Inert gas welding ofhigh-alloy materialsProcess Engineering and Selectionof Shielding Gases

The right shielding gas for the right material

Rapid developments in base and filler metalsdemand a correspondingly wide-ranging programof shielding gases. This applies equally to TIG andto MIG/MAG welding.

TIG Welding For welding, argon is the gas predominantlyused. Hydrogen admixtures raise theperformance appreciably, but some mixtures areonly suitable for automated application. Forduplex steels, the addition of nitrogen may bebeneficial in order to safeguard the austenitecontent. For full austenites, too, the addition ofnitrogen can ensure compliance with low delta-ferrite limits. Hydrogen admixtures cannot beused for duplex steels.

MIG/MAG WeldingAustenitic steels are generally welded using anargon gas mix with 2.5% CO2 added. Oxygen canalso be used, but this results in a more oxidizedweld surface. Helium admixtures of 15%, forexample, also prove extremely effective in manycases. This is true, in particular, both for duplexsteels and for full austenites.

Backing gasesAs a rule, so-called forming gases, nitrogen-hydrogen mixtures, are used. The hydrogencomponent gives more protection againstresidual atmospheric oxygen. Under constructionsite conditions, higher hydrogen contents tend tobe used than in the workshop. According toknown studies, hydrogen admixtures in thebacking gas are unlikely to lead to negativeeffects, even on duplex steels.

Shielding gases for welding of corrosionresistant materials

Welding argon 4.6 I1 TIGWelding argon special 4.8 I1 TIGHelium 4.6 I2 TIGInoxmix H2 R1 TIGInoxmix H5 R1 TIGInoxmix H7 R1 TIGInoxmix H20 R2 TIGInoxmix H35 R2 Plasma cutting

Inoxmix He3H R2 TIGInoxmix N3H R1 TIGInoxmix N1 R1 TIGInoxmix N2 SI1 TIGInoxmix He15N1 SI2 TIG

Shielding gases for MAG welding ofaustenitic steels

Inoxmix

Inoxmix He30C3H1 M11 MAG MInoxmix He30H2C M11 MAG MInoxmix He15C2 M12 MAG MInoxmix C2 M12 MAG MInoxmix X2 M13 MAG MFerromix X4 M22 MAG M

Backing gases

Forming Gas

Forming Gas H5 F2Forming Gas H8 F2Forming Gas H12 F2Forming Gas H25 F2Inoxmix H2 R1Welding argon I1

Materials Science BackgroundAustenites contain close to 20% chromium and about 10% nickel. As a rule,the typical structure has a ferrite content of 5 to 8%. Materials frequentlyused: 1.4301, 1.4541, 1.4571. Austenitic chrome-nickel steels are eitherstabilized against intercrystalline corrosion by admixtures (usually titanium) orhave a particularly low carbon content (LC qualities). Duplex steels have high corrosion resistance, especially against mediacontaining chlorides and, at the same time, have greater mechanical strength.Most important material: 1.4462. Duplex steels have a mixed structure with a50% ferrite content. Superduplex steels have an increased resistance topitting.Fully austenites have a maximum ferrite content of 2%. This leads to anincreased sensitivity to hot cracking. On the other hand, fully austenites havehigher resistance to corrosion and to heat. Because of the extremely lowferrite content, these materials are non-magnetic. Typical materials: 1.4435and 1.4439.Nickel base materials are used for maximum corrosion resistancerequirements at high temperatures of more than 1000 °C. They can no longerbe classified as steel materials and are correspondingly identified by materialnumbers beginning with 2. When working with them, extreme cleanlinessmust be observed.TIG or MAG?Extremely high weld qualities can be achieved with TIG, as the non-metallicoxygen influences are extremely small. The welding speed is comparativelyslow and the heat input high. Plasma welding, a variety of TIG welding,guarantees constant values and is used mainly for fully automatedapplications. MAG welding is often used for fillet welds. Particularly in thecase of fully automated applications, it is also increasingly used for highlystressed welds.Pulse techniqueIn TIG welding, the pulse technique is used in the context of orbitaltechnology to achieve a perfect weld, even with out-of-position welds. InMAG welding, on the other hand, the aim is low spatter or spatter freewelding, even in the lower setting range. The safety of the process withregard to the penetration is also increased. Modern power sources providecustomised programs, adapted to the shielding gases, allowing wide variationof the welding parameters. For high alloy materials, pulse welding can begenerally recommended.Backing gasA backing gas is essential for TIG welding. A backing gas is also frequentlyused for MAG welding. As a rule, a residual oxygen content of <20 ppm isrequired at the weld root. The degree of tarnish permissible depends on thepurpose of the component. Small pipes are purged, the matching of theoutlet aperture being important. In the case of larger pipes, the backing gas isdirected on to the weld by auxiliary devices. It is important to ensure that thepre-purging time is long enough.Flux-cored wiresHigh alloy steels are mostly welded with solid wire electrodes. However,there are also applications for flux-cored wires. Here, the use of the rutile slagtype dominates. Because of the slag covering, very smooth welds areformed, little pickling is necessary and there are practically no spatterproblems. A distinction is made between slowly setting slag for normalposition and rapidly setting slag for vertical welding. In special cases, metalpowder wires are used, for example inside containers where the slag wouldcause problems. The spray arc is reached more quickly with these wires thanwith solid wires.

Practical Notes

Advice, Delivery, Service

MESSER offers a comprehensive program ofgases, which is not always a matter of course.But that is far from all.

We can give advice on the choice of process oron questions of automation, we can tell youwhich type of supply – cylinder, bundle orcryogenic liquid supply – is the right one for you.We would also be glad to talk to you about thecost-saving potentials which may exist for yourfirm in welding, cutting and related processes.

Information and training material foryour company is naturally all included in the wayof technical films which you can borrow free ofcharge, technical articles, brochures and specialinfo on the many everyday questions relating todetails of welding and cutting technology.

We will be happy to provide you with anyinformation you may require, please contact:Tel.: (0 21 51) 7811-234Tel.: (0 21 51) 7811-235Tel.: (0 21 51) 7811-236

Messer Group GmbHGahlingspfad 31

47803 KrefeldTel. +49 2151 7811-0

Fax +49 2151 7811-501info@messergroup.com

www.messergroup.com

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