Heat Kinetics Treatment
Week8
Material Sciences and Engineering MatE271
1
Baking
Temp. g-Fe (FCC) austenite a a+g a-Fe (BCC) ferrite Fe
Alloying
L g+L g g+b b+L eutectic b
eutectoid a+b (pearlite) Composition wt% C
- Ingredient - Baking temperature - Baking time Time-dependent phase transformation
Material Sciences and Engineering MatE271
- Composition (wt%) - Equilibrium diagram - Cooling time (kinetics)
b: Fe3C (cementite) b: C
Week 8
rapid cooling slow cooling
2
aterial Sciences and Engineering,
Goals for this unit (Ch. 10)
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)
Material Sciences and Engineering
MatE271
Week 8
3
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
Material Sciences and Engineering
MatE271
Week 8
4
aterial Sciences and Engineering,
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
Material Sciences and Engineering MatE271 Week 8
5
Time effect
at 100% of A
Melting Temp. (Pure A) Liquid Temperature Temperature Liquidus A + Liquid Eutectic Line A+B (both solids) Time A Liquid + B Melting Temp. (Pure B)
Invariant Point B
Composition, %B
You have to drop Temp slightly to start solidification
Material Sciences and Engineering
MatE271
Week 8
6
aterial Sciences and Engineering,
Transformation
- Most transformations do not take place instantaneously e.g. to change crystal structures, atoms must diffuse Which takes time Energy is required to form phase boundaries between parent and product phases
Net energy change Liquid
Surface energy +ve Net energy
rc volume energy -ve
Week 8
solid
Material Sciences and Engineering
Nucleation and growth
MatE271
7
Transformation by Nucleation and Growth
Nucleation
The formation of very small particles of the new phase Often begins at imperfection sites especially grain boundaries
Growth
The nuclei increase in size Some or all of the parent phase disappears Complete when system reaches equilibrium
Material Sciences and Engineering
MatE271
Week 8
8
aterial Sciences and Engineering,
10.2 The TTT Diagram
at 100% of A
Melting Temp. (Pure A) Liquid Temperature Melting Temp. (Pure B)
1 50
100 % completion
of reaction
Temperature
Liquidus A + Liquid Eutectic Line A+B (both solids) Liquid + B
Invariant Point B
Time
A
Composition, %B
Time required for reaction completion
Material Sciences and Engineering
MatE271
Week 8
9
at 100% of A
Rate of Transformation
-The fraction of reaction that has occurred is measured as a function of time - Usually at a constant T - Progress is usually determined by microscopy or other physical property - Data is plotted as fraction transformed vs. log time
Time Temperature
1 50
100 % completion
of reaction
Material Sciences and Engineering
MatE271
Week 8
10
aterial Sciences and Engineering,
Phase Transformation: when?
Phase transformations occur when either Temperature is most common method to induce phase transformations Phase boundaries are crossed during heating or cooling
Material Sciences and Engineering
MatE271
Week 8
11
Phase Diagram vs. TTT Diagram
When a phase boundary is crossed, the alloy proceeds towards equilibrium according to the phase diagram Most phase transformations require a finite time Phase diagrams cannot indicate how long it takes to achieve equilibrium Many times the preferred microstructure is metastable The required transformation time is obtained from the TTT-Diagram
Material Sciences and Engineering
MatE271
Week 8
12
aterial Sciences and Engineering,
Phase Transformation
Metallic Materials are extremely versatile - They possess a wide range of mechanical properties Microstructure development occurs by phase transformations - Diffusional Transformation: - Diffusionless Transformation Properties can be tailored by changing microstructure
Material Sciences and Engineering
MatE271
Week 8
13
Diffusional Transformation
spheroidite
a-Fe (BCC) ferrite a+g a
g-Fe (FCC) austenite g
g+b
g
coarse pearlite fine pearlite upper bainite
eutectoid a+b ( 0.77% C )
g+ a+ Fe
a+Fe3C
lower bainite
3C
Fe
Composition wt%
C
Material Sciences and Engineering
MatE271
Week 8
14
aterial Sciences and Engineering,
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
Material Sciences and Engineering
MatE271
Week 8
15
Pearlite Transformation (diffusional)
Austenite grain boundary Austenite (g)
Austenite (g) Ferrite, a Fe3C cementite Pearlite
Growth direction Of Pearlite
Check Fig. 9.2 P. 306
g (0.77 wt% C) a (0.22% C) + Fe3C (6.70% C)
Austenite
Material Sciences and Engineering
Ferrite
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Cementite
Week 8
16
aterial Sciences and Engineering,
Mechanical Properties of Pearlite
Pearlite is a mix of cementite and ferrite ( - Cementite is harder but more brittle than ferrite )
Layer thickness also has an effect - Fine pearlite is harder and stronger than coarse
Material Sciences and Engineering
MatE271
Week 8
17
Fe3C in Pearlite and Bainite
Material Sciences and Engineering
MatE271
Week 8
18
aterial Sciences Engineering,
Isothermal and Diagrams
Only valid for a particular composition for a particular system - Other compositions will have different curves Only valid when the temperature is constant throughout the transformation
Material Sciences and Engineering MatE271 Week 8
19
Diffusionless Transformation: Martensitic Transformation
Crystal: g (FCC) a (BCC) FCC accommodates C easily than BCC C Fe3C ( ) - trapped in the FCC lattice Form Body center tetragonal lattice, BCT
Material Sciences and Engineering MatE271 Week 8
20
aterial Sciences and Engineering,
The Full Isothermal TTT
spheroidite
Coarse pearlite fine pearlite upper bainite lower bainite
martensite 100% martensite
Material Sciences and Engineering MatE271 Week 8
21
Mechanical Properties of Martensite
Strongest, hardest, and most brittle Hardness is dependent on C content Martensite is not as dense - therefore when it transforms it causes stress ( Tempering (heat treatment) of martensite relieves stress - makes it tougher and more ductile
Note - other alloy system experience diffusionless (or martensitic) transformation
Material Sciences and Engineering MatE271 Week 8
)
22
aterial Sciences and Engineering,
Martensite Tempering- stress reliving
Tempering temperature
martensite
Check Fig. 10-18 10P. 370
Tempered Martensite:
Material Sciences and Engineering
M
a +Fe3C
MatE271
(isolated particles)
Week 8
23
10.3 Hardenability
Hardness: surface resistance to indentation H= F/Aprojected d
F
Ap Hardneability: relative ability of steel to hardened by quenching - Related to and of Martensitic transformation
Material Sciences and Engineering
MatE271
Week 8
24
aterial Sciences and Engineering,
Jominy End-Quench
measure hardness heat to above Teutectoid - 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)
Material Sciences and Engineering MatE271 Week 8
cool
25
Material Sciences and Engineering
MatE271
Week 8
26
aterial Sciences and Engineering,
10.4 Precipitate Hardening
Al-alloy 7150-T651
(6.2Zn, 2.3Cu, 2.3Mg, 0.12 Zr)
500nm
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27
Precipitate Hardening
Al-Cu alloy (96% Al-4%Cu) T k k q +k Slow cooling k q
Time
Material Sciences and Engineering MatE271 Week 8
28
aterial Sciences and Engineering,
Age Hardening
k T k quench q +k aging
Fine dispersion of q particle
Coherent interface
Time
Material Sciences and Engineering MatE271 Week 8
29
Age Hardening
Aging time
Super saturated k-solid solution
Material Sciences and Engineering
q-phase precipitate
MatE271 Week 8
q-phase growth
30
aterial Sciences and Engineering,
GP zone and service life
Alloy load carrying capacity
coalescence growth
Aging Time
Material Sciences and Engineering MatE271 Week 8
31
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 Deformation is measured by percentage dimensional changes
Material Sciences and Engineering MatE271
Strain =
dL 100% L
32
Week 8
aterial Sciences and Engineering,
Cold-working
The degree of plastic deformation is expressed as % cold worked: Ao Af
%CW =
Ao - Af x100% Ao
Why does this occur? Dislocation-dislocation strain field interactions Disloca...