# Technological Processes for Steel Applications in Nuclear Fusion
## 4 Fusion Welding
### Effect of PWHT on Toghness  
Charpy tests were performed first with specimens from the 5- and 10-mm welds. The Charpy properties of specimens notched within the HAZ seem to tend slightly to lower upper shelf energies (USE) but better DBTT values, compared to the specimens with notches in the weld center. In some cases, the measured impact energies of welds are even higher than the USE of the base material. This is due to bended crack propagations along the HAZs, which need more energy than the usual straight fractures. Figure 14 shows the results for these welds without and after PWHT 1 (standard two-step EUROFER heat treatment, see Table 2) and PWHT 5 (low temperature single-step treatment at 700 \({}^{\circ}\)C, see Table 2). After the standard heat treatment (PWHT 1), all welds behave almost as the base material (DBTT: \(-90\) to \(-110\)\({}^{\circ}\)C, USE: 9 J), which supports a full restauration of the microstructure. Without a PWHT, all welds show a clear DBTT shift toward higher temperatures, either with specimens notched in the HAZ or weld center or both. A single-step PWHT at 700 \({}^{\circ}\)C (which would be perfectly safe for a final breeder unit assembly including Be pebbles) improves the toughness of the EB and laser welds to values comparable to the base material. The TIG welds, however, perform quite unconvincing with this low temperature one-step PWHT. Based on these results, further Charpy tests on the 12 mm welds have been performed after PWHT 9, 10, 14, and 15 (see Table 3), with specimens notched directly at the weld fusion zone (see Figure 15). This study clearly demonstrates that base material ductility of TIG welds will only be restored by tempering temperatures of at least 740 \({}^{\circ}\)C or higher. Of course, the Charpy properties may be pushed to an optimum by grain refinement with help of a 900 \({}^{\circ}\)C austenitization step. The investigated 12 mm EB weld, however, leads to acceptable DBTT values (but still higher compared to the base material) even with a one-step heat treatment at 700 \({}^{\circ}\)C. Lower tempering temperatures are not expedient. \begin{table}
\begin{tabular}{c c c c} \hline
**PWHT** & **Annealing** & **Tempering** & \
\begin{tabular}{c} **Hardness in the Fusion Zone** \\ **of the 12 mm EB Weld (HV1)** \\ \end{tabular} &
\begin{tabular}{c} **Hardness in the Fusion Zone** \\ **the 12 mm TIG Weld (HV1)** \\ \end{tabular} \\ \hline
7 & 650 \({}^{\circ}\)C/2 h & 305 & 335 \\
8 & 650 \({}^{\circ}\)C/16 h & 318 & 320 \\
9 & 680 \({}^{\circ}\)C/2 h & 312 & 310 \\
10 & 700 \({}^{\circ}\)C/2 h & 290 & 285 \\
11 & 700 \({}^{\circ}\)C/16 h & 285 & 295 \\
12 & 720 \({}^{\circ}\)C/2 h & 280 & 285 \\
13 & 720 \({}^{\circ}\)C/4 h & 275 & 315 \\
14 & 740 \({}^{\circ}\)C/2 h & 270 & 280 \\
15 & 900 \({}^{\circ}\)C/2 h & 740 \({}^{\circ}\)C/2 h & 245 & 250 \\ \hline \end{tabular}
\end{table} 
Table 3: PWHTs applied to 12 mm EB and TIG welds. Hardness is given in pairs where the left number refers to EB and the right to the TIG weld (EB/TIG). Figure 14: Charpy test results of 5 mm and 10 mm TIG, laser and EB welds. The notches of the Charpy specimens were fabricated in the top of the weld centers and HAZs. The graphs show results from the condition as welded, after a PWHT of just tempering at 700 \({}^{\circ}\)C for 2 h, and after a PWHT of 960 \({}^{\circ}\)C/30 min + air quenching + 760 \({}^{\circ}\)C for 2 h. For comparison, each graph shows the properties of the base material (EUROFER97/2) after standard heat treatment. Finally, the test results for specimens from the 40 mm EB weld are presented in Figure 16. From the DBTT of about 20 \({}^{\circ}\)C, it is obvious that this weld is far too brittle to be useful without PWHT. However, as seen in the aforementioned studies, after a standard PWHT (980 \({}^{\circ}\)C/30 min + air cooling + 750 \({}^{\circ}\)C/2 h) ductility is fully restored. The results show further that the weld properties are distributed homogeneously (i.e., the Charpy energies from specimens of the weld top are comparable to those of the root).