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Phase transformations

1600. d. L. 1400. T (°C). g. + L. g. 1200. L +Fe 3 C. 1148°C. (austenite). 1000. g. + Fe 3 C. 800. Fe 3 C (cementite). 727°C. 600. a. + Fe 3 C. 400. 0. 1. 2. 3. 4. 5. 6. 6.7. C o , wt% C. Phase transformations. • Growth of pearlite from austenite:.

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Phase transformations

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  1. 1600 d L 1400 T(°C) g +L g 1200 L+Fe3C 1148°C (austenite) 1000 g +Fe3C 800 Fe3C (cementite) 727°C 600 a +Fe3C 400 0 1 2 3 4 5 6 6.7 Co, wt% C Phase transformations

  2. • Growth of pearlite from austenite: Diffusive flow of C needed Austenite (g) cementite (Fe3C) grain a Ferrite (a) a a boundary g g a g g a pearlite a a growth a direction a 600°C (DT larger) 650°C 50 675°C y (% pearlite) (DT smaller) 0 100

  3. 727°C Transformations & Supercooling 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 800 DT a +Fe3C 600 0.022 0.76 400 0 1 2 3 4 5 6 6.7 Co, wt%C (Fe)

  4. Isothermal Transformation Diagrams 100 T = 675°C y, 50 % transformed 0 2 4 time (s) 1 10 10 T(°C) Austenite (stable) TE(727C) 700 Austenite (unstable) Pearlite 600 100% 500 50% 0%pearlite 400 time (s) 2 3 4 5 1 10 10 10 10 10

  5. 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 600 100% 500 50% 0%pearlite 400 time (s) • Eutectoid composition, Co = 0.76 wt% C.

  6. P + A Transformations with Proeutectoid Materials 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 1000 Fe3C (cementite) g +Fe3C P 600 800 727°C a DT a +Fe3C 500 600 0.022 0.76 1 10 102 103 104 1.13 400 time (s) 0 1 2 3 4 5 6 6.7 (Fe) Co, wt%C

  7. 800 Austenite (stable) TE T(°C) A P 600 pearlite/bainite boundary B 400 A 200 0% 100% 50% 10 -1 3 5 10 10 10 time (s) Non-Equilibrium Transformation Products: Fe-C • Bainite: --a strips with long needles of Fe3C --diffusion controlled. • Isothermal Transf. Diagram Fe3C (cementite) a (ferrite) 5 mm

  8. Spheroidite: Fe-C System a (ferrite) Fe3C (cementite) 60 m

  9. Martensite: Fe-C System (involves single atom jumps) x Fe atom potential 60 m x x sites C atom sites 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 t/s -1 10 3 5 10 10 10 • Martensite: --g(FCC) to Martensite (BCT) • Isothermal Transf. Diagram

  10. Phase Transformations of Alloys

  11. 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 • 50% martensite and 50% austenite

  12. 800 A + a A T (°C) 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%

  13. 800 A + a A T (°C) 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%

  14. 800 A + a A T (°C) 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%

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

  16. Hypo Hyper Hypo Hyper 90 320 fine spheroidite 60 pearlite 240 coarse Ductility (%AR) 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

  17. 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:

  18. Tempering Martensite MPa 1800 TS 1600 YS 1400 9 mm 1200 60 50 1000 %RA %RA 40 800 30 200 400 600 Tempering T(°C)

  19. 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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