Influence of atmosphere on microstructure and nitrogen content in AISI 316L fabricated by laser‐based powder bed fusion

The present work focuses on the influence of the composition of the protective gas (argon or nitrogen) used in laser‐based powder bed fusion (L‐PBF) on the nitrogen content, microstructure and hardness of AISI 316L austenitic stainless steel. L‐PBF of AISI 316L powder using Ar gas resulted in loss of nitrogen in the final part. On the other hand, L‐PBF using N₂ gas resulted inan increase in nitrogen content in the final part, showing that nitrogen is absorbed during L‐PBF manufacturing in N₂ gas. The nitrogen absorption implies that the build part is actually AISI 316LN rather than AISI 316L.The microstructures of 316L specimens manufactured in both atmospheres exhibited highly elongated γ‐austenite grains, with acellular structure. The hardness of the part manufactured in N₂ gas was systematically higher than the part manufactured in Ar. For the part manufactured in Ar, a clear gradual decrease in hardness was observed with increasing distance from the build plate, whilefor the N₂ manufactured part this hardness decrease is first observed at some distance from the build plate.For the part manufactured in Ar, a larger variation in the measured nitrogen content was observed. Moreover, a systematically lower and position dependent micro‐hardness and inhomogeneous etching response of this specimen indicate an inhomogeneous microstructure in the build.The results demonstrate that the nitrogen content of L‐PBF manufactured AISI 316L depends on the composition of the gas atmosphere used in the chamber. It is discussed qualitatively how desorption and absorption of nitrogen from the applied atmosphere play a role on the resulting composition and microstructure of the build part.