Microstructure of the WWER-440 dissimilar weld joints

The basic metallographic microstructures occurring in the welds of the WWER-440 reactor pressure vessel nozzle to the primary safe end are described and documented, and the mechanisms of in-service thermal degradation are briefly discussed. Particular attention is paid to the ferrite-austenite transition region, containing microstructural elements which can cause embrittlement of the material during a NPP operation. The potentially critical places with respect to thermal degradation during reactor operation are: the steep carbon gradient at the fusion boundary of dissimilar welds; the high-density particle cores of segregated zones in the 1st buttering layer; the δ-ferrite containing carbide precipitates as well as the σ-phase particles in the 2nd buttering layer and the spinodal decomposition of δ-ferrite and precipitation of G-phase in the filler metal. (Z.S.) 3 tabs., 18 figs., 11 refs