Shielding the weld
The primary tasks of a shielding gas are to protect the molten pool from the influence of the atmosphere, i.e. from oxidation and nitrogen absorption, and to stabilize the electric arc. The choice of shielding gas can also influence the characteristics of the arc.
In submerged-arc welding (SAW) the shield is achieved by a welding flux, completely covering the consumable, the arc and the molten pool. The flux also stabilises the electric arc. Sandvik has a range of fluxes to suit different applications.
A 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 of 20 ppm O2 at the root side can be tolerated.
This can be achieved with a purging setup 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 (N2 + 5 - 12% 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 be used for duplex steels in order to avoid nitrogen loss in the weld metal. The purity of the gas used for root protection should be at least 99.995%. In certain cases root flux can be an alternative to gas purging.