Microstructure and corrosion resistance of UNS S32750 super duplex stainless steel laser welded joint

Holding UNS S32750 SDSS as the object of study, this paper studied the influence of laser power on the microstructure and pitting behaviour of laser welded joints using optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalyzer and critical pitting temperature. The results show that increasing the laser power can significantly increase the austenite content while inhibiting the precipitation of Cr₂N in the laser welded seam. In addition, the laser welded seam has a lower pitting resistance than that of the base metal. However, with the rising of the laser power , the corrosion-resistant austenite content of the weld grows and the tendency for Cr₂N precipitation is weakened, resulting in a gradual increase in the pitting resistance. Compared with the base metal, the difference in the distribution of alloying elements such as Cr, Ni and Mo between ferrite and austenite in laser welded seam is significantly reduced, while the difference in the distribution of N atoms grows, so that the ferrite in the weld has a lower pitting resistance index than austenite. Therefore, it is thus preferentially selectively corroded. For base metal, pitting occurs mainly at the δ/γ phase boundary and inclusions, while pitting in laser welded seam is mainly caused by the large amount of Cr₂N precipitated in the ferrite where pitting happend and the rapid development of the weak phase ferrite.