Why do we study phase transformations? The tensile strength of an Fe-C alloy of eutectoid composition can bevaried between 700-2000 MPa depending on the heat treatment processadopted. This shows that the desirable mechanical properties of a material can beobtained as a result of phase transformations using the right heat treatmentprocess.In order to design a heat treatment for some alloy with desired RTproperties, time and temperature dependencies of some phasetransformations can be represented on modified phase diagrams.
phase transformationsMost phase transformations begin with the formation of numerous smallparticles of the new phase that increase in size until the transformation iscomplete. Nucleation is the process whereby nuclei (seeds) act as templates forcrystal growth.Homogeneous nucleation - nuclei form uniformly throughout the parentphase; requires considerable supercooling (typically 80-300°C).Heterogeneous nucleation - form at structural inhomogeneities (containersurfaces, impurities, grain boundaries, dislocations) in liquid phase mucheasier since stable “nucleating surface” is already present; requires slightsupercooling (0.1-10ºC ).
Thermodynamics andkinectics of PHASETRANSFORMATIONWhat does lie underneath the structure……..
phase transformation Phase transformation is predominantly controlled by TEMP. But transformation never really start at transformation temp rather it starts at a temp much below the temp predicted for the transformation to occur. Undercooling: It is the gap between the temp predicted for the transformation to occur and the temp at which the transformation actually occurs.
SupercoolingDuring the cooling of a liquid, solidification (nucleation) will begin onlyafter the temperature has been lowered below the equilibrium solidification(or melting) temperature Tm. This phenomenon is termed supercooling (orundercooling. The driving force to nucleate increases as ∆T increasesSmall supercooling slow nucleation rate - few nuclei - large crystalsLarge 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 andenthalpy 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 liquidas 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 differencebetween the solid and liquid phases (volume free energy, ΔGV); and 2) thesolid-liquid phase boundary (surface free energy, ΔGS).
phase transformationAllotropic / polymorphic transformation: No change in compositionof the structurePhase transformation: Change in crystal structure+ Change incomposition.•Surface creations always hinders the process of transformation. Thenew phase always trys to create the surface, so energy needs to besupplied. So volume free energy will try to decrease the energy butsurface free energy will try to increase the energy.
Transforming one phase into another takes time. Fe Fe C γ Eutectoid 3 transformation (cementite) (Austenite) + C α FCC (ferrite) (BCC) ∆G = ∆GS + ∆GV
phase transformation• In the previous fig it can be observed that as soon as the particles of A phase are formed the free energy of the system should decrease the new phase is developed and has lower energy than the B phase. ΔFv=VΔf V= Vol of the new crystal f=free energies of the new phase• formation of the new crystal is linked with the interface between the new and initial phases. ΔFs = sνs = surface area of the new crystalν = free energy per unit area
phase transformation• If rate kinetics of phase transformation is increased then the structure will be finer and this is indicated by the Hall - Petch equation States that decrease in grain size and with fineness in the structure the strength in increased. δo =δ + Ka (-1/2) → Hall-Petch EquationWhere, δo = Friction stress δ = in stress a = grain size K= locking parameter
Solid state transformation• During the solid state transformation still another factor acting inhibiting the nucleation transformation nuclei.• A new phase always differs from the initial one in its structure and specific volume.• Since the transformation develops an elastic crystalline medium, change in specific volume should cause an development in elastic strain energy in one or both the phases. This inhibits the transformation and kinetics the free energy.
Solid state transformation• Therefore, the certain elastic component ΔFel makes a +ve contribution to the free energy change in the solid state transformation
Martensite transformation temp is much lower than Pearlite transformation temp?? ΔTm>>ΔTp Reason: Elastic strain energy component A→ M leads to volumetric expansion which leads to straining of the lattice and hence a +ve component in the free energy. To compensate this +ve component an undercooling is there. So temp of transformation is so low.
Nucleation and Growth• Reaction rate is a result of nucleation and growth of crystals. 100 Nucleation rate increases w/T % Pearlite Growth regime Growth rate increases w/ T 50Nucleation regime 0 t50 log (time) • Examples: pearlite γ colony γ γ T just below TE T moderately belowTE T way below TE Nucleation rate low Nucleation rate medium Nucleation rate high Growth rate is high Growth rate is medium Growth rate is low 5
FRACTION OF TRANSFORMATION • Fraction transformed depends on time. Avrami Eqn. n y = 1 − e−kt fraction transformed time • Transformation rate depends on T. activation energy Ex: recrystallization of Cu °C 119°C 103°C C y (%) °C °C 2° 5 1 − 13 11 88 43 100 r= = Ae Q /RT t 0.5 50 0 1 10 102 104 log (t) min • r often small: equil not possible 2
Eutectoid Transformation rate ~ ΔT • Transformation of austenite to pearlite: Diffusion of C Austenite (γ) cementite (Fe3C) during transformation grain α Ferrite (α) boundary α γ α γ α α pearlite γ α γ growth α α direction α• For this transformation, 100 Carbon diffusion rate increases with ( ∆T) 600°C (∆T larger) % pearlite [Teutectoid – T ]. 650°C 50 675°C (∆T smaller) 0 Coarse pearlite formed at higher temperatures – relatively soft Fine pearlite formed at lower temperatures – relatively hard
PHASE TRANSFORMATIONSBased on Masstransport Diffusion less military Diffusional transformation transformation Change in No change in composition composition PHASE TRANSFORMATIONSBased on Order Ist order nucleation 2nd order entire and growth volume transforms
Diffusion-less transformation in solids Major phase transformations that occur in solid phase are due tothermally activated atomic movementsThe different types of phase transformation that is possible can bedivided 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 compositionalchanges in the formation of the product phase and consequentlyrequire 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→ αOriginal phase decomposes into one or more new phases which havethe same composition as the parent phase but different crystalstructures
Polymorphic Transformations: Typically exhibited by singlecomponent systems where different crystal structures are stable overdifferent temperature ranges. E.g. bcc-fcc transformation in Fe
Possible Transformations Martensite T Martensite Strength Ductility bainite fine pearlite coarse pearlite spheroidite General Trends
WHAT ARE TTT CURVES T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.
Time- Temperature-Transformation (TTT) Curves – Isothermal Transformation 800 Eutectoid temperature 723 Austenite Coarse Pearlite 600 Fine Eutectoid steel 500 Pearlite + Bainite T → 400 BainiteNot an isothermal 300 Ms Austenite 200 transformation Mf 100 Martensite 0.1 1 10 102 103 104 105 t (s) →
The dependance of transformation to temperature and time can beanalyzed 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 CURVES Carbon has significant effects on the nature of the TTT curves Carbon is an austenite – stabilizerHYPOEUTECTOID STEELS Ferrite is the nucleating phase on decomposition of austenite As carbon increases from 0 to 0.77% :EUTECTOID STEELS Have the maximum incubation period
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
Proeutectoid Proeutectoid phase starts to cementite starts form on this line to form on this line A+P A +FTemperature oC Ac1 Fe3C +A A F+P P Fe3C +P Ms B B Ms Ms Ms Pearlite reaction starts TTT curves for hypo , eutectoid and hyper-eutectoid steels
EFFECT OF ALLOYING ELEMENTS ON THE TTT CURVES All alloying elements (except Co) shift the S curve to the right Austenite stabilizers move the curve to the right( Mn, Ni,etc) Carbide formers shift the S curve further to the right because: Diffusion of alloying elements is too slow(substitutional elements) Diffusion of carbon is slower as carbide formers donot easily part with the carbon Allotropic change γ α is reduced by solutes Bainitic transformation is lesser affected ( no redistribution of alloying elements)
EFFECT OF GRAIN SIZE ON THE TTT CURVESAll decomposition products of austenite nucleate heterogenously at grain boundariesThus incubation period is reduced for fine grained steelS 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
Figure shows the process of Martempering and the characteristictemperatures: 800 Eutectoid temperature 723 Austenite Pearlite 600 α + Fe3C 500 Pearlite + Bainite T → 400 Bainite Martempering 300 Ms 200 Mf 100 Martensite 0.1 1 10 102 103 104 105 t (s) →
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 800 Eutectoid temperature 723 Austenite Pearlite 600 α + Fe3C 500 Pearlite + Bainite T → 400 Bainite 300 Ms Austempering 200 Mf 100 Martensite 0.1 1 10 102 103 104 105 t (s) →
Continuous Cooling Transformation (CCT) Isothermal heat treatments are not the most practical due to rapidlycooling and constant maintenance at an elevated temperature. Most heat treatments for steels involve the continuous cooling of aspecimen 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 isdelayed.The isothermal curves are shifted to longer times and lower temperatures.
In the above figure Moderately rapid and slow cooling curves aresuperimposed on a continuous cooling transformation diagram of a eutectoidiron-carbon alloy. The transformation starts after a time period corresponding to theintersection of the cooling curve with the beginning reaction curve and endsupon crossing the completion transformation curve. Normally bainite does not form when an alloy is continuously cooled toroom temperature; austenite transforms to pearlite before bainite has becomepossibleThe austenite-pearlite region (A---B) terminates just below the nose.Continued cooling (below Mstart) of austenite will form martensite
For continuous coolingof a steel alloy there existsa critical quenching ratethat represents theminimum rate of quenchingthat will produce a totallymartensitic structure. This curve will just missthe nose where pearlitetransformation begins
Continuous cooling diagram for a 4340 steel alloy and several coolingcurves 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 formartensite are►chromium,► nickel,► molybdenum► manganese► silicon► tungsten
Effect of adding other elements 4340 Steel Other elements (Cr, Ni, Mo, Si and W) may cause significant changes in the positions and shapes of the TTT curves: Change transition temperature; Shift the nose of the austenite-to- pearlite transformation to longer times; nose Shift the pearlite and bainite noses plain to longer times (decrease critical carbon cooling rate); steel Form a separate bainite nose; Plain carbon steel: primary alloying element is carbon.
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.
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 InHeat 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 cementite Pearlite Austenite Eutectoid composition transforms at a particular (Ac1) temperature 1st step: ( On heating to eutectoid temperature)Lattice changesBCC 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 α-Fe Fe3C Austenite
Homogenization of austeniteThe austenite formed from cementite and ferrite is generally not homogenousHomogenization requires high temperature/time , or bothHigh 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 transformationThe kinetics depends on: Transformation temperature and holding time Rate of heating Interface between ferrite and cementite Grain size Nature of the alloying elements present
Transformation Temperature Austenite transformation occurs at a temperature higher than Ac1 in the Fe-Cementite phase diagram – Superheating Equilibrium temperatures are raised on heating and lowered on cooling ( free energy should be negative) The rate of austenite formation increases with increase in temperature as it increases the rate of carbon diffusion and the free energy is more negative Interdependence of time and temperature : Transformation takes a shorter time at higher temperatures of transformation and vice versa
Rate of heating :For higher rates of heating, transformation starts at higher temperatures and for slower rates, at lower temperaturesFor any rate of heating transformation occurs over a range of temperatureFor transformation at a constant temperature, heating rate should extremely slowSpecial 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 theAc3 temperature and rapid rise in Acm temperature withincreasing 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 increases Austenisation of free cementite needs very high temperature as it involves the diffusion of large amount of carbon( from cementite) to become homogenous Thus as carbon content increases, amount of free cementite increases, which needs higher temperature to austenize. Thus Acm line is so steep
Austenite Grain SizeOriginal grain size- size of austenite grains as formed after nucleation and growthActual grain size – size of the austenitic grains obtained after homogenization at higher temperaturesGenerally grain size is referred to as actual grain sizeDepending 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 Grain size Inherently coarse grain Inherently fine 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 temperaturesThey 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 sizeThus 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 steel The effect of grain size on different properties are given below: YIELD STRESS The dependence is given by Hall-Petch equation : Where is the yield stress is the frictional stress opposing motion of dislocation K is the extent to which dislocations are piled at barriers D is the avg grain diameter
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)
Pearlite It is a common micro constituent of a variety of steels where is increases the strength of steel to a substantial extent. Unique micro constituent formed when austenite in iron carbon alloys is transformed isothermally at or below the eutectoid temp (723K) One of the most interesting features of austenite to pearlite transformation is that the tr product consists of entirely 2 diff phase. Consists of alternate plates of ferrite and cementite and the continuous phase is ferrite.
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.
Transformation rate ~ ΔT • Transformation of austenite to pearlite: Diffusion of C Austenite (γ) cementite (Fe3C) during transformation grain α Ferrite (α) boundary α γ α γ α α pearlite γ α γ growth α α direction α• For this transformation, 100 Carbon diffusion rate increases with ( ∆T) 600°C (∆T larger) % pearlite [Teutectoid – T ]. 650°C 50 675°C (∆T smaller) 0 Coarse pearlite formed at higher temperatures – relatively soft Fine pearlite formed at lower temperatures – relatively hard
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 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.
Morphology Mechanism The austenite to pearlite transformation occurs by nucleation and growth. Nucleation occurs heterogeneously at the grain boundaries of austenite, and if homogeneous nucleation occurs then it will occur at the carbide particles or in regions of high carbon concentration or on inclusions The nucleated pearlite grows into austenite as roughly spherically shaped nodules. Each nodule has a number of structural units In each structural unit the lamellae are largely parallel and is called a colony. The HULL-MEHL model can explain well the morphology of pearlite
HULL-MEHL MODEL The initial nucleus is a widmanstatten platelet of cementite forming at the austenite g.b. which when as grows thickens as well This occurs by the removal of carbon atoms from austenite on both sides of it till carbon decreases in the adjacent austenite to a fixed low value at which ferrite nucleates. The growth of ferrite leads to build of carbon at the ferrite austenite interface until there is enough carbon to nucleate fresh plates of cementite which then grow.
HULL-MEHL MODELThis 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
Figures showing coarse and fine pearlite - Smaller ∆T: - Larger ∆T: colonies are colonies are larger smaller
MechanismHull-Mehl Mechanism for pearlitic transformation
This equation however makes the following assumptions:(i)The , average nucleation rate is const. with time whichactually isnt true(ii)Nucleation occurs randomly, which is also not trulycorrect.(iii)The growth rate, is const. with time, which canchange from one nodule to other with time.(iv) Nodules maintain a spherical shape but nodules maynot 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.
Kinectics of transformation (contd). At lower critical temp, the free energy of austenite is equal to the free energy of pearlite. Therefore at this temperature transformation of pearlite to austenite transformation will be completed in infinite time. So the rate of transformation will be zero. So it is essential to undercool the austenite below the equilibrium (A1) temp. Below the lower critical temp, free energy of pearlite < free energy for austenite and hence it is more thermodynamically stable. Lower thr free energy more will be the stability of PEARLITE.
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
Kinectics of transformation (contd). This shows that undercooling affects the rate of transformation in 2 ways: Undercooling increased degree of increased degree of undercooling undercooling reduces the increases the transformation rate transformation rate by by providing greator difference lowering the rate of in free energies of austenite and carbon diffusion curve. pearlite.
The combined effect is shown in the curve below: Where (a) is rate of crystal growth and (b) is rate of nucleation
Kinectics of transformation (contd).The austenite to pearlite transformation is completed by nucleation and growth mechanism. The rate of transformation is governed by both. The rate of nucleation is expressed as total numbers of of nuclei appearing per unit time in unit vol of untransformed austenite. Both rate of nucleation and growth are zero at eutectoid temp. They also temd to be zero below 200 C as shown in the graph previously
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.
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 showingBainite
Mechanism of Bainitic transformationIn the TTT curve the incubation period the transformation is diffusion controlledBut 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
Diffusive theory The diffusive theory of bainites 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.
When the austenite is undercooled below the Bs temp, C atomsredistribute in the Austenite by diffusion. This redistribution leadsto formation of regions with varying carbon concentration inAustenite. Some of these regions are enriched in carbon whileothers are deficient in C. Such a difference in C concentration willresolve in the development of stresses
Displacive theory One of the theories on the specific formation mechanism for bainite is that it occurs by a shear transformation, as in martensite. The transformation is said to cause a stress-relieving effect, which is confirmed by the orientation relationships present in bainitic microstructures. There are, however, similar stress-relief effects seen in transformations that are not considered to be martensitic in nature, but the term similar does not imply identical. The relief associated with bainite is an invariant—plane strain with a large shear component. The only diffusion that occurs by this theory is during the formation of the carbide phase (usually cementite) between the ferrite plates.
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.
Upper BAINITEUPPER Bainite Known as ‘feathery bainite’ as it resembles feather of a bird Forms in temperature range of 5500C-4000C The structure consists ofi. lath or needle-like ferrite which runs parallel to the longer axis andii. 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.
Schematic growth mechanism of Upper Bainite Upper bainite in medium carbon steel
Lower Bainite Known as ‘Plate bainite’ Forms in the temperature range of 4000C-2500C The structure consists ofi. Lenticular plates of ferriteii. 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 plates Ferrite plates have smaller sub-plates with low angle boundaries between them Higher dislocation density than upper bainite Habit planes of ferrite plates are the same as martensite that forms at low temperatures of the same alloy Alloying elements do not diffuse or form their carbides during bainite transformation
Schematic representation of lower bainite structure Lower Bainite structure in medium carbon steel Stages of formation of Lower Bainite
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
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 martensiteremains macroscopically coherent toparent 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: oThe graphs along side show that magnitude of the driving force increases with decrease in the temperature of transformation The figures above demonstrate the equation given above
Crystal Structure of Martensite Martensite has ‘ Body Centered Tetragonal’ structure The tetragonality of martensite, measured by the c/a ratio is given by: c/a=1+ 0.045 X wt% C Tetragonality increases with increase in carbon percent When the fcc γ- Fe transforms to bcc α-Fe, carbon is trapped in the octahedral sites of body centered cubic structure to give body centered tetragonal (BCT) structure The trapped carbon atoms cause tetragonal distortion of bcc lattice When carbon is more than 0.2%, bct structure is formed
Kinetics of Martensite Transformation The transformation starts at a definite temperature –Ms ( Martensite start) temperature The transformation proceeds over a range of temperatures till Mf temperture The amount of martensite increases on decreasing transformation temperature between Ms and Mf At Mf not all austenite is converted to martensite, but a certain amount is present as retained austenite
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 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 MartensiteTransformation Diffusionless/Military tranformation Athermal transformation. Retained Austenite Ms – Mf temp Reversibility of transformation Habit planes Bain distortion Effect of applied stress on transformation Hardness of Martensite
Ms and Mf Temperature Martensite transformation begins as the Ms temperature is reached and ends at the Mf temperature The Ms temperature depends on the chemical composition of steel and is independent of the rate of cooling Austenizing temperature to which the steel had been heated prior to the transformation affects Ms temperature Higher the temperature creates the following two conditions: Greater dissolution of carbon and carbides, which results in lowering of Ms Larger grain size of austenite, which results in a rise of Ms
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
Reversibility of Martensite Martensite transformation is reversible . Martensite can be reverted to austenite on heating above the Ms temp. The essential condition for the reversibility of martensite is that there should not be any change in chemical composition of martensite during heating Since Martensite in steels is supersaturated Most steels dont satisfy this condition solid solution of carbon in alpha iron and it decomposes at a very rapid rate on heating
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 decreasedmartensite temp of transformation
Athermal and Isothermal MartensiteAthermal and Isothermal Martensite Athermal transformation occurs in most carbon steels Martensite transformation proceeds on continuous cooling below the Ms temperature The transformation stops when the steel is held at a particular temperature in between Ms and Mf The transformation is independent of holding time
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.
Effect of applied stress on transformation Presence of external and internal stresses affect the kinetics of martensitic transformation If external stresses are applied to austenite above Ms temperature, Ms is raised As there are a large number of habit planes, the application of stress favors martensite plate formation on any of the plates, thus the ms is raised The maximum temperature at which martensite can be formed by plastic deformation is denoted as Md The amount of martensite formed by plastic deformation is a function of plastic deformation
Bain Distortion model The model was proposed by E.C. Bain Any simple homogeneous pure disyortion of the nature which converts one lattice to another by expansion and contraction along the crystallographic axis belong to a class known as BAIN DISTORTION The model explains how bct lattice can be obtained from fcc lattice with minimum atomic movement
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 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 totransform into martensite.The different colors indicate the displacements caused when martensiteforms.
Hardness of Martensite Hardness of martensite is due to carbon content and chemical composition Strengthening effect is due to super saturation of alpha solution with carbon Hardness increases with increase in carbon content in martensite and then decreases after a certain Carbon% (0.5- 0.6%)
High carbon % lowers the Ms and Mf , so large amount of retained austenite is present Alloying elements that lower Ms and Mf temperatures, give more retained austenite Steel becomes softer as retained austenite increases Two suspected factors for enhanced hardness a) internal strains within α-Fe due to excess carbon b) the plastic deformation of austenite surrounding martensite plates Appearance of large number of twins interlayer and increase of dislocation density on martensite transformation Segregation of carbon atoms to dislocations leading to Cottrel atmospheres Precipitation of dispersed carbide particles from alpha phase Self tempering results in lowering of hardness
Morphology of Martensite Martensite transformation involves two shears: a) homogeneous lattice deformation or Bain strain b) inhomogeneous lattice deformation which makes lattice to be undistorted This shear can be slip or twin . This shear depends on composition, temperature of transformation and strain rate. Twinning is favored when the yield stress of austenite is raised carbon and alloying elements increase
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.
Types of Martensite There are two types of martensite classified according to morphology: - Lath martensite - Plate martensiteA) 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• 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
References1. 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.