# Technological Processes for Steel Applications in Nuclear Fusion
## 9 Post-Test Microstructure Analysis  
After the high heat flux testing, we wanted to identify and investigate possible microstructural changes, defects, and surface modifications in the assumed critical zones of the mock-up. For this, several cross-sections through the mock-up were cut and prepared for optical and electron microscopy, i.e., scanning electron microscopy and Back Scattering Electron (BSE) imaging. Prior to the microstructure analysis, a representative piece of the mock-up was cut off, from which further samples were extracted and prepared for microscopy. The first results are illustrated in Figure 33. The BSE images of the ODS steel/EUROFER97 weld interface were acquired close to the corners of channels A and C, as indicated in the sketch. Both are the high heat flux loaded hot areas. On the EUROFER side of the interface, an area of about 100 \(\upmu\)m depth is recognizable that contains larger grains. In the following, we denote this area as weld interface zone (WIZ). The interface temperature during fabrication was 1100 \({}^{\circ}\)C for 2 h (followed by several heat treatments), during which the WIZ certainly has formed. We estimated the interface temperature between the channels during the test sequences #3 and #4 to about 500 \({}^{\circ}\)C, based on FEM analyses. This raises the question whether the WIZ grew further during the high heat flux tests. Hence, for comparison, we examined an interface area far away from the heat load zone (i.e., this region did not exceed temperatures of about 200-300 \({}^{\circ}\)C) and observed also WIZs with depths in the range from about 70-100 \(\upmu\)m. This confirms (1) that the formation of the WIZ occurred during the solid-state bonding (ODS steel plating) and (2) that the zone did not grow during operation in the HELOKA facility. Moreover, we could successfully determine the involved phases by performing a principal component analysis (PCA) on SEM-EDX datasets of an interface region inside the heat flux zone. In this case, we identified three phases (factors): Fe/Cr matrix, Cr/W carbides (i.e., M\({}_{23}\)C\({}_{6}\)), and Ti carbo-nitrides. The PCA maps are depicted in Figure 34. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Sequence** & **Cycles** & \begin{tabular}{c} **Heat Flux** \\ **(MW/m2)** \\ \end{tabular} & \
\begin{tabular}{c} **Pulse Length** \\ **(on/off)** \\ \end{tabular} &
\begin{tabular}{c} **Surface** \\ **Temperature** \\ \end{tabular} \\ \hline \#1 & 100 & 0.7 & 2 min/2 min & \textasci{}550 \({}^{\circ}\)C \\ \hline \#2 & 100 & 0.8 & 2 min/2 min & \textasci{}600 \({}^{\circ}\)C \\ \hline \#3 & 100 & 0.9 & 2 min/2 min & \textasci{}650 \({}^{\circ}\)C \\ \hline \#4 & 7 & 0.9 & 2 h/\textasci{}2 min & \textasci{}650 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 6: High heat flux fatigue test sequences for the mock-up in the Helium Loop at KIT (HELOKA) facility. The density of Cr-rich precipitates in the ODS ferrite (white dots in the M\({}_{25}\)C\({}_{6}\) map) is increasing towards the interface. There are also Ti-rich precipitates in the ODS ferrite (white dots in the Ti (C, N) map), likely at grain boundaries. In addition to PCA, we performed an EBSD analysis of the interface region. This reveals the typical martensitic structure of the EUROFER area on the bottom and the extreme fine rolling texture of the ODS steel on top. Moreover, the coarse-grained WIZ appears now clearly in between and is fully distinguishable from the other phases. Figure 34: (**a**) Principal component analysis (PCA) of SEM-EDX data in the vicinity of channel A (see in Figure 33). It reveals the single phases, which are present in the respective sample areas. The weld interface and weld interface zone are indicated by dotted lines; (**b**) Electron backscatter diffraction (EBSD) analysis of the weld interface zone (WIZ).