M.S ch6
M.S ch6
M.S ch6
Phase Transformations
ISSUES TO ADDRESS...
• Transforming one phase into another takes time.
Fe Fe C
g Eutectoid 3
transformation (cementite)
(Austenite) +
a
C FCC (BCC)
(ferrite)
2
Solidification: Nucleation Types
• Homogeneous nucleation
• nuclei form in the bulk of liquid metal
• requires considerable supercooling
(typically 80-300ºC)
• Heterogeneous nucleation
– much easier since stable “nucleating surface” is already present —
e.g., mold wall, impurities in liquid phase
– only very slight supercooling (0.1-10ºC)
3
Homogeneous Nucleation & Energy Effects
Surface Free Energy - destabilizes
the nuclei (it takes energy to make
an interface)
GS 4r 2 g
g = surface tension
r* decreases as T increases
For typical T r* ~ 10 nm
5
Rate of Phase Transformations
Kinetics - study of reaction rates of phase
transformations
• To determine reaction rate – measure degree
of transformation as function of time (while
holding temp constant)
How is degree of transformation measured?
X-ray diffraction – many specimens required
electrical conductivity measurements –
on single specimen
6
Rate of Phase Transformation
Fraction transformed, y
transformation complete
Fixed T
fraction time
transformed
• k & n are transformation specific parameters
By convention rate = 1 / t0.5
7
Temperature Dependence of Transformation Rate
Adapted from Fig.
10.11, Callister &
Rethwisch 8e.
135C 119C 113C 102C 88C 43C (Fig. 10.11 adapted
from B.F. Decker and
D. Harker,
"Recrystallization in
Rolled Copper", Trans
AIME, 188, 1950, p.
888.)
1 10 102 104
8
Transformations & Undercooling
• Eutectoid transf. (Fe-Fe C system): g a + Fe3C
3
• For transf. to occur, must 0.76 wt% C 6.7 wt% C
cool to below 727ºC 0.022 wt% C
(i.e., must “undercool”)
T(ºC)
1600 Adapted from Fig.
d 9.24,Callister & Rethwisch
8e. (Fig. 9.24 adapted from
1400 L Binary Alloy Phase
Diagrams, 2nd ed., Vol. 1,
g g +L T.B. Massalski (Ed.-in-
1200 L+Fe3C Chief), ASM International,
Fe3C (cementite)
1148ºC
(austenite) Materials Park, OH, 1990.)
1000
a Eutectoid: g +Fe3C
ferrite 800 Equil. Cooling: Ttransf. = 727ºC
727ºC
T a +Fe3C
600
Undercooling by Ttransf. < 727C
0.022
0.76
400
0 1 2 3 4 5 6 6.7
(Fe) C, wt%C
9
The Fe-Fe•3Transformation
C Eutectoidof Transformation
austenite to pearlite:
Diffusion of C
Austenite (g) cementite (Fe3C) during transformation
grain a Ferrite (a)
a a
boundary g
g a
a g
Adapted from
a pearlite g
Fig. 9.15,
a growth a
Callister &
Rethwisch 8e. direction
a
• For this transformation, 100
Carbon
diffusion
y (% pearlite)
rate increases with 600ºC
[Teutectoid – T ] (i.e., T). (T larger)
650ºC
50 Adapted from
675ºC Fig. 10.12,
(T smaller) Callister &
Rethwisch 8e.
0
% transformed 100
T = 675ºC
y,
50
0
1 10 2 10 4 time (s)
T(ºC)
Austenite (stable)
TE (727ºC)
700 Austenite
(unstable)
1 10 10 2 10 3 10 4 10 5
time (s)
12
Transformations Involving
Noneutectoid Compositions
Consider C = 1.13 wt% C 0
T(ºC) T(ºC)
900 1600
d
A 1400 L
Fe3C (cementite)
800
A TE (727ºC) g +L
+ 1200 g L+Fe3C
700 A C (austenite)
P 1000
A
+ P a g +Fe3C
600
800
727ºC
T a +Fe3C
500 600
0.022
0.76
1 10 102 103 104 400
1.13
time (s) 0 1 2 3 4 5 6 6.7
(Fe)
C, wt%C
Adapted from Fig. 10.16, Adapted from Fig. 9.24,
Callister & Rethwisch 8e. Callister & Rethwisch 8e.
200
60 mm
Adapted from Fig. 10.19, Callister &
Rethwisch 8e. (Fig. 10.19 copyright
United States Steel Corporation,
1971.)
15
Martensite: A Nonequilibrium Transformation Product
• Martensite:
-- g(FCC) to Martensite (BCT)
60 mm
Fe atom potential
x x
sites x x C atom sites
x Adapted from Fig. 10.20,
Callister & Rethwisch 8e.
tempering
M (BCT)
17
PhaseEffectTransformations
of adding other elements
of Alloys
Change transition temp.
Conversion of isothermal
transformation diagram to
continuous cooling
transformation diagram
19
Isothermal Heat Treatment Example Problems
20
Solution to Part (a) of Example Problem
a) 42% proeutectoid ferrite and 58% coarse pearlite
Fe-Fe3C phase diagram,
Isothermally treat at ~ 680ºC for C0 = 0.45 wt% C
800 A+a
T (ºC) A
-- all austenite transforms
to proeutectoid a and
A+P
coarse pearlite. 600 P
B
C0 0.022 A+B
Wpearlite A
0.76 0.022 50%
400
M (start)
0.45 0.022 M (50%)
= = 0.58 M (90%)
0.76 0.022
200
Wa = 1 0.58 = 0.42
Adapted from 0
Fig. 10.29, 0.1 10 103 105
Callister 5e. time (s)
21
Solution to Part (b) of Example Problem
b) 50% fine pearlite and 50% bainite
Fe-Fe3C phase diagram,
for C0 = 0.45 wt% C
800 A+a
Isothermally treat at ~ 590ºC T (ºC) A
– 50% of austenite transforms
to fine pearlite. P
A+P
600
B
Then isothermally treat A+B
A
at ~ 470ºC 50%
400
– all remaining austenite M (start)
transforms to bainite. M (50%)
M (90%)
200
Adapted from 0
Fig. 10.29, 0.1 10 103 105
Callister 5e. time (s)
22
Solutions to Parts (c) & (d) of Example Problem
c) 100% martensite – rapidly quench to room
temperature Fe-Fe3C phase diagram,
for C0 = 0.45 wt% C
d) 50% martensite 800 A
A+a
T (ºC)
& 50% austenite
A+P
-- rapidly quench to 600 P
B
~ 290ºC, hold at this
A+B
temperature A
400 50%
M (start)
M (50%) d)
M (90%)
200
c)
Adapted from 0
Fig. 10.29, 0.1 10 103 105
Callister 5e. time (s)
23
Mechanical Props: Influence of C Content
Pearlite (med)
Pearlite (med) Cementite
ferrite (soft) (hard)
Adapted from Fig. 9.30, C0 < 0.76 wt% C C0 > 0.76 wt% C Adapted from Fig. 9.33,
Callister & Rethwisch 8e. Callister & Rethwisch 8e.
Hypoeutectoid Hypereutectoid
Hypo Hyper Hypo Hyper
TS(MPa) %EL 80
1100
0.76
0 0.5 0 0.5 1
wt% C wt% C
• Increase C content: TS and YS increase, %EL decreases
24
Mechanical Props: Fine Pearlite vs.
Coarse Pearlite vs. Spheroidite
Hypo Hyper 90 Hypo Hyper
320 fine
pearlite
Ductility (%RA)
spheroidite
60
Brinell hardness
240 coarse
pearlite
spheroidite
160 30 coarse
pearlite
fine
80 pearlite
0
0 0.5 1 0 0.5 1
wt%C wt%C
Adapted from Fig. 10.30, Callister &
• Hardness: fine > coarse > spheroidite Rethwisch 8e. (Fig. 10.30 based on
data from Metals Handbook: Heat
• %RA: fine < coarse < spheroidite Treating, Vol. 4, 9th ed., V. Masseria
(Managing Ed.), American Society for
Metals, 1981, pp. 9 and 17.)
25
Mechanical Props: Fine Pearlite vs. Martensite
Hypo Hyper
600
Brinell hardness
martensite
Adapted from Fig. 10.32,
Callister & Rethwisch 8e. (Fig.
400 10.32 adapted from Edgar C.
Bain, Functions of the Alloying
Elements in Steel, American
Society for Metals, 1939, p. 36;
200 and R.A. Grange, C.R. Hribal,
fine pearlite and L.F. Porter, Metall. Trans. A,
Vol. 8A, p. 1776.)
0
0 0.5 1
wt% C
• Hardness: fine pearlite << martensite.
26
Tempered Martensite
Heat treat martensite to form tempered martensite
• tempered martensite less brittle than martensite
• tempering reduces internal stresses caused by quenching
TS(MPa)
YS(MPa)
1800
1600 TS
Adapted from Adapted from Fig.
9 mm
Fig. 10.34, 1400 YS 10.33, Callister &
Callister & Rethwisch 8e. (Fig.
Rethwisch 8e. 10.33 copyright by
(Fig. 10.34 1200 60 United States Steel
adapted from Corporation, 1971.)
Fig. furnished 1000 50
courtesy of %RA %RA
Republic Steel
40
Corporation.) 800 30
200 400 600
Tempering T (ºC)
• tempering produces extremely small Fe3C particles surrounded by a.
• tempering decreases TS, YS but increases %RA
27
Summary of Possible Transformations Adapted from
Austenite (g) Fig. 10.36,
Callister &
Rethwisch 8e.
slow moderate rapid
cool cool quench
Martensite reheat
T Martensite
Strength
Ductility
bainite Tempered
fine pearlite Martensite
coarse pearlite (a + very fine
spheroidite Fe3C particles)
General Trends 28