The Evolution of Oxygen-Based Inclusions in an Additively Manufactured Super-Duplex Stainless Steel

Super-duplex stainless steel powder feedstocks specified for use in directed energy deposition additive manufacturing processes can have an oxygen composition nearly five times higher than that present in comparable wrought forms. A combination of computational thermodynamic calculations and experimental validation showed that high levels of oxygen promoted the formation of oxygen-rich inclusions during directed energy deposition additive manufacturing. These inclusions play an important role in microstructural evolution during the rapid heating and cooling cycles prevalent in additive manufacturing and impact mechanical and corrosion properties. Inclusions observed across the powder feedstock and additively manufactured and post-processed materials exhibited complex structures with a combination of amorphous, metastable, and stable phases. The powder feedstock, which experiences rapid cooling rates during the gas atomization process, yielded amorphous inclusions that were rich in manganese, chromium, silicon, and oxygen surrounded by small crystalline MnS particles. After additive manufacturing, inclusions transformed to a combination of rhodonite (MnSiO₃) and spinel (MnCr₂O₄) with amorphous regions around the exterior. Post-process hot isostatic pressing treatments, which replicate conditions most similar to equilibrium, resulted in the formation of a stable spinel oxide with MnS particles around the exterior, matching the results predicted by thermodynamic equilibrium calculations.