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Microstructural analysis of Sanicro 25 (42Fe22Cr25NiWCuNbN) after oxidation in steam for 25,000 h at 700 °C.

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Title: Microstructural analysis of Sanicro 25 (42Fe22Cr25NiWCuNbN) after oxidation in steam for 25,000 h at 700 °C.
Authors: Cempura, Grzegorz1 (AUTHOR) cempura@agh.edu.pl, Kruk, Adam1 (AUTHOR)
Source: Materials Characterization. Mar2025, Vol. 221, pN.PAG-N.PAG. 1p.
Subjects: Scanning transmission electron microscopy, Laves phases (Metallurgy), Electron microscope techniques, Three-dimensional imaging, Substrates (Materials science)
Abstract: The paper presents the results of structural studies on Sanicro 25 steel oxidation in a steam atmosphere at 700 °C over 25,000 h. Microstructural changes were analyzed in the oxide/steel interface region and at a greater distance, where microstructural changes were caused mainly by the high-temperature annealing. Microstructural analysis was performed using scanning and transmission electron microscopy techniques and FIB-SEM tomography techniques. The phase analysis of the substrate material was supported by thermodynamic simulations using the Thermo-Calc software package. The application of 3D volume imaging allowed visualization of the morphology of the oxide layer, pores, voids, and the substrate material (Cr 23 C 6 carbides, Laves phase, Z -phase, σ-phase, and Cu-rich phase). This enabled 3D spatial imaging of the distribution of voids and quantitative assessment of porosity at the oxide/metal boundary. Numerous large voids were observed in the substrate material below the metal/oxide boundary, with dimensions comparable to the grain size in the near-boundary area. Large voids with oxidized surfaces and others with clean inner surfaces were observed, which suggests the significant impact of steam on the formation of voids at the metal/oxide boundary. The structural studies did not reveal the presence of an internal oxidation zone. HRSTEM studies confirmed the presence of Cu-rich precipitates, coherent with the austenitic matrix and nucleating at dislocations and other structural elements. FIB-SEM tomography reconstruction and 3D visualization of microstructural elements with an 8 × 8 × 8.2 nm voxel size indirectly imaged dislocation networks by visualizing secondary precipitates, such as Cr 23 C 6 carbides and Laves phase precipitated at dislocations. In summary, the paper provides valuable insights into the mechanisms of high-temperature oxidation of Sanicro 25 steel in steam conditions and the effects of these processes on the material's microstructure. • FIB-SEM tomographic reconstruction of the microstructure elements in 3D using an 8 × 8 × 8.2 nm voxel allowed us to indirectly visualize dislocation systems by imaging secondary precipitates: carbides Cr 23 C 6 , Laves phase precipitates on dislocations. • The presence of steam during the high-temperature oxidation process of Sanicro 25 steel significantly affects the scale formation and void formation process in the scale/metal interface area. The formation of volatile oxide compounds leading to the formation of large voids in the material under the scale layer is possible. • Voids were found in the substrate material below the metal/oxides boundary, with dimensions of the material grain size in the border area, an oxidized inner surface, and a clean inner surface without a clear oxide layer. [ABSTRACT FROM AUTHOR]
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Abstract:The paper presents the results of structural studies on Sanicro 25 steel oxidation in a steam atmosphere at 700 °C over 25,000 h. Microstructural changes were analyzed in the oxide/steel interface region and at a greater distance, where microstructural changes were caused mainly by the high-temperature annealing. Microstructural analysis was performed using scanning and transmission electron microscopy techniques and FIB-SEM tomography techniques. The phase analysis of the substrate material was supported by thermodynamic simulations using the Thermo-Calc software package. The application of 3D volume imaging allowed visualization of the morphology of the oxide layer, pores, voids, and the substrate material (Cr 23 C 6 carbides, Laves phase, Z -phase, σ-phase, and Cu-rich phase). This enabled 3D spatial imaging of the distribution of voids and quantitative assessment of porosity at the oxide/metal boundary. Numerous large voids were observed in the substrate material below the metal/oxide boundary, with dimensions comparable to the grain size in the near-boundary area. Large voids with oxidized surfaces and others with clean inner surfaces were observed, which suggests the significant impact of steam on the formation of voids at the metal/oxide boundary. The structural studies did not reveal the presence of an internal oxidation zone. HRSTEM studies confirmed the presence of Cu-rich precipitates, coherent with the austenitic matrix and nucleating at dislocations and other structural elements. FIB-SEM tomography reconstruction and 3D visualization of microstructural elements with an 8 × 8 × 8.2 nm voxel size indirectly imaged dislocation networks by visualizing secondary precipitates, such as Cr 23 C 6 carbides and Laves phase precipitated at dislocations. In summary, the paper provides valuable insights into the mechanisms of high-temperature oxidation of Sanicro 25 steel in steam conditions and the effects of these processes on the material's microstructure. • FIB-SEM tomographic reconstruction of the microstructure elements in 3D using an 8 × 8 × 8.2 nm voxel allowed us to indirectly visualize dislocation systems by imaging secondary precipitates: carbides Cr 23 C 6 , Laves phase precipitates on dislocations. • The presence of steam during the high-temperature oxidation process of Sanicro 25 steel significantly affects the scale formation and void formation process in the scale/metal interface area. The formation of volatile oxide compounds leading to the formation of large voids in the material under the scale layer is possible. • Voids were found in the substrate material below the metal/oxides boundary, with dimensions of the material grain size in the border area, an oxidized inner surface, and a clean inner surface without a clear oxide layer. [ABSTRACT FROM AUTHOR]
ISSN:10445803
DOI:10.1016/j.matchar.2025.114751