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The effect of thermal aging on microstructural evolution and mechanical behavior was investigated via advanced instrumental analysis. The scanning electron microscope (SEM) results demonstrated that the microstructure transformed from martensite into martensite/bainite mixture in the coarse-grained heat-affected zone (CGHAZ) of 16MND5. The micro-hardness in the CGHAZ has a significantly positive correlation with thermal aging time owing to the change of microstructure and the increased number of M23C6 precipitates. Moreover, the tensile strength and elongation of the dissimilar metal welds have a positive correlation with thermal aging time. However, the fracture occurs on the Alloy 152 side and the fracture mode was the ductile fracture.