Nitrogen Effects in Additively Manufactured Martensitic Stainless Steels: Conventional Thermal Processing and Comparison with Wrought

The microstructures of additively manufactured (AM) precipitation-hardenable stainless steels 17-4 and 15-5 were investigated and compared to those of conventionally produced materials. The residual N found in N₂-atomized 17-4 powder feedstock is inherited by the additively produced material, and has dramatic effects on phase stability, microstructure, and microstructural evolution. Nitrogen is a known austenite stabilizing element, and the as-built microstructure of AM 17-4 can contain up to 90 pct or more retained austenite, compared to the nearly 100 pct martensite structure of wrought 17-4. Even after homogenization and solutionization heat treatments, AM 17-4 contains 5 to 20 pct retained austenite. In contrast, AM 15-5 and Ar-atomized AM 17-4 contain < 5 pct retained austenite in the as-built condition, and this level is further decreased following post-build thermal processing. Computational thermodynamics-based calculations qualitatively describe the observed depression in the martensite start temperature and martensite stability as a function of N-content, but require further refinements to become quantitative. A significant increase in the volume fraction of fine-scale carbide precipitates attributed to the high N-content of AM 17-4 is also hypothesized to give rise to additional activation barriers for the dislocation motion required for martensite nucleation and subsequent growth. An increase in the volume fraction of carbide/nitride precipitates is observed in AM 15-5, although they do not inhibit martensite formation to the extent observed in AM 17-4.