# Technological Processes for Steel Applications in Nuclear Fusion
## 3 Heat Treatment
### Uncontrolled Temperature Excursions  
There are many processes involved in manufacturing, which could lead, unintentionally or by accident, to uncontrolled excursions of the component's temperature. These could be, for example, welding, heat treatment, or hot forming. In the worst case, such events lead to the formation of ferrite and carbide (see phase fields F + C in Figures 1 and 2). A simple example is a slow cooling rate after austenitization. This might occur by switching of the furnace or by an electrical defect. In most cases, the microstructure of EUROFER97 ends up as F + C phases, which makes the piece of material or component very soft, as shown in Figure 5a. Depending on the EUROFER heat and austenitization temperature, this unwanted transformation occurs for cooling rates below about 3 K/min (see Figure 1 for the case of EUROFER97/2). The recovery of the work piece that in this F + C condition, would  
Figure 4: Effect of austenitization between 900 \({}^{\circ}\)C and 1050 \({}^{\circ}\)C for 30 min followed by air cooling and tempering between 700 \({}^{\circ}\)C and 770 \({}^{\circ}\)C for 2 h (**a**) on the 500 \({}^{\circ}\)C-yield-strength of the EUROFER97 steel; (**b**) on DBTT of the EUROFER97 steel (measured with ISO-V Charyp specimens, for details see [28]). The yield strength and DBTT values are extracted and re-evaluated from the Charyp results given in [28]. require solution annealing before the full heat treatment could be repeated, which is time consuming, can be costly, and in some cases is not feasible due to the high temperatures. Another effect, which can easily happen during TIG welding, for example, is based on keeping the material (or parts of the work piece) on a constant high temperature immediately after austenitization. Possible phase formation can be predicted by applying the isothermal TTT diagram in Figure 2 for such cases. When the temperature in a fusion weld (e.g., TIG weld) drops to 750 \({}^{\circ}\)C and stays about constant for a while, then already 5 min time is sufficient to start the formation of the soft F + C phase. Experimental validation for this behavior is given in Figure 5b-d. While 30 min isothermal heating reduces the hardness notably, after one hour, full transition into F + C is reached. A further exposure to 750 \({}^{\circ}\)C, e.g., for 6 h, such as in case (d)--does not lead to additional changes. Fortunately, this effect is quite sensitive to a small temperature interval around 750 \({}^{\circ}\)C. A further deviation in the range 650 \({}^{\circ}\)C to 800 \({}^{\circ}\)C shifts the starting point of the F + C formation toward higher periods. Examples for 700 \({}^{\circ}\)C are given in Figure 5e,f, where can be seen that even exposure of 30 min does not prevent fully martensite formation. After one hour at 700 \({}^{\circ}\)C, however, the onset of F + C phase reduces the hardness clearly. Before we come to a final and possibly relevant case, let us discuss the following scenario. If an EUROFER97 work piece, starting from austenitization, is cooled to 550 \({}^{\circ}\)C, and then kept at 550 \({}^{\circ}\)C for a time, maybe even days, prior to the final air cooling, what would happen? It would fully transfer into martensite, as can be predicted by applying the TTT diagrams in Figures 1 and 2. At 550 \({}^{\circ}\)C the undercooled austenite phase is stable and does not transform. Only during the final quench, it decays into martensite. But what would happen when in a similar scenario the temperature would be kept at 850 \({}^{\circ}\)C for 6 h before air quenching? Figure 2 shows that at 850 \({}^{\circ}\)C the material is still in the austenitic phase. Then, air quenching starting from 850 \({}^{\circ}\)C with typically 10 K/min leads to fully martensitic transformation (see Figure 1). The result is shown in Figure 5g. Now we assume that the work piece, after austenitization, is kept at 550 \({}^{\circ}\)C for 5 h and then the temperature is increased up to 750 \({}^{\circ}\)C for one hour time, followed by the final air cooling.