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Cold working and annealing lab
1. Cold Working and Annealing Lab
CHE 333 Engineering Materials
7 November 2013
2. Introduction
Cold working is a process performed at room temperature. This includes a
variety of processes, such as forging, rolling, drawing, and extrusion. Although these
processes can be done at elevated temperatures, they must be done at room
temperature in order to be considered a cold working process. There are a few
reasons for utilizing cold working. One of these reasons is to homogenize the
material by moving the atoms within the structure. Secondly, this process aids in
controlling grain size. The grain size is maintained through full annealing when the
cold working process is completed. In addition to this, the material is hardened
through this process because the yield strength is increased, and thus lowering the
ductility.
In order to restore ductility, a process called annealing is performed. Since
cold working increases the yield stress and lowers the ductility, cold working can
only go so far before the material cannot handle the process anymore. Although
some ductility is restored to the material, some of the yield stress gained from cold
working will be lost. Annealing is a three-step process that recovers the material to
move and remove dislocations, recrystallizes to restore its mechanical properties,
and stimulate grain growth. Together, cold working and annealing provide the
desired mechanical properties of a material.
Equipment
One annealed 70-30 brass sample, 0.108in thick x 4in long by1.25in wide
Three furnaces at temperatures at 350 C (to +L solidus at 920C for 70-30 brass)
One measuring caliper
Instron hardness tester on R30T scale and 1/16” steel ball indenter
Five pre-rolled samples of various thicknesses
Five samples consisting of 10, 20, 30, 40, and 50% cold worked 70-30 brass
Tongs
Beakers with water to cool samples
3. Experimental Procedure
The first part of the procedure is cold working. Each group was provided one
sample of brass (6 total) initially 0.0108 inches thick. The samples were rolled to
several different thicknesses, and these final thicknesses were measured to calculate
the percent cold work for each sample. Each individual sample had a different
percent cold work. Once cold working was completed, the hardness was tested
twice for each sample. Hardness was tested on the Intron hardness tester set to the
Rockwell 30T scale with a 1/16th inch ball indenter as it was done in the previous
hardness testing lab. The hardness was tested twice for each sample and the
average hardness was recorded.
Once that process was completed, each group utilized the process of
annealing their samples. Each of these samples had a different hardness and percent
cold work. The hardness was tested twice for each of the samples after being
individually placed into a 400oC furnace at 5 minutes, 10 minutes, 15 minutes, 30
minutes, and 45 minutes. The hardness was tested using the same machine as it was
previously done after cold working. The data was recorded to illustrate the time and
cold work dependencies of recrystallization.
Experimental Data
Sample # Thickness (in) % Cold Work (%) Hardness
1 0.108 0 42.2
2 0.101 6.5 53.1
3 0.094 13 62.7
4 0.087 19.4 67.2
5 0.082 24.1 69.6
6 0.075 30.6 72.2
Table 1: Percent cold-worked and hardness for each sample
5. Table 2: Average hardness of each sample
Graph 2: Hardness vs. Time at 400°C
0 5 10 15 20 25 30 35 40 45
30
35
40
45
50
55
60
65
70
75
80
Time (minutes)
Hardness(R30T)
Hardness V Time for 70-30 Brass at 400 C
Thickness= 0.108"
Thickness= 0.101"
Thickness= 0.094"
Thickness= 0.087"
Thickness= 0.082"
Thickness= 0.075"
6. Discussion
Each sample being tested during cold working initially had the same
thickness. The final thickness of each sample differed when they were rolled at
room temperature. Knowing this, we can use the following equation to calculate the
percent cold work of each sample:
% Cold Work =
Since all of the final thicknesses were different, each sample will have a different
percent cold work. Because of this, the hardness of each sample differed as well,
meaning that the percent cold work of the sample correlates to hardness and yield
strength. Cold working increases these properties while lowering ductility.
Through annealing, each sample was placed in the same conditions, which
was the 400°C furnace 5 min, 10 min, 15 min, 30 min, and 45 min and having their
hardness tested in between each amount of time. Regardless of these conditions,
each sample reacted differently, which is easy to tell from the different curves in the
graph. The reason for this is that each sample had a different percent cold work. The
percent cold work affects recrystallization, hardness, and grain size of the materials.
Because of this, we conclude that the material has a specific percent cold work that
it must reach prior to recrystallization.
Conclusion
Cold working and annealing work together in order to strengthen the
mechanical properties of a material. Cold working is performed first, which could be
a variety of processes such as rolling, forging, extruding, and drawing as long as it is
done at room temperature. The purpose of cold working is to increase the yield
stress and hardness of a material. During cold working, the stress and strain of the
material are increase, which causes plastic deformation, and work hardening occurs.
Once cold working is completed, the material will have high yield stress and
dislocation density while also having low ductility.
Some of this ductility that has been lost is recovered through annealing. This
process has three steps: recovery, recrystallization, and grain growth. During
recovery, dislocations in the material are moved and removed through thermal
processes. This is the first stage and there is not a major change in ductility and
yield stress yet. Dislocations are removed through the diffusion of atoms. Cells are
created and form the nuclei of the new grains. During recrystallization, a majority of
the material’s mechanical properties are restored. It is necessary for the right
amount of cold working to be done prior to this step in order for new grains to form.
Small grain size is desired during grain growth, which can only be achieved if the
critical percent cold work is reached. If it is not close enough to that point, there will
be larger grain size.
7. References
CHE 333 Sixth Lab: Cold Working of 7030 Brass PDF
CHE 333 Seventh Lab: Annealing of 7030 Brass PDF
CHE 333 Class 15 PowerPoint
CHE 333 Class 16 PowerPoint
