Microstructure of the NPP Kozloduy Unit 1 Weld 4 Metal Original Research Article

The microstructure of the base metal (BM), heat affected zone (HAZ) and the weld metal (WM) of NPP Kozloduy Unit1 Weld 4 after re-irradiation was investigated by scanning and transmission electron microscopy. Structure of tempered upper bainite (enlarged in the HAZ zone) was found in the BM and HAZ. The WM showed a typical fan-like radial structure with a central zone containing fine equiaxial ferrite grains divided by ferrite bands. The structure changed to fully equiaxial at the periphery towards HAZ. Fine elongated particles (up to 50 nm) were distributed irregularly along the grain boundaries in the WM. Regularly distributed structural inhomogeneities (dots) were observed in the grain volume both in WM (with diameter up to 15 nm) and in BM (with diameter up to 50 nm). An annealing experiment was undertaken in order to prove the character of the dots. The 2 h annealing of the thin foils at 560°C showed that the dots of larger size were precipitations, but the character of the smallest ones could not be determined unambiguously. The impact fracture in vacuum of a 1 mm2 specimen at −120°C developed mainly by transcrystal cleavage and quasi-cleavage.