# Technological Processes for Steel Applications in Nuclear Fusion
## 3 Heat Treatment  
### Ausentitization, Quenching, Tempering, Phase Transitions, Mechanical Properties  
Opposed to ferritic and austenitic steels, the mechanical properties of martensitic steels, such as EUROFER97, are tunable by thermal (or heat) treatments within a certain range. With an austenitization treatment (usually between 950 \({}^{\circ}\)C and 1100 \({}^{\circ}\)C for 30-60 min) the EUROFER97 alloys recrystallizes in the austenite phase above the austenite transition temperature A\({}_{\text{c1}}\). Depending on the degree of cold work and heating duration, the austenite grain size may be adjusted: the higher the temperature, the larger the grains (examples are given in [26]). However, this austenitic phase can only exist at temperatures higher than A\({}_{\text{c1e}}\). During quenching below a certain temperature M\({}_{\text{s}}\) and exceeding a critical cooling rate (for martensitic steels, the cooling rates achieved in air are sufficient), the austenite phase transforms into a martensite phase, which hardens and embrittles the steel significantly. For EUROFER97, M\({}_{\text{s}}\) is about 390 \({}^{\circ}\)C and the critical cooling rate for the formation of martensite is in the order of a few K/min. When quenching is performed below the critical cooling rate, the austenite phase decays into ferrite and carbide, which is a rather soft and ductile material state. The details of this austenite decay are compiled in so-called Time Temperature Transition (TTT) diagrams. In the case of cooling from the austenite phase with constant cooling rates, Figure 1 shows the corresponding diagram for EUROFER97/2. Figure 2 illustrates the situation for phase transitions that occur during constant tempering, immediately after austenitization. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **EUROFER97** & **EUROFER97** & **EUROFER97/2** \\
**Element** & **Heat E83698** & **Heat D83350** & **Heat 993402** \\  & **14 mm Plate** & **Ø1 mm Filler Wire** & **25 mm Plate** \\ \hline Cr & 8.82 & 8.93 & 8.95 \\ C & 0.11 & 0.11 & 0.11 \\ Mn & 0.47 & 0.39 & 0.55 \\ V & 0.20 & 0.19 & 0.20 \\ W & 1.09 & 1.09 & 1.04 \\ Ta & 0.13 & 0.14 & 0.14 \\ N & 0.020 & 0.026 & 0.038 \\ O & 0.0010 & 0.0018 & 0.0011 \\ P & 0.005 & \textless{}0.005 & 0.0025 \\ S & 0.004 & 0.004 & 0.001 \\ B & \textless{}0.001 & \textless{}0.0005 & 0.0008 \\ Ti & 0.005 & 0.006 & \\ Nb & \textless{}0.0016 & \textless{}0.001 & 0.004 \\ Mo & \textless{}0.0010 & 0.001 & 0.005 \\ Ni & \textless{}0.0200 & 0.02 & 0.03 \\ Cu & 0.0016 & 0.074 & 0.005 \\ Al & 0.009 & 0.007 & 0.005 \\ Si & 0.04 & & \\ Co & 0.006 & 0.005 & 0.009 \\ As + Sn + Sb + Zr & \textless{}0.015 & \textless{}0.02 & \textless{}0.009 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of different EUROFER heats. Values are given in weight % by the manufacturer [26; 27]. Note: Ti and Si were not analyzed for all heats. The microstructure of martensite shows distinctive features: the prior austenite grains are interspersed by blocks that are oriented differently. The blocks, in turn, consist of parallel oriented needles. Sometimes, this material condition is called hard or hardened martensite. Austenite (existing at high temperatures) has a rather high solubility for carbon and nitrogen while in ferrite (forms below A\({}_{\text{c1}}\)) the solubility of these elements is almost negligible. Therefore, quenching leads to an oversaturated (forced) solution of carbon, which, in turn, distorts the bcc unit cell into a tetragonal shape. This is also the reason for the formation of martensite blocks and needles. Hardened martensite may be softened--accompanied by increasing ductility--by a tempering heat treatment. This has to be performed below the transition temperature A\({}_{\text{c1D}}\), which is at or around 800 \({}^{\circ}\)C for EUROFER97 steel (see Figure 3).