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Chapter 10: Phase Transformations

Chapter 10: Phase Transformations. Fe 3 C. Fe. g. Eutectoid. (cementite). transformation. +. (Austenite). a. C. (BCC). FCC. (ferrite). ISSUES TO ADDRESS. • Transforming one phase into another takes time. Phase Transformation. Nucleation and Growth. 100.

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Chapter 10: Phase Transformations

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  1. Chapter 10:Phase Transformations Fe3C Fe g Eutectoid (cementite) transformation + (Austenite) a C (BCC) FCC (ferrite) ISSUES TO ADDRESS... • Transforming one phase into another takes time.

  2. Phase Transformation

  3. Nucleation and Growth 100 Nucleation rate increases with T % Pearlite Growth Growth rate increases with T regime 50 Nucleation regime t log(time) 0.5 0 pearlite colony g g g T just below TE T moderately below TE T way below TE Nucleation rate low . Nucleation rate med Nucleation rate high Growth rate high Growth rate med. Growth rate low • Reaction rate is a result of nucleation and growth of crystals. • Examples: Eutectoid reaction

  4. Nucleation nuclei (seeds) act as template to grow crystals Nucleation driving force increases as increase T • Supercooling (or undercooling) • superheating Small supercooling  few nuclei, large crystals Large supercooling  rapid nucleation, many nuclei, small crystals

  5. Fe-Fe3C phase diagram

  6. Solidification: Nucleation Processes • Homogeneous nucleation • nuclei form in the bulk of liquid metal • requires supercooling (typically 80-300°C max) • Heterogeneous nucleation • much easier since stable “nucleus” is already present • Could be wall of mold or impurities in the liquid phase • allows solidification with only 0.1-10ºC supercooling

  7. Surface Free Energy-destabilizes the nuclei (energy for makinmg surface) ,g = surface tension HS = latent heat of solidification DGT = Total Free Energy = DGS + DGV Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy) Homogeneous Nucleation & Energy Effects r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow

  8. Homogenous and Heterogeneous Nucleation

  9. Completely growth maximum rate reached – now amount unconverted decreases so rate slower rate increases as surface area increases & nuclei grow By convention r = 1 / t0.5 Rate of Phase Transformation Fixed T: Isothermal Fraction transformed, y log t

  10. Isothermal Transformation Diagrams 100 T(°C) 600°C Austenite (stable) (DT larger) TE(727C) 650°C 50 y (% pearlite) 700 Austenite 675°C (unstable) (DT smaller) Pearlite 600 0 isothermal transformation at 675°C 500 100% 50% 0%pearlite 400 time (s) 2 3 4 5 1 10 10 10 10 10 • Fe-C system, Co = 0.76 wt% C • Transformation at T = 675°C. 100 T = 675°C y, % transformed 50 0 2 4 time (s) 1 10 10 Course pearlite  formed at higher T - softer Fine pearlite  formed at lower T - harder

  11. 135C 119C 113C 102C 88C 43C 1 10 102 104 Rate of Phase Transformations % Recrystallization of Rolled Copper Percent recrystallization is function of time and temperature

  12. g Þ a + Fe3C 0.76 wt% C 6.7 wt% C 0.022 wt% C T(°C) 1600 d L 1400 g +L g 1200 L+Fe3C 1148°C (austenite) 1000 g +Fe3C a Eutectoid: Fe3C (cementite) ferrite Equil. Cooling: Ttransf. = 727ºC 800 727°C DT a +Fe3C 600 Undercooling by DTtransf. < 727C 0.022 0.76 400 0 1 2 3 4 5 6 6.7 Co, wt%C (Fe) Transformations & Undercooling • Eutectoid transformation (Fe-C System): • Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)

  13. cementite (Fe3C) Ferrite (a) a a g g a a pearlite a growth a direction Eutectoid Transformation Rate • Growth of pearlite from austenite: Diffusive flow of C needed a g g a a • Higher T give higher diffusivity

  14. Effect of Cooling History in Fe-C System 2 3 4 5 1 10 10 10 10 10 T(°C) Austenite (stable) TE (727C) 700 Austenite (unstable) Pearlite 600 g g g g g g 100% 500 50% 0%pearlite 400 time (s) • Eutectoid composition, Co = 0.76 wt% C • Begin at T > 727°C • Rapidly cool to 625°C and hold isothermally.

  15. CO = 1.13 wt% C T(°C) T(°C) 900 1600 d A L 1400 800 TE (727°C) A g +L + g 1200 L+Fe3C A C (austenite) 700 P 1000 + Fe3C (cementite) g +Fe3C a P A 600 800 DT a +Fe3C 500 600 0.022 0.76 1 10 102 103 104 1.13 400 727°C time (s) 0 1 2 3 4 5 6 6.7 (Fe) Co, wt%C Transformations with Proeutectoid Materials Hypereutectoid composition – proeutectoid cementite

  16. T-T-T of Eutectoid Composition A – Austenite P – Pearlite B – Bainite M – Martensite C – Cementite

  17. 800 Austenite (stable) TE T(°C) A P 600 100% pearlite pearlite/bainite boundary 100% bainite B 400 A 200 0% 100% 50% 10 -1 3 5 10 10 10 time (s) Non-Equilibrium Transformation Products: Fe-C • Bainite: --a lathes (strips) with long rods of Fe3C --diffusion controlled. • Isothermal Transf. Diagram Fe3C (cementite) a (ferrite) 5 mm T-T-T Diagram Time – Temperature – Transformation

  18. Spheroidite: Fe-C System a (ferrite) Fe3C (cementite) 60 m • Spheroidite: -- a grains with spherical Fe3C -- diffusion dependent. -- heat bainite or pearlite for long times near TE

  19. Martensite: Fe-C System Fe atom potential x sites 60 m C atom sites x x x x x Martensite needles 800 Austenite (stable) Austenite TE T(°C) A P 600 B 400 A 100% 50% 0% 0% 200 M + A 50% M + A 90% M + A time (s) -1 10 3 5 10 10 10 • Martensite: --g(FCC) to Martensite (BCT) • Isothermal Transf. Diagram • g to M transformation -- is rapid! -- % transformation depends on T only.

  20. quench tempering M (BCT) Martensite Formation  (FCC) P  (BCC) + Fe3C slow cooling • M = martensite is body centered tetragonal (BCT) • Diffusionless transformation BCT if C > 0.15 wt% • BCT  few slip planes  hard, brittle

  21. Phase Transformations of Alloys Effect of adding other elements Change transition temp. Cr, Ni, Mo, Si, Mn retard    + Fe3C transformation Alloy steel (type 4340)

  22. Continuous Cooling Curve

  23. Dynamic Phase Transformations On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures: • 42% proeutectoid ferrite and 58% coarse pearlite • 50% fine pearlite and 50% bainite • 100% martensite

  24. 800 A + a A A + P P 600 B A + B A 50% 400 M (start) M (50%) M (90%) 200 0 0.1 10 103 105 time (s) Example Problem for Co = 0.45 wt% • 42% proeutectoid ferrite and 58% coarse pearlite first make ferrite then pearlite course pearlite  higher T T (°C)

  25. 800 A + a A A + P P 600 B A + B A 50% 400 M (start) M (50%) M (90%) 200 0 0.1 10 103 105 time (s) Example Problem for Co = 0.45 wt% • 50% fine pearlite and 50% bainite first make pearlite then bainite fine pearlite  lower T T (°C)

  26. 800 A + a A A + P P 600 B A + B A 50% 400 M (start) M (50%) M (90%) 200 c) 0 0.1 10 103 105 time (s) Example Problem for Co = 0.45 wt% • 100 % martensite – quench = rapid cool T (°C)

  27. Pearlite (med) Pearlite (med) C ementite (hard) ferrite (soft) Co< 0.76 wt% C Co > 0.76 wt% C Hypoeutectoid Hypereutectoid Hypo Hyper Hypo Hyper TS(MPa) %EL 80 1100 YS(MPa) 100 900 hardness 40 700 Impact energy (Izod, ft-lb) 50 500 0 300 0 1 0.5 0 1 0.5 0 0.76 wt% C 0.76 wt% C Mechanical Prop: Fe-C System (1) • Effect of wt% C • More wt% C: TS and YS increase , %EL decreases.

  28. Hypo Hyper Hypo Hyper 90 320 fine spheroidite 60 pearlite 240 coarse Ductility (%RA) pearlite spheroidite Brinell hardness coarse 160 30 pearlite fine pearlite 80 0 1 1 0.5 0 0.5 0 wt%C wt%C Mechanical Prop: Fe-C System (2) • Fine vs coarse pearlite vs spheroidite fine > coarse > spheroidite • Hardness: fine < coarse < spheroidite • %RA:

  29. Hypo Hyper 600 martensite Brinell hardness 400 200 fine pearlite 0 1 0.5 0 wt% C Mechanical Prop: Fe-C System (3) • Fine Pearlite vs Martensite: • Hardness: Fine Pearlite << Martensite. • Hardness: Pearlite < Bainite.

  30. TS(MPa) YS(MPa) 1800 TS 1600 YS 1400 1200 60 50 1000 %RA %RA 40 800 30 200 400 600 Tempering T(°C) produces extremely small Fe3C particles surrounded by a. Tempering Martensite • reduces brittleness of martensite, • reduces internal stress caused by quenching. 9 mm • • decreases TS, YS but increases %RA

  31. Austenite (g) moderate rapid slow cool quench cool Pearlite Bainite Martensite (a + Fe3C plates/needles) (a + Fe3C layers + a (BCT phase diffusionless proeutectoid phase) transformation) reheat Martensite T Martensite Tempered bainite Strength Ductility Martensite fine pearlite (a + very fine coarse pearlite Fe3C particles) spheroidite General Trends Summary: Processing Options

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