# Technological Processes for Steel Applications in Nuclear Fusion
## 3 Heat Treatment
### Ausentitization, Quenching, Tempering, Phase Transitions, Mechanical Properties
Usually, tempering is performed at 700-780 \({}^{\circ}\)C for 2 h (see Figure 3) but the effect starts to show already at temperatures above 550 \({}^{\circ}\)C. During the tempering treatment, the carbon diffuses out of the oversaturated lattice to laths and prior austenitic grain boundaries and forms carbide precipitates (mainly M\({}_{23}\)C\({}_{6}\), where M stands for Cr, W, Mn). Thus, the previously tetragonal lattice relaxes into bcc. However, the distinct microstructure containing blocks and needles mainly remains the same, but now the formation of carbides at the boundaries can be recognized in addition. The tempering treatment may be stopped after the carbon diffusion is completed. Since the diffusion rate is highly temperature dependent, the material is softer after tempering at a higher compared to a lower temperature for the same duration. This material condition is sometimes referred to as tempered martensite. Figure 1: Time Temperature Transition (TTT) diagram for EUROFER97/2 after austenitization for 60 min at 1000 \({}^{\circ}\)C. The lines correspond to cooling by different rates (note the logarithmic time scale). The phases that form during cooling are denoted: austenite A, martensite M, ferrite F, carbide C. Start and end of martensite formation M\({}_{\text{s}}\) and M\({}_{\text{f}}\). The crosses symbolize data determined by dilatometry. Figure 3: Depending on the tempering treatment (temperature and duration), hardness can be adjusted after austenitization. Here, the effect on EUROFER97/2 (25 mm plates, heats 993391 and 993402) after austenitization at 1040 \({}^{\circ}\)C and 980 \({}^{\circ}\)C for 30 min, is illustrated. The tempering treatment was performed for 2 h. The temperature at the minimum of the hardness curve corresponds to the transition temperature A\({}_{\rm cIb}\). Figure 2: Time Temperature Transition (TTT) diagram for the experimental heat J361 (it is similar to EUROFER97/2 but without Mn) starting with austenitization for 60 min at 1000 \({}^{\circ}\)C, followed by isothermal tempering (i.e., at constant temperature). This diagram corresponds to placing a piece material from one furnace at austenitization temperature to another furnace with a temperature given in this diagram. While changing the furnaces, the temperature drops with a rate of 20 K/minute. The corresponding horizontal line at this temperature indicates the start and end of a phase transition whenever the line enters or leaves a phase field boundary. The phases are denoted: austenite A, martensite M, ferrite F, carbide C. The crosses symbolize data that were determined by dilatometry. Note: For EUROFER97 steels the F + C phase field is slightly shifted towards longer durations and the martensitic phase starts at about 360 \({}^{\circ}\)C.  
Heat treating EUROFER97 steels at 1100-1150 \({}^{\circ}\)C is called solution annealing, since at such high temperature, precipitates and intermetallic phases (e.g., MX, M\({}_{23}\)C\({}_{6}\), laves, and sigma phase) dissolve completely in the matrix. By solution annealing, the EUROFER97 steel is transformed into one homogeneous phase--austenite. In summary, a full heat treatment of martensitic steels consists of (1) austenitization (or solution annealing), (2) quenching (in air, water, or oil), and (3) tempering. The first and second step is also called hardening (treatment). In the following, the term \"standard heat treatment\" for EUROFER97 steels is defined to be 980 \({}^{\circ}\)C/0.5 h + quenching + 750-760 \({}^{\circ}\)C/2 h.  
EUROFER97 is in the martensitic state resistant to aging at temperatures up to 550 \({}^{\circ}\)C for periods of about 20,000 h. That is, at temperatures below 550 \({}^{\circ}\)C (and in the absence of irradiation effects) the microstructure of EUROFER97 steels remains stable on the time scale of several years. However, when exposed to 550 \({}^{\circ}\)C for longer periods, intermetallic phases (sigma and chi) begin to form in addition to carbide coarsening. Above 550 \({}^{\circ}\)C, aging accelerates.