Over 10-fold increase in creep life of heat-resistant steel using metal 3D printing

A NIMS research team fabricated heat-resistant steel test specimens using laser powder bed fusion (LPBF)—a form of metal 3D printing—and subjected them to creep testing for up to 10,000 hours. The results demonstrated that LPBF significantly extended the creep lives of the specimens, achieving at least a 10-fold increase over heat-resistant steel produced through conventional heat-treatment processes.

LPBF is an additive manufacturing technique in which a layer of metal powder is deposited, selectively melted and solidified by a laser to form a solid metal layer. Successive 2D layers are then fused to the preceding ones, building 3D components in the desired shape. LPBF allows for the production of more complex shapes than conventional manufacturing processes and has been adopted in various applications. However, adequate creep testing on heat-resistant LPBF products intended for high-temperature, high-pressure use over long periods of time remains necessary to ensure safety.

This research team fabricated heat-resistant ferritic steel (modified 9Cr-1Mo steel) test specimens—a material commonly used in thermal power plants—using LPBF and subjected them to creep testing at 650°C under 100 MPa for up to 10,000 hours (approximately one year and two months). The results showed that the creep lives of the LPBF specimens were more than 10 times longer than those of conventionally heat-treated specimens. While the conventional specimens ruptured after 400 to 800 hours, the testing of LPBF specimen still ongoing beyond 10,000 hours (see Figure).

Unlike the tempered martensitic microstructure formed in conventionally heat-treated steel, LPBF-produced steel develops a high-temperature δ-ferrite phase-based microstructure due to its extremely rapid solidification, with cooling rates estimated at around 1,000,000°C per second. This distinct microstructure is believed to be the primary factor contributing to the significantly extended creep life of LPBF steel.

The research team is continuing to test LPBF steel specimens beyond 10,000 hours, aiming to evaluate the 100,000 hours creep rupture strength, which is required to determine the allowable tensile stress for steels used in thermal power plants. The team also plans to conduct creep testing on other heat-resistant materials produced using LPBF. By generating more comprehensive creep testing data to ensure the reliability of LPBF products, the team aims to promote their widespread adoption and support the development of industry standards.

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This project was carried out by a research team led by Tomotaka Hatakeyama (Researcher, Research Center for Structural Materials (RCSM), NIMS), Kota Sawada (Group Leader, RCSM, NIMS), Masahiro Kusano (Senior Researcher, RCSM, NIMS) and Makoto Watanabe (Deputy Director, RCSM, NIMS).
This work was funded in part by Chubu Electric Power’s Nuclear Safety R&D Center and the Japan Boiler Association.

This research was published in the online version of Additive Manufacturing on September 18, 2024.