Effects of Nb on the microstructure and elevated-temperature mechanical properties of alloy 690-SUS 304L dissimilar welds

The purpose of this paper was to investigate the effects of Nb on the microstructure and elevated-temperature mechanical properties of Alloy 690-SUS 304L dissimilar welds. The results show that the welds were dominant with dendrites and interdendritic phases. In the low Nb welds, the main interdendritic phases were Nb-rich and Al-Ti-rich phases. Cr-carbides were precipitated along the grain boundaries of the root region. In the high Nb welds, there were large Nb-rich phases in the cap region, while smaller Nb-carbonitride precipitates were found in the root region. The addition of Nb increased the hardness of the fusion zones. At 300°C, though the tensile strengths of all welds were higher than that of 304L, micro-cracks were unexpectedly found in the low Nb weld. Notched specimens were tested to characterize the mechanical properties of the welds. Both of the yielding and tensile strengths of the welds increased with Nb additions. On the fracture surface, the brittle fracture features also became noticeable.