6.
Narrow Gap Welding,
Electrogas - and
Electroslag Welding
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
77
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
characteristics
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
process
mentioned
in the upper part of Figure
6.1
are frequently
con-
nected with the definition
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
for narrow gap welding. In
Disadvantages
- 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
spite of these “definition
difficulties”
all
questions
about the universally valid
advantages and disadvan-
© ISF 2002
br-er6-01e.cdr
tages of the narrow gap
Narrow Gap Welding
welding method can be
clearly answered.
Figure 6.1
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 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 highalloy
as
ferrous
well
as
metals.
nonThis
method is, however, hardly
applied in the practice.
The other processes are
more
widely spread and
are explained in detail in
submerged arc
electroslag narrow
narrow gap welding
gap welding
process with
straightened
wire electrode
(1P/L, 2P/L, 3P/L)
process with
oscillating
wire electrode
(1P/L)
process with
twin electrode
(1P/L, 2P/L)
process with
lengthwise
positioned
strip electrode
(2P/L)
flat position
br-er6-02e.cdr
the following.
process with
linearly oscillating
filler wire
process with
stripshaped
filler and
fusing feed
gas-shielded metal arc
narrow gap welding
electrogas
process with
linearly
oscillating
wire electrode
electrogas
process with
bent,
longitudinally
positioned
strip electrode
vertical up position
tungsten innert
gas-shielded
narrow gap welding
process with hot
wire addition
(1P/L, 2P/L)
MIG/MAGprocesses
(1P/L,2P/L,3P/L)
process with cold
wire addition
(1P/L, 2P/L)
all welding positions
© ISF 2006
Survey of Narrow Gap Welding Techniques
Based on Conventional Technologies
Figure 6.2
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
78
In Figure 6.3, a systematic subdivision of the
GMA narrow gap welding
no wire-deformation
various GMA narrow gap technologies is
shown. In accordance with this, the fundamen-
long-wire method
(1 P/L, 2 P/L)
thick-wire method
(1 P/L, 2 P/L)
tal distinguishing feature of the methods is
whether the process is carried out with or without wire deformation. Overlaps in the structure
GMA narrow gap welding
wire-deformation
D
result from the application of methods where a
A
twin-wire method
(1 P/L)
tandem-wire method
(1 P/L, 2 P/L, 3 P/L)
twisted wire method
(1 P/L)
rotation method
(1 P/L)
single or several additional wires are used.
corrugated wire method
with mechanical oscillator
(1 P/L)
corrugated wire method
with oscillating rollers
(1 P/L)
corrugated wire method
with contour roll (1 P/L)
zigzag wire method
(1 P/L)
wire loop method
(1 P/L)
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 accordance with the different types of arc moveexplanation:
P/L:
Pass/Layer
ment.
C
A: method without forced arc movement
B: method with rotating arc movement
C: method with oscillating arc movement
D: method with two or more filler wires
br-er6-03e.cdr
In the following, some of the GMA narrow gap
B
coiled-wire method
(1 P/L)
© ISF 2006
Survey and Structure of the Variations of
Gas-Shielded Metal Arc Narrow Gap Welding
technologies are explained:
Using the turning tube method, Figure 6.4, side
Figure 6.3
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 motions with defined dwell periods of oscillation and oscillation frequencies can be realised - independent of the filler wire properties. In contrast, when
the
corrugated
wire
corrugated wire method with mech. oscillator
1
method
1
oscillation is produced by
3
4
4
5
5
6
6
the plastic, wavy deformation of the wire electrode.
1 - wire reel
2 - mechanical oscillator
for wire deformation
3 - drive rollers
4 - inert gas shroud
5 - wire feed nozzle and
shielding gas tube
6 - contact tip
8 - 10
12 - 14
The
1 - wire reel
2 - drive rollers
3 - wire mechanism
for wire guidance
4 - inert gas shroud
5 - wire guide tube and
shielding gas tube
6 - contact tip
mechanical
oscillator is applied, arc
2
2
3
with
deformation
is
ob-
tained by a continuously
swinging oscillator which is
fixed above the wire feed
br-er 6-04e.cdr
Principle of GMA
Narrow Gap Welding
rollers. Amplitude and frequency of the wave motion
Figure 6.4
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
79
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
plate thickness:
gap preparation:
300 mm
square-butt joint, 9 mm
flame cut
elctrode diameter:
1.2 mm
amperage:
260 A
pulse frequency:
120 HZ
arc voltage:
30 V
welding speed:
22 cm/min
-1
wire oscillation:
80 min
oscillation width:
4 mm
shielding gas:
80% Ar/ 20% Co2
primery gas flow:
25 l/min
secondary gas flow: 50 l/min
number of passes: approx. 70
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
br-er6-05e.cdr
© ISF 2002
can be noticed. Thanks to the correct setting
of the oscillation parameters and the precise,
centred torch manipulation no sidewall fusion
Figure 6.5
defects occurred, in spite of the low sidewall
penetration depth. A further advantage of the weave-bead technique is the high crystal restructuring rate in the weld metal and in the basemetal adjacent to the fusion line – an advantage that gains good toughness properties.
Two
narrow-gap
welding
variations with a rotating
rotation method
1
spiral wire method
1
arc movement are shown in
Figure 6.6. When the rota-
2
3
tion method is applied, the
4
2
3
4
5
arc movement is produced
6
5
from a contact tube nozzle
which is rotating with fre-
1 - wire reel
2 - drive rollers
3 - mechanism for
nozzle rotation
4 - inert gas shroud
5 - shielding gas nozzle
6 - wire guiding tube
1 - wire reel
2 - wire mechanism for
wire deformation
3 - drive rollers
4 - wire feed nozzle and
shielding gas supply
5 - contact piece
9 - 12
ing wire electrode (1.2 mm)
13 - 14
by an eccentrically protrud-
br-er 6-06e.cdr
quencies between 100 and
Principle of
GMA Narrow Gap Welding
150 Hz. When the wave
Figure 6.6
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
80
wire method is used, the 1.2 mm solid wire is being spiralwise deformed. This happens before 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
tandem method
twin-wire method
GMA narrow gap welding
1
1
350
methods which are oper-
2
3
2
4
3
5
4
6
ated without forced arc
movement, where a reliable sidewall fusion is ob-
5
9 - 12
1 - wire reel
2 - deflection rollers
3 - drive rollers
4 - inert gas shroud
5 - shielding gas nozzle
6 - wire feed nozzle
and contact tip
1 - wire reel
2 - drive rollers
3 - inert gas shroud
4 - wire feed nozzle and
shielding gas supply
5 - contact tips
15 - 18
tained either by the wire
deflection through constant
deformation (tandem wire
method) or by forced wire
br-er 6-07e.cdr
Principle of GMA
Narrow Gap Welding
Figure 6.7
deflection with the contact
tip (twin-wire method). In
both cases, two wire electrodes
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 incline 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 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
opposite fillet is made. Solid wire as well as cored-strip electrodes with widths between 10
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
strip electrode
81
and 25 mm are used. The gap width is, depending on the number of passes per layer,
SO stick out
s
α
s
a
x
α
gap width
electrode deflection
distance of strip tip to flank
twisting angle
h
w
bead hight
bead width
so
x
a
h
f
between 20 and 25 mm. SA twin-wire welding is, in general, carried out using two electrodes (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-
w
twin-wire electrode
wards the opposite weld edge. Either a single
pass layer or a two pass layer technique are
vw
s
H
z
a
h
vw
a
H
z
weld speed
electrode deflection
stick out
distance torch - flank
s
h
w
p
gap width
bead height
bead width
penetration depth
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
p
w
shapes in relation to plate thickness for SA
br-er6-08e.cdr
Submerged Arc
Narrow Gap Welding
welding (DIN 8551 part 4) with those for GMA
welding (EN 29692) and the unstandardised,
Figure 6.8
10°
7°
mainly used, narrow gap welding. Depending
on the plate thickness, significant differences in
s
8
the weld cross-sectional dimensions occur
s
8
16
which may lead to substantial saving of material and energy during welding. For example,
when welding thicknesses of 120 mm to 200
mm with the narrow gap welding technique,
double-U butt weld
SA-DU weld preparation
(8UP DIN 8551)
8°
square-edge butt weld
SA-SE weld preparation
(3UP DIN 8551)
10
66% up to 75% of the weld metal weight are
saved, compared to the SA square edge weld.
s
s
3
6
3
The practical application of SA narrow gap
welding for the production of a flanged calotte
joint for a reactor pressure vessel cover is de-
double-U butt weld
GMA-DU weld preparation
(Indexno. 2.7.7 DIN EN 29692)
narrow gap weld
GMA-NG weld preparation
(not standardised)
br-er6-09e.cdr
picted in Figure 6.10. The inner diameter of the
Comparison of the
Weld Groove Shape
pressure vessel is more than 5,000 mm with
Figure 6.9
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
82
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.
workpiece
wire guide
electrode
shielding gas
+
arc
weld pool
Cu-shoe
weld advance
weld metal
water
designation:
gas-shielded metal arc welding (GMAW acc. DIN 1910 T.4)
position:
vertical (width deviations of up to 45°)
plate thickness: 6 - 30 mm square-butt joint or V weld seam
30 mm double-V weld seam
materials:
unalloyed, lowalloy and highalloy steels
gap width:
8 - 20 mm
electrodes:
only 1 (flux-cored wire, for slag formation between
copper shoe and weld surface) Ø 1.6 - 3.2 mm
350 - 650 A
amperage:
28 - 45 V
voltage:
weld speed:
2 - 12 m/h
shielding gas:
unalloyed and lowalloy steels
CO2 or mixed gas (Ar 60% and 40% Co2)
highalloy steels: argon or helium
br-er6-10e_sw.cdr
© ISF 2002
br-er6-11e.cdr
Electrogas Welding
Figure 6.10
Figure 6.11
Electrogas welding (EG) is characterised by a vertical groove which is bound by two watercooled 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 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.
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
83
The application of electrogas welding for lowalloyed steels is – more often than not - limited,
as the toughness of the heat affected zone with
1
2
3
4
5
6
7
8
9
the complex coarse grain formation does not
meet sophisticated demands. Long-time exposure to temperatures of more than 1500°C and
1. base metal
2. welding boom
3. filler metal
4. slag pool
5. metal pool
low crystallisation rates are responsible for this.
6. copper shoe
7. water cooling
8. weld seam
9. Run-up plate
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-
designation:
position:
plate thickness:
gap width:
materials:
electrodes:
resistance fusion welding
vertical (and deviation of up to 45°)
30 mm (up to 2,000 mm)
24 - 28 mm
unalloyed, lowalloy and highalloy steels
1 or more solid or cored wires Ø 2.0 - 3.2 mm
plate thickness range per electrode: fixed 30 - 50 mm
oscillated: up to 150 mm
amperage:
550 - 800 A per electrode
voltage:
35 - 52 V
welding speed: 0.5 - 2 m/h
slag hight:
30 - 50 mm
br-er6-12e.cdr
adjacent regions above critical temperatures
Electroslag Welding
and thus the brittleness effects are significantly
Figure 6.12
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
powder
welding current is fed over
slag
one or more endless wire
ignition with arc
powder fusion
the
slag
highly
heated
slag
bath. Molten pool and slag
molten pool
weld metal
start of welding
electrodes which melt in
bath which both form the
welding
end of welding
© ISF 2002
br-er6-13e.cdr
Process Phases During ES Welding
weld pool are, sideways
retained
by
the
groove
faces and, in general, by
Figure 6.13
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
84
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 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 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 subsequently 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
drive motor
run-off plate
workpiece
workpiece
=
~
fusing
feed nozzle
Also curved
seams can be welded with
a bent consumable elec-
position: vertical
plate thickness:
15 mm
materials: unalloyed, lowalloy
and highalloy steels
welding consumables:
the easier welding area
preparation.
Electroslag fusing
nozzle process (channel welding)
welding cable
variation is the handiness
of the welding device and
wire or strip
electrode
workpiece cable
workpiece
trode. As the groove width
run-up plate
copper shoes
workpiece
wire electrodes: Ø 2.5 - 4 mm
strip electrodes: 60 x 0.5 mm
plate electrodes: 80 x60 up to
10 x 120 mm
fusing
feed nozzle:
Ø 10 - 15 mm
welding powder: slag formation
with high electrical
conductivity
copper shoes
br-er 6-14e.cdr
can
be
duced
significantly rewhen
Electroslag Welding with
Fusing Wire Feed Nozzle
comparing
Figure 6.14
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
85
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 techniques.
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 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, Figtechnological measures
post weld heat
treatment
ure 6.15.
metallurgical measures
increase of
purity
decrease of peak temperature
and dwell times at high
temperatures
application of suitable
base and filler metals
addition of suitable alloy
and micro-alloy elements
(nucleus formation)
increase of welding speed
reduction of energy per
unit length
To avoid postweld heat
treatment the electroslag
welding process with lo-
continuous
normalisation
furnace
normalisation
increase of
deposit rate
decrease of
groove volume
application of
several wire
electrodes,
metal powder
addition
V, double-V butt
joints, multi-pass
technique
reduction of
S-, P-, H2-, N2and O2 - contents
and other
unfavourable
trace elements
C-content limits
Mn, Si, Mo, Cr, Ni,
Cu, Nb, V, Zr, Ti
cal continuous normalisation has been developed for plate thicknesses
br-er 6-15e.cdr
of up to approx. 60 mm,
Possibilities to Improve
Weld Seam Properties
Figure 6.16. The welding
temperature in the weld
Figure 6.15
region drops below the Ar1temperature and is subsequently heated to the nor-
temperature °C
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
malising
2
2000
1500
900
7
8
9
500
10
950
11
1
2
3
4
5
6
7
8
9
10
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
br-er 6-16e.cdr
temperature
ES Welding with Local
Continuous Normalisation
can
be
reached in a short time.
Figure 6.16
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
86
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 application 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, Figure 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 conventional electroslag welding, exclusively done by flame cutting. Thus, the advantage of easier weld preparation can be maintained.
12
11
1
1
2
2
3
3
4
4
9
9
5
5
6
6
7
7
8
8
4
10
br-er6-17e.cdr
4
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
Figure 6.17
© ISF 2002
10
br-er6-18e.cdr
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
ES-welding of the outer passes
© ISF 2002
Figure 6.18
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
2005
6. Narrow Gap Welding, Electrogas- and Electroslag Welding
87
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
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 phosphor in the weld centre as well as too fast a cooling of the molten pool in the proximity of the
weld seam edges.
2005