Thermal stability of hierarchical microstructural features in additively manufactured stainless steel

Additive manufacturing of austenitic stainless steel results in an unconventional hierarchical microstructure. This hierarchical microstructure was investigated in detail in the as-built condition. The hierarchical microstructure consists of elongated austenite grains and melt pool fusion boundaries with a spherical cap morphology at the largest length scale. At a smaller length scale elongated columnar cell structures exist with elemental segregation at the cell walls. The cells were found not to be a misorientation structure in themselves as often noted, but rather groups of cells with a specific orientation, which are collected in 3-5 μm domains. At even smaller length scales, amorphous spherical silicates are found along with a high dislocation density along cell walls. The thermal stability of the unconventional features as melt pool boundaries, elongated austenite grains, cell domains, cell structure, amorphous precipitates was investigated systematically in the temperature range 400-1100 °C. The dislocation network forming the cell boundaries dissolves gradually and remains thermally stable up to about 800 °C. The melt pool boundaries are more thermally stable and dissolve above 900 °C. The elongated austenite grains and crystallographic texture are fully stable within the investigated temperature range. The cell domains appear to be pinned by precipitation as the cellular structure is dissolved and gradually straighten and resemble regular low angle grain boundaries. The amorphous silicates act as heterogeneous nucleation sites for the formation of σ-phase in the temperature range 700-800 °C, while at higher treatment temperatures these silicates are replaced by large, oblong Si-Mn oxides and small, round Mn-Cr oxides.