Phase transformations in additively manufactured high carbon-bearing steel

For high-carbon steels that are particularly sensitive to thermally induced phase transformations, the rapid solidification rates inherent to laser powder bed fusion (LPBF) offer a promising pathway to develop unconventional microstructures directly in the as-printed state. This study demonstrates the formation of a supersaturated austenitic matrix – engineered through carbon meta-stabilization and rapid solidification for subsequent heat treatments to develop complex, hierarchical microstructural constituents. A predominantly austenitic high-carbon steel, decorated with cellular segregation networks, was successfully fabricated using LPBF. Post-processing through cryogenic quenching and high-temperature solutionizing treatment, followed by low-temperature tempering, yielded a wide range of microstructures and hardness values. The cryogenically quenched sample exhibited a mixed microstructure of martensite, retained austenite, and cellularly segregated regions, achieving a hardness of 737 ± 31 HV. In contrast, the combination of solutionizing, cryogenic quenching, and tempering produced a multiphase matrix consisting of martensite, bainite, and austenite, with a hardness of 700 ± 20 HV. The insights gained into phase transformations and microstructural evolution during LPBF, along with secondary hardening via heat treatment, provide a foundation for developing tailored post-processing strategies for a broad class of hardenable steels produced by additive manufacturing.