The right choice for rootprotection workBack shielding and root protectionensure perfect weld seams

For more than 40 years, root protection andforming have proven their value in weldingtechnology. They permit an increase in weldseam quality and contribute to a reduction offollow-up costs. The focus here is on reworking,pickling costs, the associated transport costs andthe not inconsiderable loss of time. With correctforming, weld seams and roots can be producedwhich need no reworking.

Forming and root protectionRoot protection is the shielding of the weld rootand the heat affected zone with speciallyformulated gases, while simultaneouslydisplacing atmospheric oxygen (DVS Data Sheet0937). When applied to pipes and tanks, it isknown as forming. This technique is used for thewelding of gas sensitive materials such as highalloyed CrNi steels, for example, to ensure thecorrosion resistance of the materials. Withoutforming, the weld root and the heat affectedzone are oxidised by the oxygen in theatmosphere. Forming gases are evenoccasionally used for the welding of unalloyedsteels in order to improve the root quality. Thecorrect welding of gas sensitive materials, suchas titanium, zirconium, molybdenum ormagnesium, for example, is actually impossiblewithout forming.

Profit from forming

Laminar flow instead of turbulenceIn order to ensure the high quality and economyof the work, a few basic rules must be observed.One of the most important concerns the feed ofthe shield gas to the weld seam region. Thisshould never be uncontrolled. In an optimumshield gas feed, the flow is laminar. If the flow isturbulent, the eddies result in mixing of theforming gas and the atmosphere. A laminar flowis generated with the help of a diffusor, usuallycomprising pipes, sheets or mouldings of sintermaterial. The sinter metal distributes the gasfeed over a large area, from which the forminggas is emitted in laminar form. Formingtechniques are divided, according to the physicalproperties of the forming gases, into:• Forming with gases lighter than air• Forming with gases heavier than air• Forming with gases of the same weight as air

Forming with gases which are lighter orheavier than airThe distinguishing factor is the difference inweight between the forming gas and air. Whengas mixtures with a weight greater than air areused, the vessel is filled from the bottom up andhas a vent at the top for escape of theatmospheric air displaced. In the case of gasmixtures with a lower weight than air, thismechanism is reversed. 

Root without root protection Root with root protection Forming gas (l.) lighter than air and (r.) heavier than air.Red marking: Weld seam zone

Laminar and turbulent flow

The choice of technique can be decided by theforming gases available locally or, in the case oflarge construction components, for examplebecause of the position of the weld area in thecomponent, they may be deliberately applied.

Forming of pipesIn the forming of pipes, problems may arise dueto mixing if the difference in weight between theforming gas and air is too great. In order toprevent this mixing, gas mixtures with the sameweight as air can be used. These areargon/nitrogen/hydrogen mixtures with a variablehydrogen content.

The right hose materialAnother important component is the shield gashose itself. Here, the hose material is crucial.Generally available PVC hoses, originally intendedfor the transport of compressed air, are not suit-able for this purpose. The hose material is capa-ble of absorbing moisture from the atmosphereand passing it on to the dry forming gas. Hosesmanufactured in accordance with DIN EN 559and marked accordingly usually meet the require-ments and can be obtained from any well-stocked welding accessory dealer. Savings madeat this stage may turn out to be very expensive!

Which gas for which application?The gas mixtures in question are based on argonor nitrogen. To reduce the residual oxygen,hydrogen is added to the gases. In addition totheir weight relative to air, other criteria may alsobe applied for selection of suitable gases:

1. The material to be formed – gas sensitivity?2. The forming task – upper or lower component    area?3. The shape of the component – sheet, tank or    pipe?

Incompatibilities may occur between materialsand gases. The constituents of the forminggases, for example, may damage the materialthrough the formation of nitrides or oxides orthrough hydrogen cracks. Particular attentionmust be paid to this when selecting the forminggases. The table gives an overview of recom-mended forming gases. The shape of the compo-nent may also have an influence on the choice ofgas. In the case of pipes and tanks with compli-cated geometries, forming with gases lighter orheavier than air is often accompanied by the com-mon problem of irregular forming results. Here, agas mixture with the same weight as air can beused for even filling of the tank or pipe. Customer-specific argon, nitrogen and hydrogen mixtureswith varying hydrogen content permit quick andreliable forming.

Rootprotection Materials

Argon/hydrogenmixtures

Austenitic Cr-Ni steels, Ni andNi-based materials

Nitrogen/hydrogen mixtures

Steels, with the exception ofhigh-strength, fine-grainedconstruction steels, austeniticCr-Ni steels

Argon

Austenitic Cr-Ni steels,austenitic-ferritic steels(duplex), gas sensitive materials(titanium, zirconium,molybdenum), hydrogensensitive materials (high-strength, fine-grainedconstruction steels, copper andcopper alloys, aluminium andaluminium alloys and other NFmetals), ferritic Cr steels

Nitrogen Austenitic Cr-Ni steels,austenitic-ferritic steels (duplex)

Choice of metal and forming gases

Forming gas

Flow in pipes

Flammable rangeThe final consideration is extremely important:"How much hydrogen do I need for my formingprocess?" According to their hydrogen content,forming gases may be combustible in theatmosphere. These must be flared off as theyleave the component being formed.The ignition limit is 4% H2 and the gases must beflared off at 10% H2 (DVS Data Sheet 0937). Adistinction is made between spontaneously andnon-spontaneously igniting forming gases. In thecase of non-spontaneously igniting mixtures, theuse of a pilot flame is necessary. A risk in the useof flammable forming gases is that of deflagra-tion. This exists if a flammable mixture of forminggas and air is still present at the start of thewelding work.

The forming gas/air mixture in the tank varies itscomposition continuously during the forming,passing through a flammable range.

Residual oxygenWhen gassing a tank or a pipe with forming gas,in spite of observing a precise working proce-dure, mixing with the atmosphere to a greater orlesser degree is inevitable. During welding, theresulting residual oxygen content leads to oxida-tion of the surface, manifesting itself as an-nealing colours. As the forming process prog-resses, the residual oxygen content in the tank isreduced. Depending on the material, a suffi-ciently low residual oxygen content must be setbefore the welding work commences. As a rule,this is approx. 20-50 ppm. An indication of theresidual oxygen content can be obtained with asuitable measuring instrument. In the case ofseries components with low manufacturingcosts, the optimum purge time can also be deter-mined empirically by trial and error.

The graph shows the flammable ranges of different N2/H2

gas mixtures.

H2 content in %

Flammability limit/Minimum content O2

Flammability limit/Minimum content H2

O2

AirN2H2

Influence of the residual oxygen content on theforming result

0 ppm 10 ppm 18 ppm 28 ppm

49 ppm 73 ppm 97 ppm 150 ppm

Determining prepurging timesWhen the correct procedure is followed, theprepurging times for the different componentsare dependent only on the required residualoxygen content. That means, the more sensitivethe material, the longer the prepurging time. Inthe case of sheet metal and irregular tanks, theresidual oxygen content can be measured or thepurging time can be determined empirically. Forthe forming of pipes, there is a graphic aid (DVSData Sheet 0937) for determination of an ade-quate purging time.Dependent on the pipe diame-ter, the purging time per metre of pipe can bedetermined.

Aids for root protection and formingFor root protection or forming, the affected zoneshould, if possible, be spatially demarcated.Numerous aids for this purpose are availablefrom dealers.

Root protection for sheet metal weldingIn the case of sheet metal welding, the root zoneis often accessible, so that a forming device canbe attached. This applies both for butt joints andfor T and corner joints. The device must com-pletely cover the root and the heat affected zone.In principle, a distinction must be made betweenbutt and T joints. On completion of the weldingwork, the component temperature must bemeasured and the cover only removed after ithas cooled sufficiently.

Sinter metal shield gas finger for the forming of pipes

Forming of pipes and tanksCompared with root protection for sheet metalwelding, the forming of pipes and tanks is morecomplicated. The roots of the joints are oftendifficult to get at. Adequate covering of the rootzone with forming gas can usually only beachieved by using special forming devices or, inextreme cases, by completely filling the pipe.

Shield gas angle profile for corner seams

Photo: Jankus

Photo: Jankus

Shield gas cylinder for the forming of pipes

Technical centres – sources for innovationFor the development of new technologies in thefield of welding and cutting, Messer operatestechnical centres in Germany, Switzerland,Hungary and China. These facilities provide idealconditions for innovative projects as well ascustomer presentations and training courses.

Portfolio of gases – comprehensive andclear    Messer offers a spectrum of gases that extendswell beyond the standard fare: it ranges from justthe right gas for each application, and clear,application-oriented product designations to thecontinuous introduction of new gas mixturesdesigned to address current trends.

Specialised on-site consulting – right whereyou need itSpecifically in the context of your particularapplication, we can show you how to optimisethe efficiency and quality of your processes.Along with process development, we supportyou with troubleshooting and processdevelopment.

Cost analyses – fast and efficient  We will be glad to analyze your existingprocesses, develop optimisation proposals,support process modifications and compare theresults with the original conditions – becauseyour success is also our success.

Messer Group GmbHGahlingspfad 31

47803 KrefeldTel. +49 2151 7811-0

Fax +49 2151 7811-503welding-technology@messergroup.com

www.messergroup.com

Training courses – always up to date For the optimal handling of our gases, we cantrain you on processes and how to use them. Our training courses illustrate the use of variousshielding gases for welding and explain how tohandle them safely. This also includes thestorage of the gases and the safe transport ofsmall quantities. Information and trainingmaterials for your plant are also part of theservice, of course.

You can also download this brochure andmany others from the Internet as a PDF file:www.messergroup.com  

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