Thermal aging of nickel-based materials, such as Alloy 690 and associated Alloy 52/152 weld metals, can lead to a decrease in their resistance to stress corrosion cracking. This effect has been attributed to aging/embrittlement effects such as the formation of an ordered Ni2Cr intermetallic phase during short- and long-range ordering reactions and/or to the precipitation of carbides during aging. Based on performed investigations ordering can be an important aging mechanism in modern NPPs. Thermally aged Alloy 690 with different aging times and treatments (heat treatment and cold working) has been investigated using versatile techniques within a thesis work . The role hydrogen plays in the material behavior is also important to assess, as the materials are used, e.g. in the hydrogen containing primary circuit of the OL3 EPR plant.
VVER-designs include high-Ni and Ni-base alloys, such as XH35BT and XH77ЮP, which are used in some high-strength applications. The thermal aging of these materials has not been investigated in Finland, and therefore there is a gap in critical national knowledge. Events where these materials have suffered from cracking have occurred over the years in VVERs, showing the importance of knowing their behavior. The work on high-Ni VVER materials was started in THELMA in 2018 in the form of a literature review .The review disclosed a lack of open literature on these materials, and the conclusions from the review were as follows: “The available literature completely lacks information about the metallurgical state of high-Ni VVER materials. That would be essential information when analyzing the long-term behavior of the components, i.e. possible changes in intermetallic phases, possible irradiation effects etc. Thus, further basic metallurgical characterization of XH35BT and XH77ЮP alloys would be of interest, in order to clarify the metallurgical state and microstructure typical of plant components of both materials.”
To tackle parts of the objectives above, the SAFIR Management Board requested that VTT prepare a “small project” on these topics - Thermal Aging of Alloy 690 and high-Ni alloys (TAAN). The two main objectives of the TAAN project are to:
• increase the national knowledge on possible issues concerning the use of high-Ni materials used in VVER design and
• make advances in the identification of microstructural explanation of the hardness increase due to thermal aging in a studied Alloy 690 heat with respect to three other heats, and by that, improving the understanding the role of short range ordering and thermal aging as a potential degradation mechanism for NPP components.