physical properties of
metals to increase the ductility and reduce its hardness. This is basically the complete
opposite of cold working. Annealing makes the metal more workable because it involves
heating a material above its recrystallization temperature, maintaining a suitable
temperature, and then cooling it. In annealing, atoms move around in the crystal lattice
and the number of dislocations decreases. This is why the ductility of the metal increases
while the hardness decreases. I hypothesis that the more cold work done to brass samples
the hardness values will increase. When the samples are annealed, I believe the samples
that had a greater cold work percentage will have a greater final hardness after the
annealing process. Overall, the main purpose of this lab is to become more familiar with
cold working and annealing and how it effects the properties of metals.
Equipment/Materials:
One annealed 7030 brass sample, 0.108in thick x 4in long by1.25in wide.
One measuring caliper
Instron hardness tester on R30T scale and 1/16th” steel ball indenter.
Five samples consisting of 10, 20, 30, 40, and 50% cold worked 7030 brass
Three furnaces set to 350C
Tongs
Beakers of water to cool the samples
- 3. Experimental Procedure:
Initially, a 7030 brass sample, 0.108in thick x 4in long by1.25in wide, was rolled out to
several different thicknesses using a rolling mill. There was a total of six different
samples all different thicknesses. The initial thickness was measured using a caliper and
the initial hardness was measured using the Instron hardness tester. The % cold work was
calculated using this formula:
%cold work = 100 x (Initial Thickness – Rolled Thickness)/ Initial Thickness
A data table was then created and a graph of the hardness to % cold work for the 7030
brass samples was constructed. Now that there is five samples consisting of around 10,
20, 30, 40, and 50% cold worked and one sample 0% cold work the annealing process
started. The six samples after the cold working rolling process were placed in a furnace at
400 C. The hardness values were tested at 5, 10, 15, 20, 30, 40 and 50 minutes. The
values were then compiled into a table. Using the values in the table a graph of hardness
of the metal versus time was created. Depending on the hardness values, the degree of
recrystallization in the 7030 brass samples can be determined.
Experimental Results:
Day 1
Sample Thickness
(inches)
Hardnes
s
(RC30T)
% Cold
Work
0 0.108 32.1 0
1 0.098 50.8 9.3
2 0.087 68.0 19.4
3 0.078 70.1 28.2
4 0.069 73.5 36.1
5 0.050 74.6 53.7
- 4. 0 10 20 30 40 50 60
0
10
20
30
40
50
60
70
80
Hardness to % Cold Work for 70-30 Brass
% Cold Work by Rolling
Hardness (RC30T)
Day 2
Sampl
e
Hardness 5
Minutes
10
Minutes
15
Minute
s
20
Minutes
30
Minutes
40
Minutes
50
Minutes
0 0.108 33.0 32.5 31.1 34.1 34.5 33.8 36.0
1 0.098 50.9 47.3 47.6 53.8 47.5 49.8 48.6
2 0.087 65.0 64.5 62.8 63.8 59.4 55.2 50.8
3 0.078 58.4 54.2 50.7 48.8 46.2 44.0 42.6
4 0.069 55.6 50.7 49.3 47.7 46.1 46.1 44.6
5 0.050 50.9 52.1 52.6 51.1 50.3 49.5 46.4
- 5. Discussion:
Looking at our data and results my hypothesis was right and wrong. For the hardness to
% cold work for 7030 brass graph the line shows us that as cold work increases the
hardness of the metal samples also increase. This means the hardness is directly
proportional to the yield stress. This is the part where my hypothesis was correct. The
line graph has a nice smooth curve from 0% to 35% cold work then it plateaus. This
means the hardness doesn’t really increase by that much after 35% cold work has been
done to the brass sample. When material is cold worked, the crystal structure of the metal
is deformed either by bending, squeezing, rolling, etc. which results in a uniform
crystalline plains. These dislocations provide further resistance to deformation. Also the
grain shape changes to a more circular and smoother grain pattern rather than having
sharp edges before rolling. The yield strength, and tensile strength also increase due to
the "locking effect" of the dislocations due to the rolling process of the brass. The 7030
brass samples have a FCC structure at room temperature because there is less than 35%
Zn in the composition. If there’s was more than 35% it would have a BCC structure. The
FCC structure blocks the dislocation motion by intersecting slip planes. The more slip
planes being blocked the more applied stress will be required to move the planes.
Therefore, increasing the hardness. The second part of the lab was the annealing process.
My hypothesis was incorrect about annealing. I assumed the more cold work previously
done to the same, the harder the metal will get after the annealing process however it was
the complete opposite. The more cold work done previously to the metal, the more the
hardness decreased after the annealing process. Looking at the graph of hardness versus
time for 7030 brass at 400C, some samples’ hardness did not fluctuate that much
throughout the 50 minutes in the furnace while some samples’ hardness drastically
decreased. Looking at the line graphs, it is around the 10 minute mark where you really
see the hardness values begin to change. I believe this is where the metal reached the
recrystallization temperature. Also the percentage cold work previously done to the
samples had an effect on the annealing process. Looking at the data I believe the more
cold work on the brass sample, the more the recrystallization took place in the metal
structure. You can tell the samples with 53.7%, 36.1% and 28.2% cold work had much
finer and smaller grain size after the annealing process because the hardness decreased
drastically compared to the samples with very little thickness change. The smaller the
grain size, the harder the metal.
Conclusion:
