6. narrow gap welding, electrogas - and electroslag welding
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6.
Narrow Gap Welding,
Electrogas - and
Electroslag Welding
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 77
2005
Up to this day, there is no universal agreement about the definition of the term “Narrow
Gap Welding” although the term is actually self-explanatory. In the international technical
literature, the process
characteristics mentioned
in the upper part of Figure
6.1 are frequently con-
nected with the definition
for narrow gap welding. In
spite of these “definition
difficulties” all questions
about the universally valid
advantages and disadvan-
tages of the narrow gap
welding method can be
clearly answered.
The numerous variations of narrow gap welding are, in general, a further development of the
conventional welding technologies. Figure 6.2 shows a classification with emphasis on sev-
eral important process characteristics. Narrow gap TIG welding with cold or hot wire addi-
tion is mainly applied as
an orbital process method
or for the joining of high-
alloy as well as non-
ferrous metals. This
method is, however, hardly
applied in the practice.
The other processes are
more widely spread and
are explained in detail in
the following.
© ISF 2002
Narrow Gap Welding
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Process characteristics:
- narrow, almost parallel weld edges. The small preparation angle has the function to compensate the distortion of the joining members
- multipass technique where the weld build-up is a constant 1 or 2 beads per pass- usually very small heat affected zone (HAZ) caused by low energy input
Advantages:
- profitable through low consumption quantities of filler material, gas and/ or powder due to the narrow gaps
- excellent quality values of the weld metal and the HAZ due to low heat input
- decreased tendency to shrink
- higher apparatus expenditure, espacially for the control of the weld head and the wire feed device
- increased risk of imperfections at large wall thicknesses due to more difficult accessibility during process control
- repair possibilities more difficult
Disadvantages
Figure 6.1
Figure 6.2
© ISF 2006
Survey of Narrow Gap Welding TechniquesBased on Conventional Technologies
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process withstraightened
wire electrode(1P/L, 2P/L, 3P/L)
process withoscillating
wire electrode(1P/L)
process withtwin electrode(1P/L, 2P/L)
process withlengthwisepositioned
strip electrode(2P/L)
process withlinearly oscillating
filler wire
process withstripshaped
filler andfusing feed
electrogasprocess with
linearlyoscillating
wire electrode
electrogasprocess with
bent,longitudinally
positionedstrip electrode
MIG/MAG-processes
(1P/L,2P/L,3P/L)
process with hotwire addition(1P/L, 2P/L)
process with coldwire addition(1P/L, 2P/L)
submerged arcnarrow gap welding
electroslag narrowgap welding
gas-shielded metal arcnarrow gap welding
tungsten innertgas-shielded
narrow gap welding
flat position vertical up position all welding positions
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 78
2005
In Figure 6.3, a systematic subdivision of the
various GMA narrow gap technologies is
shown. In accordance with this, the fundamen-
tal distinguishing feature of the methods is
whether the process is carried out with or with-
out wire deformation. Overlaps in the structure
result from the application of methods where a
single or several additional wires are used.
While most methods are suitable for single
pass per layer welding, other methods require
a weld build-up with at least two passes per
layer. A further subdivision is made in accor-
dance with the different types of arc move-
ment.
In the following, some of the GMA narrow gap
technologies are explained:
Using the turning tube method, Figure 6.4, side
wall fusion is achieved by the turning of the contact tube; the contact tip angles are set in de-
grees of between 3° and 15° towards the torch axis. With an electronic stepper motor control,
arbitrary transverse-arc oscillating motions with defined dwell periods of oscillation and oscil-
lation frequencies can be realised - independent of the filler wire properties. In contrast, when
the corrugated wire
method with mechanical
oscillator is applied, arc
oscillation is produced by
the plastic, wavy deforma-
tion of the wire electrode.
The deformation is ob-
tained by a continuously
swinging oscillator which is
fixed above the wire feed
rollers. Amplitude and fre-
quency of the wave motion
Figure 6.3
Figure 6.4
Principle of GMANarrow Gap Welding
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corrugated wire method with mech. oscillator
1 - wire reel2 - drive rollers3 - wire mechanism for wire guidance4 - inert gas shroud5 - wire guide tube and shielding gas tube6 - contact tip
1 - wire reel2 -
3 - 4 - inert gas shroud5 - wire feed nozzle and shielding gas tube6 - contact tip
mechanical oscillator for wire deformation
drive rollers
12 -
14
8 -
10
1 1
22
33
4 4
5 5
6 6
© ISF 2006
Survey and Structure of the Variations of Gas-Shielded Metal Arc Narrow Gap Welding
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D
A
C
B
long-wire method(1 P/L, 2 P/L)
thick-wire method(1 P/L, 2 P/L)
twin-wire method(1 P/L)
tandem-wire method(1 P/L, 2 P/L, 3 P/L)
twisted wire method(1 P/L)
coiled-wire method(1 P/L)
rotation method(1 P/L)
corrugated wire methodwith mechanical oscillator
(1 P/L)
corrugated wire methodwith oscillating rollers
(1 P/L)
corrugated wire methodwith contour roll (1 P/L)
zigzag wire method(1 P/L)
wire loop method(1 P/L)
GMA narrow gap weldingno wire-deformation
GMA narrow gap weldingwire-deformation
explanation:P/L:Pass/Layer
A: method without forced arc movementB: method with rotating arc movementC: method with oscillating arc movementD: method with two or more filler wires
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 79
2005
can be varied over the total amplitude of oscil-
lation and the speed of the mechanical oscil-
lator or, also, over the wire feed speed. As the
contact tube remains stationary, very narrow
gaps with widths from 9 to 12 mm with plate
thicknesses of up to 300 mm can be welded.
Figure 6.5 shows the macro section of a
GMA narrow gap welded joint with plates
(thickness: 300 mm) which has been pro-
duced by the mechanical oscillator method in
approx. 70 passes. A highly regular weld
build-up and an almost straight fusion line
with an extremely narrow heat affected zone
can be noticed. Thanks to the correct setting
of the oscillation parameters and the precise,
centred torch manipulation no sidewall fusion
defects occurred, in spite of the low sidewall
penetration depth. A further advantage of the weave-bead technique is the high crystal re-
structuring rate in the weld metal and in the basemetal adjacent to the fusion line – an advan-
tage that gains good toughness properties.
Two narrow-gap welding
variations with a rotating
arc movement are shown in
Figure 6.6. When the rota-
tion method is applied, the
arc movement is produced
by an eccentrically protrud-
ing wire electrode (1.2 mm)
from a contact tube nozzle
which is rotating with fre-
quencies between 100 and
150 Hz. When the wave
© ISF 2002br-er6-05e.cdr
plate thickness:gap preparation:
elctrode diameter:amperage:pulse frequency:arc voltage:welding speed:wire oscillation:oscillation width:shielding gas:primery gas flow:secondary gas flow:number of passes:
300 mmsquare-butt joint, 9 mmflame cut1.2 mm260 A120 HZ30 V22 cm/min80 min4 mm80% Ar/ 20% Co
25 l/min50 l/minapprox. 70
-1
2
Figure 6.5
Figure 6.6
Principle ofGMA Narrow Gap Welding
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rotation method spiral wire method
1 - wire reel2 - drive rollers3 - mechanism for nozzle rotation4 - inert gas shroud5 - shielding gas nozzle6 - wire guiding tube
1 - wire reel2 - wire mechanism for wire deformation3 - drive rollers4 - wire feed nozzle and shielding gas supply5 - contact piece
13 -
14
1 1
2
2
3
3
4 4
5
6 5
9 -
12
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 80
2005
wire method is used, the 1.2 mm solid wire is being spiralwise deformed. This happens be-
fore it enters the rotating 3 roll wire feed device. With a turning speed of 120 to 150 revs per
minute the welding wire is deformed. The effect of this is such that after leaving the contact
piece the deformed wire creates a spiral diameter of 2.5 to 3.0 mm in the gap – adequate
enough to weld plates with thicknesses of up to 200 mm at gap widths between 9 and 12 mm
with a good sidewall fusion.
Figure 6.7 explains two
GMA narrow gap welding
methods which are oper-
ated without forced arc
movement, where a reli-
able sidewall fusion is ob-
tained either by the wire
deflection through constant
deformation (tandem wire
method) or by forced wire
deflection with the contact
tip (twin-wire method). In
both cases, two wire elec-
trodes with thicknesses
between 0.8 and 1.2 mm are used. When the tandem technique is applied, these electrodes
are transported to the two weld heads which are arranged inside the gap in tandem and
which are indeFigure pendently selectable.
When the twin-wire method is applied, two parallel switched electrodes are transported by a
common wire feed unit, and, subsequently, adjusted in a common sword-type torch at an in-
cline towards the weld edges at small spaces behind each other (approx. 8 mm) and molten.
In place of the SA narrow gap welding methods, mentioned in Figure 6.2, the method with
a lengthwise positioned strip electrode as well as the twin-wire method are explained in more
detail, Figure 6.8. SA narrow gap welding with strip electrode is carried out in the multi-
pass layer technique, where the strip electrode is deflected at an angle of approx. 5° towards
the edge in order to avoid collisions. After completing the first fillet weld and slag removal the
opposite fillet is made. Solid wire as well as cored-strip electrodes with widths between 10
Principle of GMANarrow Gap Welding
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tandem method
1 - wire reel2 - deflection rollers3 - drive rollers4 - inert gas shroud5 - shielding gas nozzle6 - wire feed nozzle and contact tip
1
2
3
4
5
6
9 -
12
350
twin-wire method
1 - wire reel2 - drive rollers3 - inert gas shroud4 - wire feed nozzle and shielding gas supply5 - contact tips
1
2
3
4
5
15 -
18
Figure 6.7
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 81
2005
and 25 mm are used. The gap width is, de-
pending on the number of passes per layer,
between 20 and 25 mm. SA twin-wire weld-
ing is, in general, carried out using two elec-
trodes (1.2 to 1.6 mm) where one electrode is
deflected towards one weld edge and the
other towards the bottom of the groove or to-
wards the opposite weld edge. Either a single
pass layer or a two pass layer technique are
applied. Dependent on the electrode diameter
and also on the type of welding powder, is the
gap width between 12 and 13 mm.
Figure 6.9 shows a comparison of groove
shapes in relation to plate thickness for SA
welding (DIN 8551 part 4) with those for GMA
welding (EN 29692) and the unstandardised,
mainly used, narrow gap welding. Depending
on the plate thickness, significant differences in
the weld cross-sectional dimensions occur
which may lead to substantial saving of mate-
rial and energy during welding. For example,
when welding thicknesses of 120 mm to 200
mm with the narrow gap welding technique,
66% up to 75% of the weld metal weight are
saved, compared to the SA square edge weld.
The practical application of SA narrow gap
welding for the production of a flanged calotte
joint for a reactor pressure vessel cover is de-
picted in Figure 6.10. The inner diameter of the
pressure vessel is more than 5,000 mm with
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Comparison of theWeld Groove Shape
8
8
s
10°
s
16
7°
8°
6
3 s s
3
10
double-U butt weldSA-DU weld preparation
(8UP DIN 8551)
square-edge butt weldSA-SE weld preparation
(3UP DIN 8551)
double-U butt weldGMA-DU weld preparation
(Indexno. 2.7.7 DIN EN 29692)
narrow gap weldGMA-NG weld preparation
(not standardised)
Figure 6.9
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Submerged ArcNarrow Gap Welding
strip electrode
twin-wire electrode
α
s
xa
so
h
h
w
w
f
p
H
a
z
vw
s
v weld speed
a electrode deflectionH stick out z distance torch - flank
s gap widthh bead heightw bead widthp penetration depth
w
SO stick out
s gap widtha electrode deflectionx distance of strip tip to flank
twisting angle
h bead hightw bead width
α
Figure 6.8
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 82
2005
wall thicknesses of 400 mm and with a height of 40,000 mm. The total weight is 3,000 tons.
The weld depth at the joint was 670 mm, so it had been necessary to select a gap width of at
least 35 mm and to work in the three pass layer technique.
Electrogas welding (EG) is characterised by a vertical groove which is bound by two water-
cooled copper shoes. In the groove, a filler wire electrode which is fed through a copper noz-
zle, is melted by a shielded arc, Figure 6.11. During this process, are groove edges fused. In
relation with the ascending rate of the weld pool volume, the welding nozzle and the copper
shoes are pulled upwards. In order to avoid poor fusion at the beginning of the welding, the
process has to be started on a run-up plate which closes the bottom end of the groove. The
shrinkholes forming at the weld end are transferred onto the run-off plate. If possible, any
interruptions of the welding process should be avoided. Suitable power sources are rectifiers
with a slightly dropping static characteristic. The electrode has a positive polarity.
© ISF 2002br-er6-10e_sw.cdr
Figure 6.10
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Electrogas Welding
+-
weld advance
workpiecewire guide
electrode
shielding gas
arc
weld pool
Cu-shoe
weld metal water
designation:position:plate thickness:
materials:gap width:electrodes:
amperage:voltage:weld speed:shielding gas:
gas-shielded metal arc welding (GMAW acc. DIN 1910 T.4)vertical (width deviations of up to 45°)6 - 30 mm square-butt joint or V weld seam 30 mm double-V weld seamunalloyed, lowalloy and highalloy steels8 - 20 mmonly 1 (flux-cored wire, for slag formation betweencopper shoe and weld surface) Ø 1.6 - 3.2 mm350 - 650 A28 - 45 V2 - 12 m/hunalloyed and lowalloy steelsCO or mixed gas (Ar 60% and 40% Co )highalloy steels: argon or helium
2 2
Figure 6.11
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 83
2005
The application of electrogas welding for low-
alloyed steels is – more often than not - limited,
as the toughness of the heat affected zone with
the complex coarse grain formation does not
meet sophisticated demands. Long-time expo-
sure to temperatures of more than 1500°C and
low crystallisation rates are responsible for this.
The same applies to the weld metal. For a
more wide-spread application of electrogas
welding, the High-Speed Electrogas Welding
Method has been developed in the ISF. In this
process, the gap cross-section is reduced and
additional metal powder is added to increase
the deposition rate. By the increase of the
welding speed, the dwell times of weld-
adjacent regions above critical temperatures
and thus the brittleness effects are significantly
reduced.
Figure 6.12 shows the process principle of Electroslag Welding. Heating and melting of the
groove faces occurs in a slag bath. The temperature of the slag bath must always exceed the
melting temperature of the metal. The Joule effect, produced when the current is transferred
through the conducting
bath, keeps the slag bath
temperature constant. The
welding current is fed over
one or more endless wire
electrodes which melt in
the highly heated slag
bath. Molten pool and slag
bath which both form the
weld pool are, sideways
retained by the groove
faces and, in general, by
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Electroslag Welding
1
2
3
4
5
6
7
8
9
1. base metal2. welding boom3. filler metal4. slag pool5. metal pool
6. copper shoe7. water cooling8. weld seam9. Run-up plate
designation:position:plate thickness:gap width:materials:electrodes:
amperage:voltage:welding speed:slag hight:
resistance fusion weldingvertical (and deviation of up to 45°) 30 mm (up to 2,000 mm)24 - 28 mmunalloyed, lowalloy and highalloy steels1 or more solid or cored wires Ø 2.0 - 3.2 mmplate thickness range per electrode: fixed 30 - 50 mmoscillated: up to 150 mm550 - 800 A per electrode35 - 52 V0.5 - 2 m/h30 - 50 mm
Figure 6.12
© ISF 2002
Process Phases During ES Welding
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~
ignition with arc powder fusion
start of welding welding end of welding
powder
slag
weld metal
molten pool
slag
Figure 6.13
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 84
2005
water-cooled copper shoes which are, with the complete welding unit, and in relation with the
welding speed, moved progressively upwards. To avoid the inevitable welding defects at the
beginning of the welding process (insufficient penetration, inclusion of unmolten powder) and
at the end of the welding (shrinkholes, slag inclusions), run-up and run-off plates are used.
The electroslag welding process can be divided into four process phases, Figure 6.13. At
the beginning of the welding process, in the so-called “ignition phase”, the arc is ignited for
a short period and liquefies the non-conductive welding flux powder into conductive slag. The
arc is extinguished as the electrical conductivity of the arc length exceeds that of the conduc-
tive slag. When the desired slag bath level is reached, the lower ignition parameters (current
and voltage) are, during the so-called “Data-Increase-Phase”, raised to the values of the
stationary welding process. This occurs on the run-up plate. The subsequent actual welding
process starts, the process phase. At the end of the weld, the switch-off phase is initiated
in the run-off plate. The solidifying slag bath is located on the run-off plate which is subse-
quently removed.
The electroslag welding with consumable feed wire (channel-slot welding) proved to be
very useful for shorter welds.
The copper sliding shoes are replaced by fixed Cu cooling bars and the welding nozzle by a
steel tube, Figure 6.14. The length of the sheathed steel tube, in general, corresponds with
the weld seam length (mainly shorter than 2.500 mm) and the steel tube melts during welding
in the ascending slag bath. Dependent on the plate thickness, welding can be carried out with
one single or with several
wire and strip electrodes. A
feature of this process
variation is the handiness
of the welding device and
the easier welding area
preparation. Also curved
seams can be welded with
a bent consumable elec-
trode. As the groove width
can be significantly re-
duced when comparing Electroslag Welding withFusing Wire Feed Nozzle
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=~
drive motorwire or stripelectrode
run-off plate
welding cable
workpiece workpiece
fusingfeed nozzle
workpiece cable run-up plate
copper shoes
workpieceworkpiece
copper shoes
Electroslag fusingnozzle process (channel welding)
position: verticalplate thickness: 15 mmmaterials: unalloyed, lowalloy and highalloy steels
welding consumables:
wire electrodes: Ø 2.5 - 4 mmstrip electrodes: 60 x 0.5 mmplate electrodes: 80 x60 up to 10 x 120 mmfusingfeed nozzle: Ø 10 - 15 mm welding powder: slag formation with high electrical conductivity
Figure 6.14
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 85
2005
with conventional processes, and a strip shaped filler material with a consumable guide piece
is used, this welding process is rightly placed under the group of narrow gap welding tech-
niques.
Likewise in electrogas welding, the electroslag welding process is also characterised by a
large molten pool with a – simultaneously - low heating and cooling rate. Due to the low cool-
ing rate good degassing and thus almost porefree hardening of the slag bath is possible.
Disadvantageous, however, is the formation of a coarse-grain structure. There are, however,
possibilities to improve
the weld properties, Fig-
ure 6.15.
To avoid postweld heat
treatment the electroslag
welding process with lo-
cal continuous normali-
sation has been devel-
oped for plate thicknesses
of up to approx. 60 mm,
Figure 6.16. The welding
temperature in the weld
region drops below the Ar1-
temperature and is subse-
quently heated to the nor-
malising temperature
(>Ac3). The specially de-
signed torches follow the
copper shoes along the
weld seam. By reason of
the residual heat in the
workpiece the necessary
temperature can be
reached in a short time.
ES Welding with LocalContinuous Normalisation
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1. filler wire
2. copper shoes
3. slag pool
4. metal pool
5. water cooling
6. slag layer
7. weld seam
8. distance plate
9. postheating torch
10. side plate
11. heat treated zone
temperature °C
2000
1500
900
500
950
1
2
3
4
5
6
7
8
9
10
2
7
8
9
11
10
Figure 6.16
Possibilities to ImproveWeld Seam Properties
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technological measures metallurgical measures
increase of purity
application of suitablebase and filler metals
addition of suitable alloyand micro-alloy elements(nucleus formation)
reduction ofS-, P-, H -, N -and O - contentsand otherunfavourabletrace elements
2 2
2
C-content limitsMn, Si, Mo, Cr, Ni,Cu, Nb, V, Zr, Ti
post weld heattreatment
decrease of peak temperatureand dwell times at hightemperatures
increase of welding speed
reduction of energy perunit length
continuousnormalisationfurnacenormalisation
increase of deposit rate
decrease of groove volume
application ofseveral wireelectrodes,metal powderaddition
V, double-V buttjoints, multi-passtechnique
Figure 6.15
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 86
2005
In order to circumvent an expensive postheat weld treatment which is often unrealistic for use
on-site, the electroslag high-speed welding process with multilayer technique has been
developed. Similar to electrogas welding, the weld cross-section is reduced and, by applica-
tion of a twin-wire electrode in tandem arrangement and addition of metal powder, the weld
speed is increased, as in contrast to the conventional technique. In the heat affected zones
toughness values are determined which correspond with those of the unaffected base metal.
The slag bath and the molten pool of the first layer are retained by a sliding shoe, Fig-
ure 6.17. The weld preparation is a double-V butt weld with a gap of approx. 15 mm, so the
carried along sliding shoe seals the slag and the metal bath. Plate preparation is, as in con-
ventional electroslag welding, exclusively done by flame cutting. Thus, the advantage of eas-
ier weld preparation can be maintained.
For larger plate thicknesses (70 to 100 mm), the three passes layer technique has been
developed. When welding the first pass with a double-V-groove preparation (root width: 20
to 30 mm; gap width: approx. 15 mm) two sliding shoes which are adjusted to the weld
© ISF 2002br-er6-17e.cdr
1 magnetic screening
2 metal powder addition
3 tandem electrode
4 water cooling
5 copper shoe (water cooled)
6 slag pool
7 molten pool
8 solidified slag
9 welding powder addition
10 weld seam
ES-welding in 2 passes with sliding shoe
1
2
3
4
9
5
6
7
8
4
10
Figure 6.17
© ISF 2002br-er6-18e.cdrES-welding of the outer passes
1
11
2
3
4
9
5
6
7
8
4
10
12
1 magnetic screening
2 metal powder supply
3 three-wire electrode
4 water cooling
5 copper shoe (water cooled)
6 slag pool
7 molten pool
8 solidified slag
9 welding powder supply
10 weld seam
11 first pass
12 second pass
Figure 6.18
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 87
2005
groove are used. The first layer is welded using the conventional technique, with one wire
electrode without metal powder addition.
When welding the outer passes flat Cu shoes are again used, Figure 6.18. Three wire elec-
trodes, arranged in a triangular formation, are used. Thus, one electrode is positioned close
to the root and on the plate outer sides two electrodes in parallel arrangement are fed into the
bath. The single as well as the parallel wire electrodes are fed with different metal powder
quantities which as outcome permit a welding speed 5 times higher than the speed of the
single layer conventional technique and also leads to strong increase of toughness in all
zones of the welded joint.
If wall thicknesses of more than 100 mm are to be welded, several twin-wire electrodes with
metal powder addition have to be used to reach deposition rates of approx. 200 kg/h. The
electroslag welding process is limited by the possible crack formation in the centre of the
weld metal. Reasons for this are the concentration of elements such as sulphur and phos-
phor in the weld centre as well as too fast a cooling of the molten pool in the proximity of the
weld seam edges.
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