phasetransformationedited ppt1 130127140500 phpapp01
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PH SE TR NSFORM TION
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Why do we study phase transformations?
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 beobtained 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.
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phase transformations
Most phase transformations begin with the formation of numerous smallparticles 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 80-300C).
Heterogeneous nucleation - form at structural inhomogeneities (containersurfaces, impurities, grain boundaries, dislocations) in liquid phase much
easier since stable nucleating surface is already present; requires slight
supercooling (0.1-10C ).
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Thermodynam ics and k inect ics
o f PHASE TRANSFORMATION
What does lie underneath the structure
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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 tooccur.
Undercooling:It is the gap between the temp predicted for the
transformation to occur and the temp at which the transformation
actually occurs.
phase transformation
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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.
Thedriving force to nucleate increases as Tincreases
Small supercoolingslow nucleation rate - few nuclei - large crystals
Largesupercoolingrapid nucleation rate- many nuclei - small crystals
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Nucleation of a spherical solid particle in a liquid
The change in free energy G(a function of the internal energy and
enthalpy of the system) must be negativefor 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 phaseAlso, 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, GV); and 2) the
solid-liquid phase boundary (surface free energy, GS).
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Transforming one phase into another takes time.
Fe
g(Austenite)
Eutectoidtransformation
C FCC
Fe3C(cementite)
a(ferrite)
+
(BCC)
G = GS+ GV
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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 thenew phase is developed and has lower energy than the B phase.
Fv=Vf
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.
Fs = s
s = surface area of the new crystal
= free energy per unit area
phase transformation
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phase transformation
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If rate kinetics of phase transformation is increased then the structurewill 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.
o=+ Ka(-1/2)
Hall-Petch EquationWhere, o= Friction stress
= in stress
a = grain size
K= locking parameter
phase transformation
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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.
Solid state transformation
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Solid state transformation
Therefore, the certain elastic component Fel makes a +ve
contribution to the free energy change in the solid state
transformation
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Tm>>Tp
Reason: Elastic strain energy
component
AM 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.
Martensite transformation temp is much lower than Pearlite
transformation temp??
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5
Reaction rate is a result of nucleation and growth of crystals.
Examples:
Nucleation and Growth
T just below TE T moderately below TE T way below TE
g g g
pearlitecolony
% Pearlite
0
50
10 0
Nucleation
regime
Growthregime
log (time)t50
Growth rate increases w/ T
Nucleation rate increases w/T
Nucleation rate low
Growth rate is high
Nucleation rate medium
Growth rate is medium
Nucleation rate high
Growth r ate is low
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2
Fract ion transformed depends on time.
Transformation ratedepends on T.
roften small:equil not possible
FRACTION OF TRANSFORMATION
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Coarse pearlite formed at higher temperaturesrelatively soft
Fine pearlite formed at lower temperaturesrelativelyhard
Transformation of austenite to pearlite:
gaaaa
a
a
pearlitegrowth
direction
Austenite (g)grainboundary
cementite (Fe3C)Ferrite (a)
g
For this transformation,
rate increases with ( T)
[TeutectoidT ].675C
(Tsmaller)
0
50
%pearlite
600C
(T larger)650C
100
Diffusion of C
during transformation
a
a
gg
aCarbon
diffusion
Eutectoid Transformation rate ~ T
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Based on
Masstransport
PHASETRANSFORMATIONS
Diffusional
transformation
Diffusion less military
transformation
Based on
Order
PHASETRANSFORMATIONS
Ist order nucleation
and growth
2ndorder entire
volume transforms
No change incomposition
Change incomposition
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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 transformationPolymorphic changes
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Precipitation Transformations: Generally expressed as +
where is a metastable supersaturated solid solution
is a stable or metastable precipitate
is a more stable solid solution with the same crystal structure as but composition closer to equilibrium
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Eutectoid Transformations: Generally expressed as +
Metastable phase () replaced by a more stable mixture of +
Precipitation and eutectoid transformations require compositional
changes in the formation of the product phase and consequentlyrequire long-range diffusion
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Ordering Transformations: Generally expressed as (disordered)
(ordered) . These do not require long range diffusion
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Massive Tranformations: Generally expressed as
Original phase decomposes into one or more new phases which
havethe same composition as the parent phase but different crystal
structures
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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
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Strength
Ductility
MartensiteT Martensite
bainitefine pearlite
coarse pearlitespheroidite
General Trends
Possible Transformations
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Time Temperature
Transformation(TTT) curves
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%C
T
Fe Fe3C6.74.30.80.16
2.06
Peritectic
L + g
Eutectic
L g+ Fe3C
Eutectoidg a+ Fe3C
L
L +g
g
g
+ Fe3C
1493C
1147C
723C
0.025 %C
0.1 %C
+ Fe3C
Iron-Iron Carbide phase diagram
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WHAT ARE TTT CURVES
T (Time) T(Temperature) T(Transformation) diagram is aplot 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 apreviously 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 isachieved.
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Austenite
Austenite
Pearlite
Pearlite + Bainite
Bainite
Martensite100
200
300
400
600
500
800
723
0.1 1 10 10
2
10
3
10
4
105
Eutectoid temperature
Not an isothermal
transformation
Ms
Mf
Coarse
Fine
t (s)
T
Eutectoid steel
Time- Temperature-Transformation (TTT) CurvesIsothermal
Transformation
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The dependance of transformation to temperature and time can be
analyzed best using the diagram below:
2 solid curves are plotted:
one represents the timerequired at each
temperature for the start o
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
supercooledto below the
eutectoid temperature
(727C).
Time for process to complet
depends on the
temperature.
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WHY TTT CURVE HAS A C- SHAPE
The transformation of austenite doesnot start immediatelyon quenching the austenised sample to a constant
temperature bath
Transformation of the austenite to its product occurs after a
definite time intervalincubation period Incubation period is that period in which transformation
doesnot proceed because enough diffusion has not taken
placein austenite for the transformation to start
Larger
incubationperiod
Greaterstability ofaustenite
Sloweraustenite
decomposition
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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.
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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
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EFFECT OF CARBON ON THE TTT CURVES
Carbon has significant effects on the nature of the TTT
curves Carbon is an austenitestabilizer
HYPOEUTECTOID STEELS
Ferrite is the nucleating phase on decomposition ofaustenite
As carbon increases from 0 to 0.77% :
EUTECTOID STEELS
Have the maximum incubation period
Ferrite contentdecreases
Incubation periodincreases
Nose of S curvemove more
towards the right
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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 entirerange
Bainite transformation is uniformly retarted
Cementitecontent
increases
Incubationperiod
decreases
Nose of S curvemoves more
towards the left
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Tempe
ratureoC
Ms
Proeutectoid
phase starts to
form on this line
A +F
AF + P
Pearlite reaction starts
Ac1
Ms
Ms Ms
A+P
P Fe3
C +P
Fe3C +A
Proeutectoid
cementite starts
to form on thisline
BB
TTT curves for hypo , eutectoid and hyper-eutectoid steels
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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 is reduced by solutes
Bainitic transformation is lesser affected ( no redistribution of
alloying elements)
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EFFECT OF GRAIN SIZE ON THE TTT CURVES
All decomposition products of austenite nucleateheterogenously at grain boundaries
Thus incubation period is reduced for fine grained steel
S curve is more towards the left in fine grained steel
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MARTEMPERING
To avoid residual stresses generated during quenching
Austenized steel is quenched above Ms (20-30
o
C above Ms i.e.180250oC)
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 austenitelesser volume change
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Austenite
Pearlite
Pearlite + Bainite
Bainite
Martensite100
200
300
400
600
500
800
723
0.1 1 10 102 103 104 105
Eutectoid temperature
Ms
Mf
t (s)
T
+ Fe3C
Martempering
Figure shows the process of Martempering and the
characteristic temperatures:
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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
(300oC400oC)
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 avoidtrasformation to pearlite during quenching and holding
Steels shouldnot have a long bainitic bay ( to avoid long
transformation times)
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HOW TO DRAW TTT CURVE
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Continuous Cooling Transformation (CCT)
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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.
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I th b fi M d t l id d l li
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In the above figure Moderately rapid and slow cooling curvesare
superimposed on a continuous cooling transformation diagram of a eutectoid
iron-carbon alloy.
The transformation starts after a time period corresponding to theintersection of the cooling curve with the beginning reaction curve and ends
upon crossing the completion transformation curve.
Normallybainite 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
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For continuous cooling
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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 pearlitetransformation begins
Continuous cooling diagram for a 4340 steel alloy and several cooling
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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 thetransformations that occur during cooling.
Alloying elements used to modify the critical cooling rate for
martensite are
chromium,
nickel,
molybdenummanganese
silicon
tungsten
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Effect of adding other
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Other elements (Cr, Ni, Mo, Si andW) 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;
Shift the pearlite and bainite noses
to longer times (decrease critical
cooling rate);
Form a separate bainite nose;
4340 Steel
plain
carbon
steel
nose
Plain carbon steel: primary
alloying element is carbon.
Effect of adding other
elements
An actual isothermal heat treatment curve on the isothermal
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An actual isothermal heat treatment curve on the isothermal
transformation diagram:
rapid cooling
isothermal treatment
Eutectoid iron-carbon al loy;composition, Co= 0.76 wt% C
Begin at T > 727C
Rapidly cool to 625C and hold isothermally.
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AUSTENITEfrom where it all starts..
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AUSTENITE Austenite, also known as gamma phase iron(-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
Why is Austenizing So Important In
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Heat Treatment of Any Steel?
Austenite can transform into various products
depending on the composition and coolingrates.
Morphology of parent austenite(grain size)
decides the morphology of products and thus
Formation of Austenite
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Formation of Austenite
Austenite is formed on heating an aggregate of pearlite, pearliteand ferrite , pearlite and cementite
Pearlite Austenite
Eutectoid compositiontransforms at a particular (Ac1)temperature
1ststep: ( On heating to eutectoid temperature)
Lattice changesBCC iron (-Fe) FCC iron (-Fe)
2ndstep:
Diffusion of carbon from Cementite (6.67% carbon) to adjoingregions
o Inspite of the carbon gradient the structure is thermodynamicallystable at room temperature due to the low diffusion rate of carbonat low temperatures and occurs only at sufficiently hightemperatures
The maximum diffusion of carbon takes place from cementite
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at ferritecementite 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 ofdissolution of cementite
Thus dissolution of ferrite is complete before that of cementite
-Fe
Fe3C
Fe3C -Fe
Austenite
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Homogenization of austenite
The austenite formed from cementite and ferrite isgenerally 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
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Kinetics of Austenite Formation
The formation of austenite on heating occurs bynucleation and growth
The factors that affect nucleation rate or growthrate affect the kinetics of the transformation
The kinetics depends on:Transformation temperature and holding time
Rate of heating
Interface between ferrite and cementite
Grain sizeNature of the alloying elements present
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Transformation Temperature
Austenite transformation occurs at a temperature higher than Ac1
in the Fe-Cementite phase diagramSuperheating
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
R t f h ti
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Rate of heating :
For higher rates of heating, transformation
starts at higher temperatures and for slowerrates, 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 theregion in between the curves indicates the
majority of the tranformation.
Interface between ferrite and cementite
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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 ferriteand cementite
The closer the ferritecementite lamellae, the higher isthe rate of nucleation.
Carbon atoms have to diffuse to smaller distances fromcementite to low carbon regions to form austenite
Increasing the cementite or carbon content
This will lead to more pearlite content in steels and thusmore interfaces.
Examples :1. High carbon steels austenize faster than lowcarbon steels
2. Tempered martensite structure austenizes faster thancoarse paerlite
3. Spheroidal pearlite takes longer time to austenize due
Grain size
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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
Nature of the alloying elements present
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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 theamount and nature of alloying element
Why does the Fe-Cementite diagram show a fall in the Ac3
temperature and rapid rise in Acm temperature with
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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 areabetween ferrite and cementite
Thus Ac3 temperature line decreases continuously
with increasing carbon content
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In hypereutectoid steels , austenization processbecomes much more difficult as the amount of
carbon increases
Austenisation of free cementite needs very high
temperature as it involves the diffusion of largeamount of carbon( from cementite) to become
homogenous
Thus as carbon content increases, amount of free
cementite increases, which needs highertemperature to austenize.
Thus Acm line is so steep
Austenite Grain Size
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Original grain size- size of austenite grains as
formed after nucleation and growth
Actual grain sizesize of the austenitic grains
obtained after homogenization at higher
temperatures
Generally grain size is referred to as actualgrain size
Depending on the tendency of steel to grain
growth, steels are classified into two groups:
Inherently fine grained Inherently coarse grained
Inherently fine grain steels resist grain growth with increasing
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temperature till 1000oC1050oC
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
Grainsize
Inherently fine grain
Inherently coarse grain
Presence of ultramicroscopic particles like oxides,
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p p ,
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 workedat high temperatures without getting coarsened
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Effect of grain size on mechanical properties
Austenite grain size plays a very important role indetermining the properties of the steel
The effect of grain size on different properties are givenbelow:
YIELD STRESS
The dependence is given by Hall-Petch equation :
Where is the yield stress
is the frictional stress opposing motion ofdislocation
K is the extent to which dislocations are piled atbarriers
D is the avg grain diameter
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Grain refinement improves the strength and ductility at the sametime
IMPACT TRANSITION TEMPERATURE
Increase in grain size raises the impact transition temperature, somore prone to failure by brittle fracture
CREEP STRENGTH
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Coarse grained steel has better creep strength aboveequicohesive 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 givesless sites for effective diffusion, so martensite formation oncooling is favoured)
MACHINABILITY
Coarse grain structure has better machinability due to ease indiscontinuos chip formation(low toughness)
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PEARLITIC TRANSFORMATION
Pearlite
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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.
P li
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Ferrite has a very low carbon content whereas cementite Fe3C isan 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.
Pearlite
Transformation rate ~ T
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Coarse pearlite formed at higher temperaturesrelatively soft
Fine pearlite formed at lower temperaturesrelativelyhard
Transformation of austenite to pearlite:
gaaaa
a
a
pearlitegrowthdirection
Austenite (g)grainboundary
cementite (Fe3C)Ferrite (a)
g
For this transformation,
rate increases with ( T)
[TeutectoidT ].675C
(Tsmaller)
0
50
%
pearlite
600C
(T larger)650C
100
Diffusion of C
during transformation
a
a
gg
aCarbon
diffusion
The layer thickness depends on temperature at which the isothermal
transformation occurs For example at T just below the eutectoid
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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 thevicinity of 5400C. This structure is called fine pearlite.
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MORPHOLOGY
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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 lamell is a single crystal andsome 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 orgrains, there are net-works of dislocations or dislocation walls, at
each node of wich a cementite rod is present.
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MECHANISM
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HULL MEHL MODEL
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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.
HULL-MEHL MODEL
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This process of formation of alternate plates of ferriteand 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
HULL-MEHL MODEL
F igures showing coarse and fine pearl i te
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- Smaller T:
colonies are
larger
- Larger T:
colonies are
smaller
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Mechanism
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Hull-Mehl Mechanism for pearlitic transformation
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KINETICS
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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
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Kinectics of transformation (contd)
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Free energy of pearlite is less at lower tem and so stability isincreased by increasing T.
The decomposition of austenite to pearlite proceeds by the
redistribution of carbon atoms of austenite into ferrite and
cementite, and is essentially a diffusion controll ed process.
The rate of diffusion decreases exponentially with decreasingtemp
This shows lower the transformation temp retards the rate of
transformation.
There is a transformation temp for which diffusion of C atoms istoo small resulting in diffusion controlled transformation
Rate of diffusion of carbon atoms is negligible below 200 C
Kinectics of transformation (contd).
Kinectics of transformation (contd)
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This shows that undercooling affects the rate of transformation in2 ways:
increased degree of undercooling
increases the transformation rate
by providing greator difference
in free energies of austenite andpearlite.
increased degree of
undercooling reduces the
transformation rate by
lowering the rate ofcarbon diffusion curve.
Undercooling
Kinectics of transformation (contd).
The combined effect is shown in the curve below:
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Where (a) is rate of crystal growth and (b) is rate of nucleation
Kinectics of transformation (contd)
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The austenite to pearlite transformation is completed bynucleation 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
Kinectics of transformation (contd).
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Effect of degree of on the rates of nucleation and growthUndercooling
Hardness of pearlite increases as S decreases and also same
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Hardness of pearlite increases as S0 decreases and also same
for strength.
As S0is 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 providessites for easy nucleation of cracks
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Effect of alloying additions on
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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 transformationwill 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.
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BAINITIC TRANSFORMATION
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Bainiteis an acicularmicrostructure (not a phase) that forms in steels
at temperatures from approximately 250-550C (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 facecentered cubiccrystal 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.
http://en.wikipedia.org/wiki/Acicular_(crystal_habit)http://en.wikipedia.org/wiki/Edgar_Bainhttp://en.wikipedia.org/wiki/Austenitehttp://en.wikipedia.org/wiki/Face_centered_cubichttp://en.wikipedia.org/wiki/Face_centered_cubichttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Martensitehttp://en.wikipedia.org/wiki/Martensitehttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Face_centered_cubichttp://en.wikipedia.org/wiki/Face_centered_cubichttp://en.wikipedia.org/wiki/Austenitehttp://en.wikipedia.org/wiki/Edgar_Bainhttp://en.wikipedia.org/wiki/Acicular_(crystal_habit) -
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A fine non-lamellar structure, bainite commonly consists of
cementiteand 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 (250-550C) isbetween those forpearliteand martensite.
When formed during continuous cooling, the cooling rate to form
bainite is more rapid than that required to form pearlite, but less rapidthan is required to form martensite (in steels of the same composition).
http://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Ferritehttp://en.wikipedia.org/wiki/Pearlitehttp://en.wikipedia.org/wiki/Martensitehttp://en.wikipedia.org/wiki/Martensitehttp://en.wikipedia.org/wiki/Pearlitehttp://en.wikipedia.org/wiki/Ferritehttp://en.wikipedia.org/wiki/Cementite -
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Most alloying elements will lower the temperature required for
the maximum rate of formation of bainite, though carbonis 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 sharingmany aspects of their transformation mechanisms
However, morphological differences do exist that require
a TEMto see. Under a simple light microscope, themicrostructure of bainite appears darker than martensite due to
its low reflectivity.
http://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Transmission_electron_microscopyhttp://en.wikipedia.org/wiki/Light_microscopehttp://en.wikipedia.org/wiki/Reflectivityhttp://en.wikipedia.org/wiki/Reflectivityhttp://en.wikipedia.org/wiki/Light_microscopehttp://en.wikipedia.org/wiki/Transmission_electron_microscopyhttp://en.wikipedia.org/wiki/Carbon -
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Illustration of Continuous cooling transformation diagram showing
Bainite
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MECHANISM
Mechanism of Baini tic transformation
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In the TTT curve the incubation period
thetransformation 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 Thisshows along with diffusion some other
mechanism is responsible for the transformation to
occur
Sinceformation of bainite is accompanied by surfacedistortion so some shear mechanism is responsible
for its transformation
So it isa complex one and involves both diffusionless
and diffusion controlled phenomena are involved
Mechanism of Baini tic transformation
Mechanism of Baini tic transformation
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Two mechanisms are thought to be for theBainite formation:
1. Displacive theory
2. Diffusion theory
Bainite is considered to be formed by diffusionlessdiffusion controlled transformation.. Both play a part
in its transformation
Mechanism of Baini tic transformation
Diffusive theory
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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.
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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 inAustenite. 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
Displacive theory
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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 isconfirmed by the orientation relationships present in bainiticmicrostructures.
There are, however, similar stress-relief effects seen in transformationsthat are not considered to be martensitic in nature, but the term 'similar'does not imply identical.
The relief associated with bainite is an invariantplane strain with alarge shear component. The only diffusion that occurs by this theory isduring the formation of the carbide phase (usually cementite) between theferrite plates.
Now the low carbon austenite region transform to ferrite(Bainiticplate) by diffusionless shear process. So It is important to know
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p ) y p phere that low C Austenite which transform by shear process is itselfa diffusion controlled process.
precipitation of carbide may occur from the C enriched Austeniticregion depending on the degree of saturation.
The C depleted A region obtained by the precipitation of carbidenow 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 highsolubility of carbon, the transformed ferrite will be supersaturatedwith C
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The degree of supersaturation increases withdecrease 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 getdifferent morphologys of Bainite.
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MORPHOLOGY
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Upper Bainite
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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.
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Schematic growth mechanism of Upper Bainite
Upper bainite in medium carbon steel
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Lower Baini te
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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 ofbainite
Carbides can be cementite or -carbide, or a mixture depending on
temperature of transformation and composition of steel
Lower Baini te
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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 thatforms at low temperatures of the same alloy
Alloying elements do not diffuse or form their carbides during
bainite transformation
Lower Baini te
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Lower Bainite structure in
medium carbon steel
Stages of formation of Lower Bainite
Schematic representation of lower bainite structure
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MARTENSITICTRANSFORMATION
Martensite - BCT
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Martensite transformation is a diffusion-less transformation
Mechanism
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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 beobserved as relief structure on the surface of martensite
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Figure shows mechanism martensite plate formation
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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 coherentto the surrounding austenite.
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The figure shows how the martensite
remains macroscopically coherent to
parent austenite on transformation
A large amount of driving force is needed for the martensitictransformation
The magnitude of the driving force is provide by the free
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The magnitude of the driving force is provide by the freeenergy change accompanying the transformation
The magnitude of the driving force for nucleation ofmartensite at the Ms can be as follows:
The figures above demonstrate the equation given above
oThe graphs along side showthat magnitude of the driving
force increases with decrease
in the temperature of
transformation
Crystal Structure of Martensite
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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 - Fe transforms to bcc -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
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KINECTICS OF
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KINECTICS OF
TRANSFORMATION
Kinetics of Martensite Transformation
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The transformation starts at a definite temperatureMs ( 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
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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
Cooling below Mf doesnot change the amount of martensite.
The velocity of the martensite transformation, in general, is
independent of the transformation temperat re
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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
Important characteristics of Martensite
Transformation
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Diffusionless/Military tranformation
Athermal transformation.
Retained Austenite
MsMf temp
Reversibility of transformation
Habit planes
Bain distortion
Effect of applied stress on transformation
Hardness of Martensite
f
Ms and Mf Temperature
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Martensite transformation begins as the Mstemperature is reached
and ends at the Mf temperature
The Mstemperature 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
Ms and Mf Temperature
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The relationship between Ms temperature and the chemical
composition can be shown as:
Ms (oC)=561474(%C)33(%Mn)17(%Ni) 17(%Cr)
21(%Mo).
The above shows that nearly all elements lower the Mstemperature 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
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Effect of carbon content on Ms and Mf temp
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Effect of alloying additions on Ms temp
Reversibility of Martensite
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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
Most steels dont satisfy this condition
Since Martensite in
steels is supersaturated
solid solution of
carbon in alpha iron
and it decomposes at avery rapid rate on
heating
Retained Austenite
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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.
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Amount of retained austenite increases with decreased
martensite temp of transformation
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If while the transformation process within the Ms-Mf temperature the
ooling is stoppedthe transformation halts
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On resuming the cooling the transformation doesnot start instantly but
needs supercooling Larger amount of retained austenite formed at Mfcalled stabilized
austenite
Martensite can also form isothermally.
Isothermally transformed martensite quantity is low.
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y q y
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.
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The model was proposed by E.C. Bain
Bain Distortion model
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Any simple homogeneous pure disyortion of the nature which
converts one lattice to another by expansion and contractionalong 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
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
A l t d it ll f th b t t b d ithi t
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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
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This model explains the transformation of martensite from
austenite with minimum movement of atoms
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Thus carbon atoms are finally present only in the middle of
the edges along [001]axis and not in the middle of the edgeswhich represent the a-axis
Habit planes
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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)
An micrograph of austenite that was polished flat and then allowed totransform into martensite.
The different colors indicate the displacements caused when martensite
forms
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forms.
Hardness of Martensite
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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%)
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
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retained austenite
Steel becomes softer as retained austenite increases Two suspected factors for enhanced hardness
a) internal strains within -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
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MORPHOLOGY
Morphology of Martensite
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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
Martensitic transformations are (usually) first order,
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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.
Types of Martensite
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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
B) Plate matensite
Forms in the shape of plates or lenses (acicular or lenticular)
The structure resembles mechanical twins
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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
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Lath Martensite Plate Martensite
References
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1. Phase transformation book by Porter Estering.
2. Physical Metallurgy, by Vijendra Singh3. 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.
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