# Technological Processes for Steel Applications in Nuclear Fusion
## 9 Post-Test Microstructure Analysis
Figure 33: (**a**) Sketch indicating the cooling channel cross-sections A and C as well as the imaging areas; (**b**) BSE image acquired in the ODS steel/EUROFER interface area close to channel C. A distinct weld interface zone of about 100 μm depth can be recognized; (**c**) BSE image of the corner area in the vicinity of channel C. In this case, also a clear weld interface zone of about 100 μm has developed. Further, the intersection of the weld interface with the surface of the cooling channel (the intentionally created weak spot) does not show any defects. With these observations, we conclude the following formation mechanism of the WIZ. The carbon content of the ferritic 13%-Cr ODS steel is lower by one order of magnitude compared to the 9%-Cr EUROFER97/2 material. During solid-state bonding (but also during the austenitization steps of the heat treatments), the EUROFER alloy is in the austenitic phase (at \(T=1100\)\({}^{\circ}\)C \(\approx 0.8\)\(T_{m}\)), in which C is in solid solution with the Fe/Cr matrix, i.e., the prior M\({}_{23}\)C\({}_{6}\) carbides of the tempered martensite are fully dissolved. The ferritic ODS steel remains in the ferrite phase, which has very low solubility for carbon. Hence, there are two gradients of concentration, one for C from EUROFER (0.1 wt.%) to the ferritic ODS steel (less than 0.016 wt.%), the other for Cr from ODS (13 wt.%) to EUROFER steel (9 wt.%), which build the driving forces for diffusion. An estimation for the according diffusivities and diffusion lengths may be obtained by Equations (1) and (2) with data from [45; 46; 47]. The results are compiled in Table 7. Summarized, Cr diffusion is high in the ODS steel and low in the EUROFER zone. Therefore, the diffusivity values would predict a chromium-enriched weld zone on the EUROFER side in the order of 5 \(\upmu\)m. We have verified this prediction by an EDX scan across the weld interface. As shown in Figure 35, the extent of the Cr concentration gradient amounts to 25-30 \(\upmu\)m, which is almost in the same order of magnitude, compared to the prediction. For carbon diffusion the situation is similar, but the values predict diffusion on a length scale of several millimeters, which would not directly lead to the formation of a narrow weld interface zone. \begin{table}
\begin{tabular}{c c c c c} \hline
**1100 \({}^{\circ}\)C** & **Diffusivity \(D\)**, & \(I_{d}\) **after 2–3 h,** & **Diffusivity \(D\)**, & \(I_{d}\) **after 2–3 h,** \\
**in** & **Chromium** & **Chromium** & **Carbon** & **Carbon** \\ \hline ODS steel & 0.5 \(\upmu\)m\({}^{2}\)/s & 60–75 \(\upmu\)m & 1250 \(\upmu\)m\({}^{2}\)/s & 3–3.7 mm \\ \hline EUROFER & 0.003 \(\upmu\)m\({}^{2}\)/s & 4.5–5.5 \(\upmu\)m & 109 \(\upmu\)m\({}^{2}\)/s & 0.9–1.1 mm \\ \hline \end{tabular}
\end{table} 
Table 7: Estimated diffusion lengths and diffusivity values for chromium and carbon in EUROFER and 13% Cr ODS steel at 1100 \({}^{\circ}\)C according to Equations (1) and (2). Note: At this temperature, EUROFER steel is in the austenitic phase. For chromium diffusion in ferrite and austenite, \(D_{0}\) of 0.15 and 1.11 \(\times\) 10\({}^{-7}\) m\({}^{2}\)/s, and for \(F/(kT)\) values of 26.38 and 17.4 were used. For carbon diffusion in ferrite and austenite, \(D_{0}\) of 0.02 cm\({}^{2}\)/s, and for \(F/(kT)\) values of 7.38 and 9.8 were applied (see [45; 46; 47]). Figure 35: EDX scan through the weld interface, starting from the ODS steel region in the vicinity of channel A (see in Figure 33). The Cr concentration drops from about 14 at. % to about 10 at. % within a diffusion length of approximately 25–30 \(\upmu\)m. The extension of the previously observed weld interface zone (WIZ) is clearly higher than that. Note: The atomic percentage of the Cr content in the ODS steel amounts to about 14. If the value of 85 on then \(y\)-axis is correlated to 14, then the value of 63 corresponds to 10 at. %, which is the Cr content in EUROFER.