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Phase transformations Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 0 1234566.7 L (austenite) +L+L + Fe 3 C L+Fe 3 C C o, wt% C 1148°C T(°C) 727°C
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Diffusive flow of C needed Growth of pearlite from austenite: pearlite growth direction Austenite ( ) grain boundary cementite (Fe 3 C) Ferrite ( ) 675°C ( T smaller) 0 50 y (% pearlite) 600°C ( T larger) 650°C 100
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Transformations & Supercooling + Fe 3 C 0.76 wt% C 0.022 wt% C 6.7 wt% C Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 0 1234566.7 L (austenite) +L+L +Fe 3 C L+Fe 3 C (Fe) C o, wt%C 1148°C T(°C) ferrite 727°C Eutectoid: TT 0.76 0.022
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Isothermal Transformation Diagrams 100 50 0 110 2 4 T = 675°C y,y, % transformed time (s) 400 500 600 700 110 2 3 4 5 0%pearlite 100% 50% Austenite (stable) T E (727 C) Austenite (unstable) Pearlite T(°C) time (s)
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Eutectoid composition, C o = 0.76 wt% C. Effect of Cooling History in Fe-C System 400 500 600 700 0%pearlite 100% 50% Austenite (stable) T E (727 C) Austenite (unstable) Pearlite T(°C) 110 2 3 4 5 time (s)
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Transformations with Proeutectoid Materials C O = 1.13 wt% C T E (727°C) T(°C) time (s) A A A + C P 11010 2 10 3 10 4 500 700 900 600 800 A + P Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 0 1234566.7 L (austenite) +L+L +Fe 3 C L+Fe 3 C (Fe) C o, wt%C T(°C) 727°C TT 0.76 0.022 1.13
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Non-Equilibrium Transformation Products: Fe-C Bainite: -- strips with long needles of Fe 3 C --diffusion controlled. Isothermal Transf. Diagram Fe 3 C (cementite) 5 m (ferrite) 10 3 5 time (s) 10 400 600 800 T(°C) Austenite (stable) 200 P B TETE 0% 100% 50% pearlite/bainite boundary A A
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Spheroidite: Fe-C System (ferrite) 60 m (cementite) Fe 3 C
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Martensite: -- (FCC) to Martensite (BCT) Isothermal Transf. Diagram Martensite: Fe-C System Martensite needles Austenite 60 m 10 3 5 t/s 10 400 600 800 T(°C) Austenite (stable) 200 P B TETE 0% 100% 50% A A M + A 0% 50% 90% x x x x x x potential C atom sites Fe atom sites (involves single atom jumps)
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Phase Transformations of Alloys
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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: a)42% proeutectoid ferrite and 58% coarse pearlite b)50% fine pearlite and 50% bainite c)100% martensite d)50% martensite and 50% austenite
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Example Problem for C o = 0.45 wt% A + B A + P A + A B P A 50% 0 200 400 600 800 0.11010 3 10 5 time (s) M (start) M (50%) M (90%) T (°C)
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Example Problem for C o = 0.45 wt% A + B A + P A + A B P A 50% 0 200 400 600 800 0.11010 3 10 5 time (s) M (start) M (50%) M (90%) T (°C)
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Example Problem for C o = 0.45 wt% A + B A + P A + A B P A 50% 0 200 400 600 800 0.11010 3 10 5 time (s) M (start) M (50%) M (90%) T (°C)
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Mechanical Prop: Fe-C System (1) Effect of wt% C C o < 0.76 wt% C Hypoeutectoid Pearlite (med) ferrite (soft) C o > 0.76 wt% C Hypereutectoid Pearlite (med) Cementite (hard) 300 500 700 900 1100 YS TS(MPa) wt% C 0 0.5 1 hardness 0.76 Hypo Hyper wt% C 0 0.5 1 0 50 100 %EL Impact energy (Izod, ft-lb) 0 40 80 0.76 Hypo Hyper TS EL I zod
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Mechanical Prop: Fe-C System (2) Fine vs coarse pearlite vs spheroidite 80 160 240 320 wt%C 0 0.5 1 Brinell hardness fine pearlite coarse pearlite spheroidite HypoHyper 0 30 60 90 wt%C Ductility (%AR) fine pearlite coarse pearlite spheroidite HypoHyper 0 0.5 1
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Mechanical Prop: Fe-C System (3) Fine Pearlite vs Martensite: 0 200 wt% C 0 0.5 1 400 600 Brinell hardness martensite fine pearlite HypoHyper
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Tempering Martensite 9 m MPa 800 1000 1200 1400 1600 1800 30 40 50 60 200400600 Tempering T (°C) %RA TS YS %RA
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Summary: Processing Options Austenite ( ) Bainite ( + Fe 3 C plates/needles) Pearlite ( + Fe 3 C layers + a proeutectoid phase) Martensite (BCT phase diffusionless transformation) Tempered Martensite ( + very fine Fe 3 C particles) slow cool moderate cool rapid quench reheat Strength Ductility Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends
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