TTT diagram

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

1. TIME-TEMPERATURE-TRANSFORMATION DIAGRAM

2. Spring 2001 Dr. Ken Lewis ISAT 430 2 0.8C

3. Phase diagram and 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. Which informations are obtained from phase diagram or TTT diagram?

4. Nucleation And Growth For a eutectic reaction : L (X E ) →  +  at T E (experiment) (1) Quench the liquid from T m 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. Description of new phase from melt local atomic fluctuation formation of many small nuclei growth of nuclei with critical size or greater 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

6. For the transformation of liquid to solid ; L → S ; and for forming a spherical nucleus  G T = total free energy change r = radius of embryo or nucleus  = specific surface free energy  G V = volume free energy change Change in free energy for homogeneous nucleation Solid-liquid interface Liquid Solid

7. n * : number of spherical nucleus of radius r * at r = r * Nuclei larger than critical size (r * ) are stable and can continue to grow.

8. Nucleation rate Nucleation rate, : number of nuclei / unit volume / unit time ,where G * : energy barrier to form a nu cleus stable to grow. E D : activation energy for diffusion At T just below T m , 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.

9. Growth of nuclei Growth of Nuclei Growth of nuclei is a diffusional process , where Q D : activation energy for self diffusion Transformation rate of a phase :

10. TTT Diagram Temperature-time-transformation curve TTT diagrams represent specific thermal histories for the given microstructure.

11. Isothermal transformation of eutectoid steel

12. TTT diagram for eutectoid steel

13. Transformations of austenite :  →  + Fe 3 C 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 T t (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 ℃ pearlite

14. A. Diffusional transformations 3) 250 ℃ < T t < 500 ℃, below the nose in TTT diagram. Driving force for the transformation (  →  + Fe 3 C) is very high. Diffusion rate is very low. Nucleation rate is very high.  →  + Fe 3 C : Bainite ; cementite in the form of needle type. 495 ℃ 410 ℃ bainite

15. B. Diffusionless Transformations - Martensitic trans. : When the austenite is quenched to temp. below M s  → ’ ( 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 Fe 3 C 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. 1) Diffusionless transformation → no compositional change during transformation. 2) The trans. of  → ’ starts at M s temp. and finishes at M f temp. 3)  → ’ (BCT) ; c/a increases as the carbon content increases.

17. Spring 2001 Dr. Ken Lewis ISAT 430 18 The Time – Temperature – Transformation Curve (TTT) 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.

18. Spring 2001 Dr. Ken Lewis ISAT 430 19 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 M s and ends at M f . If cooling stopped it will transition into bainite and Martensite.

19. Spring 2001 Dr. Ken Lewis ISAT 430 20 The Time – Temperature – Transformation Curve (TTT) Composition Specific Here 0.8% carbon At different compositions, shape is different

20. Full TTT Diagram The complete TTT diagram for an iron-carbon alloy of eutectoid composition. A: austenite B: bainite M: martensite P: pearlite

21. Martensite Diffusionless transformation of FCC to BCT (more volume!) Lenticular structure Very hard & very brittle.

22. TTT Diagrams

23. TTT diagram for Hypo-eutectoid steel.

24. TTT diagram for a hypereutectoid Steel (1.13 wt% C)

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. Slow Cooling Time in region indicates amount of microconstituent!

27. Medium Cooling Cooling Rate, R, is Change in Temp / Time °C/s

28. Fast Cooling This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

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. THANKS