Authors: Hytönen, Noora
Source: Aaltodoc (Aalto University) (article, 2025-01-01)
Field: Materials Science — General Materials Science

Haurasmurtuma hitsausliitosten mikrorakenteissa This dissertation investigates the phenomenon of brittle fracture in weld microstructures, with focus on nuclear safety and material ageing during extended long-term operation (ELTO) of nuclear power plants (NPPs). The research emphasizes the role of understanding the microstructural features of welds to ensure the integrity and safety of nuclear reactor components as they age. The study highlights the significant effects of thermal ageing and neutron irradiation on the mechanical properties of low-alloy steel (LAS), in terms of maintaining safe operational margins in NPPs. The primary focus is on two critical weld components in a nuclear reactor: the welds in the reactor pressure vessel (RPV), as well as dissimilar metal welds (DMWs) in the pressurizer surge nozzle safe-end. The study on RPV welds is conducted on harvested material from a decommissioned boiling water reactor (BWR) NPP combined with representative high-dose surveillance material. The studied DMW is a representative mock-up of a repair weld used in a pressurized water reactor (PWR) with double-sided Alloy 52 buttering weld joining LAS pressure vessel and stainless steel piping. The brittle fracture initiation and crack propagation are investigated from a microstructural perspective. The methodology includes a comprehensive multiscale analysis of microstructural features through various characterization techniques, such as scanning electron microscopy and transmission electron microscopy, along with hardness and mechanical testing to evaluate the strength and toughness properties of the welds. Key findings indicate that the microstructural characteristics of welds significantly influence their mechanical properties and susceptibility to brittle fracture. Two main types of second-phase particles are identified to initiate a cleavage fracture in a LAS weld: non-metallic oxide inclusions and carbides. The type of fracture initiator varies between the welds and is dependent on the specimen type, yet no conclusion can be made if the thermal ageing and/or irradiation affect the cleavage fracture initiation, as the mechanical testing shows near negligible operation-induced embrittlement in the RPV welds and auspicious material properties for LTO. In DMWs, three main types of fusion boundary microstructures were found along the LAS-Alloy 52 interface: sharp fusion boundary, martensitic phase, and partially melted zone. The microstructures correlate with varying levels of dilution and local strength mismatch, significantly affecting the crack propagation path. The strength mismatch and carbide evolution are influenced by welding parameters, post-weld heat treatment, and LTO temperature. The dissertation highlights that understanding of the metallurgical features and microstructural evolution of welds can lead to improved assessment of material performance and safety during ELTO. Current ageing management predictions are conservative, but to safely extend the operating lifetime, small details in embrittlement trend curves can increase the valuable time of safe operation. When performing testing of the weld metal, especially with a notched or pre-cracked sample, the location of the crack tip and the process zone must be known in terms of the local weld microstructure as it determines the local fracturing resistance. Reactor pressure vessel ; Pressure vessel ; Brittleness ; Welding ; Fracture toughness ; Nuclear reactor ; Boiling water reactor ; Pressurized water reactor ; Brittle fracture Engineering and Material Science Research ; Hydrogen embrittlement and corrosion behaviors in metals ; Fatigue and fracture mechanics