Welding bimetal pipes in duplex stainless steel

Abstract

Butting bimetal pipes in duplex stainless steel are quite recent and present a set of interesting characteristics especially for oil and gas transportation, namely weight to corrosion resistance ratio. Gas tungsten arc (GTA) welding is used to join these pipes, but several problems are identified as lack of penetration and cracking resulting not only from the material itself, but also from the difficulty to weld in orbital position. In the present work, autogenous GTA pipe welding and pulse Rapid Arc gas metal arc welding of the pipes were studied. Current intensity, welding speed, electrode diameter, shielding gas and orbital positions were defined as variables. It was shown that under appropriate conditions, it is possible to obtain sound welds with proper geometry and defect free. The major limitation to penetration is the outwards flow pattern in the molten pool driven by the Marangoni effect, as a result of low sulphur content. Sulphur is an active surface element which reverses the surface tension coefficient to a positive effect. For penetrations of about 2 mm, a combination of current intensity of 170 A and a welding speed of 200 mm/min with an electrode angle of 30° under a shielding gas protection of He–25%Ar was defined for narrow groove welding of a J-bevelled pipe in flat position. No defects were detected in the inner layer in duplex stainless steel.