# Technological Processes for Steel Applications in Nuclear Fusion
## 4 Fusion Welding
### Discussion of the Charpy Test Results  
An evaluation of the Charpy test series regarding DBTT is compiled in Figure 17. Due to its stochastic nature, the values scatter by at least \(\pm\)5 K. DBTT of the base material is about \(-\)85 to \(-\)90 \({}^{\circ}\)C, as indicated by a green bar in the graphs. A comparison of the embrittlement shows that for all welds the weakest point with regard to embrittlement is in the weld center and not in the HAZ. Moreover, the upper shelf energies of the welds, in almost all cases, are comparable to or even higher than the USE of the base material. The reason for the higher energies goes back to curved crack propagations, e.g., along the HAZ. Anyway, there is a small tendency for softening, since in the HAZ the USE values are often lower by about 0.5-1.5 J compared to the base material. Surprisingly, there is one weld with acceptable embrittlement behavior even without PWHT. This is the 5 mm EB weld with a DBTT of just 20 K worse than the base material. DBTT values of all other welds without PWHT are worse by 80-160 K. On the other hand, the applied two-step PWHTs lower the DBTTs at least down to the level of the base material. That is, the embrittlement of all welds could be completely restored by two-step heat treatments, where the lowest applied annealing temperature has been 900 \({}^{\circ}\)C in the case of the 12 mm TIG weld. Another tendency which can be clearly seen is the lower embrittlement of the beam welds compared to the fusion welds. Since all 6 welds have been heat treated at 700 \({}^{\circ}\)C for 2 h, the resulting DBTT levels may be directly compared. Laser welding shows the lowest DBTT with \(-\)75 \({}^{\circ}\)C, followed by the 5 mm and 12 mm EB welds with \(-\)65 \({}^{\circ}\)C, and finally the 12 mm, 5 mm, and the 10 mm TIG welds with \(-\)35 \({}^{\circ}\)C, \(-\)15 \({}^{\circ}\)C, and 0 \({}^{\circ}\)C, respectively. That is, after the same (low temperature one-step) PWHT, beam welds show less embrittlement than the TIG welds. Further, the other PWHTs demonstrate that TIG welds require at least 740 \({}^{\circ}\)C tempering to reach a DBTT comparable to the base material. On the other hand, if the PWHT temperature of the EB weld is just slightly reduce to 680 \({}^{\circ}\)C, the DBTT is increased by 25 K. Again, this is proof for the high sensitivity of the ductilization of EUROFER97 steels on the tempering temperature. Another, more conservative method to assess embrittlement is to consider the lowest test temperature where still no brittle fracture is observed in Charpy tests. This so-called lowest temperature in the upper shelf (LTUS) is plotted in Figure 18. The LTUS values are more design-relevant, which can be seen by the fact that the LTUS values in Figure 18 are never better than those of the base material. This is not the case for the DBTT analysis given in Figure 17. However, for our welds there are no qualitative differences. Figure 17: Ductile-to-brittle transition temperature (DBTT) of 5 mm, 10 mm, and 12 mm welds after different post-welding heat treatments. The notches of the Charpy specimens were fabricated either in the weld center or in the HAZ (as indicated). For comparison, the DBTT of the base material is plotted as a horizontal line. One goal of the present investigations was to test whether the PWHT temperature can be lowered to less than 750 \({}^{\circ}\)C. The answer may now be estimated as follows: beam welds require a PWHT of about 720 \({}^{\circ}\)C to show the same embrittlement as the base material. Applying just a 700 \({}^{\circ}\)C PWHT, DBTT is worse by about 15-25 K. TIG welds, on the other hand, need a PWHT of at least 740 \({}^{\circ}\)C to be comparable to the base material. By only applying a 700 \({}^{\circ}\)C PWHT to the TIG welds, DBTT is increased by at least 40 K.  
Two step heat treatments have the potential to restore the mechanical properties of all welds. But as has already been shown in a previous section, the micro-structure along the TIG seam cannot be fully restored.