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8/11/2019 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



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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



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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.



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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 sizesize 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).



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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


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



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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



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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




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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


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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