Effects of niobium on microstructure, mechanical properties, and corrosion behaviour in weldments of alloy 690

The present work investigates the effects of adding different quantities of Nb (0·1, 1·03, 2·49, and 3·35 wt-%) to the flux of electrodes used in welding Inconel alloy 690 on the microstructure, mechanical properties, and corrosion behaviour of the resulting weldments. Inconel filler metal I–52 coated with flux was used as the welding electrode. Weldments were butt welded using a manual shielded metal arc welding process. The experimental results indicated that the subgrain structure of the fusion zone was primarily dendritic. Niobium was depleted at the dendritic cores and enriched in the interdendritic regions. A small heat affected zone with typical coarse grains, which subsequently formed ghost grain boundaries, was present. With increasing Nb, the welds tended to show a finer subgrain structure and smaller dendritic spacing. Niobium rich segregants in the form of small particles formed in interdendritic spaces, providing the sites for microvoid formation by rupture. Correspondingly, the tensile strength and microhardness of the fusion zone appeared to increase slightly whereas the ductility decreased significantly. However, corrosion tended to initiate on the interdendritic sites, and was worse with increasing Nb. With increasing Nb, the tensile rupture mode changed from slant (ductile) to flat (brittle) fracture.