Role of composition in aging additively manufactured precipitation hardening stainless steels

Abstract Moving from traditional thermomechanical processing to additive manufacturing (AM) of precipitation hardening (PH) stainless steels has introduced unexpected differences in the heat treatment response that produce wide variation mechanical properties. Changes in powder compositions and AM processing conditions have been identified as the primary causes since they produce different as-deposited microstructures within the same alloy system. In PH grade stainless steels, differences in nitrogen composition have been identified as the major contributor. For example, with low nitrogen levels on the order of 0.01 wt%, large ferritic grain structures with small amounts of a high temperature delta-ferrite existing along the grain boundaries are produced. On the other hand, high nitrogen levels on the order of 0.1 wt% or greater result in large amounts of retained austenite (81 vol.%). Higher required aging temperatures for the high nitrogen alloys has also been identified and evaluated so specific aging heat treatments can be selected based on the material composition and targeted properties. It is clear that aging heat treatments must be adapted to incoming material composition. To adequately adjust the heat treatments, a composition-based approach which guides appropriate aging parameters to control changes in copper precipitation as well as retained austenite stability is developed based on thermodynamic and empirical results. While nitrogen content varies significantly and has been the primary focus, these analyses and heat treatments are based on the entire composition.