Microstructural features in additively manufactured EUROFER97 components

By uncoupling the manufacturability from the design process, additive manufacturing of RAFM steels like EUROFER97 can open significant design freedom for components in fusion reactors, e.g., breeding blankets and divertor applications. As mechanical properties of RAFM steels are based on their specific microstructure and high number density of precipitates, good control of the microstructure of additively manufactured parts has to be ensured. As additive manufactured components are known to possess unique microstructures compared to EUROFER97 from standard technologies, the aim of this paper is to investigate additive manufactured EUROFER97 components and the influence of post-processing steps on their microstructure and in particular on the formation of precipitates. This paper covers the technological fabrication process of EUROFER97 by selective laser melting (SLM), including the production and chemical analysis of pre-alloyed EUROFER97 powder. In the initial state after fabrication, SLM-EUROFER97 components exhibit a bimodal, anisotropic microstructure with large ferritic grains instead of the desired homogeneous, fully martensitic microstructure. Further, neither M₂₃C₆ nor MX precipitates were detected in the as-built condition. A heat treatment including hot isostatic pressing, austenitization, quenching and tempering, allows achieving a fully martensitic, uniform microstructure as well as the formation of M₂₃C₆ and MX precipitates with similar size and volume density as in conventionally produced EUROFER97. In combination with the chemical analysis of the raw material, impurities stemming from the atomization process forming oxides were detected. However, due to their small size, those are not expected to influence the performance of SLM-processed EUROFER97. The comparison of the as-built condition and the condition after hot isostatic pressing and heat treatments reveals the necessity of appropriate post-processing to tailor the microstructure to one comparable to conventional EUROFER97.