2007

Hänninen, Hannu; Brederholm, Anssi; Saukkonen, Tapio; Gripenberg, Hans; Toivonen, Aki; Ehrnsten, Ulla; Aaltonen, Pertti

The operating experience of major nuclear power plant (NPP) pressure boundary components has recently shown that dissimilar metal weld joints can markedly affect the plant availability and safety because of increased incidences of environment-assisted cracking (EAC, PWSCC) of alloy 600 and corresponding weld metals (alloys 182/82). Alloy 690 and associated weld metals (alloys 152/52) are widely used for repair and replacement of the affected thick-section components. The selection of new materials relies mainly on laboratory results and short-term service experience. The long-term behavior of these materials and their performance in the plant has still to be demonstrated. Weldability and susceptibility to hot cracking of the studied nickel-base materials was evaluated based on the Varestraint test results obtained with weld metals of different chromium contents. The microstructures and microchemistry of the multi-pass nickel-base alloy welds was studied by FEG-SEM/EDS techniques and were found to be very different from those of the wrought and recrystallized nickel-base materials. Additionally, the weld residual stresses were measured and analyzed by a novel Contour method suitable for through-thickness residual stress determination. The studied nickel-base material welds were exposed to doped steam test environment and crack initiation susceptibility of them was studied. The results showed the markedly higher susceptibility to EAC of alloy 182 weld metal as compared to the other studied alloys, i.e., alloy 152, 52 and 82.