-LAB SESSION [A]
Thermo-Mechanical treatment is one of the most important strengthening mechanisms for metals. In any mechanical treatment, plastic deformation takes place, which increases the dislocation density. Dislocations pile up at the grain boundaries or interfere with each other, causing more strain hardening (work hardening) to occur. This increases the strength of the material, at the expense of its ductility. (Kalpakjian: 98)
The type and purpose of any mechanical treatment that a material is subjected to totally depends on the temperature range at which this treatment takes place. If thermo-mechanical treatment takes place at a temperature less than 0.3 Tm, the type of thermo-mechanical treatment is denoted as Cold Working (Tm: Melting temperature in degrees Kelvin). If the temperature range is between 0.3 to 0.5 Tm, the thermo-mechanical treatment is referred to as Warm Working. At temperatures above 0.6 Tm-, it is referred to as Hot Working. (Kalpakjian: 106)
The macroscopic differences between the three types of working can be spotted in the poor surface finish due to oxides developed on heating and low dimensional accuracy due to intermittent expansion and contraction in the case of hot working. On the other side, the mechanical properties can also reveal the type of treatment through the enhancement of ductility which results in hot working. (Kalpakjian: 107)
On the microscopic level, the type of thermo-mechanical treatment
is revealed through the shape of the grains. Rolling, for example, is a
bulk deformation / thermo-mechanical treatment manufacturing process aimed
at decreasing the thickness of slabs, billets or blooms to produce sheets
and structural sections. In this process, grains are flattened and elongated.
This plastic deformation is associated with increase in strength and decrease
in ductility when it is done under cold working conditions.
Each material can be cold-worked for a certain level, beyond
which it may fail during manufacturing. This depends on how much the material
is ductile. Ductility is calculated as % elongation:
.........(1)
The amount of cold work is calculated as:
..................(2)
where A0 is the initial cross section, and Af is the final cross section.
Sometimes it is also represented as (Smith: 256):
......................(3)
where t stands for the thickness, provided that the lateral dimension
does not change.
Despite the enhancement in strength that results from cold working,
the loss of ductility represents a major disadvantage. This necessitates
the use of a remedial procedure which is called annealing. [To be continued...]
1 . Measure the dimensions of the commercial copper plate
after cutting a section for hardness and microstructural investigation
of the initial material structure.
2 . Label all your specimens.
3 . Start a 10-cycles rolling process depending on the
allowable elongation % of the material.
4 . Measuring the change in dimensions after each cycle
to calculate the amount of cold working
5 . After the 10-cycles, keep the last specimen to undergo
annealing.
6 . Arrange with Mr. Zakaria to measure the corresponding
hardness.
7 . Leave all your specimens in a dry plastic bag with
Eng. Reem by October 11th for Sunday lab and 14th for Tuesday Lab, making
sure that their corresponding labels are specified on them. Eng. Reem will
mount them all but the one needed to undergo annealing.
8 . Each group has to name a person to follow the annealing
process on October 13th for Sunday Lab, and October 15th for Tuesday Lab.
Annealing is a thermo-mechanical process in which the cold worked material is reheated to restore ductility and modify its microstructure. Annealing involves heating the metal within or above the range 0.3-0.5 Tm, which is also known as the recrystallization temperature range. The process should take place in an inert atmosphere in order to avoid oxidation of the material. (Smith: 296)
Annealing is composed of three main stages: (Kalpakjian: 276)
1 . Heating the temperature of a cold-worked material
to their corresponding annealing temperature, depending on its type and
the amount of cold working.
2 . Holding the temperature for a period of time (also
known as soaking)
3 . Slow cooling
During annealing, heat energy is supplied to the dislocations that were formed during the cold working process, which consequently induces their re-arrangement in more stable energy configurations, and the annihilation of a significant amount (Smith: 298). This decreases the dislocation density from 1011-1012 per square millimeter, which is the case of a highly cold-worked material, to 107-108 after annealing (Kalpakjian: 98).
Annealing causes significant structural changes to occur, which take
place in three steps known as (1) Recovery, (2) Recrystallization, and
(3) Grain Growth. (Kalpakjian: 104)
In Recovery, stresses are relieved and ductility is restored
(recovered), however there is no enhancement in the mechanical properties.
Some dislocations are annihilated too. In recrystallization, which takes
place within the recrystallization temperature range, new equiaxed grains
are formed. More dislocations annihilate. This lowers the strength obtained
during cold working. As the temperature increases, grain growth starts,
forming new grains of even larger size (Kalpakjian: 104) & (Smith:
298).
1 . Determine the corresponding annealing temperature and
time for the specimen using the ASM handbook (use the hardness of the first
specimen (0% CW) to estimate the commercial copper alloy category and accordingly
its heat treatment specifications).
2 . Measure the hardness of the annealed specimen.
3 . Prepare all the specimens for microstructural investigations
(grinding, polishing, etching).
4 . Bring a camera film to photograph the microstructures.
1 . Microstructure of 8 specimens (1 untreated, 6 cold
worked, 1 annealed).
2 . Plot a curve of the hardness against the amount of
cold work
3 . Plot a curve of ln the hardness against ln the % elongation,
which should yield the strain hardening exponent of the equation:
................................(4)
4 . Bonus: measure the electric conductivity of each specimen
after each cold working stage, and generate a plot between the electric
conductivity and % cold work. Use an avometer (will be provided in the
lab) to measure the resistance in Ohms, and use Ohm's law to obtain the
conductivity, which is given as:
.......(5)
N.B. This is a sensitive procedure, and it may not yield meaningful
results without precise measurements and accurate representation of data.
Report: Comment on all your results in the report, including a literature
review in your introduction using the references below. Reports will be
graded according to the depth of discussion, accurate representation of
experimental data and results, as well as professional report writing aspects
[language, format, references, organization]. Make sure to support your
report with charts, curves, and tables that are relevant to the heat treatment
of copper. Report Due Date: October 25th 2001
1. ASM Handbook, Volume 4. [Pages 880], Volume 2 [Pages 216]
2. Callister, Willaim D.. Materials Science and Engineering: An Introduction.
John Wiley & Sons, Inc, 4th Ed., 1996. [Pages: 163-173]
3. Kalpakjian, Serope. Manufacturing Process for Engineering Materials.
Addison Wesley, 3rd Ed., 1997. [Pages: 98-107 & 276]
4. Smith, William F. . Principles of Materials Science and Engineering.
McGraw Hill, 3rd Ed., 1996. [Pages: 290-302]
5. Van Vlack, Lawrence H. Materials for Engineering: Concepts
& Applications. Addison Wesley, 1982. [Pages: 71-83]
6. "Annealing", http://info.lu.farmingdale.edu/depts/met/met205/annealingstages.html,
October 2nd, 2001.
7."Annealing of Cold Worked Metals", online,
http://www.eng.wdnye.edu/MSE130/Annealing.htm, October 2nd, 2001.
8. "Alpha Brass: Exp#6", Online,
http://www.seas.upenn.edu:8080/~mse250/Exp6/exp6main.html, October
2nd, 2001.
9. "Cold Work", Online, http://info.lu.farmingdale.edu/depts/met/met205/coldwork.html,
October 2nd, 2001.
10. "Lab 2", online, http://kcgl1.eng.ohio-state.edu/~ms_unocic/Lab_2.htm,
acessed October 2nd, 2001.
11. "MSE 201 Lab II A", Online, http://web.utk.edu/~rtocchet/201lab_2a
, October 2nd, 2001.
12. "MSE 201 Lab II B", Online, http://web.utk.edu/~rtocchet/201lab_2b
, October 2nd, 2001.
13. "Procedures", Online,
http://www.seas.upenn.edu:8080/~mse250/Exp6/exp6pro.html, October
2nd, 2001.
Prepared by : Moataz M. Attallah
**If you need any help you can reach me @ 010-6202029 or mizoa@aucegypt.edu
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