SEGUNDA PARTE

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

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

Figure 6.2

© ISF 2006

Survey of Narrow Gap Welding Techniques Based on Conventional Technologies br -er6-02e.cdr 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) process with linearly oscillating filler wire process with stripshaped filler and fusing feed electrogas process with linearly oscillating wire electrode electrogas process with bent, longitudinally positioned strip electrode MIG/MAG- processes (1P/L,2P/L,3P/L)

process with hot wire addition (1P/L, 2P/L)

process with cold wire addition (1P/L, 2P/L)

submerged arc narrow gap welding

electroslag narrow gap welding

gas-shielded metal arc narrow gap welding

tungsten innert gas-shielded narrow gap welding

6. Narrow Gap Welding, Electrogas- and Electroslag Welding 78 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- Figure 6.3

Principle of GMA br-er 6-04e.cdr

corrugated wire method with mech. oscillator

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

1 - wire reel 2 -

3 -

4 - inert gas shroud 5 - wire feed nozzle and shielding gas tube 6 - contact tip

mechanical oscillator for wire deformation

drive rollers 1 2 - 1 4 8 - 1 0 1 1 2 2 3 3 4 4 5 5 6 6 © ISF 2006

Survey and Structure of the Variations of Gas-Shielded Metal Arc Narrow Gap Welding

br-er6-03e.cdr 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 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)

GMA narrow gap welding no wire-deformation

GMA narrow gap welding wire-deformation

explanation: P/L: Pass/Layer

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

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 2002 br-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 mm square-butt joint, 9 mm flame cut 1.2 mm 260 A 120 HZ 30 V 22 cm/min 80 min 4 mm 80% Ar/ 20% Co 25 l/min 50 l/min approx. 70 -1 2 Figure 6.5 Figure 6.6 Principle of GMA Narrow Gap Welding

br-er 6-06e.cdr

rotation method spiral wire method

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 1 3 - 1 4 1 1 2 2 3 3 4 4 5 6 5 9 - 1 2

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 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 acommon sword-type torch at an in-

cline towards the weld edgesat 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

Principle of GMA Narrow Gap Welding

br-er 6-07e.cdr

tandem method

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 2 3 4 5 6 9 - 1 2 350 twin-wire method 1 - wire reel 2 - drive rollers 3 - inert gas shroud 4 - wire feed nozzle and shielding gas supply 5 - contact tips 1 2 3 4 5 1 5 - 1 8 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

br-er6-09e.cdr

Comparison of the Weld Groove Shape

8 8 s 10° s 16 7° 8° 6 3 s s 3 10 double-U butt weld

SA-DU weld preparation (8UP DIN 8551)

square-edge butt weld SA-SE weld preparation

(3UP DIN 8551)

double-U butt weld GMA-DU weld preparation (Indexno. 2.7.7 DIN EN 29692)

narrow gap weld GMA-NG weld preparation

(not standardised)

Figure 6.9 br-er6-08e.cdr

Submerged Arc Narrow Gap Welding

strip electrode twin-wire electrode α s x a so h h w w f p H a z vw s v weld speed a electrode deflection H stick out

z distance torch - flank s gap width h bead height w bead width p penetration depth w SO stick out s gap width a electrode deflection x distance of strip tip to flank

twisting angle h bead hight w bead width α

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.

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 2002 br-er6-10e_sw.cdr Figure 6.10 br-er6-11e.cdr Electrogas Welding + - weld advance workpiece wire 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 seam

unalloyed, lowalloy and highalloy steels 8 - 20 mm

only 1 (flux-cored wire, for slag formation between

copper shoe and weld surface) Ø 1.6 - 3.2 mm

350 - 650 A 28 - 45 V 2 - 12 m/h

unalloyed and lowalloy steels CO or mixed gas (Ar 60% and 40% Co ) highalloy steels: argon or helium

2 2

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 br-er6-12e.cdr Electroslag Welding 1 2 3 4 5 6 7 8 9 1. base metal 2. welding boom 3. filler metal 4. slag pool 5. metal pool 6. copper shoe 7. water cooling 8. weld seam 9. Run-up plate designation: position: plate thickness: gap width: materials: electrodes: amperage: voltage: welding speed: slag hight:

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 550 - 800 A per electrode 35 - 52 V 0.5 - 2 m/h 30 - 50 mm Figure 6.12 © 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 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 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-

Electroslag Welding with

br-er 6-14e.cdr = ~ drive motor wire or strip electrode run-off plate welding cable workpiece workpiece fusing feed nozzle workpiece cable run-up plate

copper shoes workpiece workpiece

copper shoes

Electroslag fusing

nozzle process (channel welding) position: vertical

plate thickness: 15 mm materials: unalloyed, lowalloy and highalloy steels welding consumables: 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

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 Local

Continuous 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 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 Improve Weld Seam Properties

br-er 6-15e.cdr

technological measures metallurgical measures

increase of purity

application of suitable base and filler metals

addition of suitable alloy and micro-alloy elements (nucleus formation) reduction of S-, P-, H -, N - and O - contents and other unfavourable trace elements 2 2 2 C-content limits Mn, Si, Mo, Cr, Ni,

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