Effects of Nb on the microstructure and corrosive property in the Alloy 690-SUS 304L weldment

A study of the microstructure and corrosion of dissimilar weldments of Alloy 690 and SUS 304L with various additions of niobium (Nb) (0.1, 1.03, 2.49, and 3.35% by weight) in the flux of coated electrodes is presented. With identical welding parameters and procedures, the weldments were butt-welded in three layers by shielding metal arc welding (SMAW), with each layer being deposited in a single pass. The results show that the dendritic microstructure in the fusion zone changed from a cellular to columnar dendrite and equiaxed dendrite with increasing Nb addition. Furthermore, the interdendritic phase increased in volume and changed in composition from an Al-Ti oxide to a mixture of Nb-Si phase and Nb-rich phase. Nevertheless, it was clearly observed that Cr7C3 precipitated at the grain boundaries of the low Nb weldment root pass. Finally, the results revealed that corrosion occurs primarily at the grain boundaries and within the interdendritic regions of the weldements irrespective of the level of Nb addition. For the region of abundant formation of Nb-rich phase, the high-Nb weldment had a relatively low corrosion resistance.