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Modified 9Cr-1Mo steel was manufactured using laser powder bed fusion (LPBF). The as-built LPBF-sample microstructure comprised columnar δ-ferrite grains and fine martensite grains. The δ-ferrite was a unique phase that formed under the significantly rapid LPBF-induced solidification. Creep testing was performed for the LPBF sample at 923 K for up to 10,000 h. The LPBF-sample time-to-rupture and minimum creep rate were at least 10 times longer and 100 times smaller, respectively, than those of conventional modified 9Cr-1Mo steels at 923 K under 100 MPa. Microstructural characterization of the creep-ruptured samples revealed that creep deformation preferentially occurred in the martensite. Thus, the δ-ferrite contributed significantly to the enhanced creep strength. The intrinsic creep resistance of the δ-ferrite phase exceeded that of the martensite. The microstructural stability of the δ-ferrite grains (attributed to dense MX-phase precipitation at the grain boundary) was a likely strengthening factor. Overall, the modified 9Cr-1Mo steel creep strength was successfully improved via unique microstructural control through LPBF. This achievement is expected to extend LPBF application as a novel microstructural control process for steels and alloys, avoiding the diffusional transformation kinetics that occur in conventional heat-treatment processes.

Rights:

• Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International
  

Keyword: Creep, Ferritic steels, Laser powder bed fusion, Precipitation, δ-ferrite

Date published: 2024-09-18

Publisher: Elsevier BV

Journal:

• Additive Manufacturing (ISSN: 22148604) vol. 93 104445

Funding:

• Chubu Electric Power Co Inc
• Japan Boiler Association

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1016/j.addma.2024.104445

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Updated at: 2024-09-26 12:30:07 +0900

Published on MDR: 2024-09-26 12:30:08 +0900