Effect of Repeated Repair Welding on the Mechanical Properties and Pitting Corrosion Resistance of Super Duplex Stainless Steel

Abstract

The study investigated the effect of repeated repair welding on the mechanical properties and pitting corrosion resistance of 25% Cr super duplex stainless steels (UNS S32750). The test coupons were prepared by gas tungsten arc welding (GTAW) with ER2594 filler metal and repaired 3-times at the same weld locations, where the remaining previous welds were 2-3 mm in depth. The number of repairs provided various reheating to the original welds (as-welded condition) and to the previously repaired welds. The effect of repair welding reheating on the microstructure, ferrite content, hardness, impact toughness and pitting corrosion was investigated. The pitting corrosion occurred in the root region of the first repair weld (R1). Secondary phases such as austenite (γ2) and sigma (σ) were observed in the reheated regions with a consumption of ferrite content, where impact toughness and pitting corrosion resistance were significantly reduced. The vicinity of the pitting corrosion region was analyzed, and showed that sigma (σ) was precipitated in the secondary austenite (γ2) and at the boundary between ferrite and austenite. The fraction of the sigma phase was determined by electron back scatter diffraction (EBSD) to be 0.3% in the vicinity of the pitting corrosion region. This study also discusses the applicability of 3-times repair welding in the offshore industry.