13 Zentrale Universitätseinrichtungen

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Institute: search.filters.UBSInstitut.Fakultätsübergreifend / Sonstige EinrichtungAuthor: search.filters.author.Weihe, StefanStart date: 2021End date: 2024Publication Type: search.filters.UBSTyp.Zeitschriftenartikel

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Now showing 1 - 5 of 5
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    Investigations of metallurgical differences in AISI 347 and their influence on deformation and transformation behaviour and resulting fatigue life
    (2024) Veile, Georg; Regitz, Elen; Smaga, Marek; Weihe, Stefan; Beck, Tillmann
    Due to variations in chemical composition and production processes, homonymous austenitic stainless steels can differ significantly regarding their initial microstructure, metastability, and thus, their fatigue behavior. Microstructural investigations and fatigue tests have been performed in order to evaluate this aspect. Three different batches and production forms of nominally one type of steel AISI 347 were investigated under monotonic tensile tests and cyclic loading under total strain and stress control in low and high cycle fatigue regimes, respectively. The deformation induced α’-martensite formation was investigated globally by means of in situ magnetic measurements and locally using optical light microscopy of color etching of micrographs. The investigation showed that the chemical composition and the different production processes influence the material behavior. In fatigue tests, a higher metastability and thus a higher level of deformation induced α’-martensite pronounced cyclic hardening, resulting in significantly greater endurable stresses in total strain-controlled tests and an increase in fatigue life in stress-controlled tests. For applications of non-destructive-testing, detailed knowledge of a component’s metastability is required. In less metastable batches and for lower stress levels, α’-martensite primarily formed at the plasticization zone of a crack. Furthermore, the formation and nucleation points of α’-martensite were highly dependent on grain size and the presence of δ-ferrite. This study provides valuable insights into the different material behavior of three different batches with the same designation, i.e., AISI 347, due to different manufacturing processes and differences in the chemical composition, metastability, and microstructure.
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    Analysis of hydrogen-induced changes in the cyclic deformation behavior of AISI 300-series austenitic stainless steels using cyclic indentation testing
    (2021) Brück, Sven; Blinn, Bastian; Diehl, Katharina; Wissing, Yannick; Müller, Julian; Schwarz, Martina; Christ, Hans-Jürgen; Beck, Tilmann; Staedler, Thorsten; Jiang, Xin; Butz, Benjamin; Weihe, Stefan
    The locally occurring mechanisms of hydrogen embrittlement significantly influence the fatigue behavior of a material, which was shown in previous research on two different AISI 300-series austenitic stainless steels with different austenite stabilities. In this preliminary work, an enhanced fatigue crack growth as well as changes in crack initiation sites and morphology caused by hydrogen were observed. To further analyze the results obtained in this previous research, in the present work the local cyclic deformation behavior of the material volume was analyzed by using cyclic indentation testing. Moreover, these results were correlated to the local dislocation structures obtained with transmission electron microscopy (TEM) in the vicinity of fatigue cracks. The cyclic indentation tests show a decreased cyclic hardening potential as well as an increased dislocation mobility for the conditions precharged with hydrogen, which correlates to the TEM analysis, revealing courser dislocation cells in the vicinity of the fatigue crack tip. Consequently, the presented results indicate that the hydrogen enhanced localized plasticity (HELP) mechanism leads to accelerated crack growth and change in crack morphology for the materials investigated. In summary, the cyclic indentation tests show a high potential for an analysis of the effects of hydrogen on the local cyclic deformation behavior.
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    Micro-twinning in IN738LC manufactured with laser powder bed fusion
    (2023) Megahed, Sandra; Krämer, Karl Michael; Kontermann, Christian; Heinze, Christoph; Udoh, Annett; Weihe, Stefan; Oechsner, Matthias
    Components manufactured with Metal Laser Powder Bed Fusion (PBF-LB/M) are built in a layerwise fashion. The PBF-LB/M build orientation affects grain morphology and orientation. Depending on the build orientation, microstructures from equiaxed to textured grains can develop. In the case of a textured microstructure, a clear anisotropy of the mechanical properties affecting short- and long-term mechanical properties can be observed, which must be considered in the component design. Within the scope of this study, the IN738LC tensile and creep properties of PBF-LB/M samples manufactured in 0° (perpendicular to build direction), 45° and 90° (parallel to build direction) build orientations were investigated. While the hot tensile results (at 850 °C) are as expected, where the tensile properties of the 45° build orientation lay between those of 0° and 90°, the creep results (performed at 850 °C and 200 MPa) of the 45° build orientation show the least time to rupture. This study discusses the microstructural reasoning behind the peculiar creep behavior of 45° oriented IN738LC samples and correlates the results to heat-treated microstructures and the solidification conditions of the PBF-LB/M process itself.
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    Fatigue behavior and lifetime assessment of an austenitic stainless steel in the VHCF regime at ambient and elevated temperatures
    (2023) Schopf, Tim; Weihe, Stefan; Daniel, Tobias; Smaga, Marek; Beck, Tilmann
    While the LCF behavior of austenitic steels used in nuclear power plants is already well investigated, the VHCF regime has not been characterized in detail so far. For this, fatigue tests on the metastable austenitic steel AISI 347/1.4550 were performed with a servo‐hydraulic testing system at test frequencies up to 980 Hz and with an ultrasonic fatigue testing system at a test frequency of 20,000 Hz. To compare these test results to the ASME standard fatigue curve (total strain amplitude vs. load cycles to failure), a fictitious‐elastic and an elastically plastic assessment method was used. The elaborated elastic-plastic assessment method generates good results, while a purely elastic assessment in the VHCF regime, commonly used in literature, leads to significantly nonconservative results. Moreover, phase transformation from metastable austenite into stable α′‐martensite can take place, and no specimen failure occurs in the VHCF regime. Consequently, for this material, a real endurance limit exists.
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    A short-time approach for fatigue life evaluation of AISI 347 steel for nuclear power energy applications
    (2021) Bill, Tobias; Acosta, Ruth; Boller, Christian; Donnerbauer, Kai; Lücker, Lukas; Walther, Frank; Heckmann, Klaus; Sievers, Jürgen; Schopf, Tim; Weihe, Stefan; Starke, Peter
    AISI 347 austenitic steel is, as an example, used in nuclear energy piping systems. Piping filled with superheated steam or cooled water is particularly exposed to high stresses, whereupon local material properties in the pipes can change significantly, especially in the case of additional corrosive influences, leading to aging of the material. In the absence of appropriate information, such local material property variations are currently covered rather blanketly by safety factors set during the design of those components. An increase in qualified information could improve the assessment of the condition of such aged components. As part of the collaborative project “Microstructure-based assessment of the maximum service life of core materials and components subjected to corrosion and fatigue (MiBaLeB)”, the short-time procedure, StrainLife, was developed and validated by several fatigue tests. With this procedure, a complete S-N curve of a material can be determined on the basis of three fatigue tests only, which reduces the effort compared to a conventional approach significantly and is thus ideal for assessing the condition of aged material, where the material is often rare, and a cost-effective answer is often very needed. The procedure described is not just limited to traditional parameters, such as stress and strain, considered in destructive testing but rather extends into parameters derived from non-destructive testing, which may allow further insight into what may be happening within a material’s microstructure. To evaluate the non-destructive quantities measured within the StrainLife procedure and to correlate them with the aging process in a material, several fatigue tests were performed on unnotched and notched specimens under cyclic loading at room and elevated temperatures, as well as under various media conditions, such as distilled water and reactor pressure vessel boiling water (BWR) conditions.