Metal Additive Manufacturing Conference - MAMC 2023

Within the framework of the development of the additive manufacturing processes carried out by the CEA, this work aims at studying the influence of atomization and functionalization treatment conditions of grade 316L(N) steel powder particles, identified as a material of interest for the components of future nuclear reactors, on the microstructure after consolidation by L-PBF. Previous work has demonstrated the impact of using different commercial 316L steel atomized powders on the microstructural characteristics of material obtained after consolidation by L-PBF process [1]. Especially, one of the tested powders led to the production of a material with a refined and quasi-equiaxial grain structure, unlike steels characterized by a columnar grain structure typically obtained by L-PBF with the same process parameters. Even if the mechanisms of this singular behavior are not clearly identified yet, the atomization conditions and the contents of minor elements and impurities in the powders seem to have a significant influence on the observed phenomenon. Indeed, TEM observations performed on the consolidated materials show that the refined microstructure has an enhanced precipitation density, due to fine chromium/iron carbides precipitates of around 20 nm diameter. These precipitates could act as additional nucleation sites, preventing the epitaxial growth of the grains during L-PBF consolidation. This specific precipitation, observed after L-PBF consolidation, is assumed to result from the atomization process conditions, inducing a low nitrogen content in particles of elaborated powders.
This study aims at identifying the influence of the nitrogen content on the 316L steel consolidated by L-PBF microstructure. First, TEM observations have been performed to the specific 316L powder precursor, which show a fine precipitation currently under identification. Secondly, powders with various nitrogen levels are produced and studied. To control the nitrogen content of the powder, two processes are investigated: (i) nitriding treatment of the reference powder, using nitrogen atmosphere, and (ii) ultrasonic atomization in controlled conditions. Afterwards, those powders are consolidated by L-PBF process with optimized and fixed parameters, in order to focus on the influence of the nitrogen content of the powder on the final microstructure (grain size, grain morphology, cell structure and precipitation structure). Finally, tensile tests and hardness tests are performed to compare the mechanical performances and identify the anisotropic behavior associated to each microstructure.