just keep some basic in mind, its give u enough information about this topic.

1. TIME-TEMPERATURETRANSFORMATION DIAGRAM

2. 0.8C
Spring 2001
Dr. Ken Lewis
ISAT 430
2

3. Phase diagram and TTT diagram
Which informations are obtained from phase diagram or
TTT diagram?
• Phase diagram :
– Describes equilibrium microstructural development that is
obtained at extremely sow cooling or heating conditions.
– Provides no information on time to take to form phase and on
shapes, size and distribution of phase → importance of kinetics
• TTT diagram
– For a given alloy composition, the percentage completion of a
given phase transformation on temperature-time axes is
described.

4. Nucleation And Growth
• For a eutectic reaction :
L (XE) → +
at TE
(experiment)
(1) Quench the liquid from Tm to some
lower temperature
(2) Measure the time for solidification, to
go to completion at that temperature
• TTT diagram
– The time required for the liquid to
transform to the eutectic
microstructure is function of time

5. Description of new phase from melt
• Homogeneous nucleation : occurs within a homogeneous
medium.
• Heterogeneous nucleation : nucleation occurs at some structural
imperfection such as foreign surface, and hence with reduced
surface energy
local atomic fluctuation
formation of many small
nuclei
growth of nuclei with
critical size or greater

6. Change in free energy for homogeneous nucleation
For the transformation of liquid to
solid ;
L→S ;
GV
H T
Tm
and for forming a spherical
nucleus
GT
4 3
r GV
3
4 r2
GT = total free energy change
r = radius of embryo or nucleus
= specific surface free energy
GV = volume free energy change
Solid
Solid-liquid
interface
Liquid

7. d GT
dr
G
n
*
*
at r =
0
16 r 3
3( GV ) 2
s exp(
r*
2
16 rTm
3 H T2
G*
)
kT
Nuclei larger than critical
size (r*) are stable and can
continue to grow.
r*
2r
GV
n* : number of spherical
nucleus of radius r*

8. Nucleation rate
.
• Nucleation rate, N : number of nuclei / unit volume / unit time
.
N
exp(
G*
) exp(
kT
ED
)
kT
,where G* : energy barrier to form a
nucleus stable to grow.
G*
• At T just below Tm,
– Diffusion rate is rapid but very few
nuclei are formed.
∵ G* ↑
• At very low T ( T ↑)
– Diffusion rate is extremely low but
many nuclei are formed
∵ G* ↓
• At intermediate T
.
– Max.
N
1
(Tm T ) 2
1
T2
ED : activation energy for diffusion

9. Growth of nuclei
• Growth of Nuclei
– Growth of nuclei is a diffusional process
.
G
ce
QD
RT
, where QD : activation energy for self diffusion
.
• Transformation rate of a phase :
.
N G

10. TTT Diagram
• Temperature-time-transformation curve
• TTT diagrams represent specific thermal
histories for the given microstructure.

11. Isothermal transformation of eutectoid steel

12. TTT diagram for eutectoid steel

13. Transformations of austenite : →
+ Fe3C
pearlite
A. Diffusional transformations
1) At slightly lower T below 727 ℃ : T <<
• Coarse pearlite
: nucleation rate is very low.
: diffusion rate is very high.
2) As the Tt (trans. temp.) decreases to
500 ℃
• Fine pearlite
: nucleation rate increases.
: diffusion rate decreases.
Strength : (MPa) = 139 + 46.4 S-1
S : intermetallic spacing
655
℃
600
℃
534
℃
487
℃

14. A. Diffusional transformations
3) 250 ℃ < Tt < 500 ℃, below the nose in TTT diagram.
• Driving force for the transformation ( → + Fe3C) is very high.
• Diffusion rate is very low.
• Nucleation rate is very high.
→ + Fe3C
: Bainite ; cementite in the form of needle type.
495 ℃
410 ℃
bainite

15. B. Diffusionless Transformations - Martensitic trans.
: When the austenite is quenched to temp. below Ms
→ ’ (martensite)
: Driving force for trans. of austenite → extremely high.
Diffusoin rate is extremely slow.
: instead of the diffusional migration of carbon atoms to produce
separate and Fe3C phases, the matensite transformation involves
the sudden reorientation of C and Fe atoms from the austenite (FCC)
to a body centered tetragonal (bct) solid solution.
→ ’ (martensite), a solid solution
: super saturated carbon atoms
in ’ + shearlike transformation
→ very hard and brittle phase
martensite

16. 1) Diffusionless transformation → no compositional change during
transformation.
2) The trans. of → ’ starts at Ms temp. and finishes at Mf temp.
3) → ’ (BCT) ;
c/a increases as the carbon content increases.

18. The Time – Temperature –
Transformation Curve (TTT)
•
•
•
Spring 2001
Dr. Ken Lewis
At slow cooling rates the trajectory
can pass through the Pearlite and
Bainite regions
Pearlite is formed by slow cooling
– Trajectory passes through Ps
above the nose of the TTT
curve
Bainite
– Produced by rapid cooling to a
temperature above Ms
– Nose of cooling curve avoided.
ISAT 430
18

19. The Time – Temperature –
Transformation Curve (TTT)
• If cooling is rapid enough
austenite is transformed into
Martensite.
– FCC > BCT
– Time dependent diffusion
separation of ferrite and iron
carbide is not necessary
• Transformation begins at Ms and
ends at Mf.
– If cooling stopped it will
transition into bainite and
Martensite.
Spring 2001
Dr. Ken Lewis
ISAT 430
19

20. The Time – Temperature –
Transformation Curve (TTT)
• Composition Specific
– Here 0.8% carbon
• At different compositions,
shape is different
Spring 2001
Dr. Ken Lewis
ISAT 430
20

21. Full TTT Diagram
The complete TTT
diagram for an ironcarbon alloy of eutectoid
composition.
A: austenite
B: bainite
M: martensite
P: pearlite

22. Martensite
• Diffusionless
transformation
of FCC to BCT (more
volume!)
• Lenticular structure
• Very hard & very brittle.

23. TTT Diagrams

24. TTT diagram for Hypo-eutectoid steel.

25. TTT diagram for a hypereutectoid Steel
(1.13 wt% C)

26. So What’s a CCT Diagram?
• Phase Transformations and Production of
Microconstituents takes TIME.
• Higher Temperature = Less Time.
• If you don’t hold at one temperature and allow time
to change, you are “Continuously Cooling”.
• Therefore, a CCT diagram’s transition lines will be
different than a TTT diagram.

27. Slow Cooling
Time in region
indicates amount of
microconstituent!

28. Medium Cooling
Cooling Rate, R, is
Change in Temp /
Time °C/s

29. Fast Cooling
This steel is very
hardenable… 100%
Martensite in ~ 1
minute of cooling!

30. Continuous cooling transformation diagram for eutectoid steels
• Normalizing : heat the steel
into region → cool it in air →
fine pearlite
• Annealing : heat the steel into
region → cool it in furnace
(power off) → coarse pearlite

31. • THANKS