Nitrogen loss from the weld metal of duplex stainless steels into the back purge during deposition of the root run is a phenomenon that is well known and has been discussed in detail elsewhere. There have been various methods proposed to counter the loss because it is recognised that nitrogen is an important element in the metallurgy and corrosion resistance of duplex stainless steels. The methods that have been used to alleviate the problems caused by nitrogen loss are, additions of nitrogen to the shielding gas, controlled air content in the backpurge and a nitrogen gas back purge.

All of the methods that have been put forward cause secondary problems and as a consequence are difficult to control and use and are therefore not readily accepted by the fabricators.

The authors have investigated the effect of different back purge gases based on argon, nitrogen and oxygen mixtures on the nitrogen and oxygen contents of the weld metal in the root. The work also investigated the effect of the gases on the properties of the weld root by corrosion testing, impact testing and microstructural examinations.

The paper reviews the issue of nitrogen loss with the effect on the properties of the weldment and discusses the use of each of the proposed resolutions to the problem.