Effect of the microstructure in unmixed zone on corrosion behavior of 439 tube/308L tube-sheet welding joint

The unmixed zone (UMZ) was found in 439 tube/308L tube-sheet weld, which was jointed with 316L filler metal. The effect of microstructure of UMZ on hardness and corrosion behavior was investigated. The mechanism of macrosegregation combined with fluid flow was applied to account for the formation of UMZ, based on that the liquidus temperature of the bulk weld metal was relatively lower than that of the base metal. The microstructure of UMZ predominantly consisted of ferrite, Widmanstätten austenite (WA) and a few martensite. Martensite fraction in UMZ varied from weldment side to 439 side. It is found that the sample with UMZ was more vulnerable to corrosion and pitting. Difference in liquidus temperature and cooling rate contributed to the different microstructure in UMZ, and resulting in poor corrosion resistance of this dissimilar metal welded joint.