1. Chapter 10: Phase Transformations
WHY STUDY PHASE TRANSFORMATION?
The development of a set of desirable mechanical characteristics for a material often results from a phase transformation. That is wrought by heat-treatment.
It is important to design a proper heat-treatment to get the desired room-temperature mechanical properties of an alloy.
Is it possible to develop other microstructural elements than pearlite for iron-carbon alloys?

2. Phase Transformations
Phase transformation may be wrought in metal alloy systems by varying temperature, composition, and the external pressure.
Temperature changes by means of heat-treatments are most conveniently utilized to induce phase transformations.
This corresponds to crossing a phase boundary on the composition-temperature phase diagram as an alloy of a given temperature is heated or cooled.
One limitation of phase diagrams is their inability to indicate the time period required for the attainment of equilibrium.
Equilibrium conditions are maintained only if heating or cooling is carried out at extremely slow and unpractical rates.

3. THE INFLUENCE OF TIME
Why it is important to study the influence of time on phase transformation?
How does the rate of transformation depend on time and temperature?
For other than equilibrium cooling, transformations are shifted to lower temperatures than indicated by the phase diagram.
Supercooling: the shift to lower temperatures for cooling
Superheating: the shift to higher temperatures for heating
Transforming one phase into another takes time Fe g
(Austenite)
Eutectoid
transformation C
FCC
Fe3C
(cementite) a
(ferrite) +
(BCC)

4. Transformations & Undercooling•
Eutectoid transf. (Fe-Fe3C system): g Þ a +
Fe3C •
For transf. to occur, must
cool to below 727ºC
(i.e., must “undercool”)
0.76 wt% C
6.7 wt% C
0.022 wt% C
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)
C, wt%C
1148ºC
T(ºC)
ferrite
727ºC
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC DT
Undercooling by Ttransf. < 727C
0.76
0.022
Adapted from Fig. 9.24,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

5. The Fe-Fe3C Eutectoid Transformation
• Transformation of austenite to pearlite:
Adapted from Fig. 9.15, Callister & Rethwisch 8e. g a
pearlite
growth
direction
Austenite (g)
grain
boundary
cementite (Fe3C)
Ferrite (a)
Diffusion of C
during transformation a g
Carbon diffusion

6. The Fe-Fe3C Eutectoid Transformation (cont.)
• For this transformation,
rate increases with
[Teutectoid – T ] (i.e., DT).
Adapted from Fig , Callister & Rethwisch 8e.
675ºC
(DT smaller) 50
y (% pearlite)
600ºC
(DT larger)
650ºC
100
S-shaped curves of the percentage transformation versus the logarithm of time at three different temperatures.
For each curve, data were collected after rapidly cooling a specimen composed of 100% austenite to the temperature indicated.
That temperature was maintained constant throughout the course of the reaction.

7. ISOTHERMAL TRANSFORMATION DIAGRAMS
A more convenient way of representing both the time and temperature dependence of this transformation.
These curves were generated from a series of plots of the percentage transformations versus the logarithm of time taken over a range of temperatures (from the S-shaped curves).

8. Generation of Isothermal Transformation Diagrams
Consider:
• The Fe-Fe3C system, for C0 = 0.76 wt% C
• A transformation temperature of 675ºC.
100
T = 675ºC 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 (727ºC)
Austenite
(unstable)
Pearlite
T(ºC)
time (s)
isothermal transformation at 675ºC
Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.)

9. Generation of Isothermal Transformation Diagrams
Consider:
• The Fe-Fe3C system, for C0 = 0.76 wt% C
• A transformation temperature of 675ºC.
Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.)

10. ISOTHERMAL TRANSFORMATION DIAGRAMS
The austenite-to-pearlite transformation will occur only if an alloy is supercooled to below the eutectoid
The time necessary for the transformation to begin and then end depends on temperature.
The start and finish curves are nearly parallel, and they approach the eutectoid line asymptotically.
To the left of the transformation start curve, only austenite (unstable) will be present
To the right of the finish curve, only pearlite will exist.
In between, the austenite is in the process of transforming to pearlite, and both microconstituents will be present.

11. Austenite-to-Pearlite Isothermal Transformation
• Eutectoid composition, C0 = 0.76 wt% C
• Begin at T > 727ºC
• Rapidly cool to 625ºC
• Hold T (625ºC) constant (isothermal treatment)
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)
TE (727ºC)
Austenite
(unstable)
Pearlite
T(ºC) 1 10 2 3 4 5
time (s)
Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) g g

12. Austenite-to-Pearlite Isothermal Transformation (cont.)
This plot is only valid for an iron-carbon alloy of eutectoid composition (different curves for other alloys).
Such a plot is called isothermal transformation diagram or time-temperature-transformation (T-T-T).
Shorter the time  higher is the rate of transformation
For instance, at temperatures just below the eutectoid, very long times are required for the 50% transformation (i.e., the action rate is very slow).
Coarse pearlite  formed at higher temperatures – relatively soft
Fine pearlite  formed at lower temperatures – relatively hard

13. Bainite: Another Fe-Fe3C Transformation Product
-- elongated Fe3C particles in a-ferrite matrix
-- diffusion controlled
• Isothermal Transf. Diagram, C0 = 0.76 wt% C
Fe3C
(cementite) a
(ferrite) 10 3 5
time (s) -1
400
600
800
T(ºC)
Austenite (stable)
200 P B TE 0%
100%
50% A
100% pearlite
100% bainite
5 mm
Adapted from Fig , Callister & Rethwisch 8e. (Fig from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)
Adapted from Fig , Callister & Rethwisch 8e.

14. Spheroidite: Another Microstructure for the Fe-Fe3C System
Adapted from Fig , Callister & Rethwisch 8e. (Fig copyright United States Steel Corporation, 1971.)
60 m a
(ferrite)
(cementite)
Fe3C
• Spheroidite:
-- Fe3C particles within an a-ferrite matrix
-- formation requires diffusion
-- heat bainite or pearlite at temperature just below eutectoid for long times
-- driving force – reduction
of a-ferrite/Fe3C interfacial area

15. Martensite: A Nonequilibrium Transformation Product
-- g(FCC) to Martensite (BCT)
Martensite needles
Austenite
60 m x
potential
C atom sites
Fe atom
sites
Adapted from Fig , Callister & Rethwisch 8e.
Adapted from Fig , Callister & Rethwisch 8e.
• Isothermal Transf. Diagram
• g to martensite (M) transformation..
-- is rapid! (diffusionless)
-- % transf. depends only on T to which rapidly cooled 10 3 5
time (s) -1
400
600
800
T(ºC)
Austenite (stable)
200 P B TE 0%
100%
50% A
M + A
90%
Adapted from Fig , Callister & Rethwisch 8e. (Fig courtesy United States Steel Corporation.)

16. Martensite Formation  (FCC)  (BCC) + Fe3C slow cooling quench
M (BCT)
tempering
Martensite (M) – single phase
– has body centered tetragonal (BCT) crystal structure
Diffusionless transformation BCT if C0 > 0.15 wt% C
BCT  few slip planes  hard, brittle

17. Continuous Cooling Transformation Diagrams
Cooling curve
Conversion of isothermal transformation diagram to continuous cooling transformation diagram
Actual processes involves cooling – not isothermal
Can’t cool at infinite speed
Adapted from Fig ,
Callister & Rethwisch 8e.

18. Isothermal Heat Treatment Example Problems
On the isothermal transformation diagram for a 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
50% martensite and 50% austenite

19. Solution to Part (a) of Example Problem
42% proeutectoid ferrite and 58% coarse pearlite
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%)
Adapted from Fig ,
Callister 5e.
Fe-Fe3C phase diagram, for C0 = 0.45 wt% C
Isothermally treat at ~ 680ºC
-- all austenite transforms
to proeutectoid a and
coarse pearlite.
T (ºC)

20. Solution to Part (b) of Example Problem
50% fine pearlite and 50% bainite
T (ºC)
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%)
Adapted from Fig ,
Callister 5e.
Fe-Fe3C phase diagram, for C0 = 0.45 wt% C
Isothermally treat at ~ 590ºC
– 50% of austenite transforms
to fine pearlite.
Then isothermally treat
at ~ 470ºC
– all remaining austenite
transforms to bainite.

21. Solutions to Parts (c) & (d) of Example Problem
100% martensite – rapidly quench to room temperature
T (ºC)
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%)
Adapted from Fig ,
Callister 5e.
Fe-Fe3C phase diagram, for C0 = 0.45 wt% C
50% martensite
& 50% austenite
-- rapidly quench to ~ 290ºC, hold at this temperature c) d)

22. Mechanical Props: Influence of C Content
Adapted from Fig. 9.33, Callister & Rethwisch 8e.
C0 > 0.76 wt% C
Hypereutectoid
Pearlite (med) C
ementite
(hard)
Pearlite (med)
ferrite (soft)
C0 < 0.76 wt% C
Adapted from Fig. 9.30, Callister & Rethwisch 8e.
Hypoeutectoid
Adapted from Fig , Callister & Rethwisch 8e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)
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
• Increase C content: TS and YS increase, %EL decreases

23. Mechanical Props: Fine Pearlite vs. Coarse Pearlite vs. Spheroidite80
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
Adapted from Fig , Callister & Rethwisch 8e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)
• Hardness:
fine > coarse > spheroidite
• %RA:
fine < coarse < spheroidite

24. Mechanical Props: Fine Pearlite vs. Martensite
200
wt% C
0.5 1
400
600
Brinell hardness
martensite
fine pearlite
Hypo
Hyper
Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p )
• Hardness: fine pearlite << martensite.

25. Tempered Martensite • •
Heat treat martensite to form tempered martensite
• tempered martensite less brittle than martensite
• tempering reduces internal stresses caused by quenching
YS(MPa)
TS(MPa)
800
1000
1200
1400
1600
1800 30 40 50 60
200
400
600
Tempering T (ºC)
%RA TS YS
Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.)
Adapted from Fig , Callister & Rethwisch 8e. (Fig copyright by United States Steel Corporation, 1971.)
9 mm •
tempering produces extremely small Fe3C particles surrounded by a. •
tempering decreases TS, YS but increases %RA

26. Summary of Possible Transformations
Adapted from Fig , Callister & Rethwisch 8e.
Austenite (g)
Pearlite
(a + Fe3C layers + a
proeutectoid phase)
slow
cool
Bainite
(a + elong. Fe3C particles)
moderate
cool
Martensite
(BCT phase
diffusionless
transformation)
rapid
quench
Strength
Ductility
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
Tempered
Martensite
(a + very fine
Fe3C particles)
reheat