###### Abstract  
The assessment of metallic materials used in power plants' piping represents a big challenge due to the thermal transients and the environmental conditions to which they are exposed. At present, a lack of information related to degradation mechanisms in structures and materials is covered by safety factors in its design, and in some cases, the replacement of components is prescribed after a determined period of time without knowledge of the true degree of degradation. In the collaborative project \"_Microstructure-based assessment of maximum service life of nuclear materials and components exposed to corrosion and fatigue (MibaLeb)_\", a methodology for the assessment of materials' degradation is being developed, which combines the use of NDT techniques for materials characterization, an optimized fatigue lifetime analysis using short time evaluation procedures (STEPs) and numerical simulations. In this investigation, the AISI 347 (X6 CrNiNb18-10) is being analyzed at different conditions in order to validate the methodology. Besides microstructural analysis, tensile and fatigue tests, all to characterize the material, a pressurized hot water pipe exposed to a series of flow conditions will be evaluated in terms of full-scale testing as well as prognostic evaluation, where the latter will be based on the materials' data generated, which should prognose changes in the material's condition,funded by the German Federal Ministry of Economic Affairs and Energy (BMWi), the austenitic stainless steel AISI 347 (X6 CrNiNb18-10), widely used for piping in German nuclear power plants, has been analyzed and characterized from a microstructural point of view. A series of constant amplitude fatigue tests (CAT), as well as load and strain increase fatigue tests (LIT and SIT), were performed under various loading, temperature and environmental conditions, where not only load and deformation had been recorded but also parameters associated with non-destructive testing (NDT), taken as a material response for the description of the material's damaging behavior. These fatigue tests were performed following short time evaluation procedures (STEP), that allow a complete S-N curve of a material to be determined with three tests only. A material's response recorded through NDT-based parameters needs the recorded signals to be analyzed in the first place before extracting features from those signals, that may be related to the different damage mechanisms that occur during the test. It is important to obtain a good understanding of how damage develops for every condition to be assessed for the material being considered. In MibaLeb, differentiation has been done concerning temperature (room temperature (RT) and elevated temperature at 240 and 300 \({}^{\circ}\)C), the fatigue regime (low (LCF) and high cycle fatigue (HCF)), and the type of control (load versus strain control). Other factors like the strain/stress amplitude or the strain/stress rate may affect the material's response as well. Based on the Ni and Cr content of the AISI 347 (X6 CrNiNb18-10) and using the Strauss-Maurer microstructure diagram, a metastable \(\gamma\)-austenite microstructure after standard heat treatment is to be expected. When the material is subjected to strain controlled cyclic loading at RT, the paramagnetic \(\gamma\)-austenite can transform into ferromagnetic \(\alpha\)'-martensite [1]. It has been shown that in the LCF regime, this phase transformation reduces the fatigue life of the specimen, mainly for two reasons: the formation involves the change from a face-centered cubic (fcc) structure to a body-centered tetragonal (bct) structure, which is a distorted form of a body-centered-cubic (bcc) structure [2], increasing its volume and producing a highly stressed structure, harder and more brittle than the austenite [3]. Furthermore, the martensite is expected to act as a preferential crack initiation site.