Structural integrity of Ni-base alloy welds

The Tekes project Structural integrity of Ni-base alloy welds (SINI) was carried out at Aalto University and VTT during the period from 2010 to 2014. In nuclear industry applications, the degradation of Ni-base alloy dissimilar metal welds (DMWs), both in pressurised water reactor (PWR) and in boiling water reactor (BWR) plants, is an extensive international problem. The project participates with ongoing international cooperation in USA (Electric Power Research Institute EPRI, Alloy 690/52/152 PWSCC Research Collaboration) and in Japan (Tohoku University) for the conduction of new Ni-base alloy research. The research need is actual, especially because indications have been found in the DMWs in the reactor pressure vessel nozzle of BWRs similar to Finnish BWR designs and European pressurised water reactor (EPR PWR) -plant under construction contains a large number of varying and new types of dissimilar metal welds of which no previous experience is available. The main focus of the project was to investigate the weldability of Alloy 690 base metal corresponding filler metal Alloy 52 and the properties of the welded joint. Comparable reactor pressure vessel (RPV) safe-end weld joint was made by narrow-gap tungsten inert gas (TIG) welding with Alloy 52 filler metal for research purposes. Also prototypical BWR reactor pressure vessel (RPV) safe-end weld joint was made with Alloy 82/182 filler metals. Determination of the mechanical properties and characterization of the microstructures of the welded joints were conducted. Crack initiation tests in autoclave were made in different environments. After PINC (Program for the Inspection of Nickel Alloy Components) round-robin program new PARENT (Program to Assess Reliability of Emerging Non-destructive Techniques for Dissimilar Metal Welds) round-robin program was started where VTT participated. Based on the PINC results, new non-destructive testing methods were developed to enhance the reliability of detection and accuracy of sizing of flaws in nickel-based alloy dissimilar metal welds. Separate reports: 1. Holmström, P. (2012), The effect of mismatch on the mechanical properties of a dissimilar metal weld. M.Sc. Thesis, Aalto University, Espoo, Finland, 162 p. + app. 4 p. 2. Mouginot, R. and Hänninen, H. (2013), Microstructures of nickel-base alloy dissimilar metal welds. Aalto University publication series SCIENCE + TECHNOLOGY, 5/2013, https://aaltodoc.aalto.fi/handle/123456789/9006, Aalto University, Espoo, Finland, 178 p. 3. Keinänen, H. (2013), Computational estimation of the risk of hot cracking in narrow gap welding. Research report VTT-R-08369-13, VTT Technical Research Centre of Finland, Espoo, Finland, 27 p. + app. 10 p.