A research team from Finland, Netherlands and Sweden has used the MAXPEEM beamline to study non-metallic inclusions in ultra-high-strength steel. Such additions to the steel matrix may have significant effects on the steel properties and are therefore important to know and control.
Ultra-high-strength steels are crucial engineering materials used in various fields ranging from the defence industry to civil applications. Examples can be found in lift crane booms or tower cranes. The steels also benefit carbon neutralization. They are supposed to substitute conventional steels in transportation tools and reduce their weights but keep the same strength. Correspondingly, lighter vehicles will consume less fuel and emit less CO2.
However, during steel manufacturing, certain impurities may get incorporated. As a result, they form particles that greatly influence the properties of the resulting steel product. The particles, or inclusions, are not easy to study with conventional methods as they sit inside a tough metal matrix. Determining the chemical components of such inclusions is challenging, with destructive methods ruining the inclusions. There may also be complications with chemical separations at the sub-micrometre scale within the inclusions. The instrument available to the researchers at the MAXPEEM beamline is, therefore, a valuable tool.
“PEEM provides simultaneous spectroscopic and microscopic determinations of chemical and morphological properties of the inclusions,” says Assoc. Prof. Wei Cao, one of the authors of the study. “Combined with the extremely brilliant synchrotron source at MAX IV, the MAXPEEM gives unprecedented merits to reach a high spatial resolution. This further helps identify the inclusion types and their interactions within the steel matrix,” continues Cao.
The ultimate conclusion from this study is to reveal the interactions among different compositions. The team found the unfavourable coexistence between hexagonal boron nitride and titanium nitride. These nitrides rather need calcium-based phases to be stable in the steel matrix. Different impurity elements in the steel mixture may influence each other to form certain inclusions.
Not all inclusions lead to weakened properties. Like good and bad knots in wood, small and coherent particles may even strengthen the steel. It is the larger and incoherent inclusions that are detrimental. For these reasons, figuring out their roles and the decisive mechanisms behind the properties are constantly in demand within industrial and academic interests.
The research team have a lot of ideas for further studies that can help the steel industry. Among many, one of the next plans is to figure out the growth mechanisms of all types of inclusions and their preference in different phases of steel.
“Knowing these mechanisms will help the metallurgical processes in the steel industry,” concludes Cao. “The team is keen to understand the fracture and corrosion mechanisms happening next to the inclusions. These will provide useful knowledge for production and application of ultra-high-strength steels.”
Harishchandra Singh, Tuomas Alatarvas, Andrey A Kistanov, S Assa Aravindh, Shubo Wang, Lin Zhu, Brice Sarpi, Yuran Niu, Alexei Zakharov, F.M.F. de Groot, Marko Huttula, Wei Cao, Timo Fabritius, Unveiling interactions of non-metallic inclusions within advanced ultra-high-strength steel: A spectro-microscopic determination and first-principles elucidation, Scr. Mater. 197, 113791 (2021) DOI: 10.1016/j.scriptamat.2021.113791
Read what University of Oulu wrote about the study here https://www.oulu.fi/en/news/unveiling-interactions-tiny-particles-advanced-steels