Phase transformation edited.ppt1

PHASE TRANSFORMATION

Why do we study phase transformations?

The tensile strength of an Fe-C alloy of eutectoid composition can be varied between 700-2000 MPa depending on the heat treatment process adopted.

This shows that the desirable mechanical properties of a material can be obtained as a result of phase transformations using the right heat treatment process.

In order to design a heat treatment for some alloy with desired RT properties, time and temperature dependencies of some phase transformations can be represented on modified phase diagrams.

phase transformations

Most phase transformations begin with the formation of numerous small particles of the new phase that increase in size until the transformation is complete.

Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth.

Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable supercooling (typically 80-300°C).

Heterogeneous nucleation - form at structural inhomogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable “nucleating surface” is already present; requires slight supercooling (0.1-10ºC ).

Thermodynamics and kinectics of PHASE TRANSFORMATION

What does lie underneath the structure……..

Phase transformation is predominantly controlled by TEMP. But transformation never really start at transformation temp rather it starts at a temp much below the temp predicted for the transformation to occur.

Undercooling: It is the gap between the temp predicted for the transformation to occur and the temp at which the transformation actually occurs.

phase transformation

Supercooling

During the cooling of a liquid, solidification (nucleation) will begin only after the temperature has been lowered below the equilibrium solidification (or melting) temperature Tm. This phenomenon is termed supercooling (or undercooling.

The driving force to nucleate increases as T increases

Small supercooling slow nucleation rate - few nuclei - large crystals

Large supercooling rapid nucleation rate - many nuclei - small crystals

Nucleation of a spherical solid particle in a liquid

The change in free energy ΔG (a function of the internal energy and enthalpy of the system) must be negative for a transformation to occur.

The Assume that nuclei of the solid phase form in the interior of the liquid as atoms cluster together-similar to the packing in the solid phase

Also, each nucleus is spherical and has a radius r.

Free energy changes as a result of a transformation: 1) the difference between the solid and liquid phases (volume free energy, ΔGV); and 2) the solid-liquid phase boundary (surface free energy, ΔGS).

Allotropic / polymorphic transformation: No change in composition of the structurePhase transformation: Change in crystal structure+ Change in composition.

•Surface creations always hinders the process of transformation. The new phase always trys to create the surface, so energy needs to be supplied. So volume free energy will try to decrease the energy but surface free energy will try to increase the energy.


Transforming one phase into another takes time.

Fe

(Austenite)

Eutectoid transformation

C FCC

Fe3C(cementite)

(ferrite)

+(BCC)

G = GS + GV

• In the previous fig it can be observed that as soon as the particles of A phase are formed the free energy of the system should decrease the new phase is developed and has lower energy than the B phase.

ΔFv=VΔf V= Vol of the new crystal f=free energies of the new phase • formation of the new crystal is linked with the interface between the

new and initial phases. ΔFs = sνs = surface area of the new crystalν = free energy per unit area


• If rate kinetics of phase transformation is increased then the structure will be finer and this is indicated by the Hall - Petch equation States that decrease in grain size and with fineness in the structure the strength in increased.

δo =δ + Ka (-1/2) → Hall-Petch Equation

Where, δo = Friction stress

δ = in stress

a = grain size

K= locking parameter


• 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

Solid state transformation• Therefore, the certain elastic component ΔFel makes a +ve

contribution to the free energy change in the solid state transformation

ΔTm>>ΔTp Reason: Elastic strain energy component

A→M leads to volumetric expansion which leads to straining of the lattice and hence a +ve component in the free energy. To compensate this +ve component an undercooling is there. So temp of transformation is so low.

Martensite transformation temp is much lower than Pearlite transformation temp??

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

pearlite colony

% Pearlite

0

50

100

Nucleation regime

Growth regime

log (time)t50

Growth rate increases w/ TNucleation rate increases w/T

Nucleation rate lowGrowth rate is high

Nucleation rate mediumGrowth rate is medium

Nucleation rate highGrowth rate is low

2

• Fraction transformed depends on time.

fraction transformed time

y1 e ktn

Avrami Eqn.

• Transformation rate depends on T.

1 10 102 1040

50

100 135°

C11

9°C

113°

C10

2°C

88°C

43°Cy (%)

log (t) min

Ex: recrystallization of Cu

r 1t0.5

Ae Q /RT

activation energy

• r often small: equil not possible

FRACTION OF TRANSFORMATION

Coarse pearlite formed at higher temperatures – relatively soft

Fine pearlite formed at lower temperatures – relatively hard

• Transformation of austenite to pearlite:

pearlite growth direction

Austenite ()grain boundary

cementite (Fe3C)Ferrite ()

• For this transformation, rate increases with ( T) [Teutectoid – T ].

675°C (T smaller)

0

50

% p

earli

te600°C

(T larger)650°C

100

Diffusion of C during transformation

Carbon diffusion

Eutectoid Transformation rate ~ ΔT

Based onMass

transport

PHASE TRANSFORMATIONS

Diffusional

transformation

Diffusion less military

transformation

Based onOrder

PHASE TRANSFORMATIONS

Ist order nucleation and growth

2nd order entire volume transforms

No change in composition

Change in composition

Diffusion-less transformation in solids

Major phase transformations that occur in solid phase are due to thermally activated atomic movements

The different types of phase transformation that is possible can be divided into 5 groups:

► Precipitation Transformation

► Eutectoid transformation

► Ordering reactions

► Massive transformation

► Polymorphic changes

Precipitation Transformations: Generally expressed as α’→ α + βwhere α’ is a metastable supersaturated solid solutionβ is a stable or metastable precipitateα is a more stable solid solution with the same crystal structure as α’ but composition closer to equilibrium

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

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

Massive Tranformations: Generally expressed as β→ α

Original phase decomposes into one or more new phases which have the same composition as the parent phase but different crystal structures

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

Str

engt

h

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends

Possible Transformations

Time Temperature Transformation(TTT) curves

%C →

T →

Fe Fe3C6.74.30.80.16

2.06

PeritecticL + →

EutecticL → + Fe3C

Eutectoid → + Fe3C

L

L +

+ Fe3C

1493ºC

1147ºC

723ºC

0.025 %C

0.1 %C

+ Fe3C

Iron-Iron Carbide phase diagram

WHAT ARE TTT CURVEST (Time) T(Temperature) T(Transformation) diagram is a

plot of temperature versus the logarithm of time for a steel alloy of definite composition.

It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy

TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

Austenite

Austenite

Pearlite

Pearlite + Bainite

Bainite

Martensite100

200

300

400

600

500

800

723

0.1 1 10 102 103 104 105

Eutectoid temperature

Not an isothermal

transformation

Ms

Mf

Coarse

Fine

t (s) →

T →

Eutectoid steel

Time- Temperature-Transformation (TTT) Curves – Isothermal Transformation

The dependance of transformation to temperature and time can be analyzed best using the diagram below:

2 solid curves are plotted: one represents the

time required at each temperature for the start of the transformation;

the other is for transformation completion.

The dashed curve corresponds to 50% completion.

The austenite to pearlite transformation will occur only if the alloy is supercooled to below the eutectoid temperature (727˚C).

Time for process to complete depends on the temperature.

WHY TTT CURVE HAS A C- SHAPE…The transformation of austenite doesnot start immediately

on quenching the austenised sample to a constant temperature bath

Transformation of the austenite to its product occurs after a definite time interval – incubation period

Incubation period is that period in which transformation doesnot proceed because enough diffusion has not taken placein austenite for the transformation to start

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.

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

EFFECT OF CARBON ON THE TTT CURVESCarbon has significant effects on the nature of the TTT

curvesCarbon is an austenite – stabilizer HYPOEUTECTOID STEELSFerrite is the nucleating phase on decomposition of

austeniteAs carbon increases from 0 to 0.77% :

EUTECTOID STEELSHave the maximum incubation period

HYPEREUTECTOID STEELSCementite is the nucleating phase As the carbon content increases more than 0.77%:

BAINITEFerrite is the nucleating phaseS curve uniformly shifts towards the right in entire

rangeBainite transformation is uniformly retarted

Tem

pera

ture

o C

Ms

Proeutectoid phase starts to form on this line

A +F

A F + P

Pearlite reaction starts

Ac1

Ms

Ms Ms

A+P

P Fe3C +P

Fe3C +A

Proeutectoid cementite starts to form on this line

BB

TTT curves for hypo , eutectoid and hyper-eutectoid steels

EFFECT OF ALLOYING ELEMENTS ON THE TTT CURVES

All alloying elements (except Co) shift the S curve to the rightAustenite stabilizers move the curve to the right( Mn, Ni,etc)Carbide formers shift the S curve further to the right because: Diffusion of alloying elements is too slow(substitutional

elements) Diffusion of carbon is slower as carbide formers donot easily

part with the carbon Allotropic change γ α is reduced by solutesBainitic transformation is lesser affected ( no redistribution of

alloying elements)

EFFECT OF GRAIN SIZE ON THE TTT CURVES

All decomposition products of austenite nucleate heterogenously at grain boundaries

Thus incubation period is reduced for fine grained steel

S curve is more towards the left in fine grained steel

MARTEMPERING

To avoid residual stresses generated during quenching Austenized steel is quenched above Ms (20-30oC above Ms i.e.

180 – 250oC) Holding in salt bath for homogenization of temperature across

the sample (large holding time is avoided to avoid forming bainite)

The steel is then quenched in air and the entire sample transforms simultaneously

Tempering follows The process is called Martempering The process is beneficial as: Steep temperature gradient is minimized Thermal and structural stresses are minimal More retained austenite – lesser volume change

AustenitePearlite

Pearlite + Bainite

Bainite

Martensite100

200

300

400

600

500

800

723

0.1 1 10 102 103 104 105


Ms

Mf

t (s) →

T →

+ Fe3C

Martempering

Figure shows the process of Martempering and the characteristic temperatures:

AUSTEMPERING To avoid residual stresses, distortion and cracks generated during

quenching in high carbon steels Austenized steel is quenched in molten salt bath above Ms

(300oC – 400oC) Held long enough for isothermal transformation to lower Bainite No tempering is done This process is termed as Austempering Equalization of temperature across cross-section minimizes the

stress development The steels should have sufficient hardenability to avoid

trasformation to pearlite during quenching and holding Steels shouldnot have a long bainitic bay ( to avoid long

transformation times)

Advantages: Improved ductility with same hardness Elimination of distortion and cracks No tempering required Impact strength is improved Uniformity in properties High endurance limit

AustenitePearlite

Pearlite + Bainite

Bainite

Martensite100

200

300

400

600

500

800

723

0.1 1 10 102 103 104 105


Ms

Mf

t (s) →

T →

+ Fe3C

Austempering

HOW TO DRAW TTT CURVE

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.

In the above figure Moderately rapid and slow cooling curves are superimposed on a continuous cooling transformation diagram of a eutectoid iron-carbon alloy.

The transformation starts after a time period corresponding to the intersection of the cooling curve with the beginning reaction curve and ends upon crossing the completion transformation curve.

Normally bainite does not form when an alloy is continuously cooled to room temperature; austenite transforms to pearlite before bainite has become possible

The austenite-pearlite region (A---B) terminates just below the nose. Continued cooling (below Mstart) of austenite will form martensite

For continuous cooling of a steel alloy there exists a critical quenching rate that represents the minimum rate of quenching that will produce a totally martensitic structure.

This curve will just miss the nose where pearlite transformation begins

Continuous cooling diagram for a 4340 steel alloy and several cooling curves superimposed in the figure below

This demonstrates the dependence of the final microstructure on the transformations that occur during cooling.

Alloying elements used to modify the critical cooling rate for martensite are

►chromium,

► nickel,

► molybdenum

► manganese

► silicon

► tungsten

Other elements (Cr, Ni, Mo, Si and W) may cause significant changes in the positions and shapes of the TTT curves:

Change transition temperature; Shift the nose of the austenite-to-

pearlite transformation to longer times;

Shift the pearlite and bainite noses to longer times (decrease critical cooling rate);

Form a separate bainite nose;

4340 Steel

plain carbonsteel

nose

Plain carbon steel: primary alloying element is carbon.

Effect of adding other elements

An actual isothermal heat treatment curve on the isothermal transformation diagram:

rapid cooling

isothermal treatment

• Eutectoid iron-carbon alloy; composition, Co = 0.76 wt% C• Begin at T > 727˚C• Rapidly cool to 625˚C and hold isothermally.

AUSTENITEfrom where it all starts…..

AUSTENITEAustenite, also known as gamma phase iron (γ-Fe),

is a metallic, non-magnetic allotrope of iron or a solid solution of iron with carbon.

It has an FCC crystal structureThe maximum solubility of carbon in austenite is 2.13

% at 1147oC

Why is Austenizing So Important In Heat Treatment of Any Steel?Austenite can transform into various

products depending on the composition and cooling rates.

Morphology of parent austenite(grain size) decides the morphology of products and thus its properties.

Formation of Austenite Austenite is formed on heating an aggregate of pearlite, pearlite

and ferrite , pearlite and cementitePearlite Austenite Eutectoid composition transforms at a particular (Ac1)

temperature 1st step: ( On heating to eutectoid temperature)Lattice changes BCC iron (α-Fe) FCC iron (γ-Fe)2nd step: Diffusion of carbon from Cementite (6.67% carbon) to adjoing

regionso Inspite of the carbon gradient the structure is thermodynamically

stable at room temperature due to the low diffusion rate of carbon at low temperatures and occurs only at sufficiently high temperatures

The maximum diffusion of carbon takes place from cementite at ferrite –cementite interface

Austenite nucleates at interfaces between ferrite and cementite, specially in between pearlitic colonies

By gradual dissolution of carbon from cementite austenite is formed

The primary austenite formed dissolve the surrounding ferrite and grow at their expense.

The growth rate of austenite is higher than the rate of dissolution of cementite

Thus dissolution of ferrite is complete before that of cementite

α-Fe

Fe3C

Fe3C α-Fe

Austenite

Homogenization of austeniteThe austenite formed from cementite and

ferrite is generally not homogenousHomogenization requires high

temperature/time , or both High temperatures if the rate of heating is

fasterShorter time spread over a large range of

temperatures if the rate of heating is slower

Kinetics of Austenite FormationThe formation of austenite on heating occurs

by nucleation and growthThe factors that affect nucleation rate or

growth rate affect the kinetics of the transformation

The kinetics depends on:Transformation temperature and holding

timeRate of heating Interface between ferrite and cementiteGrain sizeNature of the alloying elements present

Transformation TemperatureAustenite transformation occurs at a temperature higher than Ac1

in the Fe-Cementite phase diagram – SuperheatingEquilibrium 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

Rate of heating : For higher rates of heating, transformation

starts at higher temperatures and for slower rates, at lower temperatures

For any rate of heating transformation occurs over a range of temperature

For transformation at a constant temperature, heating rate should extremely slow

Special note: Austenite transformation starts as soon

as the eutectoid temperature is reached, but the region in between the curves indicates the majority of the tranformation.

Interface between ferrite and cementiteHigher the interfacial area faster is the tranformationInterfacial area can be increased by: Decreasing the inter-lamellar spacing between ferrite

and cementite The closer the ferrite – cementite lamellae, the higher

is the rate of nucleation. Carbon atoms have to diffuse to smaller distances

from cementite to low carbon regions to form austenite Increasing the cementite or carbon content This will lead to more pearlite content in steels and

thus more interfaces. Examples : 1. High carbon steels austenize faster than low

carbon steels 2. Tempered martensite structure austenizes faster

than coarse paerlite 3. Spheroidal pearlite takes longer time to austenize

due to very low interfacial area

Grain sizeThe coarser the parent grain size the

slower is the transformation rate This is because in larger grains the

interfacial area is lesserThe smaller is the parent grain size the

faster is the transformation to austenite

Nature of the alloying elements present

Alloying elements in steel are present as alloyed cementite or as alloy carbides

Alloy carbides dissolve much more slowly than alloyed cementite or cementite

The stronger the alloy carbide formed the slower is the rate of formation of austenization

Diffusion of substitutional alloying elements is much slower than the interstitial element, carbon

Thus the rate of austenization depends on the amount and nature of alloying element

Why does the Fe-Cementite diagram show a fall in the Ac3 temperature and rapid rise in Acm temperature with increasing carbon percentage?

In hypoeutectoid steels, austenisation process takes place rapidly as carbon content increases.As carbon percentage increases, the amount of

pearlite increases, which increases the interfacial area between ferrite and cementite

Thus Ac3 temperature line decreases continuously with increasing carbon content

In hypereutectoid steels , austenization process becomes much more difficult as the amount of carbon increasesAustenisation of free cementite needs very

high temperature as it involves the diffusion of large amount of carbon( from cementite) to become homogenous

Thus as carbon content increases, amount of free cementite increases, which needs higher temperature to austenize.

Thus Acm line is so steep

Austenite Grain SizeOriginal grain size- size of austenite

grains as formed after nucleation and growth

Actual grain size – size of the austenitic grains obtained after homogenization at higher temperatures

Generally grain size is referred to as actual grain size

Depending on the tendency of steel to grain growth, steels are classified into two groups:

Inherently fine grainedInherently coarse grained

Inherently fine grain steels resist grain growth with increasing temperature till 1000oC – 1050oC

Inherently coarse grain steels grow abruptly on increasing temperature

On heating above a certain temperature T1 inherently fine grain steels give larger grains than inherently coarse grain steels

Gra

in s

ize

Inherently fine grain

Inherently coarse grain

Presence of ultramicroscopic particles like oxides, carbides and nitrides present at grain boundaries prevent grain growth in inherently fine grain steels till very high temperatures

They act as barriers to grain growthSteels deoxidized with Al or treated with B,Ti and V

are inherently fine grainedAt temperatures above T1,dissolution of

ultramicroscopic particles cause sudden increase in grain size

Thus inherently fine grain steels can be hot worked at high temperatures without getting coarsened

Effect of grain size on mechanical properties Austenite grain size plays a very important role in

determining the properties of the steelThe effect of grain size on different properties are given

below: YIELD STRESS The dependence is given by Hall-Petch equation :

Where is the yield stress is the frictional stress opposing motion of

dislocation K is the extent to which dislocations are piled

at barriers D is the avg grain diameter

Grain refinement improves the strength and ductility at the same time

IMPACT TRANSITION TEMPERATURE Increase in grain size raises the impact transition temperature, so

more prone to failure by brittle fracture

CREEP STRENGTH Coarse grained steel has better creep strength above

equicohesive temperature Below this fine grain structure have better creep strength

FATIGUE STRENGTH Fine grained steel have higher fatigue strength

HARDENABILITY Coarse grained steels have higher hardenability (smaller grain boundary area in coarse grained structure gives

less sites for effective diffusion, so martensite formation on cooling is favoured)

MACHINABILITY Coarse grain structure has better machinability due to ease in

discontinuos chip formation(low toughness)

PEARLITIC TRANSFORMATION

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.

Pearlite

Ferrite has a very low carbon content whereas cementite Fe3C is an intermetallic compound of iron with 6.67 wt% of carbon.

Name pearlite is related to the fact that when it is polished and etched then the structure reveals the colorfulness of the “mother of pearl”

Ferrite and cementite are present here in the ratio 8:1.

Pearlite

Coarse pearlite formed at higher temperatures – relatively soft

Fine pearlite formed at lower temperatures – relatively hard

• Transformation of austenite to pearlite:

pearlite growth direction

Austenite ()grain boundary

cementite (Fe3C)Ferrite ()

• For this transformation, rate increases with ( T) [Teutectoid – T ].

675°C (T smaller)

0

50

% p

earli

te600°C

(T larger)650°C

100

Diffusion of C during transformation

Carbon diffusion

Transformation rate ~ ΔT

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

MORPHOLOGY

Morphology

It is a lamellar structure with cementite and ferrite.The cementite and ferrite are present in a definite ratio of 8:1.Each ferrite plate in the pearilte lamellæ is a single crystal and

some neighbouring plates in a single colony have approximately the same orientation of lattice. This holds for the cementite also.

In general, both sides of the line of discontinuity in a pearlite colony make a small angle in lattice orientation with each other.

In the ferrite region near the boundary of pearlite colonies or grains, there are net-works of dislocations or dislocation walls, at each node of wich a cementite rod is present.

MECHANISM

MorphologyThe austenite to pearlite transformation occurs by nucleation and

growth.Nucleation occurs heterogeneously at the grain boundaries of

austenite, and if homogeneous nucleation occurs then it will occur at the carbide particles or in regions of high carbon concentration or on inclusions

The nucleated pearlite grows into austenite as roughly spherically shaped nodules.

Each nodule has a number of structural units In each structural unit the lamellae are largely parallel and is called a

colony.The HULL-MEHL model can explain well the morphology of

pearlite

Mechanism

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

This process of formation of alternate plates of ferrite and cementite forms a colony.

A new cementite nucleus of different orientation may form at the surface of colony forming another colony.

The point to be noted is if austenite transforms to pearlite at a constant temp then the interlamellar spacing is same in all the colonies. The following fig will depict it clearly

HULL-MEHL MODEL

- Smaller T: colonies are larger

- Larger T: colonies are smaller

Figures showing coarse and fine pearlite

Mechanism Hull-Mehl Mechanism for pearlitic

transformation

KINETICS

Kinetics

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

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

At lower critical temp, the free energy of austenite is equal to the free energy of pearlite.

Therefore at this temperature transformation of pearlite to austenite transformation will be completed in infinite time.

So the rate of transformation will be zero.So it is essential to undercool the austenite below the equilibrium

(A1) temp.Below the lower critical temp, free energy of pearlite < free

energy for austenite and hence it is more thermodynamically stable.

Lower thr free energy more will be the stability of PEARLITE.

Kinectics of transformation (contd).

Free energy of pearlite is less at lower tem and so stability is increased by increasing ΔT.

The decomposition of austenite to pearlite proceeds by the redistribution of carbon atoms of austenite into ferrite and cementite, and is essentially a diffusion controlled process.

The rate of diffusion decreases exponentially with decreasing temp

This shows lower the transformation temp retards the rate of transformation.

There is a transformation temp for which diffusion of C atoms is too small resulting in diffusion controlled transformation

Rate of diffusion of carbon atoms is negligible below 200 C


This shows that undercooling affects the rate of transformation in 2 ways:

increased degree of undercooling increases the transformation rate by providing greator difference in free energies of austenite and pearlite.

increased degree of undercooling reduces the transformation rate by lowering the rate of carbon diffusion curve.

Undercooling


The combined effect is shown in the curve below:

Where (a) is rate of crystal growth and (b) is rate of nucleation

The austenite to pearlite transformation is completed by nucleation and growth mechanism.

The rate of transformation is governed by both.The rate of nucleation is expressed as total numbers of of nuclei

appearing per unit time in unit vol of untransformed austenite.Both rate of nucleation and growth are zero at eutectoid temp.

They also temd to be zero below 200 C as shown in the graph previously


Effect of degree of on the rates of nucleation and growthUndercooling

Hardness of pearlite increases as S0 decreases and also same for strength.

As S0 is inversely proportional to the degree of undercooling thus yield strength and also UTS is linearly related to the interlamellar spacing or degree of undercooling below eutectoid temp.

As the pearlite content increases in C steels, impact transition temp is substantially raised, decreasing ductility and toughness as the ferrite-cementite interface provides sites for easy nucleation of cracks

Effect of alloying additions on Pearlitic Transformation

Almost alloying element except Co lower both the rate of nucleation and rate of growth.

As compared to carbon other alloying element diffuse very slowly.

As the diffusion rate for metallic atom is much slower than the

carbon atom the formation of stable carbide during the transformation will be feasible only at higher transformation temp.

Partitioning of carbon gets delayed when Cr eats up C and forms carbide Cr23C6 when alloyed with austenite.

BAINITIC TRANSFORMATION

Bainite is an acicular microstructure (not a phase) that forms in steels at temperatures from approximately 250-550°C (depending on alloy content).

First described by E. S. Davenport and Edgar Bain, it is one of the decomposition products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a critical temperature of 727 °C (about 1340 °F).

Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite

In plain carbon steel Pearlite and Bainite superimpose.Bainite is not so popular and is very much difficult to get.

A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite harder than it normally would be

The temperature range for transformation to bainite (250-550°C) is between those for pearlite and martensite.

When formed during continuous cooling, the cooling rate to form bainite is more rapid than that required to form pearlite, but less rapid than is required to form martensite (in steels of the same composition).

Most alloying elements will lower the temperature required for the maximum rate of formation of bainite, though carbon is the most effective in doing so

The microstructures of martensite and bainite at first seem quite similar; this is a consequence of the two microstructures sharing many aspects of their transformation mechanisms

However, morphological differences do exist that require a TEM to see. Under a simple light microscope, the microstructure of bainite appears darker than martensite due to its low reflectivity.

Illustration of Continuous cooling transformation diagram showing Bainite

MECHANISM

In the TTT curve the incubation period the transformation is diffusion controlled

But the bainite formation takes at a temp at which diffusion is impossible X i.e. metallic atoms wont diffuse but diffusion of C atoms is important

This shows along with diffusion some other mechanism is responsible for the transformation to occur

Since formation of bainite is accompanied by surface distortion so some shear mechanism is responsible for its transformation

So it is a complex one and involves both diffusionless and diffusion controlled phenomena are involved hence it is termed as a “Diffusionless diffusion controlled transformation”

Mechanism of Bainitic transformation

Two mechanisms are thought to be for the Bainite formation:

1. Displacive theory 2. Diffusion theoryBainite is considered to be formed by

diffusionless diffusion controlled transformation.. Both play a part in its transformation

Mechanism of Bainitic transformation

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.

Diffusive theory

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

One of the theories on the specific formation mechanism for bainite is that it occurs by a shear transformation, as in martensite.

The transformation is said to cause a stress-relieving effect, which is confirmed by the orientation relationships present in bainitic microstructures.

There are, however, similar stress-relief effects seen in transformations that are not considered to be martensitic in nature, but the term 'similar' does not imply identical.

The relief associated with bainite is an invariant—plane strain with a large shear component. The only diffusion that occurs by this theory is during the formation of the carbide phase (usually cementite) between the ferrite plates.

Displacive theory

Now the low carbon austenite region transform to ferrite(Bainitic plate) by diffusionless shear process. So It is important to know here that low C Austenite which transform by shear process is itself a diffusion controlled process.

precipitation of carbide may occur from the C enriched Austenitic region depending on the degree of saturation.

The C depleted A region obtained by the precipitation of carbide now transform to ferrite by shear mechanism.

Such a condition is favourable in the upper region of the intermediate transformation temp range, as ferrite has very high solubility of carbon, the transformed ferrite will be supersaturated with C

The degree of supersaturation increases with decrease in transformation temperature

As carbon diffusion is intensive in Bainitic transformation region, Carbon may precipitate out from the supersaturated ferrite.

This happens when the bainitic transformation in the lower region in the transformation range.

Diffusion decreases exponentially so we get different morphology’s of Bainite.

MORPHOLOGY

UPPER BAINITEKnown as ‘feathery bainite’ as it resembles feather of a birdForms in temperature range of 5500C-4000CThe structure consists of

i. lath or needle-like ferrite which runs parallel to the longer axis and

ii. carbide precipitates as fine plates, parallel to the direction of growth of bainite, mainly at the lath boundaries

Carbides are present as ‘discontinuous stringers’ when carbon content is low and ‘continuous stringers’ when carbon content is high.

Upper Bainite

The ferrite laths have ‘sub laths’ with high dislocation density Decrease in temperature produces finer and closely formed

laths with smaller spacing of carbide particles The ferrite and cementite in bainite have a specific orientation

relationship with the parent austenite Diffusivity of carbon in this temperature range is high enough

to cause partition of carbon between ferrite and austenite. Structure is brittle and hard and the deposition of hard carbide

stringers on the soft ferrite makes it a completely useless structure.

Upper Bainite

Schematic growth mechanism of Upper Bainite

Upper bainite in medium carbon steel

Known as ‘Plate bainite’Forms in the temperature range of 4000C-2500CThe structure consists of

i. Lenticular plates of ferrite

ii. Fine rods or blades of carbide at an angle of 55 to 60o to the axis of bainite

Carbides can be cementite or ε-carbide, or a mixture depending on temperature of transformation and composition of steel

Lower Bainite

Carbides precipitate within the ferrite platesFerrite plates have smaller sub-plates with low angle boundaries

between themHigher dislocation density than upper bainiteHabit planes of ferrite plates are the same as martensite that

forms at low temperatures of the same alloyAlloying elements do not diffuse or form their carbides during

bainite transformation

Lower Bainite

Lower Bainite structure in medium carbon steel

Stages of formation of Lower Bainite

Schematic representation of lower bainite structure

MARTENSITIC TRANSFORMATION

Martensite - BCT

MECHANISM

Martensite transformation is a diffusion-less transformation

Martensite is formed on quenching austenite, such that the diffusion of carbon is not favored

The atoms move in an organized manner relative to their neighbours and therefore they are known as a military transformations in contrast to diffusional civilian transformations

Each atom moves by a distance less than one inter-atomic distance and also retain its neighborhood undisturbed

But the total displacement increases as one moves away from the interphase boundary which results in a macroscopic slip as can be observed as relief structure on the surface of martensite

Mechanism

Figure shows mechanism martensite plate formation

At the beginning of the transformation martensite takes the form of lens or plates spanning the entire grain diameter

The subsequent plates formed are limited by the grain boundaries and the initial martensite plates formed

Where the plates intersect the polished surface they bring about a tilting of the surface.

But, macroscopically the transformed regions appear coherent to the surrounding austenite.

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

A large amount of driving force is needed for the martensitic transformation

The magnitude of the driving force is provide by the free energy change accompanying the transformation

The magnitude of the driving force for nucleation of martensite at the Ms can be as follows:

The figures above demonstrate the equation given above

oThe graphs along side show that magnitude of the driving force increases with decrease in the temperature of transformation

Martensite has ‘ Body Centered Tetragonal’ structureThe tetragonality of martensite, measured by the c/a ratio is given

by:

c/a=1+ 0.045 X wt% CTetragonality increases with increase in carbon percent When the fcc γ- Fe transforms to bcc α-Fe, carbon is trapped in

the octahedral sites of body centered cubic structure to give body centered tetragonal (BCT) structure

The trapped carbon atoms cause tetragonal distortion of bcc latticeWhen carbon is more than 0.2%, bct structure is formed

Crystal Structure of Martensite

KINECTICS OF TRANSFORMATION

The transformation starts at a definite temperature –Ms ( Martensite start)

temperature

The transformation proceeds over a range of temperatures till Mf

temperture

The amount of martensite increases on decreasing transformation

temperature between Ms and Mf

At Mf not all austenite is converted to martensite, but a certain amount is

present as retained austenite

Kinetics of Martensite Transformation

Although the martensite transformation ends at Mf, some austenite still remains untransformed as retained austenite

Mf temperature depends on cooling rateSlower cooling rates lower the Mf temperatureMf 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 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

Diffusionless/Military tranformationAthermal transformation.Retained AusteniteMs – Mf tempReversibility of transformationHabit planesBain distortionEffect of applied stress on transformationHardness of Martensite

Important characteristics of Martensite Transformation

Martensite transformation begins as the Ms temperature is reached and ends at the Mf temperature

The Ms temperature depends on the chemical composition of steel and is independent of the rate of cooling

Austenizing temperature to which the steel had been heated prior to the transformation affects Ms temperature

Higher the temperature creates the following two conditions:Greater dissolution of carbon and carbides, which results in

lowering of MsLarger grain size of austenite, which results in a rise of Ms

Ms and Mf Temperature

The relationship between Ms temperature and the chemical composition can be shown as:

Ms (oC)=561 – 474(%C) – 33(%Mn) – 17(%Ni) – 17(%Cr) – 21(%Mo).

• The above shows that nearly all elements lower the Ms temperature except Cobalt and aluminium

• Carbon has the most profound effect on Ms temperature and an increase in carbon content cause lowering of the Ms temperature

Ms and Mf Temperature

Effect of carbon content on Ms and Mf temp

Effect of alloying additions on Ms temp

Reversibility of Martensite

Martensite transformation is reversible .Martensite can be reverted to austenite on heating above the Ms

temp.The essential condition for the reversibility of martensite is that

there should not be any change in chemical composition of martensite during heating

Most steels dont satisfy this condition

Since Martensite in steels is supersaturated solid solution of carbon in alpha iron and it decomposes at a very rapid rate on heating

Retained Austenite

Retained austenite Untransformed Austenite. It forms as Austenite to martensite transforms on quenching below

the Ms temp but above Mf temp. As Austenite to martensite never goes to completion some amount

of austenite is present in the hardened steel. Since Ms and Mf temp decrease with carbon content increase so

amount of retained austenite increases with increase in carbon content.

All alloying elements except Al and Co lower the Ms temp and enhance the amount of retained austenite.

Therefore, both high carbon steels and high alloy steels are prone to retained austenite.

Amount of retained austenite increases with decreased martensite temp of transformation

Athermal and Isothermal MartensiteAthermal transformation occurs in most carbon steelsMartensite transformation proceeds on continuous cooling

below the Ms temperature

The transformation stops when the steel is held at a particular temperature in between Ms and Mf

The transformation is independent of holding time

Athermal and Isothermal Martensite

If while the transformation process within the Ms-Mf temperature the

ooling is stopped – the transformation halts

On resuming the cooling the transformation doesnot start instantly but

needs supercooling

Larger amount of retained austenite formed at Mf called ‘stabilized

austenite’

• Martensite can also form isothermally.

• Isothermally transformed martensite quantity is low.

• In extra low carbon base alloys or high alloy steels - low

transformation temperatures and long period of transformation.

• Amount of martensite decrease with decrease in Ms- Mf

temperature.

Presence of external and internal stresses affect the kinetics of martensitic transformation

If external stresses are applied to austenite above Ms temperature, Ms is raised

As there are a large number of habit planes, the application of stress favors martensite plate formation on any of the plates, thus the ms is raised

The maximum temperature at which martensite can be formed by plastic deformation is denoted as Md

The amount of martensite formed by plastic deformation is a function of plastic deformation

Effect of applied stress on transformation

The model was proposed by E.C. BainAny simple homogeneous pure disyortion of the nature which

converts one lattice to another by expansion and contraction along the crystallographic axis belong to a class known as BAIN DISTORTION

The model explains how bct lattice can be obtained from fcc lattice with minimum atomic movement

Bain Distortion model

In the figure in the previous slide, x,y,z and x’, y’, z’ represent the

initial and final axes of fcc and bcc unit cells

An elongated unit cell of the bcc structure can be drawn within two

fcc cells

The elongated bcc unit cell has a c/a ratio of 1.40

The pure bcc unit cell has a c/a ratio of 1.0

The bct structure of martensite has c/a ratio of 1.08

Transformation to a bct unit cell is achieved by:

(a) contracting the cell 20% in the z direction and

(b) expanding the cell by 12% along the x and y axes

This results in 4% increase in volume

In the case of steels, the carbon atoms fit into z’ axis of the

bcc cell at ½<100> positions causing the lattice to elongate in

this direction

Bain distortion results in the following correspondence of

crystal planes and directions:

This model explains the transformation of martensite from austenite with minimum movement of atoms

Thus carbon atoms are finally present only in the middle of the edges along [001]axis and not in the middle of the edges which represent the a-axis

Habit planes

The transformation is characterized by a well established relationship

between the orientation of parent austenite and the transformed martensite.

Habit planes are those planes of the parent austenitic lattice on which

martensitic plates are formed and which lie parallel t the physical plane of

the martensitic plate.

A habit plane is distorted by the martensite transformation though along it

shear displacement takes place during transformation.

The habit planes for low, medium and high carbon steels are (111),(225),

(259)

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

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%)

Hardness of Martensite

High carbon % lowers the Ms and Mf , so large amount of retained austenite is present

Alloying elements that lower Ms and Mf temperatures, give more retained austenite

Steel becomes softer as retained austenite increasesTwo suspected factors for enhanced hardness

a) internal strains within α-Fe due to excess carbon

b) the plastic deformation of austenite surrounding martensite plates

Appearance of large number of twins interlayer and increase of dislocation density on martensite transformation

Segregation of carbon atoms to dislocations leading to Cottrel atmospheres

Precipitation of dispersed carbide particles from alpha phaseSelf tempering results in lowering of hardness

MORPHOLOGY

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

Morphology of Martensite

Martensitic transformations are (usually) first order, diffusionless, shear (displacive) solid state structural changes.

Their kinetics and morphology are dictated by the strain energy arising from shear displacement.

The displacement can be described as a combination of homogeneous lattice deformation, known also as “Bain Distortion”, and shuffles.

In a homogeneous lattice deformation one Bravais Lattice is converted to another by the coordinated shift of atoms.

A shuffle is a coordinated shift of atoms within a unit cell, which may change the crystal lattice but does not produce homogeneous lattice distortive strain.

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

Types of Martensite

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

percentage

Lath Martensite Plate Martensite

References

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 Sharma5. Modern physical Metallurgy by Smallman and Bishop.

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