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Potential savings in welding stainless steels using flux cored welding wires

BÖHLER

POTENTIAL SAVINGS IN WELDING STAINLESS STEELS USING FLUX-CORED WIRES H. Schabereiter, R. Dörfler, J. Ziegerhofer

1. GENERAL The use of high-alloyed flux-cored wires has undergone a remarkable upturn in recent years and more particularly in the very recent past. For cost reasons the constant aim in welding fabrication shops is to substitute established welding procedures with more up-to-date and above all more cost-effective processes. This is usually achieved by means of a close working relationship between the technicians, salesmen and welders. The factors shown in Fig. 1 represent the main considerations for this. Flux-cored welding wires are primarily measured on the properties of coated electrodes and solid wires. For many years the consumption of flux-cored wires in Europe was less than 5 % of the overall potential for high-alloyed filler metals and this has increased to almost 10 % in the last few years. An additional increase in the consumption of flux-cored wires is anticipated. This will not be so much at the cost of coated electrodes but rather as substitution of solid wires.

PRODUCTIVITY PROFITABILITY

CRITERIA FOR SELECTING WELDING PROCESSES

Selection depends mainly on 3 factors that have essentially equal significance in the decision-making process

The following sections will deal in greater detail with the most significant deciding factors for selecting welding processes and the possibilities of reducing costs by using modern filler metals in fabrication of corrosion resistant components.

Fig. 1: Deciding factors for the introduction of welding processes in various welding shops

2. SIGNIFICANT FACTORS FOR DECISION-MAKING IN THE SELECTION OF WELDING PROCESSES Filler metal

ä

Filler metal costs

Metal recovery Weight of filler metal

Joint area

ä

Weight of weld

Welding current Welding voltage

Arc burning time

ä

ä

Gas costs

Deposition rate Weld dressing

Wire extension

Duty cycle

Electricity costs

ä

Total welding time

Labour costs

ä

Capital expenditure

Total cost of weld

Generally speaking, the only welding processes and filler metals considered in practice are those where the weld metal deposits satisfy the requirements in the standards and the characteristics specified for mechanical properties and corrosion resistance. Assuming these basic requirements, cost efficiency is compared with additional factors influencing overall cost outlay in the manufacture of welded components.

Fig. 2: Typical factors in the cost calculation

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2.1

Productivity

Fig. 2 lists all the basic factors to be taken into account when determining the total costs of welded joints. In practice the main emphasis is placed on the welding time which actually only represents a fraction of the total costs. The manufacturing costs depend on a series of additional variables which may have a considerable effect in practice on a component's service life. It is surely hardly necessary to illustrate or explain in detail the economic superiority of flux-cored wires compared with coated electrodes. A detailed comparison of costs using flux-cored wires is, however, useful in cases where semi or fully automatic welding processes using solid wires are employed. Some of the factors significant for the total fabrication costs are frequently ignored when determining the costs per metre or inch of weld which is usually the method used in practice. However, it is precisely the factors which do not pertain purely to the welding time that may be crucial as reasons for process changeover. Improved welding quality and quality assurance also represents an economic factor which does not make itself felt until after the welding process has been changed over. The following sections will illustrate as briefly as possible the productivity advantages when using fluxcored wires for welding stainless steels.

2.1.1 Welds primarily in the flat and horizontal welding position Best suitable flux cored wires for these welds have a rutile slag system with a slow freezing slag. These wires operate in the spray transfer mode with minimum spatter formation. The self releasing slag covers the weld completely. This type of flux-cored wire is suitable for single and multi-pass welds in the flat and horizontal position, horizontal/vertical position as well as the slightly vertical-down position (1 o‘clock). Deposition rate assuming 100% duty cycle (kg/h)

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Fig. 3 illustrates a comparison of deposition rates for coated electrodes, solid and flux-cored wires. The superiority of the flux-cored wires compared with the other filler metals is clearly visible. The reason for this is the high current density (A/mm2) due to the current being conducted through the metal strip mainly which represents a smaller cross sectional area than solid wires. Apart from submerged arc welding, GMAW with flux cored wire belongs to the most productive welding processes.

Flux-cored wire Ø 0.9 mm Flux-cored wire Ø 1.2 mm Flux-cored wire Ø 1.6 mm Solid wire Ø 1.0 mm Solid wire Ø 1.2 mm Stick electrode

8 7 6

Welding position: horizontal (PA)

5 4 3

5 mm

2

4 mm 1

100

200

300

400

Amperage (A)

Flux-cored wire Ø 0.9 mm Argon + 18% CO2 Ø 1.2 / 1.6 mm Argon + 18% CO2

distance of contact tip 15 mm distance of contact tip 20 mm

Solid wire

distance of contact tip 12 mm

Argon + 12% CO2

Fig. 3: Comparison of deposition rates of stick electrodes, solid wires and flux-cored wires

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Fig. 4 shows the comparable lengths of fillet welds with a specified throat thickness of 3 mm and the welding time of 1 minute. The considerably faster travel speed using the flux-cored wire (800 mm) compared with the solid wire (670 mm) and the stick electrode (280 mm) speaks for itself.

a ... flux-cored wire Ø 1.2 mm = 800 mm

b ... solid wire Ø 1.0 mm

= 670 mm

c ... stick electrode Ø 3.2 mm

= 280 mm

Fig. 4: Comparison of travel speed between stick electrode, solid wire and flux-cored wire (type 316 L) in fillet welds, welding position 2F

Fig. 5 illustrates additional comparisons of travel speed. In this case different fillet weld thicknesses (3/5/7 mm) were produced using solid and flux-cored wire and the weld lengths achievable were measured. The welds obtainable using the flux-cored wires are between 19 and 50 % longer with the same welding time.

Gas: throat thickness

3 mm

5 mm

7 mm

Böhler EAS 4 M-FD, ø 1.2 mm 235 A, 33 V, 14 m/min

80 cm/min (+ 19 %)

48 cm/min (+ 33 %)

21 cm/min (+ 23 %)

Böhler EAS 4 M-FD, ø 1.2 mm 275 A, 35 V, 18 m/min

–

54 cm/min (+ 50 %)

25 cm/min (+ 39 %)

Solid wire, ø 1.0 mm 235 A, 31 V, 14 m/min

67 cm/min

36 cm/min

17 cm/min

Solid wire, ø 1.2 mm 235 A, 28 V, 8 m/min

60 cm/min

32 cm/min

15 cm/min

–

36 cm/min

18 cm/min

Solid wire, ø 1.2 mm 275 A, 31 V, 10.5 m/min

Argon +18 % CO2 for flux-cored wire Argon +12 % CO2 for solid wire

Plate 5 mm for 3 mm fillet welds thickness: 10 mm for 5 & 7 mm fillet welds Severe bead oxidation with MAG solid wire, especially at 275 A. Wetting behaviour also not good. Only slight bead oxidation with MAG flux-cored wire even at 275 A!!

Fig. 5: Comparison of travel speeds for MAG solid wire and MAG flux-cored wire type 316L, welding position 2F

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Post weld cleaning The basic requirement for achieving optimum corrosion resistance with austenitic welds is the presence of a completely clean bright metal surface. The passive layer responsible for corrosion resistance can only form in the appropriate shape under these conditions. Thus the dressing of welds is equally a variable which determines quality during the fabrication of welding components. The cost saving benefits gained by welding with flux-cored wires as opposed to solid wires are listed below. Minimum post weld cleaning due to: Ø Ø Ø Ø Ø

Flat and smooth weld finish (minimum grinding expenditure), see Fig. 6 Minimum spatter formation Lower pickling expenditure (temper coloration is minimized), see Fig. 7 Less distortion due to increased travel speeds Lower repair rates (pores, slag inclusions, fusion defects) a)

b)

Fig. 6: Component welding using flux-cored wire: smooth, notch-free welds with good wetting and uniform weld finish

100

Fig. 7: Differences in weld oxidation between: a) flux-cored wire b) solid wire

100 % savings of

90

10 % to 30 %

80 70

Actual information regarding the overall cost savings achievable depend to some extent on the cost structure and plant set-up of the individual company as well as on the particular welding work undertaken.

60 50 40 30 20 10 0

Solid wire

Flux-cored wire

Fig. 8: Cost savings when using flux-cored wire in the flat and horizontal welding position

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However, with a saving in welding time of between 19 and 50 % (see Fig. 5) and calculated industry rates of an average DM 100.-/hr for MIG/MAG welding in Western Europe, the fluxcored electrode, which is more expensive than solid wire, usually makes savings of between 10 and 30 % possible if the overall calculation includes lower-cost weld dressing. Fig. 8 graphically illustrates this financial advantage when using flux-cored wires in the flat or horizontal welding position.

Statements from fabricators: New filler metals are essentially developed by working closely with potential users. Following extensive laboratory testing, samples are used in practice to determine the products' suitability for largescale production. Intensive discussions with consumers of stainless flux-cored wires then resulted in the following practical knowledge. Cost-effectiveness (quality and through put are decisive) Ø Ø Ø Ø

Productivity of the welding process Duty cycle Downtimes Post weld cleaning

Power Sources Ø Conventional MAG welding machines with 4-roll drive and water-cooled torches Ø Average amperage when using flux-cored wires Ø 1.2 mm can be 260 – 270 A. Higher than average increase in productivity in this current range amperages compared with solid wire Ø 1.2 mm (see Fig. 5) Weld Dressing (pickling problems/costs) Ø Ø Ø Ø

Depends on design and plant set-up – but still up to 30 % lower costs using flux-cored wire Pickling baths approx. DM 150.-/hr, waste disposal extremely expensive Spray pickling – collection – neutralisation – disposal of liquid Glass bead blasting – hourly rates DM 250.- to 350.-

The following additional benefits for the user of flux cored wires are worth mentioning: Ø Ø Ø Ø Ø Ø Ø

Easier to operate than MAG solid wire No pulsed power source required (lower noise level) Lower gas costs (Argon + 18 % CO2 instead of Argon + 2 % CO2) Smooth bead appearance Less risk of fusion defects and thus lower repair rates Easier control of heat input due to higher welding speeds Good root pass welding characteristics with extra high productivity gains when using ceramic backing

2.1.2 Positional welding Positional type flux-cored wires produce a rutile slag that solidifies rapidly and are therefore suitable for all welding positions. Both, mixed gases and 100 % CO2, are used as shielding gases. The typical characteristics of such FCAW wires are: Ø Rutile slag with rapid solidification (high melting point) Excellent backing effect for the weld pool Ø Use of high current intensities possible for out-of-position welding, e.g. 160 A instead of 100 – 120 A using solid wire (3G, 3F) Ø Up to 100 % higher travel speeds Ø Excellent wetting behaviour, flat and smooth treat profile Ø Powerful penetrating arc, spray transfer, minimum spatter formation Ø Good mechanical properties

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SINGLE-V BUTT WELD, VERTICAL UP, 1ST FILLER PASS

25 Weld length with flux cored wire + 45 % compared to solid wire

20 19.5 15 13.4 10

Fig. 9 reproduces a comparison of the travel speeds achievable in single-V butt welds in the vertical-up position (filler passes). The fluxcored electrode permits a saving in welding time of 45 % compared with solid wires. Comparison welds produced on the same basis using filler metals for ferritic-austenitic duplex steels have resulted in time savings of up to 100 %.

05 00 Current intensity: Voltage: Wire speed:

*

160.0 A 25.0 V 8.0 m/min

110.0 A 19.5 V 5.0 m/min

Solid wire electrodes Ø 1.2 mm exhibit neither welding nor economic advantages over Ø 1.0 mm in out-of-position welding.

Fig. 9: Travel speed achievable in single-V butt welds in the vertical up position

Fig. 10 shows cross-sections of fillet welds which were produced using solid wires and flux-cored wires in the vertical-up (3 F) position.

Flux-cored wire Ø 1.2 mm 8.2 m/min, 25 V 155 A spray arc Bead configuration = flat

Solid wire Ø 1.0 mm 6.4 m/min, 25 V 115 A Pulsed arc Bead configuration = convex

Welding position: vertical up

Flux-cored wire Ø 1.2 mm 8.2 m/min, 25 V 155 A Spray arc Bead configuration = flat

Solid wire Ø 1.0 mm 5 m/min, 25 V 100 A Pulsed arc Bead configuration = flat

Fast freezing slag of the FCAW è 55 Ampere higher current! Fig. 10: Weld profiles of fillet welds performed using solid wire and flux-cored wire respectively in the vertical-up position

8

The upper part of the Fig.10 clearly shows that a flat bead configuration when using the fluxcored wire is easy to assess in spite of the considerably higher current intensity (155 A) compared with the solid wire electrode (115 A). It is only possible to produce a similar flat fillet weld by using a solid wire by further reducing the current intensity to 100 A. From the economic point of view, reducing the current intensity naturally has an adverse effect on the welding time and therefore on the costs. In positional welding actual cost comparisons result in overall cost savings of more than 50 % when using positional flux-cored wires despite considerably lower prices for solid wire.

In Fig. 11 it is easy to see the time saving and the cost benefit gained by using a positional type flux-cored wire. Approximately 70 to 80 % higher travel speeds can be achieved in fillet welds compared with the solid wire electrode. This is many times higher than with manual arc welding.

Flux-cored wire

Solid wire

Stick electrode

Fig. 11: Comparison of weld lengths achievable in the same amount of time

Fig. 12: Example of practical application

Fig. 13: Macro-section and side bend test specimen from a thick-walled pipe joint.

Fig. 12 illustrates a particular example of a practical application. Here dished boiler ends with wall thicknesses of 38 mm are manufactured from material AISI 316L. The double-V butt welds are produced by automatic welding in the vertical-up position. Böhler EAS 4 PW-FD Ø 1.2 mm was used as the flux-cored wire. The self-peeling slag and smooth weld finish with only slight oxidation (can be removed by brushing easily) are clearly visible.

Joining pipes in the Offshore industry gives rise to almost all the welding positions possible in practical use. As an example, Fig. 13 shows a macro-section and a side bend test specimen originating from a thick-walled 1.4462 pipe joint. The pipe dimensions are Ø 508 x 49 mm. Böhler CN 22/9 PW-FD Ø 1.2 mm was used as the flux-cored wire with optimum results regarding welding characteristics, mechanical properties and corrosion resistance.

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2.1.3 Sheet metal fabrication using 0.9 mm FCAW wires 1.2 mm is the most commonly used diameter of flux-cored wire employed in practice for wall thicknesses above 3 mm. Flux-cored wires of 0.9 mm diameter are especially suitable for joining thin metal sheets above 1.5 mm. They are ideal for welding small, well-wetted weld cross-sections at high travel speed and with less heat input. This is one aspect that has an effect on costs since there are fewer distortion problems and less post-weld straightening at the same time as visual benefits, Fig. 14.

Butt weld

Fillet weld

Lap joint

Corner joint

Fig. 14: Welded joints produced on 2 mm thick metal sheets using fluxcored wire Ø 0.9 mm

2.1.4 Lower-priced shielding gases Stainless steel flux-cored wires are welded using the commercially available shielding gases Argon + 15–25 % CO2. At approx. 16 litres per minute the gas flow rate is the same as when welding solid wires using argon + 2 % CO2. The use of shielding gas containing higher levels of CO2 for slagforming flux-cored wires is rendered possible since every single metal droplet transferred in the arc is completely covered with slag. This prevents any reaction with the shielding gas. Fig. 15 shows a metal droplet completely covered with slag at the end of the FCAW wire. Thus there is no carburisation or burn-off of elements with an oxygen affinity, such as chromium, which would be unacceptable for corrosion reasons.

Complete covering of the droplet with rutile slag

Partially peeled slag after cooling off

Fig. 15: Metal droplet on a flux-cored electrode

The cost benefits for the user result on one hand from the lower gas costs and also from the lower gas consumption which follows on from the shorter welding time.

2.2

Reliable and consistent weld quality

The basic requirement for proper performance of welding work is the use of suitably trained welding staff. In many cases a requirement is made for certified welders for the production of welded components and in certain regulations this requirement is mandatory. On the European market the procedure for testing welders is specified in standard EN 287. There is reference to the individual welding processes, such as metal active gas (MAG) welding using flux-cored wires.

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Training, testing and maintaining the validity of test certificates for the welding staff represents a considerable amount of time and money for the welding shop. Welding processes that allow the training time to be shortened and that reduce possible weld defects therefore represent a cost reduction factor from the „quality management“ point of view.

Easy to operate and reliable in processing 38

Flux-cored wires ensure a very wide range of possible setting parameters. This makes the selection of optimum welding data significantly easier – see Fig. 16.

36

Formation of spatters

34 32

FCAW wire 1.2 mm

30 Solid wire1.2 mm 28

The welding behaviour remains constant throughSolid wire1.0 mm 26 out broad ranges of current intensity and volta24 ge. The large parameter box tolerates unintenPoor bead appearance 22 tional changes to the operating point by the wel150 200 250 300 Amperage (A) der without loss of quality. Welding with a smooth, non-spatter spray arc is possible as low Fig. 16: Possible parameter ranges in the spray transfer for flux-cored and solid wires as 150 Amps, 25 Volts and 6.5 m/min wire feed with Ø 1.2 mm FCAW wires and Ø 0.9 mm wires operate in the spray arc transfer already at 110 A, 26 V and 9.5 m/min. By comparison solid wire only tolerates a very narrow operating range. This requires a high level of concentration from the welder regarding correct torch positioning especially since relatively small deviations lead to impairment of the welding behaviour.

The greater independence of parameter setting and the lower risk of welding defects compared with welding using solid wires speak for the reliability of flux-cored wires.

Solid wire ø 1.2 mm Gas: Argon + 2.5 % CO 2 Flux-cored wire ø 1.2 mm Gas: Argon + 18 % CO2 Excellent penetration Good bead appearance

Deep penetration at the bead centre, however, the lower plate is hardly penetrated

Fig. 17: Comparison of penetration profiles of solid wire and flux-cored wires

Fig. 17 illustrates the characteristic penetration profiles of the wires mentioned. Using the flux-cored wire provides more even penetration with excellent side wall fusion and a good weld profile due primarily to the wider arc. The solid wire on the other hand exhibits very deep penetration in the middle but in this case it is possible to see poor fusion of the lower side wall due to torch manipulation being a little too flat. In practical application the problems caused by lack of fusion continue to give rise to difficulties even when working with the most vigilant and experienced solid wire welders.

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2.3

Product quality and constancy

Years ago there was some scepticism regarding the use of flux-cored wires. This was due to the fact that the flux core was sometimes variable in quality or possibly even missing. Today this scepticism is no longer appropriate since flux-cored wires from well-known companies are manufactured in modern production plants. They consist of high-alloy austenitic precision steel strips and a filling of various mineral and metallic components exhibiting a special composition depending on the type of alloy. At Böhler Welding the infills are agglomerated prior to filling to ensure uniform filling and to prevent decomposition of the different raw materials during filling due to their different specific weights. A specially installed control system using up-to-date monitoring technology constantly and efficiently checks the consistance of filling. The whole production process satisfies the criteria of EN ISO 9001. At Böhler Welding every production lot is checked for the welding characteristics, feed properties and chemical composition of the weld metal during quality assurance.

Increased precautions when storing partly used spools and spools removed from their original packaging are particularly important for today's users. As is the case with high-alloyed rutile-coated stick electrodes, all rutile slag-forming flux-cored wires must also be protected from atmospheric moisture and the formation of condensation on the wire surface if the dew point falls. They are more sensitive to the formation of worm-holes than solid wires. The reason for this is that an excess of hydrogen of humid wires is unable to effuse in time prior to solidification of the weld pool due to the slag protection of the weld on one hand and the high travel speeds and simultaneously lower heat input on the other too. Therefore care must always be taken to store partly used spools correctly and always to use wires which have acclimatized. Wires which have become humid can be rebaked at 150°C.

Today thousands of tons of high-alloyed flux-cored wires are successfully used for production welding throughout the world. The wires have proven and established themselves in the construction of chemical and petrochemical plants, offshore engineering, tanker construction, the paper and pulp industry, plant construction in the food, drinks or textile industries up to the welding of highly corrosionresistant flue gas desulphurisation plants.

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Pressure vessel with heating coil from AISI 316L welded with Böhler EAS 4 M-FD

Conveyer screw from AISI 316 Ti for the pulp industry welded with Böhler EAS 4 M-FD

Pipe segment from duplex steel UNS S 31803 for the offshore industry welded with Böhler CN 22/9 PW-FD

Part segment of a scrubber & separator made of UNS S 31803 welded with Böhler CN 22/9 N-FD

Dissimilar joint weld, 3G, welded with Böhler CN 23/12 Mo PW-FD

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High-alloyed flux-cored wires are available for the most commonly used stainless CrNi and CrNiMo steels as well as for dissimilar joints or weld cladding and are standardised in the European standard EN 12073 and the American standard AWS 5.22 respectively.

3. SUMMARY The present report investigates in depth the basic factors such as productivity, processability and quality in the selection of welding processes. The significant advantages of using flux-cored wires for welding stainless steels become perfectly apparent. Despite the higher product price of flux-cored wires, there are remarkable time savings, productivity gains and potential cost savings to be made if all the cost factors relevant to fabrication are taken into consideration. Users who already have extensive practical experience of production welding using high-alloyed fluxcored wires have given the following factors as reasons for changing over: Ø High deposition rate and increased productivity Ø Easy to operate Ø Smooth welding characterisitcs & weld finish Ø Radiographically sound weld deposit Ø Lower costs for the shielding gas Ø Simple and more cost effective post weld cleaning Ø Less repair work Ø Decreased overall fabrication costs

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RANGE OF PRODUCTS

For flat and horizontal welding positions DESIGNATION

EN 12073

size in mm

AWS A 5.22

BÖHLER EAS 2-FD

T 19 9 LRM (C) 3

E 308 LT 0-4 (1)

0.9* 1.2

BÖHLER SAS 2-FD

T 19 9 Nb RM (C) 3

E 347 T 0-4 (1)

BÖHLER EAS 4M-FD

T 19 12 3 LRM (C) 3

E 316 LT 0-4 (1)

BÖHLER E 317 L-FD

TZ 19 13 4 LRM (C) 3

E 317 LT 0-4

-

1.2

-

BÖHLER CN 22/9 N-FD

T 22 9 3 NL RM (C) 3

E 2209 T 0-4 (1)

-

1.2

-

BÖHLER CN 23/12-FD

T 23 12 LRM (C) 3

E 309 LT 0-4 (1)

0.9* 1.2

1.6

BÖHLER CN 23/12 Mo-FD

T 23 12 2 LRM (C) 3

E 309 L Mo T 0-4 (1)

0.9* 1.2

1.6

BÖHLER A 7-FD

T 18 8 Mn RM (C) 3

E 307 T 0-G

-

1.2

0.9* 1.2

-

1.2

1.6 1.6

-

* size 0.9 mm can be operated in all welding positions

For positional welding DESIGNATION

EN 12073

AWS A 5.22

size in mm

BÖHLER EAS 2 PW-FD

T 19 9 LPM (C) 1

E 308 LT 1-4 (1)

1.2

BÖHLER EAS 4 PW-FD

T 19 12 3 LPM (C) 1

E 316 LT 1-4 (1)

1.2

BÖHLER CN 22/9 PW-FD

T 22 9 3 NL PM (C) 1

E 2209 T 1-4 (1)

1.2

BÖHLER CN 23/12 PW-FD

T 23 12 LPM (C) 1

E 309 LT 1-4 (1)

1.2

BÖHLER CN 23/12 Mo PW-FD

T 23 12 2 LPM (C) 1

E 309 L Mo T 1-4 (1)

1.2

BÖHLER E 308 H PW-FD

TZ 19 9 HPM (C) 1

E 308 HT 1-4 (1)

1.2

If you have special questions or if you need expert consultation and competent advice, please feel free to contact us. A team of Böhler welding experts with special knowledge in every sphere of welding engineering is at your disposal.

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Böhler Schweißtechnik Austria GmbH Böhler Welding St. 1 A-8605 Kapfenberg Tel.: ++43 (0) 3862-301-0 Fax: ++43 (0) 3862-301-95193 e-mail: [email protected] http://www.boehler-welding.com

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Böhler Schweißtechnik Austria GmbH Böhler-Welding-St. 1 8605 Kapfenberg / AUSTRIA Tel.: ++43 (0) 3862-301-0 Fax: ++43 (0) 3862-301-95193 e-mail: [email protected] http://www.boehler-welding.com

BSGA 03/2001 E2500

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