1
1
LBL-17791
Prepr
int
,I'.
d-..
Lawrence
Berkeley
Laboratory
UNIVERSITY
OF
CALIFORNIA
Materials &
Molecular
Research Division
Submitted
to
Metallurgical
Transactions
ON
THE
STABILITY
OF
PRECIPITATED AUSTENITE
AND
THE
TOUGHNESS
OF
9Ni
STEEL
B.
Fultz,
J.I.
Kim,
Y.H.
Kim,
H.J.
Kim,
G.O.
Fior,
and
J.W.
Morris,
Jr.
August
1984
R
L::
C
',-"
II'
:::
t:'
L.'
,
~"'~
L:J.,
,'\
Y
A.".J
!::::;'":t;'
::::,\1-1-5
SECTIO.
1
TWO-WEEK
LOAN
COpy
,
is
is
a Library Ci
'ichmay
be
fating Copy
for
two weeks.
7
',~!-
........
Prepared for the U.S. Department
of
Energy
under
Contract
DE-AC03-76SF00098
DISCLAIMER
This document was prepared
as
an account
of
work sponsored
by
the United States
Government. While this document is believed to contain correct information, neither the
United States Government nor any agency thereof, nor the Regents
of
the University of
California, nor any
of
their employees, makes any warranty, express or implied, or
assumes any legal responsibility for the accuracy, completeness, or usefulness
of
any
information, apparatus, product, or process disclosed, or represents that its use would not
infringe privately owned rights. Reference herein to any specific commercial product,
process, or service
by
its trade name, trademark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation, or favoring by the
United States Government or any agency thereof, or the Regents
of
the University
of
California. The views and opinions
of
authors expressed herein do not necessarily state or
reflect those
of
the United States Government or any agency thereof or the Regents
of
the
University of California.
ON
THE
STABILITY
OF
PRECIPITATED AUS'IENlTE
AND
THE
TOUGHNESS
OF
9Ni
STEEL
B.
Fultz,
~.
I.
Xim·
, Y. H.
Xim,
H.
~.
Xim,
G.
O.Fior
and
~.
w.
Morris,
~r.
Materials
and
Molecular
Research
Div.,
Lawrence
Berkeley
Laboratory,
and
the
ABSTRACT
Dept.
of
Materials
Science
and
Mineral
Engineering,
University
of
California,
Berkeley,
CA
94720
A
correlation
was
confirmed
between
the
good
low
temperature
Charpy
toughness
of
9Ni
steel
and
the
stability
of
its.
precipitated
austenite
against
the
martensitic
transformation.
Changes
in
the
microstructure
during
isothermal
tempering
were
studied
in
detail.
The
austenite/mar-
tensite
interface
is
originally
quite
coherent
over
-100
A
distances.
With
further
tempering,
however,
the
dislocation
structure
at
the
aus-
tenite/martensite
interface
changes,
and
this
change
appears
to
be
related
to
the
increased
instability
of
the
austenite
particles.
The
strains
inherent
to
the
transformation
of
austenite
particles
create
dislocation
structures
in
the
tempered
martensite.
The
energy
required
to
form
these
dislocation
structures
affects
the
thermodynamics
of
the
transformation.
Together
with
a
reduction
in
carbon
concentration
dur-
ing
tempering,
changes
in
these
dislocation
structures
may
also
reduce
the
thermodynamic
stability
of
the
austenite
particles
as
they
grow
larger.
The
large
deterioration
of
the
Charpy
toughness
of
overternpered
material
is
attributed,
in
part,
to
these
dislocation
structures,
which
resemble
the
dislocation
structures
found
in
cold-worked
material.
•
IBM
Thomas
J.
Watson
Research
Center,
Yorktown
Heights,
New
York
10598.
1
I.
IN'IllODUCl'ION
9Ni
steel
was
developed
by
the
International
Nickel
Company
in
1942
as
a
ferritic
material
for
cryogenic
service
[1,2].
After
tempering
for
about
one
hour
at
600
0
C,
9Ni
steel
exhibits
a
beneficial
suppression
of
its
ductile-to-brittle
transition
temperature
(DBTT)
by
more
than
100
o
C.
This
tempering
temperature
is
within
the
austenite
plus
ferrite
two-
phase
region
of
the
equilibrium
phase
diagram.
After
tempering,
a
few
percent
of
austenite
(y-phase)
is
found
between
the
martensite
(a'-
phase)
laths
and
along
the
prior
austenite
grain
boundaries
[3-6].
The
formation
of
austenite
is
the
most
prominent
microstructural
change
observed
after
tempering.
Furthermore,
the
presence
of
austenite
is
the
only
clear
differen~e
between
the
microstructure
of
9Ni
steel
and
the
microstructure
of
6Ni
steel,
for
which
there
is
no
improvement
in
DBTT
after
a
simple
tempering.
It
is
therefore
widely
accepted
that
precipi-
tated
austenite
can
be
beneficial
to
toughness-at
cryogenic
temperatures
[7-10].
It
is
also
recognized
that
the
mere
presence
of
austenite
is
insufficient
to
ensure
this
beneficial
effect.
C.
W.
Marschall,
et
al.
[7]
performed
a
systematic
study
of
the
effects
of
different
tempering
treatments
on
the
Charpy
toughness
of
9Ni
steel
at
77
K
and
290
K.
They
correlated
these
toughness
data
to
the
amount
of
austenite
present
at
room
temperature,
both
before
and
after
the
material
was
immersed
in
liquid
nitrogen.
In
this
way
they
found
it
necessary
for
the
austenite
to
be
thermally
stable
against
martensitic
transformation
if
good
cryogenic
toughness
is
to
be
obtained.
Tempering
for
much
longer
than
10
hrs
at
600
0
C,
or
tempering
at
higher
temperatures,
was
found
to
be
deleterious
to
both
austenite
stability
and
to
cryogenic
toughness.
Similar
systematics
have
been
reported
by
others
[11-14].
Relationships
between
the
stability
of
austenite
and
cryogenic
fracture
toughness
have
been
an
important,
albeit
a
controversial
topic
of
research
[3-5,7-23].
Early
ideas
that
the
soft
austenite
phase
served
to
blunt
a
propagating
crack,
as
well
as
ideas
that
any
fresh
untempered
martensite
near
the
crack
tip
will
promote
brittle
fracture,
have
been
ruled
out
by
observations
that
all
austenite
transforms
to
martensite
in
the
plastic
zone
ahead
of
the
crack
tip
[3,11,18,19].
A
model
has
been
proposed
in
which
the
transformation
strains
associated
with
the
austenite
to
martensite
transformation
reduce
the'
strain
energy
available
for
crack
propagation
[23].
It
has
also
been
suggested
that
the
austenite
serves
as
an
"interstitial
scavenger"
and
promotes
a
cleaner
and
more
ductile
martensite
[7].
Kim
and
Schwartz
have
sug-
gested
that
the
austenite
is
helpful
as
a
scavenger
until
a
connected
network
of
austenite
has
formed
in
thu
material,
and
then
the
toughness
deteriorates
[11].
A
transmission
electron
microscopy
(TEM)
study
of
9Ni
steel
by
Morris,
et
al.
[21,22]
has
indicated
a
qualitative
difference
between
the
martensite
which
forms
from
thermally
unstable
precipitated
auste-
nite
and
the
martensite
that
forms
from
thermally
stable
retained
austenite.
Thermally
unstable
austenite
particles
were
found
to
trans-
form
to
variants
of
martensite
with
a
close
crystallographic
alignment
to
the
surrounding
martensite
laths.
On
the
other
hand,
thermally
stable
austenite
particles
were
found
to
transform
under
mechanical
loading
to
those
crystallographic
variants
of
martensite
compatible
with
2