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JOURNAL OF IRON AND STEEL RESEARCH. INTERNATIONAL. 2010. 17(3): 34-39
Influence Specimen Geometry Hot Torsion Test on Temperature
Distribution
During
Reheating Treatment API X7
Bahman
Mirzakhani' •
Shahin Khoddarn • Hossein
Arabi ,
Mohammad
Taghi
Salehi •
Seyed Hossein
Seyedeirr •
Mohammad Reza
Aboutalebi
(1. Faculty of Engineering . Arak Univers ity, Arak 38156-879 , I ran; 2. Department of Mechanical Engineering.
Monash
University. Clayton Campus. Vic 3800. Australia , 3. Department of Mater ials and Metallurgical
Engineering.
Iran
University
of Science and
Technology. Tehran
16844-13114.
Iran)
Abstract: The commercial finite element package ANSYSTM was uti lized for prediction of temperature distrihution
during
reheating
treatment
of hot
tor sion tes t
(HTT)
samples with
different geometries
for
API-X70 microalloyed
steel. Simulation
resul ts show tha t
an
inappropriate
choice of
test
specimen
geometry
and reheating conditions before
deformation
could
lead to non-uniform
temperature distribution
within
the
gauge section of
specimen. Therefore.
fls
surnptions of
isothermal experimental
conditions and zero temperature gradient
within
the specimen cross section ap
pear unjustified and led to uncertainties of f low curve obtained. Recommendations on finding proper specimen geome
try
for reducing
temperature gradient
along the gauge
part
of specimen will be g iven to
create
homogeneous initial mi
crostructure as much as possible before
deformation
in order to avoid uncertainty in
consequent
results of HTT.
Key words: hot
tors ion tcs t
,
simulation;
specimen geometry; reheating treatment; microalloyed steel
The hot tor sion tes t (HTT) has
been
one of the
mos t p opul ar mechanical tests for
assessment
of
workability
of metals
and
alloys for
bulk
forming
processes
during
last
decades J-,; . is
often
chosen
over
the
uniaxial tension and compression tests be
cause
very
large strain and strain rates
can
be
a
chieved
without t he
problems
of
necking
and barre
l ing. respectively' 7 . In
this
test the effective strain
and effective
stain
rate are as
function
of gauge
length and
radius.
So to overcome
the te st rig
limi
tations
a wide range of specimen geometr ies and si
zes
have
been
used
in different
studies
l l
-
6
] in order
to obtain the required strain and strain rate
range.
The reheating treatment is the first stage of
thermomechanical processes
in
order
to
obtain
hom
ogeneous composition
before doing hot
rolling
or hot
forging.
The effect of pre-deformation
reheating
treatment on microstructure evolution such as initial
austenite
grain size and behavior of
microalloying
el
ements has
been
investigated in several stud
ies
L
; H - - 12 ] .
These
investigations deduce
t ha t aus ten
itizing temperature has a great influence
on
initial
microstructure and consequence deformation behav
ior of
steels.
In
simulation
of
thermomechanical
process via
HTT.
before
s ta rt ing the
hot t or sio n
testing,
the
material is usually heated to
the
given temperature
as a reheating
temperature
and soaked for a
while
and then cooled to the deformation t empera tu re . In
hot tor sion machines th e
heating energy
usually is
supplied by
an
induction
or
resistance type fur
nace[J-3.J3-J5].
The influences
of
specimen geometry
of HTT and reheating condi tions have
no t
been
in
vestigated systematically. However. no proper con
sideration of these choices may introduce some er
rors
in consequence results of
test
due to th e non-u
niform
initial
temperature
and
microstructure within
th e material.
At the
present
study. the commercial f inite ele
ment
package ANSYS™ was used to investigate th e
effects
of
specimen geometry and process
conditions
on distribution of temperature within the specimen
during reheating treatment. This approach will ena-
ble to
prevent the high
temperature gradien t within
Biography:
Bahman Mirzakhani(
1977 .
Male. Doc tor:
E-mail:
b-mirzakhani@araku.
ac. ir , Received Dale: August
15 .
200S
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Issue
3
Influence
of
Specimen Geometry
of Ho t Torsion Test on Temperature
Distribution
• 35 •
Fig. 1 Reheating cycle before hot torsion test
1000
00
Temperature/ t
Heating cycle before deformation
120
L. .
____
600
140
200
220 .
Time/s
Quenching
I
880
1200
[ 180
]
160
is
Q
740
I
Fig.2 Dilatation
IL i
a function of temperature in dilatometry test
In
this study
for
prediction
of
temperature
dis-
tribution
during
reheating treatment a nd b ef or e
ho t
torsion
testing , the
commercial
FEM
code
AN
Y T wa s used. So according to reheating treat
ments which almost isused in experiments, a rehea
ting cycle wa s
designed
as
shown
in Fig.
1. The
tem
peratures 740 an d 880 C in this figure ar e Ad an d
c
temperatures of present microalloyed steel re-
spectively
which were determined by dilatometry
test
as presented in Fig. 2. In this
range rather
low
hea ti ng r a te considered in order to give
time
for fe r-
rite and
perlite
transformation
to austenite.
Th e
chemical composition of
th e API-X70
microalloyed
s te el u se d in t hi s s tu d y is
given
in
Table 1.
Th e geometry
of
th e H T T specimen used
in
this
investigation is
shown
in Fig. 3. A tw o
dimensional
FE analysis
of heat flow ha s
been
used here to
study
th e effect of in it ial reheating cycle before
deforma
tion starts.
The
specimen dimensions
a re s ho wn pa-
rametrically in order to consider
th e
effect of each di-
mension. Th e d om ai n, b ou nd ar ie s an d the mesh
w hi ch w er e used in ANSYS™ is
also presented
in
specimen
by f inding
opti m um ge om e tr y an d
rehea
ting
conditions
to obtain homogenous initial
micro
structure
before
starting
HTT.
is worth noting
that
th e present model devel-
oped
on
th e
base of a flexible
h ot t or si on test
ma
c hi ne , h as been developed at IROST[15].
1 Model and Simulation Conditions
Table 1 Chemical composition of the steel
c
Mn
Si
p
S Nb v
Ti
N Fe
0.09
1. 63
0.32
0.009 0.003 0.04 0.05
0.01 0.002
Re m
imm
4 S3
S3
S4 52
. .
s1
•
z
SI
Fig. 3 Schematic of geometry of a solid hot torsion specimen
a)
and domain and boundaries and mesh of ANSYS
modeling for reheating treatment
b)
Fig. 3 b ) w he re o nly
one-quarter
of a
longitudinal
section
of Fig. 3 a ) is
considered
because of
th e
symmetry
of
g eo me tr y a nd t he condition with respect
to
r a n d
z axis.
Th e axisymmetric
conditions
duri ng r ehea ti ng
treatment b et we en t he bo dy
an d
its environment im-
ply
that
heat
flux along th e
boundaries
51 is zero
or
insulated.
The boundary 52 is
directly
heated by an
induction coil of 45mm length. Th e h ea ti ng w as
controlled by a pyrometer
carefully
located above
th e
center of th e specimen. The boundary
53
is exposed
to
th e surrounding
environment
where th e
energy
loss
is
considered through
b ot h b ou nd ar y
convection
an d radiation.
A simpli fi ed
approach
for
radiation
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• 36 •
Journal
of
Iron
and Steel
Research.
International
Vol. 17
(a ) GI ;
b) G2;
(c )
G3; d) G4;
(e )
G5; f G6.
Fig.4
Initial temperature distribution in GI. G2.
G3 ,
G4, G5,
and
G6 after applying reheating cycle
distribution in various
geometries
of HTT specimens
after applying reheating cycle
(Fig. 1)
and before
starting deformation at 1000 C.
This
figure
illustrates that
inductive heating
may produce non uni form temperature along the axis
of
the
specimens and caused temperature
gradient
within gauge
part of the sample.
In that,
in s pe ci
mens having longer gauge, show greater thermal
gradient. For example in G4 (52 mm gauge length)
the temperature differential
between
center and end
of
the gauge
is the
largest and
about
160 C. It
should be noted
that
for all specimens the induction
length
assumed to be constant and was
equal
to
45 mm. Sections of specimens located
outside
of in
duction
coil
absorbed the
heat
of
the
inside
sections
and led to lower temperature of ends than
center
of
gauge. This
result indicates assumption of
uniform
temperature distribution in the gauge part of the
specimen mentioned in previous works[J3 14 17 19] may
not
be valid.
This would
have a
negative impact
on
the final results of the test.
The temperature history of two points in the
gauge section of G3
during reheating
treatment is
presented in Fig. 5. t shows
that
th e desirable
re
heating cycle was only obtained at the center of s pe c
imen.
The
ends
of
gauge section not only
did
not
reach to
appropriate
temperature for
starting
deform-
120 13010
00
1000
>0
90
Table 2
Dimensional description of various Tf specimens
mm
Specimen
L l
R l
L2
R2
L3
R3
R4
L
Gl
62.0
8
25.0 6.5
8
3.35 1.0 184
G2
62.0
8
12.5 5. 5
33
3. 35 1.5 184
G3
62.0
8
7.0
6.5 42 3.35
1.5 183
G4 62.0 8
3. 0 6. 5
52
3. 35
1.5 185
G5
57.0 6. 25
12.5 4. 75
42
2. 50 1.5 184
G6
66. 5 9. 25
3. 0
8.0
41
4.25
2.0
184
heat transfer was utilized by using an equivalent con
vection
boundary
condition in
which
the
non-linearity
is considered through a temperature-dependent con
vection coefficient which will be designated here as
h, :
h,=O €[ T;+T;.) T,+T,.)]n-1 0
where,
h
is equivalent convection coefficient be
tween
the body surface 5 and the sur face of
the
envi
ronment enclosing
the
body at
the
nth
time step;
T
and
T
are the
absolute
temperatures in degrees Kel
vin
between
these surfaces; is Steffan-Boltzman
coefficient;
e
is emissivity factor of the surface.
The
value of € = 65 was determined according to the
ASTM standard test[16].
The
boundary 54 is in
contact
with grippers of
the HTT machine. For these boundaries, th e t her
mal
contact conductance
kin, of
the
interface
be
tween the specimen and
the
grips was considered and
heat exchange
modeled by convection.
The heat
transfer
characteristics
of the
steel
of
the present work
were assumed
to
be[17 18]
:
p=7 800
kg/rn , c = 6 8 0 J / k g K), k=36.8 W / m K),
k
in
, = 3 740 W / m
2
• K), = 4 8 3 K , T .=368K
0 =5.67 X
10-
8
W
/(m
2
• K
1
) ,
h= 4
W
/(m
2
•
K) .
p
and
c are
the
densi ty and hea t
capacity of ma
terial respectively.
Heat
convect ion and heat conduc
tion coefficients are denoted by
hand k respectively.
The subscripts
a,
g
refer
to
ambient and grip respec
tively.
The
HTT specimens
with
different gauge length
and
radius were des igned in
order
to investigate the
effect of specimen
geometry
on distribution of ther
mo-mechanical parameters under various deformation
conditions. t s ho ul d be n ot ed that the relationship be
tween the sizes of the specimen, as s ho wn in Fig. 3
(a),
were designed
according to the basic
stiffness
re
quirement
expressed
in[19 20]
Table 2 summarizes the
dimensions of different specimens used in
this
study.
2 Results n Discussion
Fig. 4
shows the con tour
plots of
temperature
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Issue 3
Influence of Specimen Geometry of Hot
Torsion
Test on Temperature Distribution
• 37 •
220
190
160
P
130
;l,
I'
100
-
70
40
10
5 15
25 35
45 55
Gauge length/mm
[l
.[]
100
200 300
Time/s
400
1200 r ~ ; ; ; : : : ; ~ ; : ; : : : ; ; : ; ; : = i l
o
P
800
1
o
Fig. 5 Time history of two poin ts in gauge section of
G3 during reheating treatment
Fig. 6 Effect of gauge length on maximum different ia l
temperature along gauge section
4.0
.0 3.0
r
1.0
860
9 0 0 1 ( ~ - - l O l ; o ; ; ; , ; : ; ; ; ; ; ~ ~ ~
890
870 • G5-gauge center 0 G5-gauge end
.. G3-gauge center A G3-gauge end
• G6-gauge center
0
G6-gauge end
1000
o
850
840
: ~
.
I
980
960
1
950
OJ
940
930
920
l l ~
1080
1060
a 900 C; ( b) 1000 C ; (c ) 1100 C.
Fig. 7 Radia l va riation of tempera tu re at cente r and end of
gauge section after reheating and before deformation
ation bu t also the desired
reheating
temperature,
i.
e. 1200 C, at
these points
did
not
achieved. So , it
is
expected that
the
solution
of the chemical elements
will not be occurred completely at these areas.
This
would fail achievement of a homogeneous composi-
t ion within specimen before deformation.
In
addi-
tion,
different austenitizing temperature at different
points
of
the
specimen leads to various initial
grain
sizes of austenite
which
is a c ri ti cal factor that influ-
ences subsequent deformation behavior and phase
transformation of steel[8-12].
For evaluation of
the
effec t of gauge
length
on
maximum differential temperature along the gauge
section
and obtain opt imum
sample
geometries,
the
value of temperature difference between points
and
B et tB along the gauge section
(Fig.
5
was
cal
culated
for various specimens at different
tempera-
tures and plo tt ed
in Fig. 6. This figure
indicates
as
the gauge length increased; the value of tA t B in -
creased;
so that for specimen with 52mm gauge
l ength this gradi en t
exceeded
200 C
at 1100·C.
can
be
also observed
that the
temperature gradients
prior
to start of deformation at different temperature
ar e not
similar. On
t he o the r hand, as
temperature
of final step decreased the gradient decreased.
re-
lates
to the temperature
difference between
gauge
section and o ther par t of the specimen which is less
at lower temperatures than higher temperatures.
For
specimen with short gauge l ength the t empera tu re
did not
show much influence on
temperature
gradient
along the sample.
. Fig. 7 shows the radial variation of temperature at
the center and the end of gauge section af ter reheating
and before
the s ta rt
of deformation at 900, 1000 and
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• 38 •
Journal
of
Iron
and
Steel Research.
International
Vol . 17
R es ul t s ho ws that
no
p r op e r g e om e tr y an d
di
mension selection result in no n
uniform temperature
within
specimen
an d
predicted to h av e eff ects on th e
consequence
assessment
of
m ater ial b eh avi or
during
ho t
deformation as
seen
in
Fig.
9. In
addition.
it
seems prevention
of
an y t e mp e ra t ur e g r ad ie n t d u ri n g
reheating
treatment
of H T T is un-avoidable. bu t
choosing an
o pt im um g eo me tr y
w il l m in im iz e this
p rob le m. B et we en di ff er en t
g eo me tr ie s, t he speci
mens G1
an d
G2 showed
th e
lo w
temperature
gradi
en t
in
both axis an d redial
directions of
th e ga uge
section.
Therefore it ca n be concluded
t ha t t he
opti
m u m g e om e tr y
ar e th e
samples
having between O. 2
an d
O. 7
length
of induction coil.
A numerical modelling
w as p er fo rm ed
to
analy
sis
interaction
of
geometry
of
h ot t or si on test speci
mens
an d
reheating treatment pre-deformation
of
API-X70 microalloyed
steel by
th e commercial finite
element package A NS YS ™. R es ul ts s ho we d that
th e
specimens
in
th e range
of O. 2 to 7
length
of
induc
tion
coil
e xp er ie nc ed l ow
temperature
gradient
in
both
axial an d redial directions a f te r r e he a ti ng
treat
ment.
seems
that
these
geometri.es
ar e
m o re r el ia
bl e
f or dr ivi ng
accurate constitutive
parameters of
3 Conclusion
Fig.9 Flow localization in G4 specimen deformed at 1100
and
i =
10 rad/s after applying reheating cycle
as
shown
in F ig.
6.
Fig. 7
an d
F ig . 8.
Various geom
etries experienced ~ l f f e r e n t
temperature
distribution.
In Fig. 9
th e result
of high
gradient temperature
with
in g aug e sect io n of
H T T
sample on
deformation
be
havior
of G4 is
illustrated. This
sample
deformed
at
1100 C an d 10 rad/s after applying t h e r e he a ti n g
cycle of Fig. 1. As see in
this
figure
interaction
of
sever temperature
gradient
an d
deformation condi
tions
led to flow localization
at the center
of
speci
mens.
It is
due
to
th e
mi d gauge
section experiencing
softening phenomena
i. e.
dynamic recrystallization
during deformation;
while
th e
ends of th e
gauge
sec
tion can not flow as easily as th e center.
1100
\ 0
G5-gauge center
G6-gauge end
1000
Temperature/
t
00
-- ,
15
45
35
Fig. 8 Influ en ce of fin is hi ng t em peratu re of reh eati ng
cycle on r edi al g ra di en t at c en te r and end of
gauge section in R Tf specimens
1100 C . As seen
in
th is fig ure. te mp era tu re
de
c re as ed a lo ng
th e
s pe ci me n r ad iu s be ca us e of heat
transfer
via con vecti on and r adi ati on
from
surface of
specimen to
the surrounding me di um. B ut ra di al
var
iation
of
temperature
wa s much
less at
th e
center
than
at
th e
ends
of
g aug e secti on .
is
due
to
th e
ends of ga uge s ect ion ar e
no t
o nl y a dj ac en t to
th e
en d of
i n d ~ t i o n
coil
bu t
also they
a r e c o nn e ct ed
to
th e m i ni -s h ou l de r s a nd
shoulders
which absorb
a
considerable amount
of
h e at g e ne r at e d th e gauge
section.
is
worth noting that
when
th e
g au ge r ad iu s
increased
th e
slop of temperature vs c urve s in
creased.
h o we ve r t hi s matter
is m o re o bv io us at
th e
ends
tha n the center
of
gauge. On th e other hand,
increasing
in
gauge radius
led to
th e rate
of
heating
l os s a lo ng this d ir ect i on i ncrease. B ecau se acco rd in g
to
Table
2. specimens with lar ge gau ge
diameter
have larger
mini-shoulder
an d s h ou l de r d ia m et er
in
order
to
stiffness requirement
is
carried out.
From
Fig.
7.
influence of finishing temperature of
reheating cycle
Fig.
1) on m ax imu m redial gr ad ie nt in
various H T T specimen after reheating was plotted in
Fig. 8.
W h en r eh e at in g treatment finished
at
1100
C
t he m ax im um redial gradient
at
b o th c en te r a nd e nd s
of ga uge w as greater in comparison of
1000
C an d
900 C.
However
at
th e
center
of
s p ec im e n. t em p er
ature
difference between
surface
and core
of gauge
were
less
t han 45 C
an d 15 C respectively.
Fo r
a
given reheating cycle, geometry
of
H T T
specim en h av e
a
great influence
on temperature distri
bution after reheating treatment and before
deformation
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Issue 3
Influence of Specimen Geometry of
Ho t
Torsion Test on Temperature Distribution
• 39 •
the
steel
by hot torsion tes t.
The
first
author is
grateful
to rakUniversity
for
granting
scholarship
to him
The authors
lso
acknowledge Sadid
Industrial
Group fo r providing
the steel
eferences
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