TTT Diagram

1. TIME-TEMPERATURE-
TRANSFORMATION DIAGRAM

2. Phase Transformations in Steels
• Iron has having different crystal structures at
different temperatures.
• It changes from FCC to BCC at 910o C.
• This transformation results in austenite
transforming to pearlite at eutectoid
temperature.
• This transformation of austenite is time
dependant.

3. Fe-C Equilibrium Diagram
Spring 2001 Dr. Ken Lewis ISAT 430 3

4. Fe-C Equilibrium Diagram
• Though the Fe-C equilibrium diagram is very useful, it does
not provide information about the transformation of
austenite to any structure other than equilibrium
structures, nor does it provide any details about the
influence of cooling rates on the formation of different
structures.
• In other words, Fe-C diagram does not explain the
decomposition of austenite under non-equilibrium
conditions or conditions involving faster rates of cooling
than equilibrium cooling.
• Several structures (e.g. martensite) not appearing on the
equilibrium diagram may be found in the microstructures in
steels.

5. TTT Diagram
• On the other hand, TTT diagram is a more
practical diagram.
• It shows what structures can be expected
after various rates of cooling.
• It graphically describes the cooling rate
required for the transformation of austenite to
pearlite, bainite or martensite.
• TTT diagram also gives the temperature at
which such transformations take place.

6. Phase diagram and TTT diagram
• Phase diagram :
– Describes equilibrium
microstructural development
that is obtained at extremely
slow cooling or heating
conditions.
– Provides no information on
time taken to form phase
• TTT diagram
– For a given alloy composition,
the percentage completion of
a given phase transformation
on temperature-time axes is
described.
Which information are obtained from phase diagram or
TTT diagram?

7. Transformation Diagrams
• There are two main types of transformation
diagrams that are helpful in selecting the
optimum steel and processing route to
achieve a given set of properties. These are
1. Time-temperature transformation (TTT) diagrams
2. Continuous cooling transformation (CCT) diagrams

8. Transformation Diagrams
• Time-temperature transformation (TTT)
diagrams
1. Indicates the amount of transformation at a
constant temperature.
2. Samples are austenitised and then cooled
rapidly to a lower temperature and held at that
temperature whilst the amount of transformation
is measured, for example by dilatometry.
3. Obviously a large number of experiments are
required to build up a complete TTT diagram.

9. Transformation Diagrams
• Continuous cooling transformation (CCT) diagrams
1. Indicates the extent of transformation as a function
of time for a continuously decreasing temperature.
2. Samples are austenitised and then cooled at a
predetermined rate and the degree of transformation
is measured, for example by dilatometry.
3. In this case also a large number of experiments are
required to build up a complete CCT diagram also

10. Transformation Diagrams
• CCT diagrams are generally more appropriate
for engineering applications as components
are cooled (air cooled, furnace cooled,
quenched etc.) from a processing temperature
as this is more economic than transferring to a
separate furnace for an isothermal treatment.

11. TTT Diagram

12. TTT diagram for an eutectoid carbon
steel.

13. Fe-C Equilibrium Diagram
• Austenite is stable above A1 temperature
• Below this temperature, austenite is unstable.
Spring 2001 Dr. Ken Lewis ISAT 430 13
Eutectoid
Steel

14. How Transformation Ocures?
• Transformation of austenite to pearlite ocures
by nucleation and growth mechanism.
• This transformation requires diffusion.

15. Nucleation rate
• As temperature decreases below eutectoid temperature, r* (critical size of
nucleus) decreases increasing the nucleation rate N.
• At very low temperature, nucleation rate decreases due to large decrease
in diffusion rate.
• At intermediate temperature, nucleation rate is maximum

16. Growth of nuclei
– Growth of nuclei is a diffusion controlled process
Growth Rate
where QD : activation energy for self diffusion
Growth rate decreases with decrease in temperature
RT
QD
ce
G


.

17. Transformation Rate
• Transformation rate of a phase :
• Transformation rate first increases, reaches a maximum and then starts
decreasing with decrease in temperature
.
.
G
N

18. Time for Transformation
• Time required for transformation as a function of temperature
follows a reverse trend than the rate of transformation.
Time required for transformation fist decreases, reaches a
minimum and then starts increasing with decrease in
temperature.

19. Let us do some experiment

20. Isothermal transformation of eutectoid steel
Let us take a eutectoid steel and do the following experiment
– Step 1 – Heat the sample above A1 temperature for austenitisation
– Step 2 – Transfer the sample to a salt bath kept below A1 Temp.
– Step 3 - Keep it at the bath temperature for a specified time
– Step 4 - Quench to room temperature
– Step 5 – Find out the amount of phases present

21. Isothermal transformation of eutectoid steel
below Eutectoid Temperature
• Determine the amount of pearlite formed after holding at
7050 C for different times

22. Fraction of transformation Vs the logarithm of time at
constant temperature (The S Curve).
Plot the result of the experiment
This is known as S curve

23. Fraction of transformation Vs the logarithm of time at
constant temperature (The S Curve).
The transformation starts but it takes some time before we can see a
precipitate. The time required for transformation required to initiate the
transformation is known as Incubation Period.
The rate of transformation first increases and then starts decreasing

24. Time required for completion of
transformation
• Now repeat the experiment at different temperatures below A1 temp.
• Plot the time for completion of transformation at different temperatures.

29. Time-Temperature-Transformation
Curve
• We can plot the time required for start and completion of
transformation at different temperatures or for any other amount of
transformation.

30. Let us consider eutectoid reaction as
an example
eutectoid reaction:
γ(0.8 wt% C)
↓
α (0.025 wt% C) + Fe3C

31. Let us consider eutectoid reaction as
an example
The S-shaped curves are
shifted to longer times at
higher T showing that the
transformation is
dominated by nucleation
(nucleation rate increases
with supercooling) and not
by diffusion (which occurs
faster at higher T)

32. Isothermal Transformation (or TTT) Diagrams
(Temperature, Time, and % Transformation)

33. TTT Diagrams for Eutectoid Steel
• We can plot the time for start and completion of transformation of
austenite to pearlite at different temperatures or for any other amount of
transformation.

34. Transformations of austenite to Pearlite
Transformations of austenite :
 →  + Fe3C
1) At slightly lower T below 727 ℃ : T <<
• Coarse pearlite
: nucleation rate is very low.
: diffusion rate is very high.
2) As the T (trans. temp.) decreases to
500 ℃
• Fine pearlite
: nucleation rate increases.
: diffusion rate decreases.
Strength :  (MPa) = 139 + 46.4 S-1
S : inter-lamellar spacing
655
℃
600
℃
534
℃
487
℃
pearlite

35. But at lower temperatures ….
• At lower temperatures, the austenite
transforms to bainite.
• Bainite is also a mixture of ferrite and
cementite but not in the form of alternate
layers.

36. Transformations of austenite to Bainite
3) At further lower temperatures, 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 (But not in the form of alternate layers)
: Bainite ; cementite in the form of needle type.
495 ℃ 410 ℃
bainite

37. TTT diagram for eutectoid steel
• Plot the time for start and completion of transformation at
different temperatures at still lower temperatures

38. On further decreasing the
transformation temperature
• Below a certain temperature, the austenite
changes or transforms to martensite.
• Martensite is a super saturated solid solution
of carbon in  iron.
• It is a diffusionless transformation.
• It is also known as shear transformation as the
interface between austenite and martensite
moves as a shear wave at the speed of sound.

39. Transformations of austenite to Martensite
4. When the austenite is quenched to temp. below Ms
 → ’ (martensite)
: Driving force for trans. of austenite → extremely high.
Diffusion 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 super saturated
solid solution of carbon in  iron
formed by shear transformation
(diffusionless transformation)
→ very hard and brittle phase
martensite

40. Diffusionless Transformation
1) Diffusionless transformation → no compositional change during
transformation.
2) The temperature at which the transformation of  → ’ starts is
known as at Ms temp. and finishes at Mf temp.
3) Degree of super saturation and c/a ratio increases as the carbon
content increases.

41. Complete TTT (isothermal transformation) diagram for
eutectoid steel.

42. Time Temperature Transformation
(TTT) Diagram
• Below A1 , austenite is unstable, i.e., it can transform
into pearlite, bainite or martensite.
• The phases finally formed during cooling depend upon
time and temperature.
• TTT diagram shows the time required for
transformation to various phases at constant
temperature, and, therefore, gives a useful initial guide
to likely transformations.
• In addition to the variations in the rate of
transformation with temperature, there are variations
in the structure of the transformation products also.

43. Spring 2001 Dr. Ken Lewis ISAT 430 43
The Time – Temperature –
Transformation Curve (TTT)
• 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.

44. Spring 2001 Dr. Ken Lewis ISAT 430 44
The Time – Temperature –
Transformation Curve (TTT)
• If cooling is rapid enough
austenite is transformed into
Martensite.
– FCC → BCT
– diffusion separation of carbon
and iron is not possible
• Transformation begins at Ms and
ends at Mf.
– If cooling is stopped at a
temperature between Ms and
Mf , it will transform into
martensite and bainite .

45. Full TTT Diagram
The complete TTT
diagram for an iron-
carbon alloy of eutectoid
composition.
A: austenite
B: bainite
M: martensite
P: pearlite

46. TTT Diagram
• Transformations at temperatures between
approximately 705°C and 550°C result in the
characteristic lamellar microstructure of pearlite.
• At a temperature just below A1 line, nucleation
of cementite from austenite will be very slow, but
diffusion and growth of nuclei will proceed at
maximum speed, so that there will be few large
lamellae and the pearlite will be coarse.
• However, as the transformation temperature is
lowered, i.e., it is just above the nose of the C-
curve, the pearlite becomes fine.

47. Bainite
• At temperatures between 550°C and 240°C (the approximate, Ms
temperature line), transformation becomes more sluggish as the
temperature falls, for, although austenite becomes increasingly
unstable, the slower rate of diffusion of carbon atoms in austenite
at lower temperatures outstrips the increased urge of the austenite
to transform. In this temperature range the transformation product
is bainite.
• Bainite consists (like pearlite) of a ferrite matrix in which particles of
cementite are embedded. The individual particles are much finer
than in pearlite. The appearance of bainite may vary between
– feathery mass of fine cementite and ferrite for bainite formed around
480°C and
– dark acicular (needle shaped) crystals for bainite formed in the region
of around 310°C).

48. Martensite
• At the foot of the TTT diagram,
there are two lines Ms (240°C )
and Mf (50°C).
• Ms represents the
temperature at which the
formation of martensite will
start and Mf the temperature
at which the formation of
martensite will finish during
cooling of austenite through
this range.

49. Martensite
• Martensite is formed by the
diffusionless transformation of
austenite on rapid cooling to a
temperature below 240°C
(approximately) designated as Ms
temperature.
• The martensitic transformation differs
from the other transformations in that
it is not time dependent and occurs
almost instantaneously, the
proportion of austenite transformed
to martensite depends only on the
temperature to which it is cooled.
• For example the approximate
temperatures at which 50% and 90%
of the total austenite will, on
quenching, transform to martensite
are 166°C and 116°C respectively.

50. Martensite
(i) Martensite is a metastable phase of steel,
formed by transformation of austenite below
Ms temperature.
(ii) Martensite is an interstitial supersaturated
solid solution of carbon in iron having a body-
centered tetragonal lattice.
(iii) Martensite is normally a product of
quenching.
(iv) Martensite is very hard, strong and brittle.

51. Martensite
• Diffusionless transformation
of FCC to BCT (more volume)
• Very hard & very brittle.

52. Possible transformation involving
austenite decomposition

54. Spring 2001 Dr. Ken Lewis ISAT 430 54
The Time – Temperature –
Transformation Curve (TTT)
• Composition Specific
– This curve is for 0.8%
carbon
• At different compositions,
shape is different

55. TTT Diagram
• The TTT digrams for hypo-eutectoid steels and
hyper-eutectoid steels will differ from that of
eutectoid steels
• The TTT digrams for hypo-eutectoid steels will
have an additional curve to show the
precipitation of ferrite from martensite before
transformation of remaining austenite to
pearlite

56. TTT diagram for Hypo-eutectoid steel.

57. TTT Diagram
• The TTT digrams for hyper-eutectoid steels
will differ from that of eutectoid steels
• The TTT diagrams for hyper-eutectoid steels
will have an additional curve to show the
precipitation of cementite from martensite
before transformation of remaining austenite
to pearlite

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

60. ©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 12.8 The TTT diagrams for (a) a
1050 and (b) a 10110 steel.

61. Contineous Cooling Transformation
(CCT) Diagram

62. CCT Diagram
• If you don’t hold at one temperature and allow
temperature to change with time, you are “Continuously
Cooling”.
• In continuous cooling, the constant temperature basis of
TTT diagram becomes obviously unrepresentative.
• More relevant information can, thus, be obtained from a
CCT diagram in which phase changes are tracked for a
variety of cooling rates.
• Therefore, a CCT diagram’s transition lines will be different
than a TTT diagram.
• Plotting actual cooling curves on such a diagram will show
the types of transformation product formed and their
proportions.

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

64. The CCT diagram for a low-alloy, 0.2%
C Steel

65. Effect of Cooling Rate on the Formation of
Different Reaction Products
• Very slow cooling rate (furnace cooling), typical of
conventional annealing, will result in coarse
pearlite with low hardness.
• Air cooling is a faster cooling rate than annealing
and is known as nonmalizing. It produces fine
pearlite.
• In water quenching, entire substance remains
austentic until the Ms line is reached, and
changes to martensite between the Ms and Mf
lines.

66. Effect of Cooling Rate on the Formation of
Different Reaction Products
• It is possible to form 100% pearlite or 100%
martensite by continuous cooling, but it is not
possible to form 100% Bainite.
• To obtain a bainitic structure, cool rapidly
enough to miss the nose of curve and then
holding in the temperature range at which
bainite is formed.

67. Critical Cooling Rate (CCR)
• If the cooling curve is tangent to the nose of
TTT curve, the cooling rate associated with
this cooling curve is Critical Cooling Rate (CCR)
for this steel.
• Any cooling rate equal to or faster than CCR
will form only martensite.

68. Critical Cooling Rate and Hardness of
Different Micro-Structures
Critical
cooling
rate
Hardness
of
different
structures

69. Factors Affecting Critical Cooling Rate
• Any thing which shifts the TTT diagrm towards
right will decrease the critical cooling rate
• The following factor affect the critical coolin rate
– 1. Grain size
– 2. Carbon content
– 3. Alloying elements
Increase in grain size, carbon content or alloying
elements shifts the TTT diagram towards right and
hence reduces the critical cooling rate as shown in
next slide.

70. Effect of Carbon Content and Grain
Size on Critical Cooling Rate

71. Superposition of TTT and CCT
Diagrams for Eutectoid Steel

72. The CCT diagram (solid lines) for a 1080 steel
compared with the TTT diagram (dashed lines).

73. ©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
(a)TTT and
(b) (b) CCT curves for a 4340
steel.

74. Factors Affecting TTT Diagram
• 1. Grain size
• 2. Carbon content
• 3. Alloying elements

75. Effect of Grain Size
• Fine grain steels tend to promote formation of
ferrite and pearlite from austenite.
• Hence decrease in grain size shifts the TTT
diagram towards left.
• Therefore, critical cooling rate increases with
decrease in grain size.

76. Effect of Carbon Content
• There is a significant influence of composition on
the TTT and CCT diagrams. For the
transformation diagrams we see the effect
through a shift in the transformation curves. For
example:
– An increase in carbon content shifts the CCT and TTT
curves to the right (this corresponds to an increase in
hardenability as it increases the ease of forming
martensite - i.e. the cooling rate required to attain
martensite is less severe).
– An increase in carbon content decreases the Ms
(martensite start) temperature.

77. Effect of Alloying Elements
• Different alloying elements have their
different effects on TTT diagram.
• An increase in alloy content shifts the CCT and
TTT curves to the right and
• Alloying elements also modify the shape of
the TTT diagram and separate the ferrite +
pearlite region from the bainite region making
the attainment of a bainitic structure more
controllable.

78. Effect of Alloying Elements on TTT
Diagram

79. Slow Cooling
Time in region
indicates amount of
microconstituent!
In attained
Formation of ferrite and
pearlite

80. Medium Cooling
Formation of Bainite
In attained

81. Fast Cooling
Martensite in ~ 1
minute of cooling
In attained

82. •
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