strain hardening and annealing chapter 7 – 4 th edition chapter 8 – 5 th edition
TRANSCRIPT
Strain Hardening and Annealing
Chapter 7 – 4th EditionChapter 8 – 5th Edition
Strain Hardening in Metals
When a piece of metal is deformed, the dislocations run into each otherThis traffic jam increases the material’s strengthDeforming a piece of metal also actually increases the number of dislocationsThis increases the strength too!!
Tensile Test
You can understand this better by relating it to the results of the tensile test.
John D Russ – Materials Science – A Multimedia Approach
Try it!!
Strain harden a piece of copper tubing.
Effects of Strain Hardening
Yield Strength goes upTensile Strength goes upDuctility goes down The material becomes brittle
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Strain Hardening Coefficient
Strain Hardening Coefficient is a measure of how much the metal can be strengthened by strain hardeningIt needs to have some ductility to be strain hardened.
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n is the slope of the PLASTIC portion of the curve, when graphed on a logarithmic scale
Log plot of the plastic portion of a tensile test
Log
Tru
e S
tress
, σ
Log True Strain, ε0.001
0.010
0.100
1.0
K
Slope=n
Log(σ) = Log(K) + n Log(ε)
Effect of Crystal Structure
HCP metals are already brittle Little strain hardening is possible Strain hardening coefficient around 0.05
BCC metals are less brittle than HCP Some strain hardening is possible N around 0.15
FCC metals are ductile Strain hardening is easy N around 0.5
Frank-Read Source
Strain hardening actually increases the strength of a material PAST its original tensile strengthWhy?Additional dislocations are formed as dislocations run into point defects
Yield Point
Tensile Strength
Strain Hardening – The effect of dislocation generation
From “Materials Science – A Multimedia Approach”, by John Russ
Copper 30% zinc alloy
Frank-Read Source
Point Defect Point Defect
Dislocation
http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/backbone/r5_3_2.html
Frank-Read Source
Frank-Read Source
Before deformation a typical dislocation density is about 106 cm of dislocation per cm3 of metalAfter strain hardening it may increase to as much as 1012 cm per cm3 of metal
Strain Hardening in Polymers
When you pull on a polymer, the chains line upVan der Waal bonds form between the chainsThe polymer becomes stronger
Try it with a 6-pack ring!!The mechanism for strain hardening in plastics is different from the mechanism in metals
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http://www.indiamart.com/amd-metplast/pcat-gifs/products-small/bottel-preform.jpg
PET Bottles and Preforms
Strain Hardening in Ceramics?
Ceramics are already brittle – so strain hardening is not usually possibleCeramics break because of flaws – the mechanism of deformation is differentAnnealing ceramics causes grain growth May or may not be bad
Back to MetalsCold Work
There is only a certain amount you can deform a material before it breaksCold work is strain hardening measured in % - The percent change in cross sectional area of the materialDifferent materials have varying % allowable cold work
Wire Drawing
d0
Initial diameter
Final diameter
d
Initial Cross Sectional Area - Final Cross Sectional Area% Cold Work =
Initial Cross Sectional Area
F
Rod
Deformation of a rod (or a piece of wire)Initial cross sectional area minus Final cross sectional areaOver the initial cross sectional area
100*0
0
A
AA
20
220
20
220
20
220
*
**%
d
dd
r
rr
r
rrCW
All times 100 of course
Copper is often drawn into wire
Copper rod feeding into drawing machine
Plate Rolling
h0
Initial thickness
h
Final thickness
Initial Cross Sectional Area - Final Cross Sectional Area% Cold Work =
Initial Cross Sectional Area
Cold Rolling
Metal is often rolled into sheets from thicker stockThe width of the sheet is usually kept the same, and only the thickness varies
oh
hh
Wh
hhW
A
AACW
0
0
0
0
0%
Cold Rolled Steel
Cold Work
What if you want to deform the sample more than is “possible?For example, what if you want to draw a piece of wire, from a rod of copper?You can anneal the material, and “undo” the strain hardening
Annealing is a heat treatment
Problem
Propose a series of steps to reduce a rod of copper-zinc alloy from 1 “ diameter to .1”diameter.The maximum cold work allowable for this copper zinc alloy is 85%.You will have to draw the copper, then anneal it several times.
One solution
Draw the 1” rod to 0.5”
Anneal
Draw the 0.5” rod to 0.25”
Anneal
Draw the 0.25” rod to 0.125”
Anneal
2 2
2
1 0.50.75
1CW
2 2
2
0.5 0.250.75
0.5CW
2 2
2
0.25 0.1250.75
0.25CW
The maximum cold work is not exceeded
Final Step
Draw the 0.125” rod to 0.1”
2 2
2
0.125 0.10.36
0.125CW
The final cold work is 36%
Problem
What if you want a certain tensile strength in your final product?Look at one of the graphs of properties vs. cold work from the book.Make sure that your final cold work step is the right size to give you the properties you want.
From “Materials Science”, by John Russ
Cold Work is Anisotropic
When you deform a piece of metal you elongate the grain.Slip occurs only in the favored directionsYou strengthen the material in the direction it is deformed, but properties in the other directions do not change as much.
The Science and Engineering of Materials - Askeland
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Cold Working Wire
When you draw wire, you strengthen in the longitudinal directionIt is not strengthened axiallyThis makes it easy to cut, but hard to break by pulling on it!!
Annealing
You can’t just haphazardly heat up a piece of metal to “undo” the strain hardeningIt’s a temperature dependent process
Annealing
RecoveryRecrystallizationGrain Growth
http://www.all-sourceheattreating.com/img/home_nologob.jpg
Recovery or Stress Relief
If you only add a small amount of thermal energy (heat it up a little) the dislocations rearrange themselves into networks to relieve residual stressesPolyganized subgrain structureDuctility is improvedStrength does not change
The Science and Engineering of Materials - Askeland
Three EBSD maps of the stored energy in an Al-Mg-Mn alloy after exposure to increasing recrystallization temperature. The volume fraction of recrystallized grains (light) increases with temperature for a given time.
Sometimes Residual Stresses are good
Shot PeeningTempered Glass Side and Rear Windows in Cars
http://abrasivefinishingcompany.com/images/shot_peening_1.jpg
Recrystallization
Add more heat, and new grains start to grow at the grain boundaries.The new grains have not been strain hardenedThe recrystallized metal is ductile and has low strength
The Science and Engineering of Materials - Askeland
Grain Growth
If you keep the metal hot too long, or heat it up too much, the grains become largeUsually not goodLow strengthAlso brittle
The Science and Engineering of Materials - Askeland
Check out the CD animations
Try the quiz on the CD!!On the next page explore how properties change during the annealing processThe whole process depends not only on the temperature, but on how long you keep the metal hot.
John Russ
Materials Science – A Multimedia Approach
Sometimes annealing happens by itself!!
Is cold working a good way to strengthen a metal used at high temperatures?What about a tungsten filament in a light bulb?
SEM of a tungsten filament
http://ion.asu.edu/descript_depth.htm
How hot is hot?
Most metals have a recrystallization temperature equal to about 40% of the melting point measured in Kelvin
mr TT 4.0
For Example
If a metal melts at 1000K, it’s recrystallization temperature is approximately 400KIf the metal is exposed to temperatures above the recrystallization temperature while in service, the strengthening achieved with cold work will be eliminated
Factors Contributing to Recrystallization Temperature
Melting PointOriginal Grain SizeAmount of Cold WorkPure metals recrystallize at lower temperatures than alloysTime at temperature
Typical Recrystallization Temperatures
Metal Melting Temperature 0C
Recrystallization Temperature 0C
Sn 232 -4
Pb 327 -4
Zn 420 10
Al 660 150
Mg 650 200
Ag 962 200
Cu 1085 200
Fe 1538 450
Ni 1453 600
Mo 2610 900
W 3410 1200
These metals recrystallize below room temperature – so cold work is not possible under normal conditionsThe Science and Engineering of Materials -
Askeland
What should you do if cold working isn’t applicable?
Try solid solution strengtheningTry hot working
http://www.bbc.co.uk/shropshire/content/image_galleries/friendship_through_iron_gallery.shtml
Hot Working
Shape the metal while it is hot. Above the recrystallization temperature
Blacksmiths use a combination of hot work and cold work.Can not fine tune the final properties this wayDimensional control is hardSurface finishes may be hard to produce
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
During hot working, the elongated anisotropic grains immediately recrystallize. If the hot-working temperature is properly controlled, the final hot-worked grain size can be very fine.
Hot Rolling Steel
What happens when you weld a cold worked piece of metal?
Welding affects the surrounding material
An incomplete weld of a bike frame which failed. Apparent in the image is the bright weld material in the center, the surrounding lighter heat affected zone (HAZ), and dark outer unaffected base metal. Field of view is approximately 15 mm.
http://www.mse.mtu.edu/slides/slide_2.html
Used by Permission of Ruth Kramer
The following slides show the effect of cold working on various metals