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C Phase Diagram. ➢ α-ferrite - solid solution of C in BCC Fe. • Stable form of iron at room temperature. • Transforms to FCC g-austenite at 912 °C.
Typology: Lecture notes
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3
Peritectic L
at
o
and
0.18wt%C
Eutectic
= γ +
Fe
3
at
o
and
4.3wt%C
Eutectoid
γ = α +
Fe
3
at
o
and
0.77wt%C
δ
ferrite
delta
Bcc
structure
Paramagnetic
γ
austenite
Fcc structure Non
magnetic
ductile
α
ferrite
Bcc
structure
Ferromagnetic
Fairly
ductile
Fe
3
cementite
Orthorhombic
Hard,
brittle
Max.
solubility
of
in
ferrite=0.022%
in
austenite=2.11%
3
is
an
interstitial
impurity
in
Fe.
It
forms
a
solid
solution
with
α, γ,
δ
phases
of
iron
Maximum
solubility
in
α
ferrite is
wt%
at
relatively
small
interstitial
positions
Maximum
solubility
in
austenite
is
wt%
at
has
larger
interstitial
positions
Mechanical
properties:
Cementite
(Fe
3
is
hard
and
brittle:
strengthens
steels).
Mechanical
properties
also
depend
on
microstructure:
how
ferrite
and
cementite are
mixed.
Magnetic
properties:
α
ferrite
is
magnetic
below
austenite
is
non
magnetic
types
of
ferrous
alloys:
Iron:
wt
C in
α
ferrite
at
room
Steels:
wt
(usually
wt
α
ferrite
Fe
3
at
room
Cast
iron:
wt
(usually
wt
Eutectoid
steel
Alloy
of
eutectoid
composition
wt
is
cooled
slowly:
forms
pearlite,
layered
structure
of
two
phases:
α
ferrite
and
cementite
(Fe
3
Microstructure
of
eutectoid
steel
Mechanically,
pearlite has
properties
intermediate
to
soft,
ductile
ferrite
and
hard,
brittle
cementite.
Hypoeutectoid alloys contain
proeutectoid
ferrite
(formed
above
the
eutectoid
temperature)
plus
the
eutectoid
perlite
that
contain
eutectoid
ferrite
and
cementite.
Hypereutectoid
steel
How
to
calculate
the
relative
amounts
of
proeutectoid phase
α
or
Fe
3
and
pearlite?
Use
the
lever
rule
and
a
tie
line
that
extends
from
the
eutectoid
composition
wt%
to
α
wt%
for
hypoeutectoid
alloys and
to
Fe
3
wt%
for
hypereutectoid
alloys.
Example:
hypereutectoid
alloy,
composition
1
(
)
(
)
(
)
(
)
(
)
(
)
Cementite
id
Proeutecto
of
Fraction
Pearlite
of
Fraction
1
3
1
C
Fe
P
The
problem
is
to
solve
for
compositions
at
the
phase
boundaries
for
both
α
and
β
phases
(i.e.,
and
We
may
set
up
two
independent
lever
rule
expressions,
one
for
each
composition,
in
terms
of
and
as
follows:
α
β
β
α
β
o
β
α
C
C
C
=
C
C
C C =. = W
− −
− −
60
57
0
1
1
α
β β
α
β
o
β
α
C
C C
=
C
C
C C =. = W
− −
− −
30
14
0
2
2
In
these
expressions,
compositions
are
given
in
weight
percent
Solving
for
α
and
β
from
these
equations,
yield
α
(or
wt%
wt%
β
(or
wt%
wt%
On
slowly
cooling
the
steels,
the
properties
of
the
steel
are
dependent
mainly
on
the
percentage
carbon.
Different
percentage
carbon
implies
different
percentage
of
microconstituents and
phases
•pearlite and
ferrite
pro
eutectoid
for
the
hypo
eutectoid
steels •pearlite and
cementite pro
eutectoid
for
the
hyper
eutectoid
steels.
The
temperature
is
high
enough
and
the
time
at
high
temperature
is
long
enough,
for
the
atoms
to
diffuse
and
attain
equilibrium
conditions