Chapter 10: Phase Transformations

1. Chapter 10: Phase Transformations Chapter 9 assumes equilibrium – takes too long Chapter 10 – effect of time • Transforming one phase into another takes time. • How does the rate of transformation depend on time and T? • How the rate of transformation affect the structures formed? • How are mechanical properties affected?

2. Chapter 10: Phase Transformations Classes: 1. Simple diffusion-dependent transformation: no change in the number or composition of phases 2. Diffusion-dependent with changes in compositions and/or number of phases (e.g., eutectic reaction) 3. Diffusionless transformation – produces metastable phase • Impediments to PT: • Time-dependent diffusion, • Phase boundary energy • Processes: • Nucleation, • Grain growth

3. FRACTION OF TRANSFORMATION • Fraction transformed depends on time. • Transformation rate depends on T. • Problems with phase diagrams; - rates not shown - equilibrium cooling is too slow  supercooling  transformation occurs at a lower temperature

4. Super Cooling and Eutectoid Transformation Rate • Growth of pearlite from austenite: • Reaction rate increases with DT.

5. NUCLEATION AND GROWTH • Reaction rate is a result of nucleation and growth of crystals. • Examples:

6. ISOTHERMAL COOLING • Quickly cool to below TE and keep at constant temp • Fe-C system, Co = 0.77wt%C • Transformation at T = 675C.

7. Microstructural Development: Example • Eutectoid composition, Co = 0.77wt%C • Begin at T > 727C • Rapidly cool to 625C and hold isothermally.

8. Isothermal Transformation Diagrams or TTT (time-temp-transformation) plots • Above TE only • For 540<T<726C, • two solid lines: • - left of first line -  only • - right of 2nd line – P only • - rate controlled by nucleation • - Coarse vs. fine pearlite: • - high temp: coarse P (faster diffusion  thicker layers) • - low temp: fine pearlite • Below 540C • - Bainite formation • - rate is diffusion controlled • Below 215C - Martensite

9. Coarse vs. Fine Pearlite Two cases: • Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser. • Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer.

10. NON-EQUIL TRANSFORMATION PRODUCTS: Fe-C • Bainite: - elongated Fe3C particles in a continuous a matrix - diffusion controlled

11. Spheroidite • spherelike particles of Fe3C in a continuous ferrite matrix • heating pearlite or bainite to below 726 for a long time (e.g., 700C for 18-24 hours) • Additional diffusion: reduction in surface boundary

12. Martensite • Martensite: - metastable phase - polymorphic: g(FCC) to Martensite (BCT) • Isothermal Transf. Diagram • g to M transformation - diffusionless - rapid - athermal (independent of time).

13. Isothermal Cooling: Examples

14. Isothermal Cooling: Example

15. Isothermal Cooling:

16. Continuous Cooling Transformation (CCT) diagrams • delayed start and end • results depend on cooling rate • - moderately rapid – fine P • - slow – coarse pearlite • no bainite formation

17. Continuous Cooling • three regions: pure martensite, M+P, pure P • critical cooling rate – minimum rate that produces totally martensitic structure

18. Mechanical Properties • presence of other allowing elements – (e.g., Cr, Ni, Mo, and W) – delay pearlite formation  stronger • other %C compositions – proeutectoid phase

19. Mechanical Properties

20. Mechanical Properties • Fine Pearlite vs Martensite: • Hardness: fine pearlite << martensite.

21. Tempered Martensite • • heat martensitic steel below eutectoid (250-650C) • martensite (BCT - supersaturated)  stable  and Fe3C • similar to spheroidite but smaller cementite particles. • reduces brittleness of martensite.

22. SUMMARY: PROCESSING OPTIONS