1. Strain Hardening and Annealing
Chapter 7 – 4th Edition
Chapter 8 – 5th Edition

2. Strain Hardening in Metals
When a piece of metal is deformed, the dislocations run into each other
This traffic jam increases the material’s strength
Deforming a piece of metal also actually increases the number of dislocations
This increases the strength too!!

3. Tensile Test
You can understand this better by relating it to the results of the tensile test.

4. John D Russ – Materials Science – A Multimedia Approach

5. Try it!!
Strain harden a piece of copper tubing.

6. Effects of Strain Hardening
Yield Strength goes up
Tensile Strength goes up
Ductility goes down
The material becomes brittle

7. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

8. Strain Hardening Coefficient
Strain Hardening Coefficient is a measure of how much the metal can be strengthened by strain hardening
It needs to have some ductility to be strain hardened.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

9. n is the slope of the PLASTIC portion of the curve, when graphed on a logarithmic scale

10. Log plot of the plastic portion of a tensile test
Slope=n
Log True Stress, σ K
Log(σ) = Log(K) + n Log(ε)
0.001
0.010
0.100
1.0
Log True Strain, ε

11. 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

12. Frank-Read Source
Strain hardening actually increases the strength of a material PAST its original tensile strength
Why?
Additional dislocations are formed as dislocations run into point defects

13. Strain Hardening – The effect of dislocation generation
Yield Point
Tensile Strength

14. From “Materials Science – A Multimedia Approach”, by John Russ
Copper 30% zinc alloy

15. Frank-Read Source Point Defect Dislocation

16. Frank-Read Source

17. Frank-Read Source
Before deformation a typical dislocation density is about 106 cm of dislocation per cm3 of metal
After strain hardening it may increase to as much as 1012 cm per cm3 of metal

18. Strain Hardening in Polymers
When you pull on a polymer, the chains line up
Van der Waal bonds form between the chains
The polymer becomes stronger
Try it with a 6-pack ring!!
The mechanism for strain hardening in plastics is different from the mechanism in metals

19. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

20. PET Bottles and Preforms

21. Strain Hardening in Ceramics?
Ceramics are already brittle – so strain hardening is not usually possible
Ceramics break because of flaws – the mechanism of deformation is different
Annealing ceramics causes grain growth
May or may not be bad

22. Back to Metals Cold Work
There is only a certain amount you can deform a material before it breaks
Cold work is strain hardening measured in % - The percent change in cross sectional area of the material
Different materials have varying % allowable cold work

23. Wire Drawing
Initial diameter
Final diameter d0 F d

24. Rod Deformation of a rod (or a piece of wire)
Initial cross sectional area minus Final cross sectional area Over the initial cross sectional area

25. All times 100 of course

26. Copper is often drawn into wire
Copper rod feeding into drawing machine

27. Plate Rolling
Initial thickness
Final thickness h0 h

28. Cold Rolling Metal is often rolled into sheets from thicker stock
The width of the sheet is usually kept the same, and only the thickness varies

29. Cold Rolled Steel

30. Annealing is a heat treatment
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

31. 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.

32. One solution Draw the 1” rod to 0.5” Anneal
The maximum cold work is not exceeded

33. Final Step
Draw the 0.125” rod to 0.1”
The final cold work is 36%

34. 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.

35. From “Materials Science”, by John Russ

36. Cold Work is Anisotropic
When you deform a piece of metal you elongate the grain.
Slip occurs only in the favored directions
You strengthen the material in the direction it is deformed, but properties in the other directions do not change as much.

37. The Science and Engineering of Materials - Askeland

38. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

39. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

40. Cold Working Wire
When you draw wire, you strengthen in the longitudinal direction
It is not strengthened axially
This makes it easy to cut, but hard to break by pulling on it!!

41. Annealing
You can’t just haphazardly heat up a piece of metal to “undo” the strain hardening
It’s a temperature dependent process

42. Annealing Recovery Recrystallization Grain Growth

43. 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 stresses
Polyganized subgrain structure
Ductility is improved
Strength does not change

44. The Science and Engineering of Materials - Askeland

45. 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.

46. Sometimes Residual Stresses are good
Shot Peening
Tempered Glass
Side and Rear Windows in Cars

47. Recrystallization
Add more heat, and new grains start to grow at the grain boundaries.
The new grains have not been strain hardened
The recrystallized metal is ductile and has low strength

48. The Science and Engineering of Materials - Askeland

49. Grain Growth
If you keep the metal hot too long, or heat it up too much, the grains become large
Usually not good
Low strength
Also brittle

50. The Science and Engineering of Materials - Askeland

51. Check out the CD animations
Try the quiz on the CD!!
On the next page explore how properties change during the annealing process
The whole process depends not only on the temperature, but on how long you keep the metal hot.

52. John Russ
Materials Science – A Multimedia Approach

53. 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?

55. SEM of a tungsten filament

57. How hot is hot?
Most metals have a recrystallization temperature equal to about 40% of the melting point measured in Kelvin

58. For Example
If a metal melts at 1000K, it’s recrystallization temperature is approximately 400K
If the metal is exposed to temperatures above the recrystallization temperature while in service, the strengthening achieved with cold work will be eliminated

59. Factors Contributing to Recrystallization Temperature
Melting Point
Original Grain Size
Amount of Cold Work
Pure metals recrystallize at lower temperatures than alloys
Time at temperature

60. Typical Recrystallization Temperatures
Metal
Melting Temperature 0C
Recrystallization Temperature 0C Sn
232 -4 Pb
327 Zn
420 10 Al
660
150 Mg
650
200 Ag
962 Cu
1085 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 conditions
The Science and Engineering of Materials - Askeland

61. What should you do if cold working isn’t applicable?
Try solid solution strengthening
Try hot working

62. 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 way
Dimensional control is hard
Surface finishes may be hard to produce

63. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©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.

64. Hot Rolling Steel

65. What happens when you weld a cold worked piece of metal?

66. 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.
Used by Permission of Ruth Kramer

67. The following slides show the effect of cold working on various metals