Phase transformations in additively manufactured high carbon-bearing steel

Abstract

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.

Keywords

High-carbon steel / Laser powder bed fusion / Additive manufacturing / Austenite / Martensite / Bainite

Funding

This research was sponsored by the DEVCOM Army Research Laboratory under a cooperative agreement contract, W911NF1720172.

Conflict of interest

Yongho Sohn serves as the Editorial Board Member of the journal but was not in any way involved in the editorial and peer-review process conducted for this paper, directly or indirectly. The other authors declare they have no competing interests. In addition, the views, opinions, and conclusions made in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the DEVCOM Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

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