IT Phsae transformation of metals

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

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

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

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

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

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

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

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

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

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

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

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

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

Continuous Cooling Curve Actual processes involves cooling – not isothermal Can’t cool at infinite speed

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

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

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

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

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

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

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

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

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