Impact of Manganese Composition on the Thermal Stability of Nitrides in Additively Manufactured 316L Austenitic Stainless Steel

Manganese compositions in additively manufactured 316L austenitic stainless steels typically fall in a range between (1 and 2) mass fraction (%). When coupled with oxygen contents of approximately 0.1 mass fraction (%), the precipitation of spinel oxides during solidification is promoted. Decreases in the manganese composition to a level on the order of 0.5 mass fraction (%) leads to the replacement of these spinel oxides with a-tridymite and Cr₂N phases which prove resistant to typical heat treatment procedures. The emergence of these phases activates new fatigue failure mechanisms that vary with locations within the specimen and leads to a decrease in the strain-controlled fatigue life when process porosity is mitigated. In the fine grain structures present within the contour passes at the specimen edges, intergranular failures are primarily initiated along austenite grain boundaries populated with micro- and nano-sized Cr₂N and Mn-bearing spinel oxide phases. Decreases in the build angle of individual test specimens impacts the width of this contour region and leads to an increase in the area of brittle intergranular failure and a corresponding decrease in fatigue lives. When moving into the interior of the specimens, the grain structure coarsens, and the Cr₂N phase that dominates the grain boundaries in the contour regions are replaced by nanosized monoclinic α-tridymite and spinel phases, leading to the appearance of isolated regions displaying brittle cleavage fracture features during crack propagation. Thermodynamic simulation suggests these phases form due to severe segregation upon solidification, stabilizing them to temperatures within 50 ° C of the solidus.