Chapter 10: Phase Transformations T(°C) 1600 Δ 1400 L Γ +L

Chapter 10: Phase Transformations T(°C) 1600 δ 1400 L γ +L

γ 1200 1148°C L+Fe C (austenite) 3 1000 γ +Fe3C 800

a C (cementite) 3 600 α +Fe C 3 Fe 400 0 1 2 3 4 5 6 6.7 (Fe) Co , wt%C

Chapter 10 - 1 Chapter 10: Phase Transformations

• Transforming one phase into another takes time.

Fe Fe C Eutectoid 3 γ transformation (cementite) (Austenite) + α C FCC (ferrite) (BCC)

• How does the rate of transformation depend on

time and T ? • How can we slow down the transformation so that we can engineer non-equilibrium structures? • Are the mechanical properties of non-equilibrium structures better? Chapter 10 - 2 t=time! T=temperature! ΔT= temperature difference from phase transition temperature

Chapter 10 - 3 Phase Transformations

Nucleation – nuclei (seeds) act as template to grow crystals Driving force to nucleate increases as we increase ΔT

In Δ T range close to Tm, rate of nucleation higher with higher Δ T – supercooling (eutectic, eutectoid) Growth Growth rate increases with T (thermally activated) dG/dt= C exp (-Q/kT)

Small supercooling  few nuclei - large crystals Large supercooling  rapid nucleation - many nuclei, small crystals

Chapter 10 - 4 Chapter 10 - Rate of Phase Transformations

Kinetics - measure approach to equilibrium vs. time • Hold temperature constant & measure conversion vs. time How is conversion measured? X-ray diffraction – have to do many samples electrical conductivity – follow one sample sound waves – one sample

Chapter 10 - 6 Rate of Phase Transformation y

All out of material - done Fixed T

0.5 maximum rate reached – now amount unconverted decreases so rate slows rate increases as surface area increases t0.5 & nuclei grow Fraction transformed, log t Adapted from Fig. 10.10, Callister 7e. Avrami rate equation => y = 1- exp (-ktn) fraction time transformed

– k & n fit for specific sample

By convention r = 1 / t0.5 Chapter 10 - 7 Eutectoid Transformation Rate

• Growth of pearlite from austenite: Diffusive flow of C needed Austenite (γ) cementite (Fe3C) grain α Ferrite (α) α boundary α γ γ γ α γ Adapted from α pearlite α Fig. 9.15, α growth Callister 7e. α direction α

• Recrystallization 100 600°C rate increases (ΔT larger) 650°C with ΔT. 50 Adapted from 675°C Fig. 10.12, (ΔT smaller) Callister 7e. (% pearlite)

y 0

Course pearlite  formed at higher T - softer Fine pearlite  formed at low T - harder Chapter 10 - 8 T(°C) 1600 δ 1400 L γ +L γ 1200 1148°C L+Fe C (austenite) 3 1000 γ +Fe3C 800

a C (cementite) 3 600 α +Fe C 3 Fe 400 0 1 2 3 4 5 6 6.7 (Fe) Co , wt%C

Chapter 10 - 9 Nucleation and Growth • Reaction rate is a result of nucleation and growth of crystals. 100 Nucleation rate increases with ΔT % Pearlite Growth regime Growth rate increases with T 50 Nucleation regime t Adapted from 0 0.5 log (time) Fig. 10.10, Callister 7e. • Examples: pearlite γ colony γ γ

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

Chapter 10 - 10 Isothermal Transformation Diagrams

• Fe-C system, Co = 0.76 wt% C • Transformation at T = 675°C.

100

T = 675°C , y 50

% transformed 1 10 2 10 4 time (s) T(°C) Austenite (stable) T (727°C) 700 Austenite E (unstable) 600 Pearlite isothermal transformation at 675°C 500

400 2 3 4 5 time (s) 1 10 10 10 10 10 Chapter 10 - 11 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.

T(°C) Austenite (stable) TE (727°C) 700 Austenite (unstable)

Adapted from Fig. 600 Pearlite 10.14,Callister 7e. γ γ (Fig. 10.14 adapted from H. Boyer (Ed.) Atlas of γ γ γ γ Isothermal Transformation 500 and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) 400

1 10 10 2 10 3 10 4 10 5 time (s) Chapter 10 - 12 Non-Equilibrium Transformation Products: Fe-C • Bainite: --α lathes (strips) with long rods of Fe3C --diffusion controlled. Fe3C • Isothermal Transf. Diagram (cementite) α (ferrite) 800 Austenite (stable) T T(°C) A E P 600 100% pearlite pearlite/bainite boundary 5 µm (Adapted from Fig. 10.17, Callister, 7e. (Fig. 100% bainite 10.17 from Metals Handbook, 8th ed., B Vol. 8, Metallography, Structures, and Phase 400 A Diagrams, American Society for Metals, Materials Park, OH, 1973.)

200

-1 3 5 10 10 10 10 time (s) Adapted from Fig. 10.18, Callister 7e. (Fig. 10.18 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Chapter 10 - 13 Transformation Diagrams, American Society for Metals, 1997, p. 28.) Martensite: Fe-C System • Martensite: --γ(FCC) to Martensite (BCT) (involves single atom jumps)

Fe atom x m potential µ sites x x x x C atom sites 60 x (Adapted from Fig. 10.20, Callister, 7e. • Isothermal Transf. Diagram 800 Austenite (stable) Martensite needles Austenite T(°C) A TE P (Adapted from Fig. 10.21, Callister, 7e. 600 (Fig. 10.21 courtesy United States Steel Corporation.) Adapted from Fig. 10.22, B Callister 7e. 400 A • γ to M transformation.. -- is rapid! 200 0% -- % transf. depends on T only. M + A 50% M + A 90% M + A -1 3 5 10 10 10 10 time (s) Chapter 10 - 14 Chapter 10 - Martensite Formation

slow cooling γ (FCC) α (BCC) + Fe3C quench tempering M (BCT)

M = martensite is body centered tetragonal (BCT)

Diffusionless transformation BCT if C > 0.15 wt% BCT  few slip planes  hard, brittle

Chapter 10 - 16 Spheroidite: Fe-C System

• Spheroidite: α --α grains with spherical Fe3C (ferrite) --diffusion dependent.

--heat bainite or pearlite for long times Fe3C --reduces interfacial area (driving force) (cementite)

60 µm (Adapted from Fig. 10.19, Callister, 7e. (Fig. 10.19 copyright United States Steel Corporation, 1971.)

Chapter 10 - 17 Mechanical Prop: Fe-C System (1)

• Effect of wt% C Pearlite (med) Pearlite (med) C ementite ferrite (soft) (hard) Adapted from Fig. 9.30,Callister Co < 0.76 wt% C Co > 0.76 wt% C Adapted from Fig. 9.33,Callister 7e. 7e. (Fig. 9.30 courtesy Republic (Fig. 9.33 copyright 1971 by United Steel Corporation.) Hypoeutectoid Hypereutectoid States Steel Corporation.) Hypo Hyper Hypo Hyper TS(MPa)

1100 %EL 80 YS(MPa) 100 Adapted from Fig. 10.29, Callister 7e. 900 hardness (Fig. 10.29 based on 40 data from Metals 700 Handbook: Heat 50 Treating, Vol. 4, 9th ed., V. Masseria 500 (Managing Ed.), 0 American Society for 300 Metals, 1981, p. 9.) Impact energy (Izod, ft-lb)

0 0 0.5 1 0 0.5 1

0.76 wt% C 0.76 wt% C • More wt% C: TS and YS increase , %EL decreases.

Chapter 10 - 18 Mechanical Prop: Fe-C System (2)

• Fine vs coarse pearlite vs spheroidite

Hypo Hyper 90 Hypo Hyper 320 fine spheroidite pearlite 60 240 coarse pearlite spheroidite 160 30 coarse Ductility (%AR) pearlite

Brinell hardness fine 80 pearlite 0 0 0.5 1 0 0.5 1 wt%C wt%C Adapted from Fig. 10.30, Callister 7e. • Hardness: fine > coarse > spheroidite (Fig. 10.30 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th • %RA: fine < coarse < spheroidite ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)

Chapter 10 - 19 Mechanical Prop: Fe-C System (3)

• Fine Pearlite vs Martensite:

Hypo Hyper

600 martensite Adapted from Fig. 10.32, Callister 7e. (Fig. 10.32 adapted 400 from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals,

Brinell hardness 1939, p. 36; and R.A. Grange, 200 C.R. Hribal, and L.F. Porter, fine pearlite Metall. Trans. A, Vol. 8A, p. 1776.) 0 0 0.5 1 wt% C • Hardness: fine pearlite << martensite.

Chapter 10 - 20 Tempering Martensite • reduces brittleness of martensite, • reduces internal stress caused by quenching. TS(MPa) YS(MPa) 1800 1600 TS

Adapted from Adapted from Fig. 10.34, 1400 YS Fig. 10.33, m Callister 7e. Callister 7e. µ (Fig. 10.34 (Fig. 10.33 1200 9 adapted from 60 copyright by Fig. furnished United States 1000 50 courtesy of %RA %RA Steel Republic Steel 40 Corporation, Corporation.) 1971.) 800 30 200 400 600 Tempering T (°C)

• produces extremely small Fe3C particles surrounded by α. • decreases TS, YS but increases %RA Chapter 10 - 21 Summary: Processing Options

Adapted from Austenite (γ) Fig. 10.36, Callister 7e. slow moderate rapid cool cool quench

Pearlite Bainite Martensite (α + Fe3C layers + a (α + Fe3C plates/needles) (BCT phase proeutectoid phase) diffusionless transformation) Martensite reheat

T Martensite bainite Tempered fine pearlite Martensite ( + very fine Ductility Strength coarse pearlite α Fe C particles) spheroidite 3 General Trends Chapter 10 - 22