Investigation of Microstructural Changes due to Thermal Aging in Dissimilar Metal Welds

To investigate the effect of long-term thermal aging on the fusion boundary region between low-alloy steel A533 Gr. B and weld metal Alloy 152, a representative dissimilar weld mockup composed of Alloy 690/Alloy 152/A533 Gr. B was aged in laboratory furnaces under accelerated temperature conditions. The aging time was determined using the diffusion equation. The heat treatment was performed at 450°C for 60-y equivalent time (5,500 h) to simulate thermal aging effects. An additional aging heat treatment was also performed at 400°C for 15- and 30-y equivalent times (6,450 and 12,911 h, respectively) to determine the effects of temperature on aged microstructures. The characterization was mainly conducted in the microstructure of the fusion boundary region in the weld root region using scanning electron microscopy, transmission electron microscopy, and three-dimensional atom probe tomography. It was determined that the region near the fusion boundary was generally divided into several regions, such as a dilution zone (that included a chemical gradient in the weld side), fusion boundary, and heat-affected zone in the low-alloy steel. The results of this study showed that heat treatment increased Cr content in the dilution zone, but the chemical gradient in the weld side near the fusion boundary persisted. For the microstructure, it was observed that treatment induced the formation and growth of Cr precipitates in the fusion boundary region of the dissimilar metal joints due to the thermodynamic driving force. At two heat treatment conditions (400 and 450°C), although the extent of the results described above differed, the trend in the results appeared to be the same. This microstructure information can improve the understanding of cracking-resistant change when structural changes occur. Furthermore, such data will be important for assessing the effects of aging on structural components and for evaluating the long-term operation of nuclear power plants.