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5610 |
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Tuesday, September 26, 2017, Saal M 11:30 AM Öl- und Gasindustrie |
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Hyper duplex stainless steel, structural and corrosion properties in high alloyed duplex stainless steel welds
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Gerrit Sasse / Sandvik Materials Technology, Germany Johan Salwén* / SANDVIK MATERIALS TECHNOLOGY, R&D, Schweden Johan Löthman/ SANDVIK MATERIALS TECHNOLOGY, R&D, Schweden |
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The hyper duplex stainless steel (HDSS) combines very high mechanical strength (Rm = 920-1100 MPa) with very high corrosion resistance (CPT > 50°C) in welded structures. This combination makes HDSS a cost effective solution in many applications and also an attractive alternative to nickel alloys. Heat exchanger is one application where hyper duplex stainless steels have found its use over recent years. The base material of HDSS typically consist of 50% ferrite/austenite ratio but contrary to duplex stainless steel (DSS) and super duplex stainless steel (SDSS) where the welded microstructure displays lower ferrite (30-50%), welded HDSS typically display higher ferrite compared to the base material (50-70%). The investigation was made using TIG/GTAW welding with shielding gas of Ar+2-3%N2. The tubesheet was made by depositing the HDSS consumable using hot-wire TIG welding, and the tube to tubesheet joint was made by using an orbital TIG welder. In DSS and SDSS, higher ferrite content is a sign of poorly balanced metallurgy with increasing risk for structural stability issues that lead to precipitation of secondary phases, such as sigma phase. In HDSS, the same issues have not been observed. Ferrite levels between 65-75% were examined in this investigation, no sign of sigma phase could be observed and G48A testing at 50°C showed no pitting indication. It was further noticed that there is a large discrepancy between different methods to evaluate ferrite. Where a metallographic examination using the ASTM E562 method gave the examined ferrite level of 65-75%, the magnetic method Fisher ferriscope gave values approximately 15 unit-% lower whereas the WRC-92 method gave values approximately 10 unit-% higher. Hence, it could be concluded that the existing models for estimating ferrite is not applicable to HDSS and needs further investigation. |