Thermal Aging Effects on Microstructures of Type-II Boundary in Dissimilar Metal Weld Joints

Abstract

In order to investigate the long-term thermal aging effects on the type-II boundary region in Alloy 152 weld metal, a representative dissimilar weld mock-up made of Alloy 690–Alloy 152–A533 Gr. B has been fabricated and heat treated under accelerated temperature conditions. To simulate the thermal aging effects, the heat treatment was performed at 450°C for 15, 30 and 60-yr equivalent times (1,375, 2,750 and 5,500 h). The aging time was determined by the diffusion equation based on the activation energy for chromium diffusion. The microstructure characterization was primarily conducted in the type-II boundary region of the weld root, which is a boundary parallel to fusion boundary existing within 100um from the fusion boundary and is known to be less resistant to stress corrosion cracking than other regions in the weld. The investigations were performed by scanning electron microscope, electron backscatter diffraction, and nanoindentation test. In this study, the dilution zone of the chromium content was observed at the weld metal region within a 1.5-mm range from the fusion boundary. Ferrites and high angle grain boundaries are found at the type-II boundary region of weld metal. In the narrow zone between the type-II boundary and fusion boundary, the hardness is relatively higher than that of other regions. These results show that the chromium content in the dilution zone increases with heat treatment, but the stiff chemical gradient still exists in the weld region at the narrow zone between the type-II boundary and fusion boundary.