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Experimental Characterization of Grain Boundary Fracture Properties in a FeCr12Ni26Si3 Austenitic Stainless Steel Oxidized in PWR Environment

Austenitic stainless steels are used for the core internal structures (bolts, baffles, formers) in Pressurized Water Reactors (PWR). During operational service, baffle to former bolts have been observed to undergo Irradiation-Assisted Stress Corrosion Cracking (IASCC), which is characterized by intergranular cracking. IASCC results from the material corrosion susceptibility, the microstructural changes induced by irradiation, the corrosive media and the mechanical loading. Numerous studies have been conducted to evaluate the complex interplay between the different factors, mostly focusing on InterGranular Stress Corrosion Cracking (IGSCC) of pre-irradiated samples in PWR environment. In particular, the oxidation behavior of grain boundaries and the mechanical loading of grain boundaries have been assessed in details. Depending on the oxidation time and the GB nature, oxide penetration along GB has been observed. The intergranular oxide is composed of (Nix,Fe1-x)Cr2O4 spinels. However, all grain boundaries (GBs) do not have the same oxidation behavior, and it has been reported that high angle grain boundaries show higher oxidation susceptibility than special grain boundaries. Radiation induced segregation at grain boundaries might also lead to higher susceptibility to intergranular oxidation. Irradiation also modifies the deformation mechanisms in austenitic steels resulting in strain localization which is believed to be an important factor in IASCC initiation as it can lead to local increase of the stress due to dislocation pile-ups at GB.

Product Number: ED22-17126-SG
Author: Rachma Azihari, Jérémy Hure, Marc Legros, Benoît Tanguy
Publication Date: 2022
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Irradiation-Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon observed in austenitic stainless steels Pressurized Water Reactor (PWR) internal structures, leading to the initiation and propagation of InterGranular (IG) cracks. Grain boundary (GB) fracture properties are expected to play a key role for the predictive modelling of initiation. However, experimental data for austenitic stainless steels remain scarce. Therefore, an experimental protocol is built to assess the oxidized grain boundary fracture properties of a FeCr12Ni26Si3 austenitic stainless steel. Prior to tests, the material has been exposed to PWR nominal environment for 3680h and 7470h, leading to the formation of a duplex oxide layer and micron-scale oxide penetration along some GBs. Oxide penetration depth at the grain boundaries in the 7470h oxidized sample appeared to be higher than in the 3680h sample. After Electron BackScatter Diffraction (EBSD) mapping to identify grain boundaries, micro-cantilevers containing a single grain boundary have been milled using Focused Ion Beam (FIB) and tested through in-situ micro-bending tests at room temperature inside a Scanning Electron Microscope (SEM). Intergranular cracking has been observed on 12 micro-cantilevers, either inside the GB oxide or at the interfaces between the oxide and the metal. Fracture energy has been evaluated based on energy balance, leading to values comprised between 2J/m2 to 22J/m2. In addition, beam theory has been applied in order to determine the stresses at the crack initiation point, ranging from 338MPa to 857MPa. The scatter associated with both fracture energy and critical stress is discussed.

Irradiation-Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon observed in austenitic stainless steels Pressurized Water Reactor (PWR) internal structures, leading to the initiation and propagation of InterGranular (IG) cracks. Grain boundary (GB) fracture properties are expected to play a key role for the predictive modelling of initiation. However, experimental data for austenitic stainless steels remain scarce. Therefore, an experimental protocol is built to assess the oxidized grain boundary fracture properties of a FeCr12Ni26Si3 austenitic stainless steel. Prior to tests, the material has been exposed to PWR nominal environment for 3680h and 7470h, leading to the formation of a duplex oxide layer and micron-scale oxide penetration along some GBs. Oxide penetration depth at the grain boundaries in the 7470h oxidized sample appeared to be higher than in the 3680h sample. After Electron BackScatter Diffraction (EBSD) mapping to identify grain boundaries, micro-cantilevers containing a single grain boundary have been milled using Focused Ion Beam (FIB) and tested through in-situ micro-bending tests at room temperature inside a Scanning Electron Microscope (SEM). Intergranular cracking has been observed on 12 micro-cantilevers, either inside the GB oxide or at the interfaces between the oxide and the metal. Fracture energy has been evaluated based on energy balance, leading to values comprised between 2J/m2 to 22J/m2. In addition, beam theory has been applied in order to determine the stresses at the crack initiation point, ranging from 338MPa to 857MPa. The scatter associated with both fracture energy and critical stress is discussed.