51316-7856-Achieving Success during Welding of Lean Duplex Stainless Steel Tubes to Carbon Steel Tubesheets

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Product Number: 51316-7856-SG

ISBN: 7856 2016 CP

Author: Ivan Morales

Publication Date: 2016

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A relatively new material class namely lean duplex stainless steel (LDX) (e.g. UNS S32101 or UNS S82011) is commercially available in tube form and can be used in fresh and treated cooling water exchanger with carbon steel tubesheets were corrosion is not very aggressive. Joining of these two dissimilar metals can readily be accomplished with a suitable filler metal achieving reliable joints in service. These materials combination are also a life cycle cost effective solution to carbon steel tube bundle replacements. While qualifying a tube to tubesheet welding procedure with an unexperienced fabricator cross sections of the mockup showed incomplete fusion and penetration as well as cracking. The fabricator attributed the cracking to the materials being unweldable. The results of our investigation showed that Incomplete fusion and penetration resulted from inadequate welder technique. Since the mockup was also intended to qualify the welder the results obtained showed the inability of the welder to perform sound welds. Cracking resulted from a combination of high hardness and contamination within the annular space between the tube OD and tubesheet hole. The presence of porosity in the root of the welds further confirmed contamination. We believe that cutting fluids used for machining the holes were not thoroughly cleaned prior to welding. This caused the trapped fluid (moisture or hydrocarbon) to decompose and generate atomic hydrogen within the arc that subsequently diffused into the weld. A second attempt was carried out correcting the deficiencies identified above and successful welds were obtained.