Phase transformations 1600 d L 1400 g 1200

Phase transformations 1600 d L 1400 g 1200 g +L 1000 g + Fe 3 C 800 727°C 600 400 L+Fe 3 C 1148°C (austenite) Fe 3 C (cementite) T(°C) a + Fe 3 C 0 1 2 3 Co , wt% C 4 5 6 6. 7

• Growth of pearlite from austenite: Austenite (g) grain boundary a a g cementite (Fe 3 C) Ferrite (a) (DT larger) y (% pearlite) g a 600°C 650°C 675°C (DT smaller) 0 a a pearlite growth direction 100 50 Diffusive flow of C needed g

Transformations & Supercooling g Þ a + Fe 3 C 0. 76 wt% C 6. 7 wt% C 0. 022 wt% C T(°C) 1600 d 1200 L+Fe 3 C 1148°C 1000 g +Fe 3 C Eutectoid: 800 DT 0. 76 400 0 (Fe) 1 727°C a +Fe 3 C 600 0. 022 a ferrite g+L g (austenite) 2 3 4 5 6 Fe 3 C (cementite) L 1400 6. 7 Co , wt%C

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

Effect of Cooling History in Fe-C System • Eutectoid composition, Co = 0. 76 wt% C. T(°C) 700 Austenite (stable) TE (727 C) Austenite (unstable) Pearlite 600 % 100 50% pea 0% 500 rlite 400 1 10 10 2 10 3 time (s) 10 4 10 5

Transformations with Proeutectoid Materials CO = 1. 13 wt% C T(°C) 900 d A C A + P 1200 g L+Fe 3 C 1000 P g +Fe 3 C 800 a 600 500 1 g +L (austenite) 10 102 103 time (s) 104 DT 400 0 (Fe) 0. 76 600 A TE (727°C) A 1 1. 13 + 700 L 1400 0. 022 800 727°C a +Fe 3 C 2 3 4 5 6 Fe 3 C (cementite) 1600 6. 7 Co , wt%C

Non-Equilibrium Transformation Products: Fe-C • Bainite: --a strips with long needles of Fe 3 C --diffusion controlled. • Isothermal Transf. Diagram 800 Austenite (stable) T(°C) A 400 TE P 600 Fe 3 C (cementite) pearlite/bainite boundary B A 10 103 % 100 10 -1 50% 0% 200 105 time (s) a (ferrite) 5 m

Spheroidite: Fe-C System a (ferrite) Fe 3 C (cementite) 60 m

Martensite: Fe-C System • Martensite: --g(FCC) to Martensite (BCT) Fe atom sites x x x 60 m (involves single atom jumps) potential C atom sites x x x • Isothermal Transf. Diagram 800 Austenite (stable) T(°C) A P 600 400 A 200 10 -1 B 5 0% 0% 10 0% 0% 50% 90% M+A M+A 10 TE 103 105 t/s Martensite needles Austenite

Phase Transformations of Alloys

Dynamic Phase Transformations On the isothermal transformation diagram for 0. 45 wt% C Fe-C alloy, sketch and label the timetemperature paths to produce the following microstructures: a) b) c) d) 42% proeutectoid ferrite and 58% coarse pearlite 50% fine pearlite and 50% bainite 100% martensite 50% martensite and 50% austenite

Example Problem for Co = 0. 45 wt% 800 T (°C) A A+a P B 600 A+P A+B A 400 50% M (start) M (50%) M (90%) 200 0 0. 1 10 time (s) 103 105

Example Problem for Co = 0. 45 wt% 800 T (°C) A A+a P B 600 A+P A+B A 400 50% M (start) M (50%) M (90%) 200 0 0. 1 10 time (s) 103 105

Example Problem for Co = 0. 45 wt% 800 T (°C) A A+a P B 600 A+P A+B A 400 50% M (start) M (50%) M (90%) 200 0 0. 1 10 time (s) 103 105

Mechanical Prop: Fe-C System (1) • Effect of wt% C TS(MPa) 1100 Co < 0. 76 wt% C Hypoeutectoid Hypo Hyper Co > 0. 76 wt% C Hypereutectoid Hypo %EL 100 900 TS Hyper 80 Izod hardness 40 700 50 EL 500 0. 5 1 wt% C 0 0 0. 5 0. 76 0 YS 0. 76 300 0 1 Impact energy (Izod, ft-lb) Pearlite (med) ferrite (soft) Pearlite (med) Cementite (hard) wt% C

Mechanical Prop: Fe-C System (2) • Fine vs coarse pearlite vs spheroidite Hypo Hyper 90 Hypo Hyper fine pearlite 240 160 coarse pearlite spheroidite 80 0 0. 5 1 wt%C Ductility (%AR) Brinell hardness 320 spheroidite 60 coarse pearlite fine pearlite 30 0 0 0. 5 1 wt%C

Mechanical Prop: Fe-C System (3) • Fine Pearlite vs Martensite: Brinell hardness Hypo 600 Hyper martensite 400 200 0 fine pearlite 0 0. 5 1 wt% C

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

Summary: Processing Options Austenite (g) slow cool moderate cool rapid quench Bainite Martensite (a + Fe 3 C layers + a proeutectoid phase) (a + Fe 3 C plates/needles) (BCT phase diffusionless transformation) Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends reheat Ductility Strength Pearlite Tempered Martensite (a + very fine Fe 3 C particles)