TIG welding is usually performed with pure argon as the shielding gas. Resistance to, in particular, pitting corrosion can be considerably raised by the addition of up to 2% nitrogen. However, because the risk of pores increases with increased nitrogen content, the latter should not exceed 2%. The addition of around 30% helium markedly increases arc energy and thus enables a considerable increase (20–30%) in welding speed. In the welding of duplex steels, the addition of hydrogen is not to be recommended. In combination with the high ferrite content (over 70%), this can lead to hydrogen embrittlement. Single-sided root beads must be welded with a backing gas. This is normally the same as the shielding gas. However, Formier gas (90% N2 + 10% H2) is a good alternative that also provides first-class root protection while also being cheaper than pure argon. Because only a negligible quantity of the hydrogen penetrates the weld metal, no negative effect has been demonstrated. A backing gas should also be used for tack welding all the way up until weld thickness is at least 8 mm.
FCAW is most suitably performed using argon with an addition of 16–25% carbon dioxide as the shielding gas. Welding with pure carbon dioxide is also possible, but arc stability and weld pool control are noticeably poorer. However, compared with a mixed gas, one advantage is that penetration is slightly better. Also compared with a mixed gas, the voltage should be increased by 2–3 volts when welding with pure carbon dioxide. This prevents the arc being too short.
Shielding gases for MIG, TIG, FCAW, Plasma and Laser welding
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of P.Rao
Sent: Wednesday, March 23, 2011 7:21 PM
To: materials-welding@googlegroups.com
Subject: [MW:10465] Welding duplex material UNS 31803
Dear All
For welding subject material, what will be shielding gases for GTAW & FCAW process.
regards
P.Rao
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