cold working is actually strain hardening n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Cold Working is Actually Strain Hardening PowerPoint Presentation
Download Presentation
Cold Working is Actually Strain Hardening

Loading in 2 Seconds...

play fullscreen
1 / 17

Cold Working is Actually Strain Hardening - PowerPoint PPT Presentation


  • 220 Views
  • Uploaded on

s. large hardening. s. y 1. s. small hardening. y 0. e. Cold Working is Actually Strain Hardening. Basic equation relating flow stress (strain hardening) to structure is: s o = s i + a Gb r 1/2. s o is the yield stress

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Cold Working is Actually Strain Hardening' - benjamin


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
cold working is actually strain hardening

s

large hardening

s

y1

s

small hardening

y0

e

Cold Working is Actually Strain Hardening

Basic equation relating flow stress (strain hardening) to structure is:

so = si +aGbr1/2

  • so is the yield stress
  • si is the “friction stress” – overall resistance of lattice to dislocation motion
  • a is numerical constant 0.3 – 0.6
  • G shear modulus
  • b is the burger’s vector
  • r is the dislocation density

• Yield stress increases

as r increases:

effects of cold work
Effects of Cold Work

As cold work is increased

  • Yield strength (sy) increases
  • Tensile strength (TS) increases
  • Ductility (%EL or %AR) decreases
other cold work effects
Other Cold Work Effects
  • Usually a small decrease in density (few 10ths of a percent)
  • An appreciable decrease in electrical conductivity (increased number of scattering centers)
  • Small increase in the thermal coefficient of expansion
  • Because of increased internal energy – chemical reactivity is increased (decreased resistance to corrosion)
s e behavior vs temperature
s-e Behavior vs. Temperature

800

-200C

600

-100C

Stress (MPa)

400

25C

200

0

0

0.1

0.2

0.3

0.4

0.5

Strain

3

. disl. glides past obstacle

2. vacancies

replace

atoms on the

obstacle

disl. half

1. disl. trapped

plane

by obstacle

• Results for

polycrystalline iron:

• sy and TS decrease with increasing test temperature.

• %ELincreases with increasing test temperature.

• Why? Vacancies

help dislocations

move past obstacles.

Climb of

Edge Dislocations

Never Screw

Positive Climb

strain energy related to cold work
Strain Energy Related to Cold Work
  • Mentioned that ~10% of the energy imparted during cold working is stored as strain energy
  • Amount of strain energy is increased by increasing the severity of deformation, lowering the deformation temperature, and by impurity additions
  • The strain energy increase is stored in the highly deformed microstructure – dislocation tangles
  • Metastable microstructure!

Figure: Stored energy of cold work and fraction of the total work of deformation remaining as stored energy for high purity copper

Source: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition,

PWS Publishing Company, 1994.

annealing
Annealing
  • Can we release the stored strain energy? YES!
  • The material is in an unstable state – but there is an activation energy barrier to releasing that energy
  • By heating the material and adding energy to the system we can increase the probability of moving past the activation barrier
  • Heat treating cold worked material is called Annealing
release of stored energy
Release of Stored Energy
  • What happens as we heat up cold worked material?
  • Curve to the left is an anisothermal anneal curve
  • Two samples – one cold worked and the other not
  • Samples are heated continuously from low temperature to a higher temperature
  • Energy release is determined as a function of temperature
  • Difference in power to heat the specimens at same rate

Figure: Anisothermal anneal curve for electrolytic copper

Source: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

annealing stages

Annealing Temperature (ºC)

100

200

300

400

500

600

700

600

60

tensile strength

50

500

ductility (%EL)

tensile strength (MPa)

40

400

30

ductility

20

300

Recovery

Grain Growth

Recrystallization

Annealing Stages
  • The cold worked state is thermodynamically unstable.
  • With increasing temperature it becomes more and more unstable
  • Eventually the metal softens and returns to a strain-free condition
  • Complete process is known as Annealing
  • Annealingis easily divided into 3 distinct processes:
  • Recovery
  • Recrystallization
  • Grain Growth
recovery
Recovery
  • Defined as:Restoration of physical properties of a cold worked metal without any observable change in microstructure
    • Electrical conductivity increases and lattice strain is reduced
    • Strength properties are not affected
  • Involves:
    • Dislocation Annihilation
    • Polygonization:
      • Removal of grain curvature created during deformation
      • Regrouping of edge dislocations into low angle boundaries within grains
      • Reduces the energy of system by creating reduced energy subgrains

Source 1: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

Source 2: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition,

PWS Publishing Company, 1994.

recrystallization
Recrystallization

Recrystallization is:

  • The replacement of the cold worked structure by the nucleation and growth of a new set of strain free grains
    • Density of dislocations is reduced
    • Strain hardening is eliminated
    • The hardness and strength is reduced and the ductility is increased
    • Driving force for recrystallization is the release of stored strain energy
      • Note this is also the driving force for recovery and therefore they are sometimes competing processes

Source 1: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

how does it work
How does it work?
  • Nucleation of strain free grains occurs at points of high lattice curvature
    • Slip line intersections
    • Deformation twin intersections
    • Areas close to grain boundaries
  • Several models (unproven) that propose mechanisms for nucleation:
    • Grain boundary bulging due to a local variance in strain energy
    • Sub-boundary rotation and coalescence

Source 2: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition,

PWS Publishing Company, 1994.

recrystallization1
Recrystallization

0.6 mm

0.6 mm

33% cold

worked

brass

New crystals

nucleate after

3 sec. at 580C.

• New grains are formed that:

-- have a small dislocation density

-- are small

-- consume cold-worked grains.

further recrystallization
Further Recrystallization

0.6 mm

0.6 mm

After 8

seconds

After 4

seconds

• All cold-worked grains are consumed.

slide14

TR

º

TR = recrystallization temperature

º

variables for recrystallization
Variables for Recrystallization

Six main variables influence recrystallization behavior:

  • The amount of prior deformation
  • Temperature
  • Time
  • Initial grain size
  • Composition
  • Amount of recovery or polygonization prior to the start of recrystallization

Because the temperature at which recrystallization occurs depends 

Recrystallization temperature is not a fixed temperature like melting point

The practical definition for recrystallization temperature is:

The temperature at which a given alloy in a highly cold worked state completely

recrystallizes in 1 hour.

Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

affect of variables on recrystallization
Affect of Variables on Recrystallization
  • Minimum amount of deformation is required
  • The smaller the deformation, the higher the temperature required for recrystallization
  • Increasing annealing time decreases required recrystallization temperature. Temperature is more important than time. Doubling annealing time is approximately equivalent to increasing annealing temperature 10oC
  • Final grain size depends most on the degree of deformation and to lesser extent on the annealing temperature. The greater the deformation & the lower the annealing temp., the smaller the recrystallized grain size.
  • The larger the original grain size, the greater the amount of cold work required to produce same recrystallization temp.

Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

affect of variables on recrystallization1
Affect of Variables on Recrystallization
  • The recrystallization temperature decreases with increasing purity of the metal. Solid solution alloying additions ALWAYS raise the recrystallization temperature.
  • The amount of deformation required to produce equivalent recrystallization behavior increases with increased working temperature
  • For a given reduction in cross-section – different metal working processes produce different effective deformations. Therefore, identical recrystallization behavior may not be obtained.

Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.