TWI Industrial Member Report Summary 506/1995
P C J Anderson and F J Blunt
In addition to a high strength/weight ratio, duplex stainless steels have excellent resistance to stress corrosion cracking and to general and pitting corrosion. The root pass of a welded joint in duplex stainless steel is normally made using the tungsten inert gas (TIG) welding process, with argon shielding gas. However, the corrosion resistance of the weld can be adversely affected by the loss of nitrogen from the weld pool. Furthermore, the productivity is limited by the low joint completion rate inherent to the TIG welding process.
A small addition of nitrogen to the shielding gas helps to maintain uniform corrosion resistance, mechanical properties and the austenite-ferrite phase balance. Further, the joint completion rate could be increased by the replacement of argon in the shielding gas with helium, which produces a hotter arc. However, little information is available on the effect of helium and nitrogen additions to the shielding gas on either the welding characteristics of duplex stainless steel, or the gain or loss of nitrogen in the weld metal.
The aim of this project is, for the TIG welding of duplex stainless steel plate, meeting UNS S31803, to investigate the effect of the proportion of helium in Ar-He-N2 shielding gas on the following:
- Joint completion rate.- Phase balance of the weld metal.- Corrosion resistance of the weldment.- Nitrogen content of the weld metal.