Influence of microstructure and elemental partitioning on pitting corrosion resistance of duplex stainless steel welding joints

By Zhang, Zhiqiang; Jing, Hongyang; Xu, Lianyong; Han, Yongdian; Zhao, Lei; Zhang, Jianli
Published in Applied Surface Science NULL 2017

Abstract

Abstract The influences of microstructure and elemental partitioning on pitting corrosion resistance of duplex stainless steel joints welded by gas tungsten arc welding (GTAW) and flux-cored arc welding (FCAW) with different shielding gas compositions were studied by optical microscopy, electron backscatter diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, electron probe microanalysis, and potentiostatic and potentiodynamic polarization methods The adding 2% {N2} in shielding gas facilitated primary austenite formation in {GTAW} weld metal (WM) and suppressed Cr2N precipitation in {GTAW} weld root. In the HAZ, the banded microstructure disappeared while the coarse ferrite grains maintained same orientation as the banded ferrite in the BM. In the WM, the ferrite had one single orientation throughout a grain, whereas several families of austenite appeared. The austenite both in {BM} and {WM} enriched in Ni and nitro`gen, while Cr and Mo were concentrated in the ferrite and thus no element showed clear dendritic distribution in the {WM} (ER2209 and E2209T1). In addition, the secondary austenite had higher Ni content but lower Cr and Mo content than the primary austenite. The N2-supplemented shielding gas promoted nitrogen solid-solution in the primary and secondary austenite. Furthermore, the secondary austenite had relatively lower pitting resistance equivalent number (PREN) than the ferrite and primary austenite, thereby resulting in its preferential corrosion. The Cr2N precipitation led to relatively poor resistance to pitting corrosion in three {HAZs} and pure Ar shielding {GTAW} weld root. The N2-supplemented shielding gas improved pitting corrosion resistance of {GTAW} joint by increasing {PREN} of secondary austenite and suppressing Cr2N precipitation. In addition, the {FCAW} {WM} had much poorer resistance to pitting corrosion than the {GTAW} {WM} due to many O-Ti-Si-Mn inclusions. In the BM, since the austenite with lower {PREN} compared to the ferrite, the pitting corrosion occurred at the ferrite and austenite interface or within the austenite.

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