The document discusses phase transformations in materials and heat treatments. It explains that phase transformations can be used to vary the mechanical properties of alloys between 700-2000 MPa depending on the heat treatment. Time-temperature-transformation (TTT) diagrams are used to determine when phase transformations start and end during isothermal heat treatments. TTT diagrams have a characteristic C-shape due to the competing factors of nucleation and diffusion rates during transformations. The position and shape of TTT curves are affected by variables like carbon content, alloying elements, and grain size of the material.

1. PHASE TRANSFORMATION

2. Why do we study phase transformations?
 The tensile strength of an Fe-C alloy of eutectoid composition can be
varied between 700-2000 MPa depending on the heat treatment process
adopted.
 This shows that the desirable mechanical properties of a material can be
obtained as a result of phase transformations using the right heat treatment
process.
In order to design a heat treatment for some alloy with desired RT
properties, time and temperature dependencies of some phase
transformations can be represented on modified phase diagrams.

3. phase transformations
Most phase transformations begin with the formation of numerous small
particles of the new phase that increase in size until the transformation is
complete.
 Nucleation is the process whereby nuclei (seeds) act as templates for
crystal growth.
Homogeneous nucleation - nuclei form uniformly throughout the parent
phase; requires considerable supercooling (typically 80-300°C).
Heterogeneous nucleation - form at structural inhomogeneities (container
surfaces, impurities, grain boundaries, dislocations) in liquid phase much
easier since stable “nucleating surface” is already present; requires slight
supercooling (0.1-10ºC ).

4. Thermodynamics and
kinectics of PHASE
TRANSFORMATION
What does lie underneath the structure……..

5. phase transformation
 Phase transformation is predominantly controlled by TEMP. But
transformation never really start at transformation temp rather it starts at
a temp much below the temp predicted for the transformation to occur.
 Undercooling: It is the gap between the temp predicted for the
transformation to occur and the temp at which the transformation
actually occurs.

6. Supercooling
During the cooling of a liquid, solidification (nucleation) will begin only
after the temperature has been lowered below the equilibrium solidification
(or melting) temperature Tm. This phenomenon is termed supercooling (or
undercooling.
 The driving force to nucleate increases as ∆T increases
Small supercooling  slow nucleation rate - few nuclei - large crystals
Large supercooling  rapid nucleation rate - many nuclei - small crystals

7. Nucleation of a spherical solid particle in a liquid
 The change in free energy ΔG (a function of the internal energy and
enthalpy of the system) must be negative for a transformation to occur.
 The Assume that nuclei of the solid phase form in the interior of the liquid
as atoms cluster together-similar to the packing in the solid phase
 Also, each nucleus is spherical and has a radius r.
Free energy changes as a result of a transformation: 1) the difference
between the solid and liquid phases (volume free energy, ΔGV); and 2) the
solid-liquid phase boundary (surface free energy, ΔGS).

8. phase transformation
Allotropic / polymorphic transformation: No change in composition
of the structure
Phase transformation: Change in crystal structure+ Change in
composition.
•Surface creations always hinders the process of transformation. The
new phase always trys to create the surface, so energy needs to be
supplied. So volume free energy will try to decrease the energy but
surface free energy will try to increase the energy.

9. Transforming one phase into another takes time.
Fe Fe C
γ Eutectoid 3
transformation (cementite)
(Austenite) +
C
α
FCC (ferrite) (BCC)
∆G = ∆GS + ∆GV

10. phase transformation
• In the previous fig it can be observed that as soon as the particles of A
phase are formed the free energy of the system should decrease the
new phase is developed and has lower energy than the B phase.
ΔFv=VΔf
V= Vol of the new crystal
f=free energies of the new phase
• formation of the new crystal is linked with the interface between the
new and initial phases.
ΔFs = sν
s = surface area of the new crystal
ν = free energy per unit area

12. phase transformation

13. phase transformation
• If rate kinetics of phase transformation is increased then the structure
will be finer and this is indicated by the Hall - Petch equation States that
decrease in grain size and with fineness in the structure the strength in
increased.
δo =δ + Ka (-1/2) → Hall-Petch Equation
Where, δo = Friction stress
δ = in stress
a = grain size
K= locking parameter

14. Solid state transformation
• During the solid state transformation still another factor acting
inhibiting the nucleation transformation nuclei.
• A new phase always differs from the initial one in its structure and
specific volume.
• Since the transformation develops an elastic crystalline medium,
change in specific volume should cause an development in elastic
strain energy in one or both the phases. This inhibits the transformation
and kinetics the free energy.

15. Solid state transformation
• Therefore, the certain elastic component ΔFel makes a +ve
contribution to the free energy change in the solid state
transformation

16. Martensite transformation temp is much lower than Pearlite
transformation temp??
ΔTm>>ΔTp
Reason: Elastic strain energy
component
A→ M leads to volumetric expansion
which leads to straining of the lattice and
hence a +ve component in the free
energy. To compensate this +ve
component an undercooling is there. So
temp of transformation is so low.

17. Nucleation and Growth
• Reaction rate is a result of nucleation and growth of crystals.
100
Nucleation rate increases w/T
% Pearlite Growth
regime Growth rate increases w/ T
50Nucleation
regime
0 t50 log (time)
• Examples:
pearlite
γ colony γ γ
T just below TE T moderately belowTE T way below TE
Nucleation rate low Nucleation rate medium Nucleation rate high
Growth rate is high Growth rate is medium Growth rate is low
5

18. FRACTION OF TRANSFORMATION
• Fraction transformed depends on time.
Avrami Eqn.
n
y = 1 − e−kt
fraction
transformed time
• Transformation rate depends on T.
activation energy Ex: recrystallization of Cu
°C
119°C
103°C
C
y (%)
°C
°C
2°
5
1 −
13
11
88
43
100
r= = Ae Q /RT
t 0.5 50
0
1 10 102 104
log (t) min
• r often small: equil not possible
2

19. Eutectoid Transformation rate ~ ΔT
• Transformation of austenite to pearlite:
Diffusion of C
Austenite (γ) cementite (Fe3C) during transformation
grain α Ferrite (α)
boundary α γ α
γ α
α pearlite γ
α γ
growth α
α
direction
α
• For this transformation, 100
Carbon
diffusion
rate increases with ( ∆T) 600°C
(∆T larger)
% pearlite
[Teutectoid – T ]. 650°C
50
675°C
(∆T smaller)
0
Coarse pearlite  formed at higher temperatures – relatively soft
Fine pearlite  formed at lower temperatures – relatively hard

20. PHASE TRANSFORMATIONS
Based on
Mass
transport Diffusion less military Diffusional
transformation transformation
Change in No change in
composition composition
PHASE TRANSFORMATIONS
Based on
Order
Ist order nucleation 2nd order entire
and growth volume transforms

21. Diffusion-less transformation in solids
 Major phase transformations that occur in solid phase are due to
thermally activated atomic movements
The different types of phase transformation that is possible can be
divided into 5 groups:
► Precipitation Transformation
► Eutectoid transformation
► Ordering reactions
► Massive transformation
► Polymorphic changes

22. Precipitation Transformations: Generally expressed as α’→ α + β
where α’ is a metastable supersaturated solid solution
β is a stable or metastable precipitate
α is a more stable solid solution with the same crystal structure as α’
but composition closer to equilibrium

23. Eutectoid Transformations: Generally expressed as γ→ α + β
Metastable phase (γ) replaced by a more stable mixture of α + β
Precipitation and eutectoid transformations require compositional
changes in the formation of the product phase and consequently
require long-range diffusion

24. Ordering Transformations: Generally expressed as α (disordered) →
α’ (ordered) . These do not require long range diffusion

25. Massive Tranformations: Generally expressed as β→ α
Original phase decomposes into one or more new phases which have
the same composition as the parent phase but different crystal
structures

26. Polymorphic Transformations: Typically exhibited by single
component systems where different crystal structures are stable over
different temperature ranges. E.g. bcc-fcc transformation in Fe

27. Possible Transformations
Martensite
T Martensite
Strength
Ductility
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends

28. Time Temperature
Transformation(TTT) curves

29. Iron-Iron Carbide phase diagram Eutectic
L → γ + Fe3C
Peritectic L
L+δ→γ 1493ºC
δ
L+γ
0.1 %C γ 2.06 1147ºC
Eutectoid γ + Fe3C
γ → α + Fe3C
723ºC
α
0.025 %C α + Fe3C
T →
Fe Fe3C
0.16 0.8 4.3 6.7
%C →

30. WHAT ARE TTT CURVES
 T (Time) T(Temperature) T(Transformation) diagram is a
plot of temperature versus the logarithm of time for a steel
alloy of definite composition.
 It is used to determine when transformations begin and end
for an isothermal (constant temperature) heat treatment of a
previously austenitized alloy
 TTT diagram indicates when a specific transformation
starts and ends and it also shows what percentage of
transformation of austenite at a particular temperature is
achieved.

31. Time- Temperature-Transformation (TTT) Curves – Isothermal
Transformation
800 Eutectoid temperature
723 Austenite
Coarse
Pearlite
600 Fine
Eutectoid steel
500 Pearlite + Bainite
T →
400 Bainite
Not an isothermal
300
 Ms
Austenite
200
transformation
Mf
100
Martensite
0.1 1 10 102 103 104 105
t (s) →

32. The dependance of transformation to temperature and time can be
analyzed best using the diagram below:
2 solid curves are plotted:
 one represents the time
required at each
temperature for the start of
the transformation;
 the other is for
transformation completion.
 The dashed curve
corresponds to 50%
completion.
The austenite to pearlite
transformation will occur
only if the alloy is
supercooled to below the
eutectoid temperature
(727˚C).
Time for process to complete
depends on the
temperature.

33. WHY TTT CURVE HAS A C- SHAPE…
 The transformation of austenite doesnot start immediately
on quenching the austenised sample to a constant
temperature bath
 Transformation of the austenite to its product occurs after a
definite time interval – incubation period
 Incubation period is that period in which transformation
doesnot proceed because enough diffusion has not taken
placein austenite for the transformation to start


34.  Thus the C shape shows that the stability of austenite first
decreases sharply to the minimum then increases again
 Thus the rate of austenite transformation is:
 Nil at Ac1 temperature (free energy change is 0)
 As temperature falls, it first increases and reaches maximum
(free energy change increases with increase in undercooling)
 Nucleation rate increases as critical nucleus size decreases
 Rate is maximum at nose
 Below the nose the rate of increase in the transformation duc to
nucleation rate is ofset by in rate of diffusion at low
temperatures
 The rate further decreases with the increase in undercooling
( diffusion rate)
• Thus the TTT curve has a characteristic C shape.

35. Different types of Time- Temperature-Transformation
(TTT) Curves
 Three types of curves are there depending on the carbon content of steel:
► TTT for hypereutectoid steel
► TTT for eutectoid steel
► TTT for hypo eutectoid steel

36. EFFECT OF CARBON ON THE TTT CURVES
 Carbon has significant effects on the nature of the TTT
curves
 Carbon is an austenite – stabilizer
HYPOEUTECTOID STEELS
 Ferrite is the nucleating phase on decomposition of
austenite
 As carbon increases from 0 to 0.77% :
EUTECTOID STEELS
 Have the maximum incubation period

37. HYPEREUTECTOID STEELS
Cementite is the nucleating phase
 As the carbon content increases more than 0.77%:
BAINITE
Ferrite is the nucleating phase
S curve uniformly shifts towards the right in entire
range
Bainite transformation is uniformly retarted

38. Proeutectoid Proeutectoid
phase starts to cementite starts
form on this line to form on this
line
A+P
A +F
Temperature oC
Ac1 Fe3C +A
A
F+P P Fe3C +P
Ms B B
Ms Ms
Ms
Pearlite reaction starts
TTT curves for hypo , eutectoid and hyper-eutectoid steels

39. EFFECT OF ALLOYING ELEMENTS ON THE TTT
CURVES
 All alloying elements (except Co) shift the S curve to the right
 Austenite stabilizers move the curve to the right( Mn, Ni,etc)
 Carbide formers shift the S curve further to the right because:
 Diffusion of alloying elements is too slow(substitutional
elements)
 Diffusion of carbon is slower as carbide formers donot easily
part with the carbon
 Allotropic change γ α is reduced by solutes
 Bainitic transformation is lesser affected ( no redistribution of
alloying elements)

40. EFFECT OF GRAIN SIZE ON THE TTT CURVES
All decomposition products of austenite nucleate
heterogenously at grain boundaries
Thus incubation period is reduced for fine grained
steel
S curve is more towards the left in fine grained steel

41. MARTEMPERING
 To avoid residual stresses generated during quenching
 Austenized steel is quenched above Ms (20-30oC above Ms i.e.
180 – 250oC)
 Holding in salt bath for homogenization of temperature across
the sample (large holding time is avoided to avoid forming
bainite)
 The steel is then quenched in air and the entire sample
transforms simultaneously
 Tempering follows
 The process is called Martempering
 The process is beneficial as:
 Steep temperature gradient is minimized
 Thermal and structural stresses are minimal
 More retained austenite – lesser volume change

42. Figure shows the process of Martempering and the characteristic
temperatures:
800
Eutectoid temperature
723
Austenite
Pearlite
600
α + Fe3C
500 Pearlite + Bainite
T → 400 Bainite
Martempering 300
Ms
200
Mf
100 Martensite
0.1 1 10 102 103 104 105
t (s) →

43. AUSTEMPERING
 To avoid residual stresses, distortion and cracks generated during
quenching in high carbon steels
 Austenized steel is quenched in molten salt bath above Ms
(300oC – 400oC)
 Held long enough for isothermal transformation to lower Bainite
 No tempering is done
 This process is termed as Austempering
 Equalization of temperature across cross-section minimizes the
stress development
 The steels should have sufficient hardenability to avoid
trasformation to pearlite during quenching and holding
 Steels shouldnot have a long bainitic bay ( to avoid long
transformation times)

44.  Advantages:
 Improved ductility with same hardness
 Elimination of distortion and cracks
 No tempering required
 Impact strength is improved
 Uniformity in properties
 High endurance limit
800
Eutectoid temperature
723
Austenite
Pearlite
600
α + Fe3C
500 Pearlite + Bainite
T →
400 Bainite
300
Ms
Austempering 200
Mf
100 Martensite
0.1 1 10 102 103 104 105
t (s) →

45. HOW TO DRAW TTT CURVE

57. Continuous Cooling Transformation (CCT)
 Isothermal heat treatments are not the most practical due to rapidly
cooling and constant maintenance at an elevated temperature.
 Most heat treatments for steels involve the continuous cooling of a
specimen to room temperature.
TTT diagram (dotted curve) is modified for a CCT diagram (solid curve).
For continuous cooling, the time required for a reaction to begin and end is
delayed.
The isothermal curves are shifted to longer times and lower temperatures.

59.  In the above figure Moderately rapid and slow cooling curves are
superimposed on a continuous cooling transformation diagram of a eutectoid
iron-carbon alloy.
 The transformation starts after a time period corresponding to the
intersection of the cooling curve with the beginning reaction curve and ends
upon crossing the completion transformation curve.
 Normally bainite does not form when an alloy is continuously cooled to
room temperature; austenite transforms to pearlite before bainite has become
possible
The austenite-pearlite region (A---B) terminates just below the nose.
Continued cooling (below Mstart) of austenite will form martensite

61.  For continuous cooling
of a steel alloy there exists
a critical quenching rate
that represents the
minimum rate of quenching
that will produce a totally
martensitic structure.
 This curve will just miss
the nose where pearlite
transformation begins

62.  Continuous cooling diagram for a 4340 steel alloy and several cooling
curves superimposed in the figure below
 This demonstrates the dependence of the final microstructure on the
transformations that occur during cooling.
 Alloying elements used to modify the critical cooling rate for
martensite are
►chromium,
► nickel,
► molybdenum
► manganese
► silicon
► tungsten

64. Effect of adding other
elements
4340 Steel

Other elements (Cr, Ni, Mo, Si and
W) may cause significant changes
in the positions and shapes of the
TTT curves:
 Change transition temperature;

Shift the nose of the austenite-to-
pearlite transformation to longer
times; nose

Shift the pearlite and bainite noses
plain
to longer times (decrease critical carbon
cooling rate); steel

Form a separate bainite nose;
 Plain carbon steel: primary
alloying element is carbon.

65. An actual isothermal heat treatment curve on the isothermal
transformation diagram:
rapid cooling
isothermal treatment
• Eutectoid iron-carbon alloy; composition, Co = 0.76 wt% C
• Begin at T > 727˚C
• Rapidly cool to 625˚C and hold isothermally.

66. AUSTENITE
from where it all starts…..

67. AUSTENITE
Austenite, also known as gamma phase iron (γ-Fe),
is a metallic, non-magnetic allotrope of iron or a solid
solution of iron with carbon.
It has an FCC crystal structure
The maximum solubility of carbon in austenite is 2.13
% at 1147oC

68. Why is Austenizing So Important In
Heat Treatment of Any Steel?
Austenite can transform into various products depending
on the composition and cooling rates.
Morphology of parent austenite(grain size) decides the
morphology of products and thus its properties.

69. Formation of Austenite
 Austenite is formed on heating an aggregate of pearlite, pearlite
and ferrite , pearlite and cementite
 Pearlite Austenite
 Eutectoid composition transforms at a particular (Ac1)
temperature
 1st step: ( On heating to eutectoid temperature)
Lattice changes
BCC iron (α-Fe) FCC iron (γ-Fe)
 2nd step:
Diffusion of carbon from Cementite (6.67% carbon) to adjoing
regions
o Inspite of the carbon gradient the structure is thermodynamically
stable at room temperature due to the low diffusion rate of carbon
at low temperatures and occurs only at sufficiently high
temperatures

70.  The maximum diffusion of carbon takes place from cementite
at ferrite –cementite interface
 Austenite nucleates at interfaces between ferrite and
cementite, specially in between pearlitic colonies
 By gradual dissolution of carbon from cementite austenite is
formed
 The primary austenite formed dissolve the surrounding ferrite
and grow at their expense.
 The growth rate of austenite is higher than the rate of
dissolution of cementite
 Thus dissolution of ferrite is complete before that of cementite
α-Fe Fe3C α-Fe
Fe3C Austenite

71. Homogenization of austenite
The austenite formed from cementite and ferrite is generally
not homogenous
Homogenization requires high temperature/time , or both
High temperatures if the rate of heating is faster
Shorter time spread over a large range of temperatures if the
rate of heating is slower

72. Kinetics of Austenite Formation
The formation of austenite on heating occurs by nucleation
and growth
The factors that affect nucleation rate or growth rate affect
the kinetics of the transformation
The kinetics depends on:
 Transformation temperature and holding time
 Rate of heating
 Interface between ferrite and cementite
 Grain size
 Nature of the alloying elements present

73.  Transformation Temperature
 Austenite transformation occurs at a temperature higher than Ac1
in the Fe-Cementite phase diagram – Superheating
 Equilibrium temperatures are raised on heating and lowered on
cooling ( free energy should be negative)
 The rate of austenite formation increases with increase in
temperature as it increases the rate of carbon diffusion and the free
energy is more negative
 Interdependence of time and temperature :
Transformation takes a shorter time at higher temperatures of
transformation and vice versa

74. Rate of heating :
For higher rates of heating, transformation starts at higher
temperatures and for slower rates, at lower temperatures
For any rate of heating transformation occurs over a range
of temperature
For transformation at a constant temperature, heating rate
should extremely slow
Special note:
Austenite transformation starts as soon as the eutectoid
temperature is reached, but the region in between the
curves indicates the majority of the tranformation.

75. Interface between ferrite and cementite
Higher the interfacial area faster is the tranformation
Interfacial area can be increased by:
 Decreasing the inter-lamellar spacing between ferrite and cementite
The closer the ferrite – cementite lamellae, the higher is the rate of
nucleation.
Carbon atoms have to diffuse to smaller distances from cementite to
low carbon regions to form austenite
 Increasing the cementite or carbon content
This will lead to more pearlite content in steels and thus more
interfaces.
 Examples :
1. High carbon steels austenize faster than low carbon steels
2. Tempered martensite structure austenizes faster than coarse
paerlite
3. Spheroidal pearlite takes longer time to austenize due to very low
interfacial area

76. Grain size
The coarser the parent grain size the slower is the
transformation rate
This is because in larger grains the interfacial area is lesser
The smaller is the parent grain size the faster is the
transformation to austenite

77. Nature of the alloying elements present
 Alloying elements in steel are present as alloyed cementite
or as alloy carbides
 Alloy carbides dissolve much more slowly than alloyed
cementite or cementite
 The stronger the alloy carbide formed the slower is the
rate of formation of austenization
 Diffusion of substitutional alloying elements is much
slower than the interstitial element, carbon
 Thus the rate of austenization depends on the amount and
nature of alloying element

78. Why does the Fe-Cementite diagram show a fall in the
Ac3 temperature and rapid rise in Acm temperature with
increasing carbon percentage?
In hypoeutectoid steels, austenisation process takes place rapidly
as carbon content increases.
As carbon percentage increases, the amount of pearlite increases,
which increases the interfacial area between ferrite and cementite
 Thus Ac3 temperature line decreases continuously with increasing
carbon content

79. In hypereutectoid steels , austenization process becomes
much more difficult as the amount of carbon increases
Austenisation of free cementite needs very high temperature as
it involves the diffusion of large amount of carbon( from
cementite) to become homogenous
Thus as carbon content increases, amount of free cementite
increases, which needs higher temperature to austenize.
Thus Acm line is so steep

80. Austenite Grain Size
Original grain size- size of austenite grains as formed after
nucleation and growth
Actual grain size – size of the austenitic grains obtained
after homogenization at higher temperatures
Generally grain size is referred to as actual grain size
Depending on the tendency of steel to grain growth, steels
are classified into two groups:
Inherently fine grained
Inherently coarse grained

81.  Inherently fine grain steels resist grain growth with increasing
temperature till 1000oC – 1050oC
 Inherently coarse grain steels grow abruptly on increasing
temperature
 On heating above a certain temperature T1 inherently fine
grain steels give larger grains than inherently coarse grain
steels
Grain size
Inherently coarse grain
Inherently fine grain

82. Presence of ultramicroscopic particles like oxides,
carbides and nitrides present at grain boundaries
prevent grain growth in inherently fine grain steels
till very high temperatures
They act as barriers to grain growth
Steels deoxidized with Al or treated with B,Ti and V
are inherently fine grained
At temperatures above T1,dissolution of
ultramicroscopic particles cause sudden increase in
grain size
Thus inherently fine grain steels can be hot worked
at high temperatures without getting coarsened

83. Effect of grain size on mechanical properties
 Austenite grain size plays a very important role in
determining the properties of the steel
 The effect of grain size on different properties are given
below:
 YIELD STRESS
 The dependence is given by Hall-Petch equation :
 Where is the yield stress
 is the frictional stress opposing motion of
dislocation
 K is the extent to which dislocations are piled
at barriers
 D is the avg grain diameter

84.  Grain refinement improves the strength and ductility at the same
time
 IMPACT TRANSITION TEMPERATURE
 Increase in grain size raises the impact transition temperature, so
more prone to failure by brittle fracture

85.  CREEP STRENGTH
 Coarse grained steel has better creep strength above
equicohesive temperature
 Below this fine grain structure have better creep strength
 FATIGUE STRENGTH
 Fine grained steel have higher fatigue strength
 HARDENABILITY
 Coarse grained steels have higher hardenability
 (smaller grain boundary area in coarse grained structure gives
less sites for effective diffusion, so martensite formation on
cooling is favoured)
 MACHINABILITY
 Coarse grain structure has better machinability due to ease in
discontinuos chip formation(low toughness)

86. PEARLITIC TRANSFORMATION

87. Pearlite
 It is a common micro constituent of a variety of steels where is increases
the strength of steel to a substantial extent.
 Unique micro constituent formed when austenite in iron carbon alloys is
transformed isothermally at or below the eutectoid temp (723K)
 One of the most interesting features of austenite to pearlite transformation
is that the tr product consists of entirely 2 diff phase.
 Consists of alternate plates of ferrite and cementite and the continuous
phase is ferrite.

88. Pearlite
 Ferrite has a very low carbon content whereas cementite Fe3C is
an intermetallic compound of iron with 6.67 wt% of carbon.
 Name pearlite is related to the fact that when it is polished and
etched then the structure reveals the colorfulness of the “mother
of pearl”
 Ferrite and cementite are present here in the ratio 8:1.

89. Transformation rate ~ ΔT
• Transformation of austenite to pearlite:
Diffusion of C
Austenite (γ) cementite (Fe3C) during transformation
grain α Ferrite (α)
boundary α γ α
γ α
α pearlite γ
α γ
growth α
α
direction
α
• For this transformation, 100
Carbon
diffusion
rate increases with ( ∆T) 600°C
(∆T larger)
% pearlite
[Teutectoid – T ]. 650°C
50
675°C
(∆T smaller)
0
Coarse pearlite  formed at higher temperatures – relatively soft
Fine pearlite  formed at lower temperatures – relatively hard

90.  The layer thickness depends on temperature at which the isothermal
transformation occurs. For example at T just below the eutectoid,
relatively thick layers of both ferrite and cementite phases are
produced. This structure is called coarse pearlite. At lower T,
diffusion rates are slower, which causes formation of thinner layers at
the vicinity of 5400C. This structure is called fine pearlite.

91. MORPHOLOGY

92. Morphology
 It is a lamellar structure with cementite and ferrite.
 The cementite and ferrite are present in a definite ratio of 8:1.
 Each ferrite plate in the pearilte lamellæ is a single crystal and
some neighbouring plates in a single colony have approximately
the same orientation of lattice. This holds for the cementite also.
 In general, both sides of the line of discontinuity in a pearlite
colony make a small angle in lattice orientation with each other.
 In the ferrite region near the boundary of pearlite colonies or
grains, there are net-works of dislocations or dislocation walls, at
each node of wich a cementite rod is present.


93. MECHANISM

94. Morphology
Mechanism
 The austenite to pearlite transformation occurs by nucleation and
growth.
 Nucleation occurs heterogeneously at the grain boundaries of
austenite, and if homogeneous nucleation occurs then it will occur at
the carbide particles or in regions of high carbon concentration or on
inclusions
 The nucleated pearlite grows into austenite as roughly spherically
shaped nodules.
 Each nodule has a number of structural units
 In each structural unit the lamellae are largely parallel and is called a
colony.
 The HULL-MEHL model can explain well the morphology of
pearlite

95. HULL-MEHL MODEL
 The initial nucleus is a widmanstatten platelet of cementite forming at the
austenite g.b. which when as grows thickens as well
 This occurs by the removal of carbon atoms from austenite on both sides
of it till carbon decreases in the adjacent austenite to a fixed low value at
which ferrite nucleates.
 The growth of ferrite leads to build of carbon at the ferrite austenite
interface until there is enough carbon to nucleate fresh plates of cementite
which then grow.

96. HULL-MEHL MODEL
This process of formation of alternate plates of ferrite
and cementite forms a colony.
A new cementite nucleus of different orientation may
form at the surface of colony forming another colony.
The point to be noted is if austenite transforms to pearlite
at a constant temp then the interlamellar spacing is same
in all the colonies. The following fig will depict it clearly

97. Figures showing coarse and fine pearlite
- Smaller ∆T: - Larger ∆T:
colonies are colonies are
larger smaller

99. Mechanism
Hull-Mehl Mechanism for pearlitic transformation

100. KINETICS

101. Kinetics

102. This equation however makes the following assumptions:
(i)The , average nucleation rate is const. with time which
actually isnt true
(ii)Nucleation occurs randomly, which is also not truly
correct.
(iii)The growth rate, is const. with time, which can
change from one nodule to other with time.
(iv) Nodules maintain a spherical shape but nodules may
not be truly spherical

103.  However when f(t) plotted against is shown in the curve
shown is which illustrates that the basic kinetic behaviour of pearlite
formation is nucleation and growth.

104. Kinectics of transformation (contd).
 At lower critical temp, the free energy of austenite is equal to
the free energy of pearlite.
 Therefore at this temperature transformation of pearlite to
austenite transformation will be completed in infinite time.
 So the rate of transformation will be zero.
 So it is essential to undercool the austenite below the equilibrium
(A1) temp.
 Below the lower critical temp, free energy of pearlite < free
energy for austenite and hence it is more thermodynamically
stable.
 Lower thr free energy more will be the stability of PEARLITE.

105. Kinectics of transformation (contd).
 Free energy of pearlite is less at lower tem and so stability is
increased by increasing ΔT.
 The decomposition of austenite to pearlite proceeds by the
redistribution of carbon atoms of austenite into ferrite and
cementite, and is essentially a diffusion controlled process.
 The rate of diffusion decreases exponentially with decreasing
temp
 This shows lower the transformation temp retards the rate of
transformation.
 There is a transformation temp for which diffusion of C atoms is
too small resulting in diffusion controlled transformation
 Rate of diffusion of carbon atoms is negligible below 200 C

106. Kinectics of transformation (contd).
 This shows that undercooling affects the rate of transformation in
2 ways:
Undercooling
increased degree of increased degree of undercooling
undercooling reduces the increases the transformation rate
transformation rate by by providing greator difference
lowering the rate of in free energies of austenite and
carbon diffusion curve. pearlite.

107.  The combined effect is shown in the curve below:
 Where (a) is rate of crystal growth and (b) is rate of nucleation

108. Kinectics of transformation (contd).
The austenite to pearlite transformation is completed by
nucleation and growth mechanism.
 The rate of transformation is governed by both.
 The rate of nucleation is expressed as total numbers of of nuclei
appearing per unit time in unit vol of untransformed austenite.
 Both rate of nucleation and growth are zero at eutectoid temp.
 They also temd to be zero below 200 C as shown in the graph
previously

109. Effect of degree of on the rates of nucleation and growthUndercooling

110.  Hardness of pearlite increases as S0 decreases and also same
for strength.
 As S0 is inversely proportional to the degree of undercooling
thus yield strength and also UTS is linearly related to the
interlamellar spacing or degree of undercooling below
eutectoid temp.
 As the pearlite content increases in C steels, impact
transition temp is substantially raised, decreasing ductility
and toughness as the ferrite-cementite interface provides
sites for easy nucleation of cracks

112. Effect of alloying additions on
Pearlitic Transformation
 Almost alloying element except Co lower both the rate of nucleation and
rate of growth.
 As compared to carbon other alloying element diffuse very slowly.
 As the diffusion rate for metallic atom is much slower than the
 carbon atom the formation of stable carbide during the transformation
will be feasible only at higher transformation temp.
 Partitioning of carbon gets delayed when Cr eats up C and forms carbide
Cr23C6 when alloyed with austenite.

113. BAINITIC TRANSFORMATION

114.  Bainite is an acicular microstructure (not a phase) that forms in steels
at temperatures from approximately 250-550°C (depending on alloy
content).
 First described by E. S. Davenport and Edgar Bain, it is one of the
decomposition products that may form when austenite (the
face centered cubic crystal structure of iron) is cooled past a critical
temperature of 727 °C (about 1340 °F).
 Davenport and Bain originally described the microstructure as being
similar in appearance to tempered martensite
 In plain carbon steel Pearlite and Bainite superimpose.
 Bainite is not so popular and is very much difficult to get.

115.  A fine non-lamellar structure, bainite commonly consists of cementite
and dislocation-rich ferrite. The high concentration of dislocations in
the ferrite present in bainite makes this ferrite harder than it normally
would be
 The temperature range for transformation to bainite (250-550°C) is
between those for pearlite and martensite.
 When formed during continuous cooling, the cooling rate to form
bainite is more rapid than that required to form pearlite, but less rapid
than is required to form martensite (in steels of the same composition).

116.  Most alloying elements will lower the temperature required for
the maximum rate of formation of bainite, though carbon is the
most effective in doing so
 The microstructures of martensite and bainite at first seem quite
similar; this is a consequence of the two microstructures sharing
many aspects of their transformation mechanisms
 However, morphological differences do exist that require a TEM
to see. Under a simple light microscope, the microstructure of
bainite appears darker than martensite due to its low reflectivity.

117. Illustration of Continuous cooling transformation diagram showing
Bainite

118. MECHANISM

119. Mechanism of Bainitic transformation
In the TTT curve the incubation period  the
transformation is diffusion controlled
But the bainite formation takes at a temp at which diffusion is
impossible X i.e. metallic atoms wont diffuse but diffusion of C
atoms is important
 This shows along with diffusion some other mechanism is
responsible for the transformation to occur
 Since formation of bainite is accompanied by surface distortion so
some shear mechanism is responsible for its transformation
 So it is a complex one and involves both diffusionless and diffusion
controlled phenomena are involved hence it is termed as a
“Diffusionless diffusion controlled transformation”

120. Mechanism of Bainitic transformation
 Two mechanisms are thought to be for the Bainite
formation:
1. Displacive theory
2. Diffusion theory
Bainite is considered to be formed by diffusionless diffusion
controlled transformation.. Both play a part in its
transformation

121. Diffusive theory
 The diffusive theory of bainite's transformation process is based
on short range diffusion at the transformation front.
 Here, random and uncoordinated thermally activated atomic
jumps control formation and the interface is then rebuilt by
reconstructive diffusion.
 The mechanism is not able to explain the shape nor surface relief
caused by the bainite transformation.
 Here redistribution of carbon atoms takes place from regions
enriched with carbon to the regions deficient in carbon
concentration.

122. When the austenite is undercooled below the Bs temp, C atoms
redistribute in the Austenite by diffusion. This redistribution leads
to formation of regions with varying carbon concentration in
Austenite. Some of these regions are enriched in carbon while
others are deficient in C. Such a difference in C concentration will
resolve in the development of stresses

123. Displacive theory
 One of the theories on the specific formation mechanism for bainite is
that it occurs by a shear transformation, as in martensite.
 The transformation is said to cause a stress-relieving effect, which is
confirmed by the orientation relationships present in bainitic
microstructures.
 There are, however, similar stress-relief effects seen in transformations
that are not considered to be martensitic in nature, but the term 'similar'
does not imply identical.
 The relief associated with bainite is an invariant—plane strain with a
large shear component. The only diffusion that occurs by this theory is
during the formation of the carbide phase (usually cementite) between the
ferrite plates.

124.  Now the low carbon austenite region transform to ferrite(Bainitic
plate) by diffusionless shear process. So It is important to know
here that low C Austenite which transform by shear process is itself
a diffusion controlled process.
 precipitation of carbide may occur from the C enriched Austenitic
region depending on the degree of saturation.
 The C depleted A region obtained by the precipitation of carbide
now transform to ferrite by shear mechanism.
 Such a condition is favourable in the upper region of the
intermediate transformation temp range, as ferrite has very high
solubility of carbon, the transformed ferrite will be supersaturated
with C

125. The degree of supersaturation increases with decrease in
transformation temperature
 As carbon diffusion is intensive in Bainitic transformation
region, Carbon may precipitate out from the supersaturated
ferrite.
This happens when the bainitic transformation in the lower
region in the transformation range.
Diffusion decreases exponentially so we get different
morphology’s of Bainite.

126. MORPHOLOGY

127. Upper BAINITE
UPPER Bainite
 Known as ‘feathery bainite’ as it resembles feather of a bird
 Forms in temperature range of 5500C-4000C
 The structure consists of
i. lath or needle-like ferrite which runs parallel to the longer axis and
ii. carbide precipitates as fine plates, parallel to the direction of growth
of bainite, mainly at the lath boundaries
 Carbides are present as ‘discontinuous stringers’ when carbon
content is low and ‘continuous stringers’ when carbon content is
high.

128. Upper Bainite
 The ferrite laths have ‘sub laths’ with high dislocation density
 Decrease in temperature produces finer and closely formed
laths with smaller spacing of carbide particles
 The ferrite and cementite in bainite have a specific orientation
relationship with the parent austenite
 Diffusivity of carbon in this temperature range is high enough
to cause partition of carbon between ferrite and austenite.
 Structure is brittle and hard and the deposition of hard carbide
stringers on the soft ferrite makes it a completely useless
structure.

129. Schematic growth mechanism of Upper Bainite
Upper bainite in medium carbon steel

131. Lower Bainite
 Known as ‘Plate bainite’
 Forms in the temperature range of 4000C-2500C
 The structure consists of
i. Lenticular plates of ferrite
ii. Fine rods or blades of carbide at an angle of 55 to 60o to the axis of
bainite
 Carbides can be cementite or ε-carbide, or a mixture depending on
temperature of transformation and composition of steel

132. Lower Bainite
 Carbides precipitate within the ferrite plates
 Ferrite plates have smaller sub-plates with low angle boundaries
between them
 Higher dislocation density than upper bainite
 Habit planes of ferrite plates are the same as martensite that
forms at low temperatures of the same alloy
 Alloying elements do not diffuse or form their carbides during
bainite transformation

133. Schematic representation of lower bainite structure
Lower Bainite structure in medium
carbon steel
Stages of formation of Lower Bainite

135. MARTENSITIC TRANSFORMATION
Martensite - BCT

136. MECHANISM

137. Mechanism
 Martensite transformation is a diffusion-less transformation
 Martensite is formed on quenching austenite, such that the diffusion of
carbon is not favored
 The atoms move in an organized manner relative to their neighbours
and therefore they are known as a military transformations in contrast
to diffusional civilian transformations
 Each atom moves by a distance less than one inter-atomic distance and
also retain its neighborhood undisturbed
 But the total displacement increases as one moves away from the
interphase boundary which results in a macroscopic slip as can be
observed as relief structure on the surface of martensite

138. Figure shows mechanism martensite plate formation

139.  At the beginning of the transformation martensite takes the
form of lens or plates spanning the entire grain diameter
 The subsequent plates formed are limited by the grain
boundaries and the initial martensite plates formed
 Where the plates intersect the polished surface they bring
about a tilting of the surface.
 But, macroscopically the transformed regions appear coherent
to the surrounding austenite.

140. The figure shows how the martensite
remains macroscopically coherent to
parent austenite on transformation

141.  A large amount of driving force is needed for the martensitic
transformation
 The magnitude of the driving force is provide by the free
energy change accompanying the transformation
 The magnitude of the driving force for nucleation of
martensite at the Ms can be as follows:
oThe graphs along side show
that magnitude of the driving
force increases with decrease
in the temperature of
transformation
 The figures above demonstrate the equation given above

142. Crystal Structure of Martensite
 Martensite has ‘ Body Centered Tetragonal’ structure
 The tetragonality of martensite, measured by the c/a ratio is given
by:
c/a=1+ 0.045 X wt% C
 Tetragonality increases with increase in carbon percent
 When the fcc γ- Fe transforms to bcc α-Fe, carbon is trapped in
the octahedral sites of body centered cubic structure to give body
centered tetragonal (BCT) structure
 The trapped carbon atoms cause tetragonal distortion of bcc lattice
 When carbon is more than 0.2%, bct structure is formed

144. KINECTICS OF TRANSFORMATION

145. Kinetics of Martensite Transformation
 The transformation starts at a definite temperature –Ms ( Martensite start)
temperature
 The transformation proceeds over a range of temperatures till Mf
temperture
 The amount of martensite increases on decreasing transformation
temperature between Ms and Mf
 At Mf not all austenite is converted to martensite, but a certain amount is
present as retained austenite

146.  Although the martensite transformation ends at Mf, some austenite
still remains untransformed as retained austenite
 Mf temperature depends on cooling rate
 Slower cooling rates lower the Mf temperature
 Mf temperatures are also lowered by increase in carbon content

147. • Cooling below Mf doesnot change the amount of martensite.
• The velocity of the martensite transformation, in general, is
independent of the transformation temperature.
• The velocity of transformation is extremely fast almost 10-7 s. This is
associated with a crying sound.
• Martensitic transformation is independent of holding time

148. Important characteristics of Martensite
Transformation
 Diffusionless/Military tranformation
 Athermal transformation.
 Retained Austenite
 Ms – Mf temp
 Reversibility of transformation
 Habit planes
 Bain distortion
 Effect of applied stress on transformation
 Hardness of Martensite

149. Ms and Mf Temperature
 Martensite transformation begins as the Ms temperature is reached
and ends at the Mf temperature
 The Ms temperature depends on the chemical composition of steel
and is independent of the rate of cooling
 Austenizing temperature to which the steel had been heated prior
to the transformation affects Ms temperature
 Higher the temperature creates the following two conditions:
Greater dissolution of carbon and carbides, which results in
lowering of Ms
Larger grain size of austenite, which results in a rise of Ms

150. Ms and Mf Temperature
 The relationship between Ms temperature and the chemical
composition can be shown as:
Ms (oC)=561 – 474(%C) – 33(%Mn) – 17(%Ni) – 17(%Cr) –
21(%Mo).
• The above shows that nearly all elements lower the Ms
temperature except Cobalt and aluminium
• Carbon has the most profound effect on Ms temperature and an
increase in carbon content cause lowering of the Ms temperature

151. Effect of carbon content on Ms and Mf temp

152. Effect of alloying additions on Ms temp

153. Reversibility of Martensite
 Martensite transformation is reversible .
 Martensite can be reverted to austenite on heating above the Ms
temp.
 The essential condition for the reversibility of martensite is that
there should not be any change in chemical composition of
martensite during heating
Since Martensite in
steels is supersaturated
 Most steels dont satisfy this condition
solid solution of
carbon in alpha iron
and it decomposes at a
very rapid rate on
heating

154. Retained Austenite
 Retained austenite Untransformed Austenite.
 It forms as Austenite to martensite transforms on quenching below
the Ms temp but above Mf temp.
 As Austenite to martensite never goes to completion some amount
of austenite is present in the hardened steel.
 Since Ms and Mf temp decrease with carbon content increase so
amount of retained austenite increases with increase in carbon
content.
 All alloying elements except Al and Co lower the Ms temp and
enhance the amount of retained austenite.
 Therefore, both high carbon steels and high alloy steels are prone
to retained austenite.

155. Amount of retained austenite increases with decreased
martensite temp of transformation

156. Athermal and Isothermal Martensite
Athermal and Isothermal Martensite
 Athermal transformation occurs in most carbon steels
 Martensite transformation proceeds on continuous cooling
below the Ms temperature
 The transformation stops when the steel is held at a particular
temperature in between Ms and Mf
 The transformation is independent of holding time

157.  If while the transformation process within the Ms-Mf temperature the
ooling is stopped – the transformation halts
 On resuming the cooling the transformation doesnot start instantly but
needs supercooling
 Larger amount of retained austenite formed at Mf called ‘stabilized
austenite’

158. • Martensite can also form isothermally.
• Isothermally transformed martensite quantity is low.
• In extra low carbon base alloys or high alloy steels - low
transformation temperatures and long period of transformation.
• Amount of martensite decrease with decrease in Ms- Mf
temperature.

159. Effect of applied stress on transformation
 Presence of external and internal stresses affect the kinetics of
martensitic transformation
 If external stresses are applied to austenite above Ms
temperature, Ms is raised
 As there are a large number of habit planes, the application of
stress favors martensite plate formation on any of the plates, thus
the ms is raised
 The maximum temperature at which martensite can be formed
by plastic deformation is denoted as Md
 The amount of martensite formed by plastic deformation is a
function of plastic deformation

160. Bain Distortion model
 The model was proposed by E.C. Bain
 Any simple homogeneous pure disyortion of the nature which
converts one lattice to another by expansion and contraction
along the crystallographic axis belong to a class known as
BAIN DISTORTION
 The model explains how bct lattice can be obtained from fcc
lattice with minimum atomic movement

161.  In the figure in the previous slide, x,y,z and x’, y’, z’ represent the
initial and final axes of fcc and bcc unit cells
 An elongated unit cell of the bcc structure can be drawn within two
fcc cells
 The elongated bcc unit cell has a c/a ratio of 1.40
 The pure bcc unit cell has a c/a ratio of 1.0
 The bct structure of martensite has c/a ratio of 1.08

162.  Transformation to a bct unit cell is achieved by:
(a) contracting the cell 20% in the z direction and
(b) expanding the cell by 12% along the x and y axes
 This results in 4% increase in volume
 In the case of steels, the carbon atoms fit into z’ axis of the
bcc cell at ½<100> positions causing the lattice to elongate in
this direction
 Bain distortion results in the following correspondence of
crystal planes and directions:

163.  This model explains the transformation of martensite from
austenite with minimum movement of atoms
 Thus carbon atoms are finally present only in the middle of
the edges along [001]axis and not in the middle of the edges
which represent the a-axis

164. Habit planes
 The transformation is characterized by a well established relationship
between the orientation of parent austenite and the transformed martensite.
 Habit planes are those planes of the parent austenitic lattice on which
martensitic plates are formed and which lie parallel t the physical plane of
the martensitic plate.
 A habit plane is distorted by the martensite transformation though along it
shear displacement takes place during transformation.
 The habit planes for low, medium and high carbon steels are (111),(225),
(259)

165. An micrograph of austenite that was polished flat and then allowed to
transform into martensite.
The different colors indicate the displacements caused when martensite
forms.

166. Hardness of Martensite
 Hardness of martensite is due to carbon content and chemical
composition
 Strengthening effect is due to super saturation of alpha
solution with carbon
 Hardness increases with increase in carbon content in
martensite and then decreases after a certain Carbon% (0.5-
0.6%)

167.  High carbon % lowers the Ms and Mf , so large amount of retained
austenite is present
 Alloying elements that lower Ms and Mf temperatures, give more
retained austenite
 Steel becomes softer as retained austenite increases
 Two suspected factors for enhanced hardness
a) internal strains within α-Fe due to excess carbon
b) the plastic deformation of austenite surrounding martensite
plates
 Appearance of large number of twins interlayer and increase of
dislocation density on martensite transformation
 Segregation of carbon atoms to dislocations leading to Cottrel
atmospheres
 Precipitation of dispersed carbide particles from alpha phase
 Self tempering results in lowering of hardness

168. MORPHOLOGY

169. Morphology of Martensite
 Martensite transformation involves two shears:
a) homogeneous lattice deformation or Bain strain
b) inhomogeneous lattice deformation which makes lattice to
be undistorted
 This shear can be slip or twin .
 This shear depends on composition, temperature of
transformation and strain rate.
Twinning is favored when
the yield stress of austenite is raised
carbon and alloying elements increase

170.  Martensitic transformations are (usually) first order,
diffusionless, shear (displacive) solid state structural changes.
 Their kinetics and morphology are dictated by the strain energy
arising from shear displacement.
 The displacement can be described as a combination of
homogeneous lattice deformation, known also as “Bain
Distortion”, and shuffles.
 In a homogeneous lattice deformation one Bravais Lattice is
converted to another by the coordinated shift of atoms.
 A shuffle is a coordinated shift of atoms within a unit cell, which
may change the crystal lattice but does not produce
homogeneous lattice distortive strain.

171. Types of Martensite
 There are two types of martensite classified according to
morphology:
- Lath martensite
- Plate martensite
A) Lath martensite
• Has shape of a strip , length is greatest dimension
• Are grouped together in the form of parallel packets
• Lath martensite has high dislocation density and low angle
boundaries
• Slip is the main mode of dislocation
• Formed when Ms temperature is high
• Formed in medium or low carbon steels

172. B) Plate matensite
• Forms in the shape of plates or lenses (acicular or lenticular)
• The structure resembles mechanical twins
• Twinning is predominant form of dislocation
• Formed at low Ms temperature
• Formed in high carbon or high alloy steels.
• High Carbon steels shows such martensite having carbon
percentage

173. Lath Martensite Plate Martensite

174. References
1. Phase transformation book by Porter Estering.
2. Physical Metallurgy, by Vijendra Singh
3. Material Science and Engineering, by Callister.
4. Heat treatment, principle and techniques, by Rajan Sharma
and Sharma
5. Modern physical Metallurgy by Smallman and Bishop.