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The fracture toughness in the ductile-to-brittle transition region is determined for the heat-affected zone (HAZ) adjacent to the fusion boundary between a low alloy steel (LAS) and the weld metal of narrow-gap Alloy 52 dissimilar metal weld (DMW) after 15 000 h of thermal aging at 400 °C and of an Alloy 52 DMW with buttering in reference condition. The fracture toughness testing is done according to ASTM E1921, and fractography and cross-section metallography are applied to characterize the crack paths, crack locations and fracture type. The T0 transition temperature for the DMW with buttering is −117 °C, indicating marginally higher toughness compared to the narrow-gap DMW. The cracks close to the fusion boundary (approximately 200 μm) in both DMWs deviate from the HAZ towards the fusion boundary. The thermal aging treatment of the narrow-gap Alloy 52 DMW does not significantly affect the fracture toughness properties of the fusion boundary. Further research is needed to better understand the lower boundary fracture toughness behavior at approximately 300 μm from the fusion boundary. The results contribute to long-term operation assessment of nuclear power plants, and development of analysis and characterization methods for DMWs related to the effect of crack path and location.

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