Materials Engineering – Day 13
More About Steel
Chapter 10 -
Steel and Carbon Conent
• 1015 steel – plain carbon – 0.15%C
• 1090 steel – plain carbon – 0.90%C
• What happens as carbon content increases? In
general, we see more and more pearlite in slow
cooled steels. More and more cementite
available in all steels. Strength  up. Ductility 
down.
• BUT, AT A GIVEN CARBON CONTENT, WIDELY
VARYING PROPERTIES ARE AVAILABLE
DEPENDING ON PROCESSING.
Chapter 10 -
Chapter 10:
Phase Transformations
ISSUES TO ADDRESS...
• Transforming one phase into another takes time.
Fe
g
(Austenite)
C
FCC
Fe C
3
Eutectoid
transformation (cementite)
+
a
(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?
• Are the mechanical properties of non-equilibrium
structures better?
Chapter 10 - 3
The Austenite to Pearlite
Transformation
• This takes time. Two things must happen.
1. Nucleation of Pearlite. (Best nucleation sites are
in or near grain boundaries.) Nuclei are baby
crystals. The thermodynamics of nucleation get
stronger with supercooling.
2. Growth of Pearlite. This takes diffusion. Carbon
atoms must diffuse away from the g to the Fe3C
layers which are starting. Diffusion is stronger at
high temperatures and weakens as the
temperature decreases.
• These two aspects of the transformation work
against each other.
Chapter 10 -
Eutectoid Transformation Rate
• Growth of pearlite from austenite:
Adapted from
Fig. 9.15,
Callister 7e.
a
a
g a
a
a
a
• Recrystallization
rate increases
with DT.
g
cementite (Fe3C)
Ferrite (a)
a
g
a
pearlite
growth
direction
g
a
100
y (% pearlite)
Austenite (g)
grain
boundary
Diffusive flow
of C needed
600°C
(DT larger)
50
650°C
675°C
(DT smaller)
Adapted from
Fig. 10.12,
Callister 7e.
0
Course pearlite  formed at higher T - softer
Fine pearlite
 formed at low T - harder
Chapter 10 - 5
C-Curves
• Think of some austenite, lowered suddenly to a
temperature below 727C and allowed to transform
at that temperature.
• At high temperature, atoms can diffuse rapidly
BUT, nucleation rates are very low due to only
slight undercooling. Therefore, the overall
transformation tends to be lengthy.
• At low temperature, nucleation is very speedy,
but diffusion is slow. Therefore the
transformation tends to be lengthy.
• At some intermediate temperatures there must be
an optimum. Thus we get a C-shaped curve.
Chapter 10 -
Nucleation and Growth
• Reaction rate is a result of nucleation and growth
of crystals.
100
% Pearlite
Nucleation rate increases with DT
Growth
regime
50 Nucleation
Growth rate increases with T
regime
t 0.5
0
log (time)
Adapted from
Fig. 10.10, Callister 7e.
• Examples:
g
pearlite
colony
T just below TE
Nucleation rate low
Growth rate high
g
T moderately below TE
Nucleation rate med .
Growth rate med.
g
T way below TE
Nucleation rate high
Growth rate low
Chapter 10 - 7
TTT Curves for a Euctectoid Steel
Note the axes:
1. Time – always log
scale.
2. Temperature
Note the curves:
1. Start time
2. 50% Completion
(dashed)
3. End time. (after
crossing this there is no
more austenite left to
transfform.
Chapter 10 -
Isothermal Transformation Diagrams
y,
% transformed
• Fe-C system, Co = 0.76 wt% C
Note the axes:
• Transformation at T = 675°C.
1. Time – always log
100
scale.
T = 675°C
50
2. Temperature
0
Note the curves:
1. Start time
T(°C)
Austenite (stable)
2. 50% Completion
T
(727C)
E
700 Austenite
(dashed)
(unstable)
Pearlite
3. End time. (after
600
isothermal transformation at 675°C
crossing this there is
500
no more austenite
400
left to transform.
10 2
1
1
10
10 4
time (s)
10 2 10 3 10 4 10 5
time (s)
Chapter 10 - 9
The Pearlite Spectrum
• The kind of pearlite that you get depends
on the transformation temperature.
1. High temperature. Low nucleation. Higher
growth. Larger coarser more separated
layers in the pearlite.  Not as strong,
more ductile.
2. Low temperature. (Nearer to the “nose.”)
High nucleation. Little diffusion. Finer
thinner closer spaced layers in the
pearlite.  Stronger, less ductile.
3. Intermediate temperatures – an
intermediate pearlite.
Chapter 10 -
Two Pearlites. Which formed at
the higher Temperature?
Which is
stronger?
Which is
more
ductile?
Chapter 10 -
What Happens as we Transform at
Lower and Lower Temperatures?
T (C)
727
Fine P
???
Coarse
P
Log time
Chapter 10 -
The Answer is “Bainite”
• Bainite is relatively rare. The austenite is changed to
pearlite before the bainite transformation can start in
most practical cooling processes. But it is good stuff.
Strong, hard, some
ductility.
Upper (above 300)

Lower (below 300)

Upper B. Needles of ferrite
with strips of cementite
Lower B. Very thin ferrite
plates. Ultra thin cementite
rods
Chapter 10 -
What Happens if we Transform at
an even lower Temperature?
• The answer is that we get a phase known as
martensite.
• Martensite is BC tetragonal with C interstitially.
• It forms instantaneously. No nucleation or
diffusion is required.
• Martensite is one of the hardest substances
known to man. It is also totally brittle like a
ceramic.
• The production of martensite is an important
step, but not the final step, in the production of
high strength steels. (Quench and tempered)
Chapter 10 -
Here is the TTT Curve for a
Euctectoid Steel.
Coarse P
Fine P
Upper B
Lower B
M (Martensite)
Chapter 10 -
Martensite: Fe-C System
• Martensite:
--g(FCC) to Martensite (BCT)
Fe atom
sites
x
x
x
x
x
60 m
(involves single atom jumps)
potential
C atom sites
x
(Adapted from Fig.
10.20, Callister, 7e.
• Isothermal Transf. Diagram
800
Austenite (stable)
T(°C)
A
400
10-1
(Adapted from Fig. 10.21, Callister, 7e.
(Fig. 10.21 courtesy United States
Steel Corporation.)
• g to M transformation..
B
A
200
TE
P
600
Adapted from
Fig. 10.22,
Callister 7e.
Martensite needles
Austenite
0%
50%
90%
M+A
M+A
M+A
10
103
105
-- is rapid!
-- % transf. depends on T only.
time (s)
Chapter 10 - 16
Martensite Formation
g (FCC)
slow cooling
a (BCC) + Fe3C
quench
M (BCT)
tempering
M = martensite is body centered tetragonal (BCT)
Diffusionless transformation
BCT  few slip planes
BCT if C > 0.15 wt%
 hard, brittle
Chapter 10 - 17
Tempering Martensite
• reduces brittleness of martensite,
• reduces internal stress caused by quenching.
TS(MPa)
YS(MPa)
1800
Adapted from
1400
Fig. 10.34,
Callister 7e.
(Fig. 10.34
1200
adapted from
Fig. furnished
1000
courtesy of
Republic Steel
Corporation.)
800
200
TS
YS
60
50
%RA
40
30
%RA
400
9 m
1600
Adapted from
Fig. 10.33,
Callister 7e.
(Fig. 10.33
copyright by
United States
Steel
Corporation,
1971.)
600
Tempering T (°C)
• produces extremely small Fe3C particles surrounded by a.
• decreases TS, YS but increases %RA
Chapter 10 - 18
Mechanical Prop: Fe-C System (1)
• Effect of wt% C
Adapted from Fig. 9.30,Callister
7e. (Fig. 9.30 courtesy Republic
Steel Corporation.)
TS(MPa)
1100
YS(MPa)
Co < 0.76 wt% C
Hypoeutectoid
Hypo
Hyper
Co > 0.76 wt% C Adapted from Fig. 9.33,Callister 7e.
9.33 copyright 1971 by United
Hypereutectoid (Fig.
States Steel Corporation.)
%EL
Hypo
Hyper
80
100
900
hardness
40
700
50
500
0
0.5
1
0
Adapted from Fig.
10.29, Callister 7e.
(Fig. 10.29 based on
data from Metals
Handbook: Heat
Treating, Vol. 4, 9th
ed., V. Masseria
(Managing Ed.),
American Society for
Metals, 1981, p. 9.)
0.76
0
0.76
300
Impact energy (Izod, ft-lb)
Pearlite (med)
ferrite (soft)
Pearlite (med)
Cementite
(hard)
1
0.5
0
wt% C
wt% C
• More wt% C: TS and YS increase, %EL decreases.
Chapter 10 - 19
Mechanical Prop: Fe-C System (2)
• Fine vs coarse pearlite vs spheroidite
Hypo
Hyper
90
Hypo
Hyper
fine
pearlite
240
coarse
pearlite
spheroidite
160
80
0
• Hardness:
• %RA:
0.5
1
wt%C
Ductility (%AR)
Brinell hardness
320
spheroidite
60
coarse
pearlite
fine
pearlite
30
0
0
fine > coarse > spheroidite
fine < coarse < spheroidite
0.5
1
wt%C
Adapted from Fig. 10.30, Callister 7e.
(Fig. 10.30 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.)
Chapter 10 - 20
Mechanical Prop: Fe-C System (3)
• Fine Pearlite vs Martensite:
Brinell hardness
Hypo
600
Hyper
martensite
Adapted from Fig. 10.32,
Callister 7e. (Fig. 10.32 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.
1776.)
400
200
fine pearlite
0
0
0.5
1
wt% C
• Hardness: fine pearlite << martensite.
Chapter 10 - 21
Summary: Processing Options
Austenite (g)
slow
cool
moderate
cool
Adapted from
Fig. 10.36,
Callister 7e.
rapid
quench
Bainite
Martensite
(a + Fe3C layers + a
proeutectoid phase)
(a + Fe3C plates/needles)
(BCT phase
diffusionless
transformation)
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
reheat
Ductility
Strength
Pearlite
Tempered
Martensite
(a + very fine
Fe3C particles)
Chapter 10 - 22
Spheroidite: Fe-C System
• Spheroidite:
a
(ferrite)
--a grains with spherical Fe3C
--diffusion dependent.
--heat bainite or pearlite for long times
Fe3C
--reduces interfacial area (driving force) (cementite)
60 m
(Adapted from Fig. 10.19, Callister, 7e.
(Fig. 10.19 copyright United States
Steel Corporation, 1971.)
Chapter 10 - 26
Phase Transformations of Alloys
Effect of adding other elements
Change transition temp.
Cr, Ni, Mo, Si, Mn
retard g  a + Fe3C
transformation
Adapted from Fig. 10.23, Callister 7e.
Chapter 10 - 27
Cooling Curve
plot temp vs. time
Adapted from
Fig. 10.25,
Callister 7e.
Chapter 10 - 28
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:
a) 42% proeutectoid ferrite and 58% coarse
pearlite
b) 50% fine pearlite and 50% bainite
c) 100% martensite
d) 50% martensite and 50% austenite
Chapter 10 - 29
Example Problem for Co = 0.45 wt%
a) 42% proeutectoid ferrite and 58% coarse pearlite
first make ferrite 800
then pearlite T (°C)
A
P
B
600
course pearlite
 higher T
A+a
A+P
A+B
A
400
50%
M (start)
M (50%)
M (90%)
200
Adapted from
Fig. 10.29,
Callister 5e.
0
0.1
10
103
time (s)
105
Chapter 10 - 30
Example Problem for Co = 0.45 wt%
b) 50% fine pearlite and 50% bainite
800
first make pearlite
T (°C)
then bainite
A
P
B
600
fine pearlite
 lower T
A+a
A+P
A+B
A
400
50%
M (start)
M (50%)
M (90%)
200
Adapted from
Fig. 10.29,
Callister 5e.
0
0.1
10
103
time (s)
105
Chapter 10 - 31
Example Problem for Co = 0.45 wt%
c) 100 % martensite – quench = rapid cool
d) 50 % martensite
800
A+a
and 50 %
A
T (°C)
austenite
P
B
600
A+P
A+B
A
400
50%
M (start)
M (50%)
M (90%)
d)
200
Adapted from
Fig. 10.29,
Callister 5e.
c)
0
0.1
10
103
time (s)
105
Chapter 10 - 32