Microstructure of the NPP Kozioduy Unit I Weld 4 Metal

The microstructure of the base metal (BM), heat affected zone (HAZ) and the weld metal (WM) of NPP Kozioduy Unit 1 Weld 4 after re-irradialion 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 centra) zone containing fine equiaxial fcrritc grains divided by ferrite bands. The structure changed to fully equiaxial at the periphery towards HAZ. Fine elongated particles (up to 50 iim) were distributed irregularly along lhe 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 precipilations. but the character of the smallest ones could not be determined unambiguously. The impact fracture in vacuum of a I mm" specimen at - 120°C developed mainly by transcrystal cleavage and quasi-cleavage. © 1999 Elsevicr Science S.A. All rights reserved.