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TTT diagram
 

TTT diagram

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just keep some basic in mind, its give u enough information about this topic.

just keep some basic in mind, its give u enough information about this topic.

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    TTT diagram TTT diagram Presentation Transcript

    • TIME-TEMPERATURETRANSFORMATION DIAGRAM Prof. H. K. Khaira Professor in MSME Deptt. MANIT, Bhopal
    • Phase Transformations in Steels • Iron has having different crystal structures at different temperatures. • It changes from FCC to BCC at 910o C. • This transformation results in austenite transforming to pearlite at eutectoid temperature. • This transformation of austenite is time dependant.
    • Fe-C Equilibrium Diagram Spring 2001 Dr. Ken Lewis ISAT 430 3
    • Fe-C Equilibrium Diagram • Though the Fe-C equilibrium diagram is very useful, it does not provide information about the transformation of austenite to any structure other than equilibrium structures, nor does it provide any details about the influence of cooling rates on the formation of different structures. • In other words, Fe-C diagram does not explain the decomposition of austenite under non-equilibrium conditions or conditions involving faster rates of cooling than equilibrium cooling. • Several structures (e.g. martensite) not appearing on the equilibrium diagram may be found in the microstructures in steels.
    • TTT Diagram • On the other hand, TTT diagram is a more practical diagram. • It shows what structures can be expected after various rates of cooling. • It graphically describes the cooling rate required for the transformation of austenite to pearlite, bainite or martensite. • TTT diagram also gives the temperature at which such transformations take place.
    • Phase diagram and TTT diagram Which information are obtained from phase diagram or TTT diagram? • Phase diagram : – Describes equilibrium microstructural development that is obtained at extremely slow cooling or heating conditions. – Provides no information on time taken to form phase • TTT diagram – For a given alloy composition, the percentage completion of a given phase transformation on temperature-time axes is described.
    • Transformation Diagrams • There are two main types of transformation diagrams that are helpful in selecting the optimum steel and processing route to achieve a given set of properties. These are 1. Time-temperature transformation (TTT) diagrams 2. Continuous cooling transformation (CCT) diagrams
    • Transformation Diagrams • Time-temperature transformation (TTT) diagrams 1. Indicates the amount of transformation at a constant temperature. 2. Samples are austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the amount of transformation is measured, for example by dilatometry. 3. Obviously a large number of experiments are required to build up a complete TTT diagram.
    • Transformation Diagrams • Continuous cooling transformation (CCT) diagrams 1. Indicates the extent of transformation as a function of time for a continuously decreasing temperature. 2. Samples are austenitised and then cooled at a predetermined rate and the degree of transformation is measured, for example by dilatometry. 3. In this case also a large number of experiments are required to build up a complete CCT diagram also
    • Transformation Diagrams • CCT diagrams are generally more appropriate for engineering applications as components are cooled (air cooled, furnace cooled, quenched etc.) from a processing temperature as this is more economic than transferring to a separate furnace for an isothermal treatment.
    • TTT Diagram
    • TTT diagram for an eutectoid carbon steel.
    • Fe-C Equilibrium Diagram • Austenite is stable above A1 temperature • Below this temperature, austenite is unstable. Eutectoid Steel Spring 2001 Dr. Ken Lewis ISAT 430 13
    • How Transformation Ocures? • Transformation of austenite to pearlite ocures by nucleation and growth mechanism. • This transformation requires diffusion.
    • Nucleation rate • As temperature decreases below eutectoid temperature, r* (critical size of nucleus) decreases increasing the nucleation rate N. • At very low temperature, nucleation rate decreases due to large decrease in diffusion rate. • At intermediate temperature, nucleation rate is maximum
    • Growth of nuclei – Growth of nuclei is a diffusion controlled process . QD Growth Rate G ce RT where QD : activation energy for self diffusion Growth rate decreases with decrease in temperature
    • Transformation Rate . . • Transformation rate of a phase : N G • Transformation rate first increases, reaches a maximum and then starts decreasing with decrease in temperature
    • Time for Transformation • Time required for transformation as a function of temperature follows a reverse trend than the rate of transformation. Time required for transformation fist decreases, reaches a minimum and then starts increasing with decrease in temperature.
    • Let us do some experiment
    • Isothermal transformation of eutectoid steel Let us take a eutectoid steel and do the following experiment – – – – – Step 1 – Heat the sample above A1 temperature for austenitisation Step 2 – Transfer the sample to a salt bath kept below A1 Temp. Step 3 - Keep it at the bath temperature for a specified time Step 4 - Quench to room temperature Step 5 – Find out the amount of phases present
    • Isothermal transformation of eutectoid steel below Eutectoid Temperature • Determine the amount of pearlite formed after holding at 7050 C for different times
    • Fraction of transformation Vs the logarithm of time at constant temperature (The S Curve). Plot the result of the experiment This is known as S curve
    • Fraction of transformation Vs the logarithm of time at constant temperature (The S Curve). The transformation starts but it takes some time before we can see a precipitate. The time required for transformation required to initiate the transformation is known as Incubation Period. The rate of transformation first increases and then starts decreasing
    • Time required for completion of transformation • Now repeat the experiment at different temperatures below A1 temp. • Plot the time for completion of transformation at different temperatures.
    • Time-Temperature-Transformation Curve • We can plot the time required for start and completion of transformation at different temperatures or for any other amount of transformation.
    • Let us consider eutectoid reaction as an example eutectoid reaction: γ(0.8 wt% C) ↓ α (0.025 wt% C) + Fe3C
    • Let us consider eutectoid reaction as an example The S-shaped curves are shifted to longer times at higher T showing that the transformation is dominated by nucleation (nucleation rate increases with supercooling) and not by diffusion (which occurs faster at higher T)
    • Isothermal Transformation (or TTT) Diagrams (Temperature, Time, and % Transformation)
    • TTT Diagrams for Eutectoid Steel • We can plot the time for start and completion of transformation of austenite to pearlite at different temperatures or for any other amount of transformation.
    • Transformations of austenite to Pearlite pearlite Transformations of austenite : → + Fe3C 1) At slightly lower T below 727 ℃ : T << • Coarse pearlite : nucleation rate is very low. : diffusion rate is very high. 2) As the T (trans. temp.) decreases to 500 ℃ • Fine pearlite : nucleation rate increases. : diffusion rate decreases. Strength : (MPa) = 139 + 46.4 S-1 S : inter-lamellar spacing 655 ℃ 600 ℃ 534 ℃ 487 ℃
    • But at lower temperatures …. • At lower temperatures, the austenite transforms to bainite. • Bainite is also a mixture of ferrite and cementite but not in the form of alternate layers.
    • Transformations of austenite to Bainite 3) At further lower temperatures, 250 ℃ < Tt < 500 ℃, below the nose in TTT diagram. • Driving force for the transformation ( → + Fe3C) is very high. • Diffusion rate is very low. • Nucleation rate is very high. → + Fe3C (But not in the form of alternate layers) : Bainite ; cementite in the form of needle type. 495 ℃ 410 ℃ bainite
    • TTT diagram for eutectoid steel • Plot the time for start and completion of transformation at different temperatures at still lower temperatures
    • On further decreasing the transformation temperature • Below a certain temperature, the austenite changes or transforms to martensite. • Martensite is a super saturated solid solution of carbon in iron. • It is a diffusionless transformation. • It is also known as shear transformation as the interface between austenite and martensite moves as a shear wave at the speed of sound.
    • Transformations of austenite to Martensite 4. When the austenite is quenched to temp. below Ms → ’ (martensite) : Driving force for trans. of austenite → extremely high. Diffusion rate is extremely slow. : Instead of the diffusional migration of carbon atoms to produce separate and Fe3C phases, the matensite transformation involves the sudden reorientation of C and Fe atoms from the austenite (FCC) to a body centered tetragonal (bct) solid solution. → ’ (martensite), a super saturated solid solution of carbon in iron formed by shear transformation (diffusionless transformation) → very hard and brittle phase martensite
    • Diffusionless Transformation 1) Diffusionless transformation → no compositional change during transformation. 2) The temperature at which the transformation of → ’ starts is known as at Ms temp. and finishes at Mf temp. 3) Degree of super saturation and c/a ratio increases as the carbon content increases.
    • Complete TTT (isothermal transformation) diagram for eutectoid steel.
    • Time Temperature Transformation (TTT) Diagram • Below A1 , austenite is unstable, i.e., it can transform into pearlite, bainite or martensite. • The phases finally formed during cooling depend upon time and temperature. • TTT diagram shows the time required for transformation to various phases at constant temperature, and, therefore, gives a useful initial guide to likely transformations. • In addition to the variations in the rate of transformation with temperature, there are variations in the structure of the transformation products also.
    • The Time – Temperature – Transformation Curve (TTT) • • • Spring 2001 Dr. Ken Lewis At slow cooling rates the trajectory can pass through the Pearlite and Bainite regions Pearlite is formed by slow cooling – Trajectory passes through Ps above the nose of the TTT curve Bainite – Produced by rapid cooling to a temperature above Ms – Nose of cooling curve avoided. ISAT 430 43
    • The Time – Temperature – Transformation Curve (TTT) • If cooling is rapid enough austenite is transformed into Martensite. – FCC → BCT – diffusion separation of carbon and iron is not possible • Transformation begins at Ms and ends at Mf. – If cooling is stopped at a temperature between Ms and Mf , it will transform into martensite and bainite . Spring 2001 Dr. Ken Lewis ISAT 430 44
    • Full TTT Diagram The complete TTT diagram for an ironcarbon alloy of eutectoid composition. A: austenite B: bainite M: martensite P: pearlite
    • TTT Diagram • Transformations at temperatures between approximately 705°C and 550°C result in the characteristic lamellar microstructure of pearlite. • At a temperature just below A1 line, nucleation of cementite from austenite will be very slow, but diffusion and growth of nuclei will proceed at maximum speed, so that there will be few large lamellae and the pearlite will be coarse. • However, as the transformation temperature is lowered, i.e., it is just above the nose of the Ccurve, the pearlite becomes fine.
    • Bainite • At temperatures between 550°C and 240°C (the approximate, Ms temperature line), transformation becomes more sluggish as the temperature falls, for, although austenite becomes increasingly unstable, the slower rate of diffusion of carbon atoms in austenite at lower temperatures outstrips the increased urge of the austenite to transform. In this temperature range the transformation product is bainite. • Bainite consists (like pearlite) of a ferrite matrix in which particles of cementite are embedded. The individual particles are much finer than in pearlite. The appearance of bainite may vary between – feathery mass of fine cementite and ferrite for bainite formed around 480°C and – dark acicular (needle shaped) crystals for bainite formed in the region of around 310°C).
    • Martensite • At the foot of the TTT diagram, there are two lines Ms (240°C ) and Mf (50°C). • Ms represents the temperature at which the formation of martensite will start and Mf the temperature at which the formation of martensite will finish during cooling of austenite through this range.
    • Martensite • • • Martensite is formed by the diffusionless transformation of austenite on rapid cooling to a temperature below 240°C (approximately) designated as Ms temperature. The martensitic transformation differs from the other transformations in that it is not time dependent and occurs almost instantaneously, the proportion of austenite transformed to martensite depends only on the temperature to which it is cooled. For example the approximate temperatures at which 50% and 90% of the total austenite will, on quenching, transform to martensite are 166°C and 116°C respectively.
    • Martensite (i) Martensite is a metastable phase of steel, formed by transformation of austenite below Ms temperature. (ii) Martensite is an interstitial supersaturated solid solution of carbon in iron having a bodycentered tetragonal lattice. (iii) Martensite is normally a product of quenching. (iv) Martensite is very hard, strong and brittle.
    • Martensite • Diffusionless transformation of FCC to BCT (more volume) • Very hard & very brittle.
    • Possible transformation involving austenite decomposition
    • The Time – Temperature – Transformation Curve (TTT) • Composition Specific – This curve is for 0.8% carbon • At different compositions, shape is different Spring 2001 Dr. Ken Lewis ISAT 430 54
    • TTT Diagram • The TTT digrams for hypo-eutectoid steels and hyper-eutectoid steels will differ from that of eutectoid steels • The TTT digrams for hypo-eutectoid steels will have an additional curve to show the precipitation of ferrite from martensite before transformation of remaining austenite to pearlite
    • TTT diagram for Hypo-eutectoid steel.
    • TTT Diagram • The TTT digrams for hyper-eutectoid steels will differ from that of eutectoid steels • The TTT diagrams for hyper-eutectoid steels will have an additional curve to show the precipitation of cementite from martensite before transformation of remaining austenite to pearlite
    • TTT diagram for a hypereutectoid Steel (1.13 wt% C)
    • ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a 10110 steel.
    • Contineous Cooling Transformation (CCT) Diagram
    • CCT Diagram • If you don’t hold at one temperature and allow temperature to change with time, you are “Continuously Cooling”. • In continuous cooling, the constant temperature basis of TTT diagram becomes obviously unrepresentative. • More relevant information can, thus, be obtained from a CCT diagram in which phase changes are tracked for a variety of cooling rates. • Therefore, a CCT diagram’s transition lines will be different than a TTT diagram. • Plotting actual cooling curves on such a diagram will show the types of transformation product formed and their proportions.
    • Continuous cooling transformation diagram for eutectoid steels • Annealing : heat the steel into region → cool it in furnace (power off) → coarse pearlite • Normalizing : heat the steel into region → cool it in air → fine pearlite • Hardening : heat the steel into region → quench it in water → Martensite
    • The CCT diagram for a low-alloy, 0.2% C Steel
    • Effect of Cooling Rate on the Formation of Different Reaction Products • Very slow cooling rate (furnace cooling), typical of conventional annealing, will result in coarse pearlite with low hardness. • Air cooling is a faster cooling rate than annealing and is known as nonmalizing. It produces fine pearlite. • In water quenching, entire substance remains austentic until the Ms line is reached, and changes to martensite between the Ms and Mf lines.
    • Effect of Cooling Rate on the Formation of Different Reaction Products • It is possible to form 100% pearlite or 100% martensite by continuous cooling, but it is not possible to form 100% Bainite. • To obtain a bainitic structure, cool rapidly enough to miss the nose of curve and then holding in the temperature range at which bainite is formed.
    • Critical Cooling Rate (CCR) • If the cooling curve is tangent to the nose of TTT curve, the cooling rate associated with this cooling curve is Critical Cooling Rate (CCR) for this steel. • Any cooling rate equal to or faster than CCR will form only martensite.
    • Critical Cooling Rate and Hardness of Different Micro-Structures Critical cooling rate Hardness of different structures
    • Factors Affecting Critical Cooling Rate • Any thing which shifts the TTT diagrm towards right will decrease the critical cooling rate • The following factor affect the critical coolin rate – 1. Grain size – 2. Carbon content – 3. Alloying elements Increase in grain size, carbon content or alloying elements shifts the TTT diagram towards right and hence reduces the critical cooling rate as shown in next slide.
    • Effect of Carbon Content and Grain Size on Critical Cooling Rate
    • Superposition of TTT and CCT Diagrams for Eutectoid Steel
    • The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).
    • ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. (a)TTT and (b) (b) CCT curves for a 4340 steel.
    • Factors Affecting TTT Diagram • 1. Grain size • 2. Carbon content • 3. Alloying elements
    • Effect of Grain Size • Fine grain steels tend to promote formation of ferrite and pearlite from austenite. • Hence decrease in grain size shifts the TTT diagram towards left. • Therefore, critical cooling rate increases with decrease in grain size.
    • Effect of Carbon Content • There is a significant influence of composition on the TTT and CCT diagrams. For the transformation diagrams we see the effect through a shift in the transformation curves. For example: – An increase in carbon content shifts the CCT and TTT curves to the right (this corresponds to an increase in hardenability as it increases the ease of forming martensite - i.e. the cooling rate required to attain martensite is less severe). – An increase in carbon content decreases the Ms (martensite start) temperature.
    • Effect of Alloying Elements • Different alloying elements have their different effects on TTT diagram. • An increase in alloy content shifts the CCT and TTT curves to the right and • Alloying elements also modify the shape of the TTT diagram and separate the ferrite + pearlite region from the bainite region making the attainment of a bainitic structure more controllable.
    • Effect of Alloying Elements on TTT Diagram
    • Slow Cooling In attained Time in region Formation of ferrite and indicates amount of pearlite microconstituent!
    • Medium Cooling In attained Formation of Bainite
    • Fast Cooling In attained Martensite in ~ 1 minute of cooling
    • • THANKS