Cold Rolling
October 25, 2010 [LAB REPORT- MET 2200] 2200 ]
And Tensile Testing
Metropolitan State College of Denver Mechanical Engineering Technology Department By: Sean Bise MET 2200 Professor John Chart October 25, 2010
[LAB REPORT- MET 2200] 2200] Abstract: The design of this experiment is intended to show the difference in mechanical properties of samples of common engineering materials in fully rolled, half rolled, and non-rolled states. The intent is to highlight the advantages or disadvantages of cold working on samples of CDA 360, AISI AI SI 1018, and 6061 T6.
October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
I.
Introduction:
Cold working metals changes the mechanical property with respect to yield strength, tensile strength, ductility, and hardness. The effect of co ld working can vary from alloy to alloy. This experiment is designed to display the effects of c old rolling on steel, aluminum, and a copper alloy sample and determine the resulting changes in mechanical properties. The rolling press exerts compressive stress, deforming the material at a temperature below that which re crystallization occurs. II.
Objective:
The objective of this experiment is to determine mechanical property changes in an array of cold rolled samples materials under uniform temperature. Tensile tests will be conducted after cold rolling, and shaping into tensile test samples. The end result will provide the mechanical affects of the cold working procedure at the non-rolled, half, and full-rolled state. III.
Equipment and Materials:
Hardness testing machine in Rockwell B scale with 1/16” ball penetrator. Samples were cold rolled on a cold rolling machine. Tensile tests were conducted using and Instron tensile testing machine and 3 samples each of the following specimens of engineering alloys: 1) AISI 1018 cold formed steel- A relatively soft steel with low (less than .2%) carbon content 2) CDA 360 free cutting brass- A soft material used often as a sacrificial material or in pipe fittings and machining. The composition of CDA 360 makes it relatively easy to machine, having (2.5-3.7% lead) free lead in the alloy leads to small chips that do not impede machining. 3) AA 6061-T6 aluminum alloy- A relatively soft but lightweight material with important applications in the aerospace industry. IV.
Procedures: a. Samples of each material were marked as control, half or full rolled. b. Samples were tested for their pre cold rolling hardness figures. c. Fully rolled samples were put through the rolling press until their edges displayed separation indicative of a brittle fracture at their edge. d. Half rolled samples were then rolled to half as many cycles through the rolling machine. 2
October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200] e. Samples were allowed to cool, and were measured for their change in length and thickness. Empirical data was recorded for these changes.
f. Samples were then hardness tested for their post cold-work hardness figures. g. Samples were re-shaped into tensile test samples, and tested for tensile strength.
h. All data was recorded and tabulated. V.
Data Analysis:
In this experiment the intent was to show the affects of cold working on samples of common engineering alloys with respect to their tensile strength. During the cold rolling process, the material is mechanically deformed at relatively low heat. The amount cold work is defined by the reduction in cross sectional area of the sample; this gives the amount of plastic deformation in the sample, defined by the formula: d 0
-
d 1 d 0
X100% = % Cold Work
Samples were Tested in Half and fully rolled states. From the figures of elongation, the % cold work was calculated as follows.
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
Figure 1- Hardness figures given in Rockwell B scale. % cold work derived from half and full rolled values.
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
Strain Figure 2- CDA 360 tensile testing. Stress figures given in lb./f., Strain values given as elongation in inches.
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200] Figure 3- 6061 T6 tensile testing. Stress figures given in lb./f., Strain values given as elongation in inches.
Figure 4- 1018 Sample tensile testing. Stress values are given in lb/f., Strain values given as elongation in inches.
Spacial changes caused by rolling process.
Cross section 6061-T6 C360 Control 50% 100%
.32495 dia
.32495 dia
1018 .292 dia
W
0.14335
0.22665
0.1497
H W
0.36005 0.1224
0.3676 0.2121
H
0.37125
0.2616
0.3435 0.1321 0.4437 5
Change in length 6061-T6 C360 50% Broke in Range 0.115
1018 0.95 6
October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200] 100%
Sample 50% length (as) Dia Finish Thickness
Finish Width roll cycles 100% length (as) Dia Finish Thickness Finish Width roll cycles Control Dia roll cycles
VI.
0.105
0.08
6061-T6 5.995
C360 6.011
7.25
6.5
7.4375
inches
0.4999
0.4382
0.4987
inches
0.359
0.365
0.337
inches
. 392+R.1 03 6.25 6 17.81 0.4998
. 267+R.1 02 3 6.011 8.875 0.4382
. 401+R.0 53 inches 6 6.004 inches 16.6875 inches 0.4986 inches
0.125 0.556 12.5 5.985 0.5004 0
0.265 0.371 6 6.005 0.4382 0
0.132 inches 0.565 inches 12 5.995 inches 0.4987 inches 0
0.02
1018 5.985 inches
Hardness testing: 1) Rockwell hardness : is determined by the formula:
HR = E - e
HR, the Rockwell hardness number is determined by the following, F0 = preliminary minor load in kgf F1 = additional major load in kgf F = total load in kgf e = permanent increase in depth of penetration due to major load F1 measured in units of 0.002 mm E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenter D = diameter of steel ball
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
Rockwell Hardness Scales
Minor Load Major Load Total Load Value of F0 F1 F E kgf kgf kgf
Scale
Indenter
A
Diamond cone
10
50
60
100
B
1/16" steel ball
10
90
100
130
C
Diamond cone
10
140
150
100
2) Brinell hardness testing:
Hardness tests: 8
October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200] Pre-worked samples: Materia l
Rockwell Scale
AVG.
B 1018 C 50% Full CDA 360 C 50% Full 6061 C 50% Full
Brinel l 500k g
C 88 63 63
91 64 64
92 64 64
90 64 64
73.5 74 73 49 53.5 51.5
74 72 72 51 53 51.5
73.5 71 72.5 48 51 51
74 72 73 49 53 51
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Vicke rs 3000 kg
157 101 101
185 114 114
118 114 116 82 86 84
135 130 132
136 192
After Cold-working: Materia l
Rockwell Scale
AVG.
B 1018 C 50% Full CDA 360 C 50% Full 6061 C 50% Full
VII.
Brinel l 500k g
C 88 96
91 96
92 95.5
90 96 98
73.5 83 90 48 64.5 69
73.5 84 90 49 65 70
74 83.5 91 51 67.5 70
74 84 90 49 66 70
9 18 21
3 9
Vicke rs 3000 kg
136
157 179 189
185 216 228
192 230 243
118 140 157 82 104 110
135 162 185
170 192
117 125
Discussion:
Strengthening by strain hardening or cold working occurs as a result of being plastically deformed below their melting point. The intent strain hardening is to increase the density of dislocations, the increased number of dislocations block the motion of other dislocations. The values given during the testing came as some surprise. In 1018 steel, the increase in hardness was 6.5% for the half rolled sample, and only 8.1% for the fully rolled sample. These figures represent a minimal increase in hardness compared to 9
October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
the amount of cold working. During Tensile testing, the fully rolled steel sample had the expected increase in tensile and yield strength values, with the expected reduction in ductility. The half rolled sample had an unexpected result in that it had reduced values of tensile and yield strengths compared to the control. The discrepancy could have been the result of a difference in the dimensions of the sample. Brass samples displayed a higher increase in hardness under cold work. The half rolled sample displayed a13.5% increase in hardness, and the fully rolled sample exhibited a 21.5% increase. The half rolled sample of brass had the expected increase in yield strength values, as well as the overall increase in tensile strength. The fully rolled sample yielded unexpected results, having both lower yield strength and tensile strength values, compared to the control sample. The difference from expected values I attribute to surface imperfections that may have been introduced during the rolling process. Aluminum displayed the greatest increase in hardness. The half rolled sample increased its hardness by 34.6%. The fully rolled sample displayed an increase of 42.8%. While Aluminum had an increased hardness, it displayed sharp reductions in both tensile and yield strength of the sample. The fact that this occurred at multiple degrees of cold w orking, it suggests that the difference in properties caused by the cold working process is inherent in the composition of the material. One expected result was that the rate of increase in tensile strength vs. the rate of change in yield strength did not occur at the same rate. This manifested in the steel and brass samples, but did not show in the aluminum sample. Additionally, the increase in hardness occurs most rapidly at the initial cold working. These results were ev ident as the first 50% cold working yielded more significant changes in hardness than fully working the samples.
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
Figure 5- cold work diagram showing the reduction in grain size caused by cold rolling.
VIII.
Conclusion:
The mechanical properties are changed significantly during the cold working process. With the increase in Tensile and Yield strengths as well as hardness, there are significant decreases in the ductility, electrical conductivity, and corrosion resistance of the material, due to the increase in grain density. In BCC materials such as the 1018, the yield and tensile strength of the materials is significantly increased through cold working. Caution must be observed in cold rolling BCC materials so as not to induce catastrophic failure in the material during the cold working process. With the increase in cold working, there is a decrease in grain size, corresponding to an increase in hardness with a subsequent decrease in ductility over all samples. Materials exhibiting unexpected early failure could have the advantages of cold working restored if the material were to be annealed, allowing grain structures to reform, restoring a degree of their ductility, before further cold working can occur. This recovery period allows recrystallization and grain growth, allowing the formation of strain free dislocations to reorganize themselves into a stronger structure.
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
Figure 6- Displays the reduction in re-crystallization temperature as % CW increases
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200]
Figure 7- Showing the re-crystallization process and the effects of heat
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October 25, 2010 [LAB REPORT-COLD ROLLING MET 2200] Sources: Shackelford, James F. , (2009) Introduction to Materials Science for Engineers 7th edition, Pearson International Edition, New Jersey Low Carbon Steel, Matweb. AISI 1018 cold formed steel. Retrieved 23 Sept., 2010, from
http://www.matweb.com/search/DataSheet.aspx? MatGUID=3a9cc570fbb24d119f08db22a53e2421 Brass, Matweb, CDA 360 Free Machine Bras. Retrieved 23 Sept., 2010, fromhttp://www.matweb.com/search/DataSheet.aspx? MatGUID=dde0019dc689451b9567bfdd2407d9de Rockwell Hardness Testing , Gordon England, Rockwell hardness, Retrieved 24 Sept. 2010, http://www.gordonengland.co.uk/hardness/rockwell.htm Rockwell Conversion tables , Carbide depot, Hardness conversion chart , Retrieved 24 Sept. 2010, http://www.carbidedepot.com/formulas-hardness.htm Properties of 1018 Steel , MNSU.edu, Metal properties, Retrieved 25 Sept. 2010. http://solarteam.mnsu.edu/UserFilesShared/DATA_ACQUISITION/mts/1020HotRolled1018Col d.pdf Properties of CDA 360 Brass , Alloy threaded rods, Properties of brass, Retrieved 22 Sept. 2010, http://threaded-rods.com/alloy_360_brass.htm Properties of 6061 T6, MATWEB Retrieved 5 Oct. 2010, http://www.matweb.com/search/DataSheet.aspx? MatGUID=1b8c06d0ca7c456694c7777d9e10be5b ,
Cold Work , Lab 4, Retrieved 30 Oct. 2010, http://www.chadhuard.com/wp-content/uploads/2010/03/Lab_4-ColdWork.pdf Strengthening , Mechanisms in cold working, Retrieved 26 Oct. 2010, http://web.pdx.edu/~pmoeck/phy381/coldworking.pdf
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