Abstract text (incl. figure legends and references)

1. Introduction

Current geopolitical events as well as the racing climate change demand for sustainable structural materials for CO₂-neutral energy. Future fusion reactors require large quantities (1000-ton range) of high-performance structural materials such as EF97 that can withstand elevated temperatures up to 550°C as well as a high neutron damage larger than 20 dpa.

2. Objectives

Joints of martensitic steels, specifically after liquid phase welding processes like W inert gas or laser beam welds, typically require post-welding heat treatments. The fabrication of breeding blankets for fusion reactors may involve several production steps with consecutive welding. Therefore, the structural material EF97 would be exposed to a series of heat treatments. The objective of this analysis is to study the effect of repeated heat treatments on the microstructure and mechanical properties of EF97-type steel, in the context of materials technology for nuclear fusion reactor applications.

3. Materials & methods

The samples have been heat treated (details see below) and analyzed regarding Vickers hardness. Subsequently, scanning electron microscopy with electron backscatter diffraction (SEM-EBSD in a Zeiss Merlin) scanning and transmission electron microscopy in combination with energy-dispersive X-ray analysis (STEM-EDX in a Thermofisher Talos F200X) has been performed on selected samples to determine grain sizes as well as size and composition of precipitates.

HT1, as received: 980°C/0.45h+air quenching (AQ)+760°C 2.5h HT1: 980°C/0.5h+AQ HT2: 980°C/0.5h+AQ+750°C/2h

4. Results

Figure 1: Development of Vickers hardness with the number of heat treatments.

Figure 1 shows the development of the Vickers hardness for the different applied heat treatments compared to the as received state of the material. For HT1 a slight decrease of the Vickers hardness is observed whereas for HT2 it is quite constant compared to the as received state.

The SEM-EBSD data was evaluated regarding the martensitic lath size a well as for the prior austenitic grain (PAG) size in dependence on the applied number of heat treatments. Both quantities were found to remain constant up to 20 heat treatment repetitions, only small variations were observed in Table 1.

Table 1: Martensitic lath sizes and PAG sizes summarized for selected heat treatments.

Figure 2: STEM-EDX elemental maps after applying 20x HT1 and HT2.

Figure 2 shows two STEM-EDX elemental mappings after 20 repetitions of each heat treatment. In case of HT1 there are no M₂₃C₆-type precipitates observed because without tempering, C remains mainly dissolved in the Fe-Cr matrix, whereas the MX-type precipitates (TaC & VN) form. The microstructure after 20x HT2 is comparable to other EUROFER-type steels. From these elemental maps, precipitate grain sizes, number densities and compositions were extracted and compared to thermodynamic calculations.

5. Conclusion

The repetition of heat treatments did not seem to have any real influence on the evolution of the microstructural dimensions, through the grain size of the prior austenite grains and the martensitic packets. A careful TEM analysis selected samples revealed that there are some indications of ageing on the dimensions and number densities of MX- and M₂₃C₆-type precipitates present in the material. However, based on the currently available data a general aging trend in microstructure and mechanical properties could not be identified.