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Technical article Topics Overall presentation of thermal welding processes and systems, and of their applications in shipbuilding and offshore plant construction Author Dipl.-Ing. Gerd Trommer / Marianne Walz M.A. rgt redaktionsbüro gerd trommer Johannishofweg 7, 64579 Gernsheim Tel. +49 6258 9320-30, Fax: -32 trommer.gerd@rgt-gg.de Sources Christoph Kammerhuber, Fronius International GmbH; Internal documents from Fronius Number of characters Manuscript FA 559 approx.: 14,420 Picture captions approx.: 4,200 ______________________________________________________________

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Power and system performance are what is called for

System solutions weld more economically

In shipbuilding, as in other metalworking industries, weld processes have to combine

top-quality results with high cost efficiency. What users find most helpful is system

solutions that are specifically geared to the needs of the shipbuilding and offshore

plant construction sector. This calls for system performance, meaning the interplay of

functionality, efficiency and reliability. Although it would go beyond the scope of this

article to look in detail at all possible processes, versions and aspects, it does

describe a number of significant solutions by way of example.

Digital welding processes have brought about great improvements in economical

processing of metals, and in the technical quality of the joins made between them.

The applicational range of gas metal arc (GMA) welding systems covers everything

from steel and its alloys to aluminium and other metals.

Being in contact with seawater, ships’ hulls are mainly made from Grade A steel. For

less aggressive environments, Grade B, D, and E steels are also commonly used.

High-strength steels such as A 32, E36 or E 40 are typical in welded constructions. In

fact, the grades of steel used are as diverse as the many different requirements they

address: high-temperature steels are suitable for steam and pressure boilers; heat-

treated fine-grained structural steels and nickel-alloy steels meet the requirements for

high toughness at low temperatures; and austenitic steels are the material of choice

for cargo tanks. The plate thicknesses range between 4 mm and 40 mm.

A ship’s steel pipework may have wall thicknesses of as much as 25 mm. These

pipes, too, are usually made of high-strength steel; unalloyed grades are sufficient for

normal thermal loading, while enhanced thermal loads call for alloyed grades. In the

same way as for the hull, the ambient conditions influence the choice of materials in

this case as well: high-strength alloyed steels with manganese, molybdenum and/or

chromium for high toughness, and nickel-alloy or austenitic steels for low

temperatures. Piping made of high-alloy steels can be found on cargo tanks and

pressure boilers. Slide-bearing components on the rudder are also made of high-alloy

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steel, as are the wetted parts of chemical tankers and all ship components needing to

withstand corrosive attack. The most frequently welded non-ferrous metals are

aluminium and its alloys, in both cast, rolled and drawn forms. These are used for

e.g. the hulls of yachts and speedboats, and in air-inlet and exhaust systems

(‘funnels’), ship superstructures or pipework. Other non-ferrous materials include

Invar (36% Ni), copper and its alloys, and nickel alloys. These materials are

particularly in demand in the construction of liquefied-gas (LNG/LPG) carriers, and

pressure lines for capacitors and heat exchangers.

Various considerations

Steel, then, is the predominant material. As such, it calls for a variety of special

solutions, as necessitated by different technical and economic aspects and situation-

or user-specific requirements. Although the fabrication of ship panels has some of the

features of series production, the assembly operations – especially those inside the

hull structure – are still entirely manual in nature. The ambient conditions are also

very different in each case: Unlike in enclosed shipbuilding hangars, when working

outdoors there are great variations in wind, moisture and temperature. The

interaction of these influencing factors with aggressive seawater is what causes the

particular set of conditions that typify the maritime environment and that severely test

the endurance of both man and machine.

Steel Transfer Technology and TIG welding systems

Most of the steel joins made in shipbuilding are welded by manual welders. They

need high-performing ‘all-rounder’ systems whose components are designed for

complete interoperability. The VarioStar, VarioSynergic, TransSynergic and

TransPulsSynergic MIG/MAG systems from Fronius incorporate several decades’

worth of empirical knowledge. A noteworthy innovation here is the digital,

microprocessor-controlled inverter power source TransSteel. Its developers have

fitted it out with a package of characteristics, known as ‘Steel Transfer Technology’,

that is specifically tailored to the needs of steel welding. The main benefit for users in

their everyday work performing metal-active gas (MAG) welding is that it lets them

concentrate on the essential aspects of their workflows. These machines’ intuitive

operator functions, innovative wirefeed, ergonomically shaped torch and ruggedly

designed housing are all deliberately focused on steel welding. The TransSteel

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systems stand apart for the stable arc that they deliver regardless of which

characteristic has been selected. Depending on the task to be tackled, the user either

chooses one of the three characteristics for flux-cored wire – rutile, basic or metal

powder – or for solid wire. Specifically for the conditions commonly found in the

shipbuilding industry, Fronius has developed the following additional characteristics:

‘Steel Prime’ is designed to facilitate welding over primer coatings, ‘Steel Root’ is for

good gap-bridging ability and roots, while ‘Steel Dynamic’ is ideal for deep

penetration and small included angles. A powerful filter protects sensitive vital

components, enhancing system availability in dust-laden environments.

Specially designed for shipyard use, the Yard Edition of the TransSteel comes with

welding programs which are all tailored to highly productive joining with standard to

high-alloy steels, with the usual filler metals and types of gas, flux-cored wires and

electrodes.

A very helpful innovation here is the integral media guidance, from the power source

via the VR 5000 Yard wirefeeder all the way through to the easy-change torch. The

feeder unit has a ‘sleigh’ dragging base on one side and a built-in gas-flow regulator;

it is detachable, and designed for mobile deployment. These features provide

maximum manoeuvrability even in changing, hard-to-access locations. The welder

can switch over to MMA operation at the push of a button.

Solutions for aluminium with TransPuls Synergic

A task that is specific to the welding of aluminium is to prevent an oxide skin from

forming during the fusion process. This layer would hinder and even stop the entire

arc process. Among the other challenges posed by aluminium are that it has nearly

twice the thermal elongation of steel, and thermal conductivity that is three to four

times as high.

From funnel installations for cruise liners all the way to entire yachts made of

aluminium, high-grade weld-seams are needed throughout. When it comes to careful

joining of aluminium materials, the MIG arc-welding systems of the TransPuls

Synergic series, with their special programs, are ideally suited. Together with these

machines, Fronius provides their users in the fields of shipbuilding and offshore

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platform construction with characteristics for the most commonly used alloys and filler

metals.

The MagicWave series

For many applications in the shipbuilding, yachtbuilding or boatmaking fields,

tungsten inert-gas (TIG) welding is often the ideal method for joining aluminium.

Equipped with ActiveWave technology, the all-digitised systems of the MagicWave

series provide a stable arc with low noise emissions, all while being ‘lightweights’ in

their class.

High-performance welding systems: TimeTwin

Tandem welding is a high-performance process in which two wire electrodes melt

simultaneously into a single weld-pool under a shielding-gas atmosphere. This basic

idea, of significantly boosting productivity and efficiency by using two wires instead of

one, is put into practice very successfully by the TimeTwin Digital welding system.

Welding with two wires, one following the other, has the further qualitative benefit that

the second arc improves dilution in the fluid pool, greatly reducing fusion defects and

porosity. On small fillet welds of dimension a = 3 mm to 4 mm, it permits a doubling

of welding speeds in the horizontal-vertical welding position. Another advantage will

be found useful during multi-pass welds: At the end of the seam, when the torch path

reverses direction, the TimeTwin Digital control system automatically switches over

the leading and trailing wire electrodes. The perfect start-up phase and optimum

crater-filling both help to shorten the cycle times.

In the TimeTwin process, each power source has its own control and adjustment unit,

and a separate wirefeeder. The welding system is made up of two separate

TransPuls Synergic GMA systems that are coupled together. The high welding

speeds keep the thermal input relatively low, minimising distortion and reducing the

amount of post-weld machining needed. Deposition rates of up to 30 kg/h are

possible.

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Laser beam combined with GMA arc

The laser-hybrid process is suitable for joining both steel and aluminium. It is ideal for

long seams where great welding depth and extremely solid joins are required. In

these cases, this combination of a digital GMA process and a laser beam puts up a

convincing performance. The welding speed of the Fronius-developed LaserHybrid

process is two to three times higher than in GMA welding alone. The laser beam

delivers concentrated – i.e. locationally tightly restricted – thermal input, great weld

penetration depth and high speed. The GMA process which follows the laser

provides good gap-bridging ability and simple weld-seam preparation. The relatively

high power requirements that are typical of lasers are limited to the deep-weld effect,

which supports the joining of thick steel plates. This means that the investment

volume needed for the expensive laser system is smaller than that for an all-laser

welding installation. Both processes concentrate their energy on the same process

zone and thus greatly increase the welding depth and speed as compared to either of

the processes used on its own. Thanks to its lower energy input, the hybrid process

minimises weldment distortion and causes far less spatter. This provides marked

benefits in panel production, for example. The optimised characteristics, and modified

hybrid welding heads with up to 10 kW of laser power, make work simpler and easier

for the welding practitioner.

Thermal overlaying: conditioning and cladding

A typical process in the industry, overlay welding is used either for upgrading

heavily stressed surfaces or for repairing damaged areas. In both cases, the

overlaid material enters into a metallurgically intimate intermixture with the

base metal. In the first application, also known as ‘cladding’, a higher-grade

layer is welded onto a lower-grade base metal. The second application,

known as ‘conditioning’ and used mainly in repair work, involves like-on-like

overlaying.

Cladding high-alloy (and thus more expensive) steel onto less expensive low-

alloy steel saves on both materials and expense. As well as assuring a

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protective function in aggressive environments such as salt water, cladding is

also used on sealing faces and slide-bearing surfaces. A typical application is

the overlaying of weld filler metal (S-CU 6100 and S-CU 6327 to DIN EN

14640) onto the copper alloys of ship propellers.

With conventional thermal overlay processes, the problem has always been

one of controlling and compensating for the distortion resulting from one-

sided warming. Compared to conventional GMA processes, an innovative,

‘cooler’ process has been proving beneficial to users: CMT (Cold Metal

Transfer) from Fronius. The greatly reduced heat input in this process leads,

firstly, to less distortion yet sufficient penetration with the same deposition

rate, and secondly, to significant resource savings.

CMT has also proven highly advantageous in cladding-applications, either for

upgrading surfaces or for enhancing the quality of sealing/sliding faces. In

order to achieve the stipulated purity in the applied high-grade material,

welders using the conventional GMA process have to clad the relevant

places repeatedly (as many as 5 times). This is because of the mingling of

the base and filler metals in the molten zone. With the ‘cooler’ CMT process,

by contrast, there is less melting of the base metal, and – right from the

surface zone of the very first layer – it leaves behind a base-metal

component approaching zero.

Mechanisation of the welding operations in the longitudinal and radial directions

In fields such as vehicle manufacturing and general mechanical engineering, higher

productivity and greater efficiency can be achieved through robot-based automation.

In shipbuilding, where the ‘lot-size’ is just one, this approach is not feasible. Instead,

intelligent mechanisation solutions for reproducible travel paths (i.e. welding-tracks)

offer some attractive possibilities here. Two typical fields of use are in the panel-

production and pipework-mounting operations.

The battery-powered traversing units are ideal for mechanised utilisation of the GMA

process on longitudinal fillet welds in the horizontal-vertical and vertical positions, and

also with integrated oscillation. Their compact, lightweight design makes them

particularly suitable for use in panel production or block construction. These

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appliances can be combined with the TransSteel Yard and a conventional manual

welding torch. Program buttons for the travel path, for segment welding and for crater

filling provide a high degree of flexibility and operator convenience.

In the pipework fabrication and mounting operations, single- to multi-pass

circumferential seams need to be welded, as necessitated by the wall thicknesses of

the pipes to be joined. Orbital welding systems with suitability for steel, CrNi and Cu

materials are ideal for these tasks. The use of intelligent control systems and power

sources, coupled with weld-data monitoring, makes for maximum process reliability

and outstanding welding results.

Conclusion and outlook

Shipbuilding is undergoing great change, throughout the world. This process is being

driven by the need for efficiency-gains in the building of large carrier vessels, and by

the boom in cruise liners and luxury yachts. The market for waterborne craft is also

being transformed by the large numbers of ever more diverse special vessels needed

in the offshore field. The sophisticated and differentiated types of demand generated

in this way create a need for excellent, well-thought-out weld processes and

applications. The challenge for the manufacturers of such welding equipment is to

foster this growth trend from both the technical and business angles, in an

ecologically sustainable manner. In welding, it is system solutions that dictate the

overall direction and application, and this, in turn, is the reason for the increasing

prevalence of comprehensive, integrated offerings.

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Pictures: Selection

1: The gas- or water-cooled welding system TransSteel 3500 Standard Yard is predestined for steel welding in shipyards and the offshore sector.

2: The ‘Yard’ is a portable, ultra-rugged wirefeeder with very many features to provide protection from the hazardous environmental influences encountered in shipyards and offshore. It also has an integral gas-flow regulator.

3: The Fronius System Connector (FSC) provides a central torch connector for all media. Its defined current transfer ensures the very highest process reliability. It is part of the equipment included as standard in the Yard Edition of TransSteel.

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4a: Butt-weld joints on offshore pipes require thorough fusion of the root (steel) around the entire circumference of the pipe. The solution with the Steel Root characteristic makes it easier to achieve reliable root fusion here.

4b: With the Steel Prime characteristic, the TransSteel can be relied upon to always produce a top-quality weld root. In this way, the many fillet seams that have to be welded on the primered steel plates used in shipbuilding give these plates the required strength, yet while minimising spatter formation.

5: Variability: the user can combine the TransPuls Synergic 4000/5000 Yard with a VR 4000 Yard wirefeeder (weighing only 11.4 kg) or a VR 2000 for small wirespools (only 9 kg) – whichever is more suitable for the application.

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6a: TransPuls Synergic 2700-5000 MIG/MAG systems with SynchroPuls can even weld seams in the PF (vertical-up) position on aluminium weldments, without oscillation, delivering perfect weld appearance.

6b: With the TransPuls Synergic 5000 CMT welding power source, users enjoy extraordinarily high gap-bridging ability.

7: In the field of crane construction – as here at Liebherr – very thick steel plates have to be joined. TransPuls Synergic 5000 MIG/MAG systems are ideally suited to these applications.

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8 ((Selection)): Repair-welds on propellers are a regular part of shipbuilders’ day-to-day business. Manual welding predominates here, particularly when smaller areas need repair.

9a: A typical requirements profile in shipbuilding is for long seams, a high welding speed of e.g. 150 cm/min with an ‘a’-dimension of 3.5 mm, and high cost-efficiency. The technology of the TimeTwin Digital tandem process from Fronius is putting this all into practice in the panel line at P+S Werften’s yard in Wolgast, Germany. The basic idea here is “two wires, twice the welding speed”.

9b: The micrograph of this panel-join shows the fillet welds on the bulb profile.

10a: Welding installations with mechanised sequences – like TransPuls Synergic 5000 MIG/MAG systems equipped with Fronius welding automation – apply the metal layers very efficiently.

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10b: The valve faces of valve housings and valve tappets are given their wear-resistant functional surfaces by overlay welding. This valve and valve housing are made of steel, with Inconel 625 filler metal.

11: Using LaserHybrid systems with up to 10 kW of laser power, users can weld fillet seams up to a bulb-profile thickness of 8 mm, or butt-welds up to a material thickness of 12 mm, in a single traverse.

12a: The 10 kW fillet-welding head of the LaserHybrid system, for the panel line, amply meets the requirement for through-welding fillet welds from one side in the horizontal-vertical (PB) position, on sheets of between 4 mm and 8 mm.

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12b: The 10 kW butt-welding head of the LaserHybrid system, for the panel line, fulfils the requirement for through-welding butt-welds from one side in the flat (PA) position without any manipulation of the components.

CrNi Kupfer

Stahl 19d.jpg

13: Users of Fronius Orbital welding systems weld reproducible circular weld-seams of consistently high quality.

14a: Mechanised welding-tasks with GMA systems such as the TransSteel Yard are assisted by battery-powered traversing units. Program buttons for the travel path, for segment welding, for crater filling and for oscillation greatly enhance operator convenience.

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14b: When combined with the FDV 15/22 self-travelling carriage, the TransSteel 3500 welds multi-pass seams from rutile flux-cored wires in the vertical-up and horizontal-vertical (PF+PB) positions.