6. narrow gap welding, electrogas - and electroslag weldingdantn/wt/wt1-c6.pdf · narrow gap...

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


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


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-


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

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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

22

33

4 4

5

6 5

9 -

12


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


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


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

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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


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


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


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


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


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.


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


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.

6. narrow gap welding, electrogas - and electroslag weldingdantn/wt/wt1-c6.pdf · narrow gap...

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