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# TTT diagram

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### TTT diagram

1. 1. TIME-TEMPERATURETRANSFORMATION DIAGRAM
2. 2. 0.8C Spring 2001 Dr. Ken Lewis ISAT 430 2
3. 3. Phase diagram and TTT diagram Which informations are obtained from phase diagram or TTT diagram? • Phase diagram : – Describes equilibrium microstructural development that is obtained at extremely sow cooling or heating conditions. – Provides no information on time to take to form phase and on shapes, size and distribution of phase → importance of kinetics • TTT diagram – For a given alloy composition, the percentage completion of a given phase transformation on temperature-time axes is described.
4. 4. Nucleation And Growth • For a eutectic reaction : L (XE) → + at TE (experiment) (1) Quench the liquid from Tm to some lower temperature (2) Measure the time for solidification, to go to completion at that temperature • TTT diagram – The time required for the liquid to transform to the eutectic microstructure is function of time
5. 5. Description of new phase from melt • Homogeneous nucleation : occurs within a homogeneous medium. • Heterogeneous nucleation : nucleation occurs at some structural imperfection such as foreign surface, and hence with reduced surface energy local atomic fluctuation formation of many small nuclei growth of nuclei with critical size or greater
6. 6. Change in free energy for homogeneous nucleation For the transformation of liquid to solid ; L→S ; GV H T Tm and for forming a spherical nucleus GT 4 3 r GV 3 4 r2 GT = total free energy change r = radius of embryo or nucleus = specific surface free energy GV = volume free energy change Solid Solid-liquid interface Liquid
7. 7. d GT dr G n * * at r = 0 16 r 3 3( GV ) 2 s exp( r* 2 16 rTm 3 H T2 G* ) kT Nuclei larger than critical size (r*) are stable and can continue to grow. r* 2r GV n* : number of spherical nucleus of radius r*
8. 8. Nucleation rate . • Nucleation rate, N : number of nuclei / unit volume / unit time . N exp( G* ) exp( kT ED ) kT ,where G* : energy barrier to form a nucleus stable to grow. G* • At T just below Tm, – Diffusion rate is rapid but very few nuclei are formed. ∵ G* ↑ • At very low T ( T ↑) – Diffusion rate is extremely low but many nuclei are formed ∵ G* ↓ • At intermediate T . – Max. N 1 (Tm T ) 2 1 T2 ED : activation energy for diffusion
9. 9. Growth of nuclei • Growth of Nuclei – Growth of nuclei is a diffusional process . G ce QD RT , where QD : activation energy for self diffusion . • Transformation rate of a phase : . N G
10. 10. TTT Diagram • Temperature-time-transformation curve • TTT diagrams represent specific thermal histories for the given microstructure.
11. 11. Isothermal transformation of eutectoid steel
12. 12. TTT diagram for eutectoid steel
13. 13. Transformations of austenite : → + Fe3C pearlite A. Diffusional transformations 1) At slightly lower T below 727 ℃ : T << • Coarse pearlite : nucleation rate is very low. : diffusion rate is very high. 2) As the Tt (trans. temp.) decreases to 500 ℃ • Fine pearlite : nucleation rate increases. : diffusion rate decreases. Strength : (MPa) = 139 + 46.4 S-1 S : intermetallic spacing 655 ℃ 600 ℃ 534 ℃ 487 ℃
14. 14. A. Diffusional transformations 3) 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 : Bainite ; cementite in the form of needle type. 495 ℃ 410 ℃ bainite
15. 15. B. Diffusionless Transformations - Martensitic trans. : When the austenite is quenched to temp. below Ms → ’ (martensite) : Driving force for trans. of austenite → extremely high. Diffusoin 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 solid solution : super saturated carbon atoms in ’ + shearlike transformation → very hard and brittle phase martensite
16. 16. 1) Diffusionless transformation → no compositional change during transformation. 2) The trans. of → ’ starts at Ms temp. and finishes at Mf temp. 3) → ’ (BCT) ; c/a increases as the carbon content increases.
17. 17. 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 18
18. 18. The Time – Temperature – Transformation Curve (TTT) • If cooling is rapid enough austenite is transformed into Martensite. – FCC > BCT – Time dependent diffusion separation of ferrite and iron carbide is not necessary • Transformation begins at Ms and ends at Mf. – If cooling stopped it will transition into bainite and Martensite. Spring 2001 Dr. Ken Lewis ISAT 430 19
19. 19. The Time – Temperature – Transformation Curve (TTT) • Composition Specific – Here 0.8% carbon • At different compositions, shape is different Spring 2001 Dr. Ken Lewis ISAT 430 20
20. 20. Full TTT Diagram The complete TTT diagram for an ironcarbon alloy of eutectoid composition. A: austenite B: bainite M: martensite P: pearlite
21. 21. Martensite • Diffusionless transformation of FCC to BCT (more volume!) • Lenticular structure • Very hard & very brittle.
22. 22. TTT Diagrams
23. 23. TTT diagram for Hypo-eutectoid steel.
24. 24. TTT diagram for a hypereutectoid Steel (1.13 wt% C)
25. 25. So What’s a CCT Diagram? • Phase Transformations and Production of Microconstituents takes TIME. • Higher Temperature = Less Time. • If you don’t hold at one temperature and allow time to change, you are “Continuously Cooling”. • Therefore, a CCT diagram’s transition lines will be different than a TTT diagram.
26. 26. Slow Cooling Time in region indicates amount of microconstituent!
27. 27. Medium Cooling Cooling Rate, R, is Change in Temp / Time °C/s
28. 28. Fast Cooling This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!
29. 29. Continuous cooling transformation diagram for eutectoid steels • Normalizing : heat the steel into region → cool it in air → fine pearlite • Annealing : heat the steel into region → cool it in furnace (power off) → coarse pearlite
30. 30. • THANKS