The effect of PURGING GAS on 308L TIG root pass FERRITE CONTENT

Abstract

Argon is commonly used as root shielding gas in TIG-welding of stainless steels. As argon world-wide is in short supply and prices are rising, it is of interest to investigate what effects changing to alternative root shielding gases might have on weld metal microstructure. Manual TIG welding was used for root and hot pass welding in 304L pipe and plate material using Ar2 as shielding gas and Ar2, N2+10%H2 or N2 as root shielding gas. A significant increase in the nitrogen content of root beads was found when changing the root shielding gas from pure argon to a nitrogen-rich gas. Typically the root bead nitrogen content increased from 500–600 ppm for argon shielding to 1 100 ppm with nitrogen as root shielding gas. As a result the ferrite content decreased with up to 5 FN from about 8–9 FN to 3–5 FN. However, no indication of hot cracking was found and all root beads solidified as predicted by the WRC-92 diagram with ferrite as the leading phase. It is suggested that typical 304L steel and 308L consumable compositions will permit use of nitrogen-rich gases for root shielding without a significantly increased risk of hot cracking. However, the increased nitrogen level must be considered in the choice of steel and consumable. It is advised to use the WRC-92 diagram to make sure there is sufficient safety margin for actual compositions and, if possible, check root bead ferrite content.