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The
ize of the ensitization Zone in
304
tainless teel Welds
N .
S TSAI an d T.
W
EAGAR
Factors influencing the size and shape of the sensitization zone in 3 4 stainless steel w elds
have be en studied through a statistically designed ex perim ent. The results indicate that the
width of the sensitization zone is in proportion to the magnitude of the heat input except
when very little fusion occ urs. Th e minimum sensitization size can be obtained with
a
broadly distributed heat source traveling at high speed and the maximum size can be
obtained with low speed and a narrow heat distribution.
Under certain welding con dition s one can obtain welds which are free of sensitization
on the surface and hence this process m ay lead to a new method o preventing
sensitization-induced intergranular corrosion.
By
depositing surface sensitization-free
welds on either side of a previously sensitized w eld the chrom ium carbide precipitates
disso lve and the sensitization on the plate surface is eliminated.
INTRODUCTION
Sensitization-induced intergranular corrosion in 304 stain-
less steel weldments has bec ome a major problem in recent
years. Factors controlling the sensitization are alloy con tent,
carbon content, and weld thermal cycles. Among these, the
metallurgical factors are well known, * and a number of
methods of preventing intergranula r corrosion a re available:
Employing solution heat treatment
Lowering carbon content
Adding stabilizers
Employing internal cooling of pipes to produce fa-
vorable compressive residual stress on the surface
However, these methods are either costly or difficult to
W ly , particularly to weldments that have already been sen-
sitized. A practical and inexpensive method of reducing
sensitization of these existing welds is desirable. This paper
studies the ef fec ts of the welding process parameters on the
location and size of the sensitization zone and presents a
N
TSAI
and
T
W. EAGAR are
with
the Department of
Materials Science and Engineering, Massachuset ts Institute of
Technology, Cambridge,
M A
02139
new method of preventing sensitization-induced inter-
granular corrosion.
EXPERIMENT L PROCEDURE
A Box-Behnken experimental design was used with current,
arc length, and travel speed employed as the primary vari-
ables. Single-pass, bead-on-plate welds were made by gas
tungsten arc welding on 30-cm/30-cm /1.3-cm 304 stainless
steel plates using argon shielding gas. The chemical com-
position of the stainless steel is given in Table 1
The weld cross section was etched in 10 pet oxalic acid
(per ASTM-262 practice
A).
The location and the siz e of the
sensitization zone were measured under an optical micro-
scope at 200-times ma gnification, and the sensitization zone
is defined as the region in which more than 30 pet of the
grain boundary is attacked.
RESULTS
ND
DISCUSSIONS
Sensitization occurs in the region which experiences peak
temperatures of 650Â to 900 OC for a number of seconds
during the welding process. Two primary variables deter-
mining the location and width of the sensitization zone are
~~~~~~~~ FOR
ENER GY
SYSTEMS 1984 MERIC N
SOCIETY
FOR METALS
VOL..
6 NO I
J L N E
1984
33
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Table I. Chemical Composition of the
3 4
Stainless Steel
peak
temperature distribution and the time spent between
these temperatures. Although there will always
be
a region
in the heat-affected zone of any weldment that reaches these
temperatures, it is possible to adjust the time at temperature
by appropriate choice of the welding parameters.
Th e experimental data of the location and the width of the
sensitization zone produced in this study are given in
Table 11. The sensitization width is in proportion to the heat
input per unit length, as shown in Figure
1,
except when
little or no melting occurs. It is seen that the width of the
sensitization zone can vary from nil to over o ne millimeter
at heat inputs of less than
100
J /mm, and hence, the area
of the sensitization can be very large at low heat inputs when
melting is minimized. This is because much of the thermal
energy
is
expended in heating the steel to the sensitization
range when melting is minimized.
A I n t h r o ug h plate t h i c k n e s s 4 i r i ~ L o n
I n t r a ns v e r se d i r e c t l c n
0 3 0 0 0 6 0 0 0 9 0 0 . 0 1 2 0 0 .0
Heat
input
CJ rn rn
5
Fig. 1 -Th e sensitization width is in proportion
to
the heat input p r unit
length. except when little or no melting occurs.
Table 11 Values of the Welding Process Variables and the Location and Size of the Sensitization Zone
Heat
Weld
AL
Speed Voltage Input
N iY 1\71
SY
5YW
S P
No.
AA
rnrn) rnrn/s) V ) W/mrn) mm ) mm ) rnrn) rnrn) rnrn)
34 VOL
6 N O
1.
J U N
1984
J
MATERIALS FOR
ENERGY SYSTEMS
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No 3
290A
10mm
2mmI
s
No
4
142A
1
u
No
12
131A
§ u
9 m l
No 8
21 A
5mm
2mmI s
Fig. 2 Configurations of the sensitized heat affected zon s
MATERIALS FOR ENERGY SYSTEMS
P
X .
c :
I
No
1
2
90A
lOmm
15mm/
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c )
d
Fig.
3
B y depositing high-speed, low-current welds surface-sensitization-freeweld s), the previously sensitized region can
be
eliminated.
a)
The relative
location of the surface-sensitization-freeweld,
with
respect to the previously sensitized weld;
b )
the region between the large weld and the small weld; c) the
sensitization to the left of the large weld, extending all the way up to the surface; d ) he sensitization. magnified.
Above
300
J/mm the width of the sensitized zone is
proportional to the heat input. It will be noted that the
sensitization width is about twice as large in the through-
plate thickness direction as in the transverse direction. This
is due to the finite thickness of the plate which effectively
reflects the heat from the bottom surface and raises the
temperature
in
the through-plate thickness direction.
In welding it is often assumed that the location of the
sensitization zone is determined solely by the peak tem-
perature distribution. This assumption suggests that the
cooling rates are approximately the same at either the inner
or the outer regions of the heat-affected zone. To a first
approximation this simplification describes the region of
peak temperatures between
650
to
950
OC which produce
sensitization. The study of Solomon3supports the peak tem-
perature as the most important variable influencing the de-
gree of sensitization.
Figure
2
shows the configurations of the sensitized heat-
affected zones. Those in the right column have been en-
larged four times. It is interesting to note that some welds
viz.
1
5 6 13 14 and 15 in Figure
2
have no sensi-
tization in the transverse direction. It seems that under the
conditions of high travel speed and low heat input one can
produce a surface-sensitization-freeweld. Under these con-
ditions the simplification that peak temperature alone
controls sensitization breaks down. Under these condi-
tions the time in the sensitization range is insufficient to
cause sensitization.
The discovery of such surface-sensitization-free welds
leads to an important possible application in preventing
sensitization-induced intergranular corrosion.
It
is possible
to deposit two surface-sensitization-freewelds on either side
of a previous weld at the location of the sensitized region.
This assumes a knowledge of the location of these sensitized
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regions. Fortunately, several heat-flow model^^ ^ are avail-
able from which to predict the peak temperature distribu-
tions for a given set of welding parameters. The simplified
prediction of the sensitized region, based solely on peak
temperature distribution, can give a position of the sensi-
tized heat-affected zone which is sufficient for purposes of
locating these surface-sensitization-free weld beads.
I f a
high-spee d, low-heat-input pass is deposited near these sen-
sitized regions, it eliminates the sensitization, as demon-
strated in Figure 3.
The heat of the high-speed weld causes the dissolution of
the chromium-carbide precipitates. without producing suf-
ficient time for new surface sensitization to occur. The sen-
sitization in the bulk of the plate will not influence the
corrosion resistance, since it is not in contact with the corro-
sive environment. In some cases, this may be an inexpen-
sive and practical method for repair of field welds that have
been sensitized,
Figure 3 (a) indicates the relative location of the surface -
sensitization-free weld with respect to the previously sensi-
tized weld. Note that the small weld is not located exactly
at the sensitization zone of the large weld . Th e sensitization
to the left of the large weld extends all the way up to the
surface, w hile the sensitization on the right, w here the repair
weld has been m ade, does not. Figure 3(b ) shows the region
between the large and small weld at higher magnification. I t
is clear that the sensitization is eliminated near the surface.
Figure 3(c ) shows that the sensitization to the left of the
large weld extends all the way up to the surface. Figure 3( d)
shows the magnified sensitization region.
ACKNOWLEDGMENTS
The authors are grateful for support of this work by both the
U.S. Department of Energy (Basic Energy Sciences) under
contract DE-AC02-78ER04799 and the Office of Naval
Research under contract N00014-80-C-0384.
REFERENCES
I .
M
G . Fontana and N . D. Greene:
C or ro si on h g i n e e r i n ~ .
. 61
McGraw-Hill. 1978.
2 14
H. Uhlig: Corrosion ami Corro.\Iun C t~titrol, . 305 Wile). New
York. 1971.
3. H .
D. Solomon: Variables Influe nci n~Weld Sensitization
of
Austenitic
Stainless Steel.
Weldrnmls: P /~~,vIcaletallurgy-
and
F ai lu re P / I C I I O I ~ I
emi, Pro cee din p of the Fifth Bollon Landi118Conference.
1979.
p.
149.
4 .
N S. Tsai and T. W. Eagar: Temperature Field Produced by a Distrib-
uted Heat Source.
Weld.
J 1983. vol.
62. no
12. p.
346.
5 N . S. Tsai and T. W. Eagar: Change u Weld ool Shape
by
Variations
in the Distribution
of
Heat Source in Arc Welding,
Mo~lell i t i f ;
Cust
I I I R
iind W~ld111gr oces s e .~ .
enniker. New Hampshire, 1983.
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