For duplex and superduplex steels, nitrogen plays an essential role in obtaining good quality welds both from the mechanical point of view and in respect to resistance to corrosion. Moreover, an original feature of this element is that it can be present simultaneously in the filler metals and in the shielding gases: it is then possible to conceive a range of specific gases intended for welding duplex steels in which the nitrogen added by them is optimised.

The first section deals with the transfer of nitrogen in TIG welds dependent on its content in the shielding gas. We show that a 2% addition is a good compromise between the metallurgical value and satisfactory behaviour of the tungsten electrode. Figures for ferrite contents, pitting temperatures and endurance tests on electrodes illustrate this choice. The effect of adding helium is also discussed.

The plasma method brings into play a orifice gas and an shielding gas. The nitrogen transfer curves in relation to its proportion in each of the gases are shown and solutions proposed in terms of mechanical properties and resistance to corrosion. Further, the high dilution in pipe girth welds had led us to compare the properties found with a duplex wire with those associated with other grades (Inconel or superaustenitic).

Finally, the MIG method is more rarely used for welding duplex steels since the role of the nitrogen in their metallurgy only strengthens fears of poor compactness often associated with this process. Characterisation of assemblies is done and shows that the level of properties found is quite satisfactory. Good mastery of the process enables the risk of porosity to be greatly limited: the influencing parameters as well as the origin of them are discussed

Greater understanding of the role of nitrogen in the metallurgy of duplex steels over the last ten years has contributed to their spectacular development on the corrosion resistant steels market.

So it appeared desirable to consider developing shielding gases intended for welding which would not only have the rôle traditionally allocated to them on the morphology of fillets or the stability of transfers but also a metallurgical role.

The purpose of this paper is to show how developments of gaseous mixtures intended for TIG, Plasma and Mig welding of duplex and superduplex steels have reconciled the optimisation of the operating and metallurgical properties of joints. Each of these techniques has its own specific features: TIG (tungsten inert gas) has to cover a wide field of applications: welds made automatically or manually, with or without filler metal, in various positions etc. Then, the two factors governing the metallurgical qualities of the joints, which are the heat cycle and the analysis of the welded metal, are subject to great variations which have to be taken into account without losing sight of the technological side of the process (handability, productivity). Plasma welding, due to the existence of 2 gas circuits, has an extra degree of freedom. As far as MIG (metal inert gas) is concerned, it is associated with good productivity but also very often with mediocre quality. We shall endeavour to show in this paper that using duplex and superduplex steels is possible quite satisfactorily with this method.