Post on 18-Feb-2018
2003
6.
Narrow Gap Welding,
Electrogas - and
Electroslag Welding
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 73
Up to this day, there is no universal agreement about the definition of the term “Nar-
row 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 connected with the definition for narrow gap welding. In spite of
these “definition
difficulties” all
questions about
the universally
valid advantages
and disadvantages
of the narrow gap
welding method
can be clearly an-
swered.
The numerous variations of narrow gap welding are, in general, a further develop-
ment of the conventional welding technologies. Figure 6.2 shows a classification with
emphasis on several important process characteristics. Narrow gap TIG welding
with cold or hot wire addition is mainly applied as an orbital process method or for the
joining of high-
alloy as well as
non-ferrous met-
als. This method
is, however, hardly
applied in the prac-
tice. The other
processes are
more widely
spread and are
explainedin detail
in the following.
© ISF 2002
Narrow Gap Welding
br-er6-01e.cdr
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
Survey of Narrow Gap Welding TechniquesBased on Conventional Technologies
br-er6-02e.cdr
process withstraightened
wire electrode(1R/L, 2R/L, 3R/L)
process withoscillating
wire electrode(1R/L)
process withtwin electrode(1R/L, 2R/L)
process withlengthwisepositioned
strip electrode(2R/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
(1R/L,2R/L,3R/L)
process with hotwire addition(1R/L, 2R/L)
process with coldwire addition(1R/L, 2R/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
Figure 6.2
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 74
In Figure 6.3, a systematic subdivision
of the various GMA narrow gap
technologies is shown. In accor-
dance with this, the fundamental dis-
tinguishing feature of the methods is
whether the process is carried out
with or without 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 suit-
able for single layer per pass welding,
other methods require a weld build-up
with at least two layers per pass. A
further subdivision is made in accor-
dance with the different types of arc
movement.
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 degrees of between 3° and 15°
towards the torch axis. With an electronic stepper motor control, arbitrary transverse-
arc oscillating mo-
tions with defined
dwell periods of os-
cillation and oscilla-
tion frequencies can
be realised - inde-
pendent of the filler
wire properties. In
contrast, when the
corrugated wire
method with me-
chanical oscillator is
© ISF 2002br-er6-03e.cdr
D
A
C
B
long-wire method(1 B/P, 2 B/P)
thick-wire method(1 B/P, 2 B/P)
twin-wire method(1 B/P)
tandem-wire method(1 B/P, 2 B/P, 3 B/P)
twisted wire method(1 B/P)
coiled-wire method(1 B/P)
rotation method(1 B/P)
corrugated wire methodwith mechanical oscillator
(1 B/P)corrugated wire methodwith oscillating rollers
(1 B/P)corrugated wire methodwith contour roll (1 R/L)
zigzag wire method(1 B/P)
wire loop method(1 B/P)
GMA narrow gap weldingno wire-deformation
GMA narrow gap weldingwire-deformation
explanation:B/ P:Bead/ Pass
A: method without forced arc movementB: method with rotating arc movementC: method with oscillating arc movementD: method with two or more filler wires
Figure 6.3
Figure 6.4
Principle of GMANarrow Gap Welding
br-er 6-04e.cdr
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
3 3
4 4
5 5
6 6
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 75
applied, arc oscillation is produced by
the plastic, wavy deformation of the
wire electrode. The deformation is
obtained by a continuously swinging
oscillator which is fixed above the wire
feed rollers. Amplitude and frequency
of the wave motion can be varied over
the total amplitude of oscillation and
the speed of the mechanical oscillator
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 produced 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 cor-
rect setting of the
oscillation parame-
ters and the pre-
cise, centred torch
manipulation no
sidewall fusion de-
fects occurred, in
spite of the low
sidewall penetration
depth. A further ad-
vantage of the
weave-bead tech-
© 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% Co25 l/min50 l/minapprox. 70
-1
2
Figure 6.5
Figure 6.6
Principle ofGMA Narrow Gap Welding
br-er 6-06e.cdr
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
22
33
4 4
5
6 5
9 -
12
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 76
nique is the high crystal restructuring rate in the weld metal and in the basemetal ad-
jacent to the fusion line – an advantage that gains good toughness properties.
Two narrow-gap welding variations with a rotating arc movement are shown in Fig-
ure 6.6. When the rotation method is applied, the arc movement is produced by an
eccentrically protruding wire electrode (1.2 mm) from a contact tube nozzle which is
rotating with frequencies between 100 and 150 Hz. When the wave wire method is
used, the 1.2 mm solid wire is being spiralwise deformed. This happens before it en-
ters 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 operated with-
out forced arc movement, where a reliable sidewall fusion is obtained 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 tech-
nique 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 ap-
plied, two parallel
switched elec-
trodes are trans-
ported by a com-
mon wire feed unit,
and, subsequently,
adjusted in a
common sword-
type torch at an
incline towards the Principle of GMA
Narrow Gap Welding
br-er 6-07e.cdr
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 77
weld edges at small spaces behind
each other (approx. 8 mm) and mol-
ten.
In place of the SA narrow gap weld-
ing methods, mentioned in Figure
6.2, the method with a lengthwise po-
sitioned 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 multipass 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 oppo-
site fillet is made. Solid wire as well as
cored-strip electrodes with widths be-
tween 10 and 25 mm are used. The
gap width is, depending on the number
of passes per layer, between 20 and
25 mm. SA twin-wire welding is, in
general, carried out using two elec-
trodes (1.2 to 1.6 mm) where one elec-
trode is deflected towards one weld
edge and the other towards the bottom
of the groove or towards the opposite
weld edge. Either a single pass layer
or a two pass layer technique are ap-
plied. Dependent on the electrode di-
br-er6-08e.cdr
Submerged ArcNarrow Gap Welding
strip electrode
twin-wire electrode
αs
xa
so
h
h
w
w
f
p
H
az
vw
s
v weld speeda 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
br-er6-09e.cdr
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
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 78
ameter and also on the type of weld-
ing 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. De-
pending on the plate thickness, sig-
nificant differences in the weld cross-
sectional dimensions occur which
may lead to substantial saving of ma-
terial 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 pres-
sure vessel cover is depicted in Fig-
ure 6.10. The inner diameter of the
pressure vessel is more than
5,000 mm with 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 neces-
© ISF 2002br-er6-10e_sw.cdr
Figure 6.10
br-er6-11e.cdr
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 79
sary to select a gap width of at least 35 mm and to work in the three pass layer tech-
nique.
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 nozzle, 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 fu-
sion 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 drop-
ping static characteristic. The electrode has a positive polarity.
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 exposure to tem-
peratures of more than 1500°C and
low crystallisation rates are responsi-
ble 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.
br-er6-12e.cdr
Electroslag Welding
123456789
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
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 80
Figure 6.12 shows the process principle of Electroslag Welding. Heating and melt-
ing 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 elec-
trodes 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 water-cooled copper shoes which are, with the complete welding unit, and in rela-
tion with the welding speed, moved progressively upwards. To avoid the inevitable
welding defects at
the beginning of
the welding proc-
ess (insufficient
penetration, inclu-
sion 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 conductive 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 proc-
ess 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 subsequently
removed.
© ISF 2002
Process Phases During ES Welding
br-er6-13e.cdr
~
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 81
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 noz-
zle 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 thick-
ness, welding can be carried out with one single or with several wire and strip elec-
trodes. 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
electrode. As the
groove width can
be significantly
reduced when
comparing with
conventional proc-
esses, and a strip
shaped filler mate-
rial with a con-
sumable guide
piece is used, this
welding process is
rightly placed un-
der the group of
narrow gap weld-
ing techniques.
Likewise in elec-
trogas welding, the
electroslag welding
Electroslag Welding withFusing Wire Feed Nozzle
br-er 6-14e.cdr
=~
drive motorwire or stripelectrode
run-off plate
welding cable
workpiece workpiece
fusingfeed nozzle
workpiece cable run-up plate
copper shoesworkpieceworkpiece
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
Possibilities to ImproveWeld Seam Properties
br-er 6-15e.cdr
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 82
process is also characterised by a large molten pool with a – simultaneously - low
heating and cooling rate. Due to the low cooling 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, Figure 6.15.
To avoid postweld heat treatment the electroslag welding process with local con-
tinuous normalisation has been developed 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 subsequently heated to the normalising temperature (>Ac3). The
specially designed
torches follow the
copper shoes
along the weld
seam. By reason of
the residual heat in
the workpiece the
necessary tem-
perature can be
reached in a short
time.
In order to circumvent an expensive postheat weld treatment which is often unrealis-
tic 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 application of a twin-wire electrode in tandem arrangement and
addition of metal powder, the weld speed is increased, as in contrast to the conven-
tional 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, Figure 6.17. The weld prepara-
tion 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 conventional elec-
ES Welding with LocalContinuous Normalisation
br-er 6-16e.cdr
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 torch10. side plate11. heat treated zone
temperature °C
2000
1500
900
500
950
1 2 3 4 5 6 7 8 910
2
7
8
9
11
10
Figure 6.16
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 83
troslag welding, exclusively done by
flame cutting. Thus, the advantage of
easier weld preparation can be main-
tained.
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 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 electrodes, 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 quanti-
ties which as outcome permit a weld-
ing 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.
© 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 addition10 weld seam
ES-welding in 2 passes with sliding shoe
1
2
3
49
5
6
7
8
4
10
Figure 6.17
© ISF 2002br-er6-18e.cdrES-welding of the outer passes
1
11
2
3
49
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 supply10 weld seam11 first pass12 second pass
Figure 6.18
6. Narrow Gap Welding, Electrogas- and Electroslag Welding 84
If wall thicknesses of more than 100 mm are to be welded, several twin-wire elec-
trodes 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 phosphor in the weld centre as well as too fast a cool-
ing of the molten pool in the proximity of the weld seam edges.