Consumables – Backing Gas
Inert backing gas (such as Ar, CO2, or possibly N2 for stainless steel vessels) is required with GMAW and GTAW processes for root passes of one-sided welds on alloys with more than 3% Cr content. Inert backing gas is required to prevent high temperature oxidation of Cr (sometimes called “sugaring”) on the inner surface of the weld. To use inert backing gas for pressure vessel fabrication is usually difficult, and can entail purging large volumes. Special precautions are usually necessary to localize the purging area, and one-sided welds should only be used where absolutely necessary.
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