## 4 Materials and Methods  
Due to their high material toughness and corrosion resistance, austenitic Cr-Ni steels are usually specified in nuclear safety standards to be used in pipes of the primary systems of nuclear power plants. The material used for this investigation was the Nb stabilized austenitic stainless steel AISI 347 (X6 CrNiNb18-10), delivered in the form of round bars with a diameter of 30 mm. Its chemical composition according to the inspection certificate 3.1 is shown in Table 1.  
As reported by the manufacturer, the material used was continuously casted as a squared bar of 205 \(\times\) 205 mm using an electromagnetic stirring device along the casting height to prevent the creation of strong axial segregation. The difference of solidification structure was erased afterward by hot rolling to a round bar with a diameter of 31.20 mm, where a full recrystallization occurs due to the reduction in the cross section. However, a visual trace always remains. After air cooling, the bars were heat treated in a furnace at a temperature of 1058 \({}^{\circ}\)C for 30 min, followed by water quenching. In the end, the turning of the bars to a diameter of 30 mm and final straightening was performed.  
Metallographic investigations of the cross section including light microscopy show a squared stirring mark in the center of the bar, present, as explained before, due to its manufacturing process. Outside this area towards the sample edge, a strongly deformed and fine-grained austenitic structure is present, while inside, a strongly inhomogeneous multiphase structure can be recognized [10].  
The specimens used in this investigation are presented in Figure 1. Figure 1a shows the unnotched specimen's geometry, and Figure 1b the notched specimens. There were two types of notched specimens, whose only difference was the notch radius \(R_{N}\), the first one with a radius of \(R_{N}\) = 0.35 mm and the second with a radius of \(R_{N}\) = 0.5 mm. The circles present in the center of the micrograph in Figure 1c represent the size of the specimens used. As can be seen, the gauge length cross section of the specimens, represented by the  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **C** & **Si** & **Mn** & **Ni** & **Cr** & **S** & **P** & **Nb** \\ \hline Min & & & & 9.000 & 17.000 & & & \\ \hline Max & 0.080 & 1.000 & 2.000 & 11.500 & 19.000 & 0.015 & 0.045 & 1.00 \\ \hline Product & 0.025 & 0.401 & 0.577 & 10.064 & 18.147 & 0.001 & 0.24 & 0.402 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of the AISI 347 (X6 CrNiNb18-10).  
smallest circle, was mostly in the internal microstructure of the bar, whereas the shafts have been composed of both of the microstructure types.  
Fatigue tests were conducted on unnotched specimens at different conditions. This includes results of tests performed at room temperature (RT) and at elevated temperature (240 \({}^{\circ}\)C) both in air. All tests were run at fully reversed loading \(R=-1\) and a triangular load function to keep the strain or stress rate constant was applied.  
Strain controlled tests in unnotched specimens were carried out with a constant total strain rate of \(\dot{e}_{a,t}\) = 4 \(\times\) 10\({}^{-3}\) s\({}^{-1}\) and stress controlled tests with notched specimens at a constant nominal stress rate of \(\dot{S}\) = 800 MPa\(\cdot\)s\({}^{-1}\) respectively.