• Brederholm, Anssi
• Aaltonen, Pertti
• Hänninen, Hannu
• Ivanchenko, Mykola
• Karlsen, Wade
• Toivonen, Aki
• Saukkonen, Tapio
• Ehrnstén, Ulla

Abstract

The Tekes project PERDI (Performance and ageing ofdissimilar metal joints) was carried out at TKK and VTTduring the period 2006 to 2009. It was primarily drivenby the needs of the nuclear power plant (NPP) industry,but the oil refining, conventional energy production andthe pulp and paper industries also benefitted from theresults and expertise developed in the project. Theextensive studies carried out in the project on weldingtechnology, microstructural characterization,non-destructive examination (NDE) and environmentallyassisted cracking (EAC) revealed a number of openresearch issues regarding the dissimilar metal welds(DMW) and pure Ni-based weld metals of modern nuclearpower plants. Alloy 690 and its associated weld metalsAlloy 152 and Alloy 52 are nowadays widely used forrepair and replacement of thick-section components and inthe construction of new reactors like the EuropeanPressurized Water Reactor (EPR). Some aspects of theproject were focused on the repair welding of BWRdissimilar metal welds, where the repair method and thefiller material were chosen taking into account theresidual stresses produced in the structure, possiblematerial property changes of the remaining materials, andthe resistance of the new weld metal (Alloy 82 or 52) toEAC. In addition to examining the materials propertiesand weldability of DMWs, the studies also covered NDEinspection and residual stress measurement and modellingin DMWs.The weldability research utilized a modern, versatileVarestraint test system for hot crack susceptibilitytesting of the Ni-based alloys used for nuclear and oilrefinery DMWs. Weld metals and hot cracks were examinedusing a modern FEG-SEM/EBSD system and by ATEM.Additionally the solidification and precipitationprocesses were characterized by differential scanningcalorimetry (DSC) methods. It was found that hot crackingis generally associated with the segregation of Nb, Siand Mn in the final melt of the solidifying weld. TheVarestraint test technique was also used for producingcontrolled amounts of hot cracking type defects in thetest samples for subsequent EAC studies carried out atVTT and at Tohoku University, Japan. A full scale DMWmock-up of a thick-wall weld was prepared for an oilrefinery application, and the whole manufacturingprocedure was documented on video film. The mock-up wasthen methodically characterized by both non-destructiveand destructive methods. About 2/3 of the pipe weld wasthen used for a NDE performance demonstration mock-upsample, where all possible defects expected in the oilrefinery conditions were present. Extensive NDE studieswere made of the mock-up sample, and the sample is nowused by Neste Oil for NDE performance demonstrations(separate report [1]).To assess the relative EAC susceptibilities of varioustypes of Alloy 182, 82, 52 and 152 pure weld metals andDMW configurations with and without hot cracks,four-point bend tests were carried out in ahigh-temperature steam environment that was doped withimpurities to enhance crack initiation. The resultsshowed a relatively high susceptibility of Alloys 182 and82 to EAC, while Alloys 152 and 52 showed no crackinitiation. The hot cracks and other weld defects presentin the samples did not show any signs of extension in thedoped steam test, in any of the studied materials. Thecracking mechanism of Alloy 182 and 82 weld metals aswell as Alloy 600 was studied by FEG-SEM and discussedbased on high-temperature electrochemistry and oxidation.The dynamic strain aging (DSA) behaviour of the weldmetals was determined. In an international round robin,crack growth rate data obtained by participants forAlloys 600 and 182 were compared. VTT results were foundto be like those of the others [2].Through the PINC project, an international NDE roundrobin was held using DMW mock-ups provided by NRC, USA.The test results obtained by VTT were analysed andpublished in the PINC final report (separate report [3]).The roles of the stress state and the accessibility ofthe DMWs in the inspection results were especiallyevaluated, and guidance was given on the detectability ofvarious defects by the ultrasonic techniques used forDMWs, as well as on the sizing techniques. The techniquesused in the international round robin were also employedfor the NDE inspections of the Neste Oil mock-up sample.DMWs are at present a major research topic in the nuclearindustry internationally, because a number of crackindications have been detected in DMWs in both BWR andPWR plants. During the PERDI project cooperationagreements were made and joint research was performed orstarted with the following international researchactivities:International Cooperative Research Project onNon-destructive Examinatio...

Topics

• impedance spectroscopy
• nickel
• scanning electron microscopy
• melt
• crack
• precipitation
• ultrasonic
• differential scanning calorimetry
• aging
• electron spin resonance spectroscopy
• electron backscatter diffraction
• susceptibility
• solidification
• aging
• ultraviolet photoelectron spectroscopy