The perfect welding result, without impairment of corrosion resistance and mechanical properties, can only be obtained when using a backing gas with very low oxygen content. For best results, a maximum 20 ppm O2 at the root side can be tolerated. This can be achieved with a purging set-up and can be controlled with a modern oxygen meter.
Pure argon is by far the most common gas for root protection of stainless steels. Formiergas (90% N2 + 10% H2) is an excellent alternative for conventional austenitic steels. The gas contains an active component, H2, which brings down the oxygen level in the weld area. Nitrogen can e used for duplex steels in order to avoid nitrogen loss in the weld metal.