A research team from the National Institute for Materials Science (NIMS) has achieved a significant advancement in heat-resistant steel by using laser powder bed fusion (LPBF)—a type of metal 3D printing.
The team fabricated test specimens and subjected them to creep testing for up to 10,000 hours, revealing that LPBF significantly extended the creep life of the material, achieving at least a 10-fold increase compared to steel produced through traditional heat-treatment processes. These groundbreaking findings are detailed in the journal Additive Manufacturing.
LPBF is an additive manufacturing technique where metal powder is deposited and selectively melted layer by layer using a high-powered laser, eventually forming solid 3D metal components. Unlike conventional manufacturing methods, LPBF can create complex shapes with more precision, and it has seen applications across various industries. However, ensuring that LPBF-produced materials can withstand high-temperature, high-pressure environments over extended periods is crucial, especially for safety-critical applications like thermal power plants.
For this study, the research team fabricated heat-resistant ferritic steel specimens (modified 9Cr-1Mo steel), commonly used in thermal power plants, via LPBF. These specimens were then subjected to creep testing at 650°C under a constant pressure of 100 MPa for up to 10,000 hours (about one year and two months).
The results showed that LPBF specimens had a remarkable 10-fold increase in creep life compared to conventionally heat-treated specimens. While conventional specimens ruptured after 400 to 800 hours of testing, the LPBF specimens continued to endure beyond 10,000 hours, with testing still ongoing.
The key to the extended lifespan of LPBF steel lies in its microstructure. Unlike the tempered martensitic structure formed in conventionally heat-treated steel, LPBF steel develops a high-temperature δ-ferrite phase due to the rapid solidification process, with cooling rates estimated at around 1,000,000°C per second.
This unique microstructure is believed to play a critical role in enhancing the material’s resistance to creep, making it more durable under high-stress, high-temperature conditions.
The research team is continuing to test the LPBF steel specimens beyond the 10,000-hour mark, with plans to evaluate the material’s creep rupture strength up to 100,000 hours. This long-term data is essential for determining the allowable tensile stress for materials used in thermal power plants, where reliability and safety are paramount.
Additionally, the team intends to conduct similar creep tests on other heat-resistant materials fabricated using LPBF. By generating a comprehensive dataset of creep resistance, the researchers aim to demonstrate the reliability of LPBF products and pave the way for broader adoption in high-performance industries, such as energy generation and aerospace.
The goal is not only to expand the use of LPBF in demanding applications but also to establish industry standards for the technology. With further validation and testing, LPBF could become a game-changer for producing high-performance materials that are both complex and reliable under extreme conditions, making it a promising candidate for the future of manufacturing in sectors requiring high durability and safety.
By Impact Lab