Kinetics
Heat Treatment
Material Sciences and Engineering
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1
Alloying
Baking
Temp.
L
g-Fe
(FCC)
austenite
g+L
g+b
a a+g
a-Fe
(BCC)
ferrite
Composition wt%
- Ingredient
- Composition (wt%)
- Baking temperature
- Equilibrium diagram
- Baking time
- Cooling time (kinetics)
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eutectic b
eutectoid a+b
(pearlite)
Fe
Time-dependent phase transformation
b+L
g
b: Fe3C (cementite)
b: C
Week 8
C
rapid cooling
slow cooling
2
1
Goals for this unit (Ch. 10)
Ø Understanding how temperature and cooling can
be used to alter properties (e.g. Fe-C system).
- The TTT-diagram (Ch. 10.1-2)
- Applications: (Ch. 10.3-5)
- Hardening (Steel alloys)
- Precipitate hardening (Aluminum alloys)
- Annealing (recrystallization and grain growth)
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Nonequilibrium Cooling
- All previous discussion has been for “slow” cooling
- Many times, this is TOO slow, and unnecessary
- Nonequilibrium effects
- Phase changes at T other than predicted
- The existence of nonequilibrium phases at room
temperature
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2
10.1 Time, the third dimension
- Phase diagrams only represent what should happen in
equilibrium (e.g. slow cooling)
- Most materials are not processed under such conditions
-
- Time - temperature history required to generate a certain
microstructure
- Time - temperature - transformation (TTT) diagrams
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5
Week 8
Time effect
at 100% of A
Melting Temp.
(Pure A)
Melting Temp.
(Pure B)
Temperature
Temperature
Liquid
Liquidus
A + Liquid
A+B
(both solids)
Time
Liquid + B
Eutectic Line
A
Invariant Point
Composition, %B
B
You have to drop Temp slightly to start solidification
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3
Transformation
- Most transformations do not take place instantaneously
e.g. to change crystal structures, atoms must diffuse
Which takes time
Net energy change
Energy is required to form phase boundaries between parent
and product phases
Liquid
solid
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Surface energy +ve
Net energy
rc
Nucleation and growth
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volume energy -ve
Week 8
7
Transformation by Nucleation and Growth
Ø Nucleation
The formation of very small particles of the new phase
Often begins at imperfection sites – especially grain
boundaries
Ø Growth
The nuclei increase in size
Some or all of the parent phase disappears
Complete when system reaches equilibrium
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4
10.2 The TTT Diagram
at 100% of A
Melting Temp.
(Pure A)
Melting Temp.
(Pure B)
1 50
100 % completion
of reaction
Temperature
Temperature
Liquid
Liquidus
A + Liquid
Liquid + B
Eutectic Line
A+B
(both solids)
A
Time
Invariant Point
Composition, %B
B
Time required for reaction completion
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9
Week 8
at 100% of A
-The fraction of reaction that has
occurred is measured as a
Temperature
Rate of Transformation
1 50
100 % completion
of reaction
function of time
- Usually at a constant T
Time
- Progress is usually determined
by microscopy or other physical property
- Data is plotted as fraction transformed vs. log time
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5
Phase Transformation: when?
Ø Phase transformations occur when either
Ø Temperature is most common method to induce phase
transformations
Ø Phase boundaries are crossed during heating or cooling
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11
Phase Diagram vs. TTT Diagram
ØWhen a phase boundary is crossed, the alloy proceeds
towards equilibrium according to the phase diagram
Ø Most phase transformations require a finite time
ØPhase diagrams cannot indicate how long it takes to achieve
equilibrium
Ø Many times the preferred microstructure is metastable
Ø The required transformation time is obtained from the
TTT-Diagram
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6
Phase Transformation
Ø Metallic Materials are extremely versatile
- They possess a wide range of mechanical properties
Ø Microstructure development occurs by phase transformations
- Diffusional Transformation:
- Diffusionless Transformation
Ø Properties can be tailored by changing microstructure
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Diffusional Transformation
13
Week 8
g-Fe
(FCC)
austenite
a-Fe
(BCC)
ferrite
a+g
spheroidite
g
g+b
a
g
coarse pearlite
fine pearlite
eutectoid a+b
( 0.77% C )
upper bainite
g+
a+
Fe
a+Fe3C
lower bainite
3C
Fe
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Composition wt%
C
14
7
Diffusional Transformation (Pearlite)
- Consider the eutectoid reaction
g (0.77 wt% C) ® a (0.22% C) + Fe3C (6.70% C)
Austenite transforms to ferrite and cementite – through
Carbon diffuses away from ferrite to cementite
Temperature affects the rate:
Construct isothermal transformation diagrams from
% transformation diagrams
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15
Week 8
Pearlite Transformation (diffusional)
Austenite grain
boundary
Austenite (g)
Growth direction
Of Pearlite
Austenite (g)
Ferrite, a
Check Fig. 9.2
P. 306
Fe3C
cementite
Pearlite
g (0.77 wt% C) ® a (0.22% C) + Fe3C (6.70% C)
Austenite
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Ferrite
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Cementite
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16
8
Mechanical Properties of Pearlite
Ø Pearlite is a mix of cementite and ferrite (
)
- Cementite is harder but more brittle than ferrite
Ø Layer thickness also has an effect
- Fine pearlite is harder and stronger than coarse
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Fe3C in Pearlite and Bainite
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Isothermal Diagrams
Ø Only valid for a particular composition for a particular system
- Other compositions will have different curves
Ø Only valid when the temperature is constant throughout the transformation
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Diffusionless Transformation: Martensitic Transformation
Ø Crystal: g (FCC)
a (BCC)
Ø FCC accommodates C easily than BCC
ØC
Fe3C (
)
- trapped in the FCC lattice
Ø Form Body center tetragonal lattice, BCT
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10
The Full Isothermal TTT
spheroidite
Coarse pearlite
fine pearlite
upper bainite
lower bainite
martensite
100% martensite
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Mechanical Properties of Martensite
Ø Strongest, hardest, and most brittle
ØHardness is dependent on C content
Ø Martensite is not as dense - therefore when it
transforms it causes stress (
)
Ø Tempering (heat treatment) of martensite relieves
stress - makes it tougher and more ductile
Note - other alloy system experience diffusionless (or martensitic)
transformation
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11
Martensite Tempering- stress reliving
Tempering temperature
Check Fig. 1010-18
P. 370
martensite
Tempered
Martensite:
a +Fe3C
M
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(isolated particles)
23
Week 8
F
10.3 Hardenability
Hardness: surface resistance to indentation
d
H= F/Aprojected
Ap
Hardneability: relative ability of steel to hardened
by quenching
- Related to
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and
of Martensitic transformation
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24
12
Jominy End-Quench
measure hardness
heat to
above Teutectoid
cool
- Cylindrical specimen is cooled from the end by
a spray or water
- Specimen size, shape is specified
- Water spray and time is specified
- The hardness is measured with respect to the distance
from the quenched end
- Rockwell hardness measured (a hardness scale)
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10.4 Precipitate Hardening
Al-alloy
7150-T651
(6.2Zn, 2.3Cu,
2.3Mg, 0.12 Zr)
500nm
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Week 8
Precipitate Hardening
Al-Cu alloy (96% Al-4%Cu)
T
k
k
Slow cooling
k
q
q +k
Time
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14
Age Hardening
k
T
Fine dispersion
of q particle
k
Coherent interface
quench
q +k
aging
Time
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Week 8
Age Hardening
Aging time
Super saturated
k-solid solution
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q-phase
growth
q-phase
precipitate
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15
GP zone and service life
Alloy load
carrying capacity
coalescence
growth
Aging Time
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Week 8
10.5 Annealing
- Loss of hardness at high temperature
- relief of residual stresses
- reduction of dislocation density
Force
Stress = Area
- Link between deformation and microstructure
- Cold work
- Recovery
- Recrystallization
- Grain growth
ØDeformation is measured by percentage
dimensional changes
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Strain =
Week 8
dL
100%
L
32
16
Cold-working
The degree of plastic deformation is expressed as % cold worked:
Ao
Af
%CW =
Ao - Af
x100%
Ao
Why does this occur?
Ü
Dislocation-dislocation strain field interactions
Ü
Dislocation density increases with cold working so the average separation between dislocations decreases
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Cold-working-cont.
Ü
Strain hardening may be removed by annealing
(heating to higher T to allow dislocations to move)
Brass
Cu-Zn
CW
3 sec at 580oC
8 sec
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4 sec
1 hr
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17
Recovery, Recrystallization
ÜPlastic deformation results in changes in
microstructure and properties
- Grain shape
- Strain hardening
- Increased dislocation density
ÜOriginal properties can be regained by
appropriate heat treatment
Recovery, recrystallization, grain growth
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Recovery
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Recrystallization
temperature
Brass
Ü
Some of the stored strain energy is relieved by movement of
dislocations at high T
- Number of dislocations is reduced
- Configuration of dislocation is altered
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18
Recrystallization
- Even after recovery, grains are still in a high energy state
(they have been deformed)
- Recrystallization is the formation of a new set of
strain-free equiaxed grains.
- New grains form by nucleation and growth
Short range diffusion
- Requires time and temperature
- Recrystallization temperature: Temperature at which
recrystallization reaches completion in 1 hr.
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Stages of Recrystallization
• Cold Worked
• Initial Stage
• Intermediate Stage
• Complete
Recrystallization
• Grain Growth
• Grain Growth,
higher temperature
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Grain Growth
- Occurs in all crystalline materials - why?
- Energy is associated with grain boundaries – As grain size increases, total boundary area decreases
-All grains can’t grow
– Large ones grow at the expense of small ones
-Fine grains
superior properties
- How to produce fine grain structure???
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Reading Assignment
READ Class Notes & relevant portions of
Shackelford, 2001(5th Ed)
– Chapter 10, pp 354-389
-HW5 will be available on Friday, Oct 19
Due Friday Oct 26
Will not accept HW stashed under my door
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20