# Technological Processes for Steel Applications in Nuclear Fusion
## 4 Fusion Welding
### Post-Welding Heat Treatment (PWHT)  
One of the motivations to produce these different welds with 5-12 mm plates was to investigate PWHTs for the final assembly of breeding units. That is, after filling the breeder units with beryllium pebbles, temperatures have to stay below 750 \({}^{\circ}\)C, which excludes the austenitization step and, therefore, omits the application of the EUROFER standard heat treatment. In a first series six different two- and single-step PWHTs (see Table 2) have been applied to 5- and 10-mm welds in vacuum to examine their basic behavior. With respect to hardness all welds respond comparable to the PWHT (see Table 2). That is, both two-step heat treatments (PWHT 1 and 2) remove the coarse-grained weld microstructure and lead to hardness levels around or below 250 HV1. All single step treatments (PWHT 4-6) do not change the microstructure but soften the weld down to hardness levels of 290-350 HV1. Tempering below 700 \({}^{\circ}\)C does not lead to significant changes. PWHT 3 was just applied to verify a complete alpha-gamma transformation, which was the case, as can be clearly seen from the hardness of about 450 HV1. The EUROFER standard heat treatment PWHT 1 restores nearly the same low hardness in the weld zone of all welds as in the base material, which is about 200 \(\pm\) 10 HV1. Only the microstructure of the 5 mm EB and laser welds are almost restored, while a significant part of the dendritic, coarse grained melt structures of the TIG welds still remains. An overview on the weld microstructures after the different heat treatments is given in Figure 13. These findings have been taken into account for a second series of PWHTs that was applied to the 12 mm TIG and EB welds (see Table 3). The results of single-step heat treatments clearly show that longer tempering periods do not affect the hardness (PWHT 8, 11, and 13). Furthermore, below tempering temperatures of 700 \({}^{\circ}\)C, the hardness decreases not below 300 HV1 (PWTH 7-9) and for temperatures of 700 \({}^{\circ}\)C and above, there is not much difference (PWHT 10-14). Only a two-step heat treatment (PWHT 15) softens the fusion zone to values of about 250 HV1. Before presenting the effect of PWHTs on the mechanical properties, the microstructural results from the heat treatment studies are summarized. During fusion welding, parts of the base material, as well as the filler wires during TIG welding, melt together. Therefore, especially during TIG welding, but also during high energy beam welding of thick components, the resulting microstructures of these welds are similar to those observed after casting, that is, the grains are extremely coarse, arranged in dendritic shapes, and in the case of EUROFER97, the martensitic laths are disordered and unevenly distributed. Adjacent to the weld zones are heat affected zones (HAZs) with much finer grained structures. Without further treatment, EUROFER97 welds are rather hard and brittle. Therefore, a sufficient post-welding heat treatment--with regard to the demands of the corresponding design--has to be applied. The hardness level of all EUROFER welds could be almost restored by the standard heat treatment (980 \({}^{\circ}\)C, 0.5 h + air quenching + 760 \({}^{\circ}\)C, 2 h) as well as the microstructure of the 5 mm EB and laser welds. For the other welds, it would probably need higher austenitization temperatures, much longer annealing times, or repeated heat treatments to regain the original microstructure. Another result is the observation that the fine-grained HAZs can only be remove by austenitization. Moreover,