Mechanical performance and microstructure of the grade 91 stainless steel produced via Directed Energy deposition laser technique

The grade 91 ferritic/martensitic steel is considered a promising structural or cladding material for various nuclear reactor applications. Here, grade 91 was fabricated via the Directed Energy Deposition Laser technique. This alternative manufacturing process potentially enables tailoring of the mechanical properties through increased control of the product's microstructure. Aimed at linking fabrication to performance via defining the process-structure–property relationships, the current research includes macro and up to nano-scale mechanical testing using microhardness, tensile, and in-situ nanoindentation hardness, coupled with electron diffraction-based microstructure characterization. Mapping of the product structure and properties was conducted by testing miniature-sized samples, parallel to the built direction (‘Z’ direction) and perpendicular (‘X’ direction) at constant distances. We found the majority of the microstructure consists of fine and coarsened-size lath-type martensite grains, with up to 15% δ-phase, preferentially observed at the melt pool boundaries. Most intriguing was the gradual decrease found in observed metallurgical pores alongside softening at farthest distances from the cold build platform. Here, these changes were successfully explained in terms of phase composition, ‘grain-like’ size effects of the lath-type martensite, and geometry necessary dislocation density. In finalizing this work, several competing strengthening mechanisms were addressed, and their activity was considered owing to fabrication-related mechanisms.