CHAPTER 10: Phase transformations

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1 CHPTER 10: Phase transformations ISSUES TO DDRESS... Transforming one phase into another takes time. Fe C FCC Eutectoid transformation (ustenite) Fe3C (cementite) + α (ferrite) (BCC) How does the rate of transformation depend on time and T? How can we slow down the transformation so that we can engineering non-equilibrium structures? re the mechanical properties of non-equilibrium structures better? Chapter 10-1

2 Rates of solid state reactions Phase transformation proceeds through 2 stages I- Nucleation Formation of very small crystals (nuclei) of the new phase, which are capable of growing II- Growth Nuclei increase in size with disappearance of parent phase. Growth continues until equilibrium fraction (predicted by phase diagram) is reached Fraction of transformation, y nucleation 0 log (t) t 0.5 Growth Chapter 10-

3 Fraction transformed depends on time. fraction transformed Fraction of transformation vrami Eqn. y = 1 e kt n time 1 y t 0.5 Fixed T log (t) Rate = 1 t 0.5 Transformation rate depends on T thermally activated activation energy r = 1 t 0.5 = e Q /RT y (%) C 119 C 113 C 102 C 88 C Ex: recrystallization of Cu 43 C log (t) min r often small (rate so slow): equilibrium not possible! Chapter 10-2

4 Eutectoid transf. (Fe-C System): Can make it occur at:...727ºc (cool it slowly)...below 727ºC ( undercool it!) T( C) α ferrite Transformations & undercooling L austenite (Fe) α Eutectoid: 0.77 L +Fe3C α +Fe 3 C 0.77wt%C0.022wt%C 6.7wt%C L+Fe3C Equil. cooling: Ttransf. = 727ºC 727 C T α+fe3c Undercooling by T: Ttransf. < 727ºC Co, wt% C 6.7 dapted from Fig. 9.21,Callister 6e. (Fig adapted from Binary lloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), SM International, Materials Park, OH, 1990.) Fe3C cementite Chapter 10-3

5 Eutectoid transformation rate ~ T Growth of pearlite from austenite: ustenite () grain boundary α α αα α α Reaction rate increases with T. y (% pearlite) cementite (Fe3C) ferrite (α) 100 pearlite growth direction 600 C ( T larger) C Diffusive flow of C needed C ( T smaller) time (s) 100 α α α % austenite dapted from Fig. 10.3, Callister 6e. dapted from Fig. 9.13, Callister 6e. Chapter 10-4

6 Reaction rate is a result of nucleation and growth of crystals. 100 % Pearlite 50 0 Examples: Nucleation and growth Nucleation regime t 50 Growth regime Nucleation rate increases w/ T Growth rate increases w/ T log (time) dapted from Fig. 10.1, Callister 6e. pearlite colony T just below TE T moderately below TE T way below TE Nucleation rate low Growth rate high Nucleation rate med. Growth rate med. Nucleation rate high Growth rate low Chapter 10-5

7 Isothermal transformation diagrams Fe-C system, C o = 0.77wt%C Transformation at T = 675C. y, % transformed T( C) T=675 C ustenite (unstable) 100% 0%pearlite ustenite (stable) Pearlite time (s) TE (727 C) isothermal transformation at 675 C time (s) dapted from Fig. 10.4,Callister 6e. (Fig adapted from H. Boyer (Ed.) tlas of Isothermal Transformation and Cooling Transformation Diagrams, merican Society for Metals, 1977, p. 369.) Chapter 10-6

8 Ex: Cooling history Fe-C system Eutectoid composition, C o = 0.77wt%C Begin at T > 727C Rapidly cool to 625C and hold isothermally. T( C) 700 ustenite (stable) TE (727 C) % 0%pearlite Pearlite dapted from Fig. 10.5,Callister 6e. (Fig adapted from H. Boyer (Ed.) tlas of Isothermal Transformation and Cooling Transformation Diagrams, merican Society for Metals, 1997, p. 28.) time (s) Chapter 10-7

10µm dapted from Fig. 10.6 (a) and (b),callister 6e. (Fig. 10.6 from R.M. Ralls et al.

9 Two cases: Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser. Pearlite morphology Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer. 10µm dapted from Fig (a) and (b),callister 6e. (Fig from R.M. Ralls et al., n Introduction to Materials Science and Engineering, p. 361, John Wiley and Sons, Inc., New York, 1976.) - Smaller T: colonies are larger - Larger T: colonies are smaller Chapter 10-8

10 Non-equil transformation products: Fe-C Bainite: --α lathes (strips) with long rods of Fe 3 C --diffusion controlled. Isothermal Transf. Diagram 800 T( C) ustenite (stable) P 100% pearlite 100% bainite B 0% TE pearlite/bainite boundary 100% time (s) Fe3C (cementite) Bainite reaction rate: r bainite = e dapted from Fig. 10.9,Callister 6e. (Fig adapted from H. Boyer (Ed.) tlas of Isothermal Transformation and Cooling Transformation Diagrams, merican Society for Metals, 1997, p. 28.) α (ferrite) 5 µm (dapted from Fig. 10.8, Callister, 6e. (Fig from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, merican Society for Metals, Materials Park, OH, 1973.) Q / RT Chapter 10-9

11 Other products: Fe-C system (1) Spheroidite: --α crystals with spherical Fe 3 C --diffusion dependent. --heat bainite or pearlite for long times --reduces interfacial area (driving force) Isothermal Transf. Diagram 800 T( C) ustenite (stable) P 0% B TE Spheroidite 100% spheroidite 100% spheroidite 100% α (ferrite) Fe3C (cementite) 60 µm (dapted from Fig , Callister, 6e. (Fig copyright United States Steel Corporation, 1971.) dapted from Fig. 10.9,Callister 6e. (Fig adapted from H. Boyer (Ed.) tlas of Isothermal Transformation and Cooling Transformation Diagrams, merican Society for Metals, 1997, p. 28.) time (s) Chapter 10-10

Martensite: --(FCC) to Martensite (BCT) Isothermal Transf. Diagram dapted from Fig. 10.13, Callister 6e.

12 Martensite: --(FCC) to Martensite (BCT) Isothermal Transf. Diagram dapted from Fig , Callister 6e. Other products: Fe-C system (2) (involves single atom jumps) Fe atom sites 800 T( C) x 10-1 x x x x x potential C atom sites ustenite (stable) 0% (dapted from Fig , Callister, 6e. P B 100% S TE 0% M + M + 90% M time (s) 60 µm Martentite needles ustenite (dapted from Fig , Callister, 6e. (Fig courtesy United States Steel Corporation.) to M transformation.. -- is rapid! -- % transf. depends on T only. Chapter 10-11

13 Cooling ex: Fe-C system (1) Co = Ceutectoid Three histories T( C) % 200 ustenite (stable) Case I 0% P B 100% S 100%B M + M + M Rapid cool to: 350 C 250 C 650 C 0% 90% dapted from Fig , Callister 6e. 100% Bainite time (s) Hold for: 10 4 s 10 2 s 20s Rapid cool to: 400 C Hold for: 10 4 s 10 2 s 10 3 s Rapid cool to: Chapter 10-12

14 Co = Ceutectoid Three histories T( C) Cooling ex: Fe-C system (2) ustenite (stable) 100% Case II 0% P B 100% S M + M + M Rapid cool to: 350 C 250 C 650 C 0% 90% M + trace of time (s) Hold for: 10 4 s 10 2 s 20s dapted from Fig , Callister 6e. Rapid cool to: 400 C Hold for: 10 4 s 10 2 s 10 3 s Rapid cool to: Chapter 10-13

15 Cooling ex: Fe-C system (3) Co = Ceutectoid Three histories T( C) 100% P, ustenite (stable) Case III P, P S B 0% P, B 100% 0% 200 M + M + 90% M P, B Rapid cool to: 350 C 250 C 650 C Hold for: 10 4 s 10 2 s 20s dapted from Fig , Callister 6e. time (s) Rapid cool to: 400 C Hold for: 10 4 s 10 2 s 10 3 s Rapid cool to: Chapter 10-14

Mechanical prop: Fe-C system (1) Effect of wt%c Pearlite (med) ferrite (soft) dapted from Fig. 9.27,Callister 6e. (Fig. 9.27 courtesy Republic Steel Corporation.

50 Pearlite (med) Cementite (hard) Co>0.77wt%C Hypereutectoid Hypo 0.77 dapted from Fig. 9.30,Callister 6e. (Fig. 9.30 copyright 1971 by United States Steel Corporation.

16 Mechanical prop: Fe-C system (1) Effect of wt%c Pearlite (med) ferrite (soft) dapted from Fig. 9.27,Callister 6e. (Fig courtesy Republic Steel Corporation.) TS(MPa) 1100 YS(MPa) Hypo Co<0.77wt%C Hypoeutectoid Hyper hardness %EL wt%c wt%c More wt%c: TS and YS increase, %EL decreases. 50 Pearlite (med) Cementite (hard) Co>0.77wt%C Hypereutectoid Hypo 0.77 dapted from Fig. 9.30,Callister 6e. (Fig copyright 1971 by United States Steel Corporation.) Hyper Impact energy (Izod, ft-lb) dapted from Fig , Callister 6e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), merican Society for Metals, 1981, p. 9.) Chapter 10-15

17 Mechanical prop: Fe-C system (2) Fine vs coarse pearlite vs spheroidite Brinell hardness Hypo Hyper Hypo Hyper fine pearlite wt%c coarse pearlite spheroidite Ductility (%R) Hardness: fine > coarse > spheroidite %R: fine < coarse < spheroidite spheroidite 30 coarse pearlite fine pearlite wt%c dapted from Fig , Callister 6e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), merican Society for Metals, 1981, pp. 9 and 17.) Chapter 10-16

18 Mechanical prop: Fe-C system (3) Fine Pearlite vs Martensite: Hypo Hyper Brinell hardness martensite fine pearlite wt%c dapted from Fig , Callister 6e. (Fig adapted from Edgar C. Bain, Functions of the lloying Elements in Steel, merican Society for Metals, 1939, p. 36; and R.. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans., Vol. 8, p ) Hardness: fine pearlite << martensite. Chapter 10-17

Tempering martensite reduces brittleness of martensite, reduces internal stress caused by quenching. TS(MPa) YS(MPa) 1800 dapted from Fig. 10.25, Callister 6e. (Fig. 10.25 adapted from Fig.

19 Tempering martensite reduces brittleness of martensite, reduces internal stress caused by quenching. TS(MPa) YS(MPa) 1800 dapted from Fig , Callister 6e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.) YS %R TS %R Tempering T ( C) 9 µm dapted from Fig , Callister 6e. (Fig copyright by United States Steel Corporation, 1971.) produces extremely small Fe3C particles surrounded by α. decreases TS, YS but increases %R Chapter 10-18

20 Summary: processing options slow cool ustenite () moderate cool rapid quench dapted from Fig , Callister 6e. Pearlite (α + Fe3C layers + a proeutectoid phase) Strength Bainite (α + Fe3C plates/needles) Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends Ductility Martensite (BCT phase diffusionless transformation) reheat Tempered Martensite (α + very fine Fe3C particles) Chapter 10-19

21 Effect of alloying Chapter 10-

22 NNOUNCEMENTS Reading: Read all of Chapter 10; Review the learning objectives on page 299 Core Problems: 10: 1, 10, 1432 Self-help Problems: Review Example problem 10.1 Chapter 10-0

Chapter Outline: Phase Transformations in Metals

Chapter Outline: Phase Transformations in Metals Heat Treatment (time and temperature) Microstructure Mechanical Properties Kinetics of phase transformations Multiphase Transformations Phase transformations