# Technological Processes for Steel Applications in Nuclear Fusion
## 3 Heat Treatment
### Repeated Heat Treatments  
A component, or parts of it, sometimes have to undergo several heat treatments or thermal loads, either deliberately (e.g., post-veld heat treatments) or due to a fabrication process (e.g., diffusion bonding). Thus, the impact of repeated heat treatments on the material properties is of general interest for the component design and for the planning of efficient production steps. In a case study, the full standard heat treatment was applied to EUROFER97/2 pieces for 20 times. For a second set, austenitization and quenching was repeated 20 times, followed by one tempering treatment. Then the impact on the mechanical properties was determined by tensile, miniaturized instrumented Charpy, and Vickers hardness tests. The microstructure was analyzed by optical microscopy. The tensile test results from the study are depicted in Figure 6. Yield and tensile strength do not show a significant influence from the repeated heat treatments. Also, uniform and total elongation as well as the Charpy upper shelf energy (see Figure 7) is rather similar within a typical scatter band. Only the DBTT for the condition 20 \(\times\) (A + Q) + 1 \(\times\) T seems to show a slight trend toward lower temperatures, but a more precise statement would require a much higher number of tests. Thus, we can draw the conclusion that repeated heat treatments do not significantly affect the mechanical properties of EUROFER steel, at least, if restricted to the conditions and parameters of this study. Figure 8 shows the microstructural evolution during 5-, 10-, 15-, and 20-times repetition of the full standard heat treatment A + Q + T and of A + Q. Peculiar differences in the martensitic microstructures are not recognizable for both series. This is also supported by the hardness. Neither repetitions of A + Q + T nor A + Q led to significant deviations from the initial states, which are 207 and about 400 HV30. This further confirms the above-drawn conclusion that repeated applications of heat treatments do not alter the mechanical properties of EUROFER97, at least not significantly. Figure 6: The effect of multiple heat treatments on the tensile properties of EUROFER97/2 (25 mm plate, heat 993402). The reference samples (circles) were austenitized at 980 \({}^{\circ}\)C for 30 min (A), air quenched (Q), tempered at 750 \({}^{\circ}\)C for 2 h (T), and finally air cooled. For comparison, another set of samples was 20 times heat treated the same way, i.e., 20 \(\times\) (A + Q + T). Finally, another set was prepared by 20 times repeating only austenitization and quenching, i.e., 20 \(\times\) (A + Q), which was then followed by one tempering treatment (1 \(\times\) T). The tensile tests were performed according to DIN EN ISO 6892-1 using cylindrical specimens (gauge: diameter 3 mm, length 18 mm). The tensile axis was always oriented parallel to the rolling direction. Figure 8: The effect of multiple heat treatments on the microstructure of EUROFER97/2 (25 mm plate, heat 993402). The optical micrographs show cross-sections of the plane T-S. Vickers hardness was measured on T-S cross-sections, too. The values are given in the lower left corner of the images. The heat treatment is indicated as follows: austenitization at 980 \({}^{\circ}\)C for 30 min (A), air quenching (Q), and tempering at 750 \({}^{\circ}\)C for 2 h (T). Figure 7: The effect of multiple heat treatments on the Charpy impact energy (KLST) of EUROFER97/2 (25 mm plate, heat 993402). The reference samples (circles) were austenitized at 980 \({}^{\circ}\)C for 30 min (A), air quenched (Q), tempered at 750 \({}^{\circ}\)C for 2 h (T), and finally air cooled. For comparison, another set of samples was 20 times heat treated the same way, i.e., 20 \(\times\) (A + Q + T). Finally, another set was prepared by 20 times repeating only austenitization and quenching, i.e., 20 \(\times\) (A + Q), which was then followed by one tempering treatment (1 \(\times\) T). The instrumented Charpy tests were performed according to the EU standard for small size specimens (KLST type).