Abstract
The influences of microstructure and elemental distribution on pitting corrosion resistance and passive film in different regions of super duplex stainless steel welded joint were investigated. The banded microstructure disappeared after welding and transformed to the coarse equiaxed ferrite grains in the heat-affected zone. Three types of austenitic microstructure formed in the weld centre, only two types occurred in the weld cap and weld root. It is found that the elements Ni and N were enriched in austenite, while Cr and Mo were concentrated in the ferrite. The Cr2N was found precipitating in some specific regions with a lower austenite content, which led to the relatively poorer resistance to pitting corrosion in the heat-affected zone, weld root, and weld cap. The ferrite observed in each studied region of the joint was prone to experience selective corrosion due to its lower pitting resistance equivalent number. The Cr-depletion region around the Cr2N was disappeared as the original location of pitting. The electrochemical impedance spectroscopy results also showed that there is no difference exists in all of the regions of the joint in the passive film structure and regular microstructure was helpful to the passive film.
Highlights
The wide application of steel cannot be separated from the welding process
Morphologies could be explained through the fact that the weld cap and root could not experience preheating or reheating respectively, which led to less nucleation and growing time for austenite
The difference of intragranular austenite (IGA) in the weld cap and root was mainly caused by the heat input
Summary
The wide application of steel cannot be separated from the welding process. Only when the quality of the welding joint has assurance, so do duplex stainless steels (Lu, 2010; Leif, 2012). Duplex stainless steel (DSS) is a family of two-phase alloys, consisting of austenite (A) and ferrite (F) with various compositions. The DSS combines the excellent properties of ferrite stainless steel and austenitic stainless steel. It exhibits superior strength and stress-corrosion cracking resistance of the former, as well as good ductility and toughness of the latter (Ha et al, 2014). Due to its high alloy contents and excellent corrosion resistance, it is widely used in petrochemical and offshore platforms as pressure vessels and pipelines which have a high requirement for Microstructure, Corrosion UNS S32750 Weldment corrosive properties (Barteri et al, 1987; El-Yazgi and Hardie, 1998; Taban, 2008; Taban and Kaluc, 2011; Chail and Kangas, 2016)
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