Environment-assisted cracking and hot cracking susceptibility of nickel-base alloy weld metals

The Tekes project PERDI (Performance and ageing of dissimilar metal joints) was carried out at TKK and VTT during the period 2006 to 2009. It was primarily driven by the needs of the nuclear power plant (NPP) industry, but the oil refining, conventional energy production and the pulp and paper industries also benefitted from the results and expertise developed in the project. The extensive studies carried out in the project on welding technology, microstructural characterization, non-destructive examination (NDE) and environmentally assisted cracking (EAC) revealed a number of open research issues regarding the dissimilar metal welds (DMW) and pure Ni-based weld metals of modern nuclear power plants. Alloy 690 and its associated weld metals Alloy 152 and Alloy 52 are nowadays widely used for repair and replacement of thick-section components and in the construction of new reactors like the European Pressurized Water Reactor (EPR). Some aspects of the project were focused on the repair welding of BWR dissimilar metal welds, where the repair method and the filler material were chosen taking into account the residual stresses produced in the structure, possible material property changes of the remaining materials, and the resistance of the new weld metal (Alloy 82 or 52) to EAC. In addition to examining the materials properties and weldability of DMWs, the studies also covered NDE inspection and residual stress measurement and modelling in DMWs. The weldability research utilized a modern, versatile Varestraint test system for hot crack susceptibility testing of the Ni-based alloys used for nuclear and oil refinery DMWs. Weld metals and hot cracks were examined using a modern FEG-SEM/EBSD system and by ATEM. Additionally the solidification and precipitation processes were characterized by differential scanning calorimetry (DSC) methods. It was found that hot cracking is generally associated with the segregation of Nb, Si and Mn in the final melt of the solidifying weld. The Varestraint test technique was also used for producing controlled amounts of hot cracking type defects in the test samples for subsequent EAC studies carried out at VTT and at Tohoku University, Japan. A full scale DMW mock-up of a thick-wall weld was prepared for an oil refinery application, and the whole manufacturing procedure was documented on video film. The mock-up was then methodically characterized by both non-destructive and destructive methods. About 2/3 of the pipe weld was then used for a NDE performance demonstration mock-up sample, where all possible defects expected in the oil refinery conditions were present. Extensive NDE studies were made of the mock-up sample, and the sample is now used by Neste Oil for NDE performance demonstrations (separate report [1]). To assess the relative EAC susceptibilities of various types of Alloy 182, 82, 52 and 152 pure weld metals and DMW configurations with and without hot cracks, four-point bend tests were carried out in a high-temperature steam environment that was doped with impurities to enhance crack initiation. The results showed a relatively high susceptibility of Alloys 182 and 82 to EAC, while Alloys 152 and 52 showed no crack initiation. The hot cracks and other weld defects present in the samples did not show any signs of extension in the doped steam test, in any of the studied materials. The cracking mechanism of Alloy 182 and 82 weld metals as well as Alloy 600 was studied by FEG-SEM and discussed based on high-temperature electrochemistry and oxidation. The dynamic strain aging (DSA) behaviour of the weld metals was determined. In an international round robin, crack growth rate data obtained by participants for Alloys 600 and 182 were compared. VTT results were found to be like those of the others [2]. Through the PINC project, an international NDE round robin was held using DMW mock-ups provided by NRC, USA. The test results obtained by VTT were analysed and published in the PINC final report (separate report [3]). The roles of the stress state and the accessibility of the DMWs in the inspection results were especially evaluated, and guidance was given on the detectability of various defects by the ultrasonic techniques used for DMWs, as well as on the sizing techniques. The techniques used in the international round robin were also employed for the NDE inspections of the Neste Oil mock-up sample. DMWs are at present a major research topic in the nuclear industry internationally, because a number of crack indications have been detected in DMWs in both BWR and PWR plants. During the PERDI project cooperation agreements were made and joint research was performed or started with the following international research activities: International Cooperative Research Project on Non-destructive Examination for Primary Water Stress Corrosion Cracking in Nickel-base Materials and Dissimilar Metal Welds, PINC, The United States Nuclear Regulatory Commission, 2005-2009, Tohoku University, Japan, MIT/CALTECH, USA, and TKK/VTT, Dissimilar Metal Weld Project, 2005-2010 (supported by MEXT, Ministry of Education, Culture, Sports, Science and Technology, Japan), EPRI Alloy 690/152/52 Expert Panel Collaboration, 2008-2015, EPRI/NRC Weld Residual Stress Program: Phase 1 and 2 (2009-2011). One purpose of the PERDI project was to transfer the high level knowledge in the field of DMWs and their long-term behavior in operation from the nuclear industry, to conventional industries. In return, the powder metallurgy knowledge from multi-material components was transferred to the nuclear industry. The common goal for all the partners was to develop reliable testing methods and to obtain high-level research results on the new materials and their dissimilar metal joint (thermo)-mechanical testing, their structural evaluation and design, control of residual stresses and performance of NDE. An extensive study was made on the possible use in nuclear applications of materials/components made by the Hot Isostatic Pressing (HIP) method. The purpose would be to replace the present forged products with HIP-products, but the HIP technique is not at present approved by the codes, such as ASME. The study clarified the requirements for ASME approval and the procedure for obtaining a code case for HIP products. The same aim is under process also elsewhere, and at present it is expected that in a couple of year's time there will be enough data for obtaining the ASME code case for HIP products (separate report [4]).