MP.
137
220
An Integrated Evaluation on the Mechanical Strength of Graphite Electrocle at Elevaterl Ílenperature,
Sennosuke SAT0, FacuJ.ty of Engineering, The University
of Ibaraki,
Bitachi,
ancl Jun-ichi K0N,
Sho¡ra Denko Co., Omachi, Nagano/Japan.
A graphite'
as well known' ha.s very
f.Jgtroduction
high thernal shock repiqtance to compare wíth othe¡
power
Recent ultrp-\igh
refractory materials.\1/
are, however'
operations in arc furnace steel-naking\1t
close to the li¡nit of nechanical strength of graphite
which cause the
in the the:ma1 stress conditions,
The status in the
frequently.
spalling
fracture
ordinary operations of graphite electrode +a{ be
These
in sone perlods during a batch.\'/
classifiecl
periods are repeated ser¡eral" tens of batch cluring the
Throughout these periocls' the electrod.es are
life.
due to
fluctuations
subjected to very severe electric
scrap ion, short circuj,tr
and rapid
the irregular
heating and cooLing due to the cu¡rent interrrrptions
treatnents
and so on. Accordj.ngly the
of netallurgical
electrodes are subjected to the repeated thermal shock'
inpact force due to the fall down of scraps, inpulsive
force due to short circuit
bencling by electro-magnetic
anong the electrodes and continuous bend.ing cycles due
These stress
to three phases al-ternating currents.
during all over
are loaded. repeateclly and fluetuatingly
the steefunaking operations'
of
0n the nechanical and thernaL properties
graphites at elevated temperature, nany papers have
characteristic
been presented and the indivisual
0n the other
features have been alnost naked clear.
of nany physical prope¡hand, soroe conposed quantities
or spalling
ties, for instance, ther:naI shock resista¡ce
is widely being used to the evaluation of
factordk/M(
In the then¡al stress problen, however' the
naterials.
capable strain in the ¡nateriaLs is the priúary factor.
it is necessary to introduce conprehensiv+
Ibrthemore,
eLectrj"c property and
condition'
1y the heat t¡ansfer
fatigue and the like.
This paper deales with the expennents on the
of four grades of
tensile and thernal properties
electrode graphite up.to 2!00uC and these experirnental
val-ues are introduced and conpared with conprehensively
in two eval-uation equations on the naxinum tenperature
for the therrnal shock fatigue and the na:rinrú
difference
current carring capacity for the heat generatelectric
ing therual stress fatigue.
The specirnens used in these experi2.E¡periments
to the extrusion a¡is of
nents were cut in parallel
four grades of connercial electrod.e graphite having 20
the physical and nechanical propeF
inches in diameter.
ties in rooro temperature are shown in TabLe 1. The
test piece is a contour type as shown
shape of tensile
0he minj-mum d.larneters of specimen were
in Fig.l.
tests by a micromeasured. before antl after of tensile
strain and
and. then the true fracture
scopic projecter
Fig.2 shows
stress were obtained precisely.
fracture
section of high tenperature furnace equipetl
the vertical
material testing nachine having
for Instron universal
capacity of 5 tons.
Fí9.1, 4 and 5 show the tensile
strength ñ,
elasLic noduLus E and true fracture
strain D respectiveIy, neasured in the cross-head speed t mn/n:-n., as a
The na¡inum values of dl and
of tenperature.
function
Á are found at about TtOO-74OOoC and 1800-20O0oC reFig.6 shows the tenperature tlepenrlences of
spectively.
of thermal expansion 0( of four graphibs.
the coefficient
The others tenperature clependences of thermal conduck, specific resistance R and.Poisson's ratio ¡¡
tivity
¡¡ere deduced on the basis of these values in ¡oon
tenperalu"e.
The maximun strain to be
S.Evaluation equations
shodr
the naxinum stress due to the:nal
derivetl tlividing
range
moclulus E ' a¡rd the ma¡inum strain
by the elastic
of Coffin-Larrger on the Iow-cyc1e
in the fopylg
r\2./ s¡s placed in equal.
Then the characterr
fatigus\ai
istic
tenperature d.ifference on the:mal shock fatigue
y'T is derived as an approxinate
the
equation involving
paraneters as follows:
inportant
¡a{a+f - cqp(fi}ffi+zn)
(r)
where a, b, c and. il are constant on the shapes anrl
ther:maL contlitions
of naterials.
For instance, they are
a=2,O¡ b=4.3, c=0.5 and¡d=16 in the case of rapid cooling of cylind.rical
borlylo/. p is the non-d.imenslonal
heat t¡ansfer r¡hat is called Siot nu¡ober. N is the
nunber of cycles to failure.
It roay be introduged in üre
consiaieration that the cr¡nulatj-ve da,nage effect(7)
in
the case of fluctuating
thenal
shock fatigue.
and
I
p" are para^meters as follows¡
E-D(t-Yt,/a
, E,- f <t-uS/¿o¡
(z)
where f is ad.opted the tensiLe strength fi
instead of
the endurance linit
6ü .
The other hand an evaluation equation is related
with tlepal
stress problem r¡ith internal
heat generThat is to say, the rnari¡¡un strain developed
ation.\o,/
in cylinclrical
body of iliameter cI with heat generatj.on
per unit volu-üe q, and the naxi-mr¡n strain
range in the
fornula of low-cycle fatigue a¡e also placed in equel.
Then, the naximun heat genetation capacity
is
l*,
derived as follows:
c- ..- 4(
Á, *z
' - h- t
l.t
dlr 2N,
v)
bñ^L
where
?,-DA<t-y)/d ,
h- dlQ-v¡/¿d
At the sane tirne, ?s
is related
current óarring capacity
electric
t*A.l
- o.e¿
r;:R
with
t-*
t/A
whe¡e A=?((/+ a¡ta f is a length of cylindrical
Then eq.(1] is replaced as follows;
tT--T-
t*: u/"1/ffi(ffi,+et' )
(+)
the úaxinun
as follows;
(¡)
electrode.
(0
Fj.g.? shows the
of graphite el"ectrodeg
4.Evaluation
j.nfluences
N=ldanit
fairuré
ir' the caseiffi
dependence of llT ir
Siot nunberp =10 on the tenperature
eq.(t) ror four graphites.
Fig.8 and 9 show the teoperature dependences of
in eq.(O) for four graphites in the cases of N=1/4
fs
In consequence of these figures'
and 10, respective]y.
teoperate found. in a certain
the nininue values of IN
At this tj¡ne, on the assurnption that the erperiature.
of 20 inches in dj-aneter
for the graphites
nental ¡esults
to the other sizes of
are applicable approxjnately
to
graphite.
fn* in eq.(6) is generally proportional
the cliameter cl. I'i.nding the current carring capacÍtÍes
for
the value of f¡s
to rnininize
I in the tenperatures
varj-ous values of N' these values of I correspond with
value of allowab1e current carring
the upper critical
Fig.10 shows the a1lowab1e cuttent carríng
capacities.
capacity I for four graales of graphite in N=1/4 and 10'
as a function of electrode dianeter tt. According to tle
A, 3 and D
the orcler of capacity is graphite-C,
figure,
These
tor I/4 and is replaced B with D for N=10'.
are alsorcgnpared with the fracture
velues of cepacities
and the no¡oinal cprEnt
experinent by Kon and g¡sn¡¡s\9/
carring ra¡ges of steel.naking eLectric alc furnace\¿vl
The allowable
as a f\¡nction of electrode dianeter.
of four graphites at N=101 obtained
carring capacities
approxifound to be equivalent
here are resultantly
po$er ope"ation'
nately to the range of the ultrahigb
Concludingly,
the evaluation equatio¡s derived
to bring out the bases
uere are not only interesting
the reasonintroducing
of naterials
in the selection
cycl'es of the practical
able values of fluctuating
but also they are very usefulr we thinkt
operations,
of current
lioit
to nake the reasonable applicable
carring capacity of electrotle graphite in ultrahigh
power operation of arc fu¡nace steeJ-nakíng'
Table I
MP.
137
Properties
of testetl Electrode
Graphites at Roon Eenperature
221
59ac¡ ñan
O.ñtit y
A
1 . 6¿
0
900
2.05
r.90 619
ct.5
r9.9
r.613
961
515
t . 4 9 0.9¿
2.0t
r.a0 580
8A¿
t699 ltt.6
a
r.?50
It2t
828
2.66
1.85
1.10
¿ . t8
u
t . 7 59
r$o
14¿
¿.3
r.96
3.42
Ih¡tic
td.
t ñdñg Str
( l(.,/ñrz)
r.38
r.0t
Cmlon
R.riat¡v¡ty lñcqú
63.1 rü5
79.5
493
H*-?,
ot0
ll,'
oBo
r6ts r379
2.63
l6t.l
0.cs
nJ6
Fig.I
Shape of Tensile
Test Piece
+A
--G-B
-FC
--G-
O
o\
0
Fie.5
50
too
rs00
20@
,5@
0
¡00
. rfc)
Tensile Strength as a
tr\rnction of Eenperature
300
to00
I
|
FiS.4
t!00 2o0
('c '
\
tlo
ooo
Elastic lloclulus as s
tr\-¡nction of Tenps¡s¡rt
Fig.2
Vertical- Section of
High Tenperatu¡e tr\:,¡nace
l|=10
N',|0&yctc
<-A
--o--B
{-c
--o--D
U
t
¡'
l-.
r (.c)
Fl.g.6
rsm
T
Fig.5
?00
The¡nal Expansion Coefficient
as a Fuoction of Tenperature
(.C)
Fig.f
True F¡acture Strain as
a Function of Tenpereture
g
a
t
t:
I
b
J
J
t(.c)
Fig,8
T(t)
}faxi-uun Tenperature Difference on The:nal Shock
Fatigue as a tr\rnction of
Ternperatu¡e
ltaximun Current Carring
Capacity as a Function
(r=r/+)
of Benperatur"
É
_1 Cc.I
Fig.P
Maxinun Current Carriag
Capacity as a tr'unctiqn
(lt=191)
of fenpárature
Fig. IO Critical^Current
u?rr].nq uapaclry
rtamp.J as a Iunction
of ELectrode Diane¡er