The magnetic secret inside steel finally explained

Published in Physical Review Letters, the work sheds new light on how carbon affects the internal grain structure of steel, a key factor in its strength and performance.

Why Steel Processing Uses So Much Energy

Steel, made by combining iron and carbon, is one of the most widely used construction materials in the world. Shaping its internal structure requires extremely high temperatures, which is why steel production consumes so much energy. Decades ago, scientists observed that some steels performed better when heat treated in the presence of a magnetic field, but the explanations at the time were largely theoretical. Without a clear physical understanding, engineers had no reliable way to predict or control the effect.

"The previous explanations for this behavior were phenomenological at best," said Dallas Trinkle, the Ivan Racheff Professor of Materials Science and Engineering and the senior author of the paper. "When you're designing a material, you need to be able to say, 'If I add this element, this is how (the material) will change.' And we had no understanding of how this was happening; there was nothing predictive about it."

To address this long-standing question, Trinkle applied his expertise in diffusion modeling as part of a research team supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy. In iron-carbon alloys such as steel, carbon atoms occupy small octahedral "cages" formed by surrounding iron atoms. By simulating how carbon atoms move from one cage to another, the team was able to pinpoint what causes magnetic fields to slow that motion.

Simulating Magnetism and Atomic Motion

Using a computational approach known as spin-space averaging, Trinkle ran simulations that accounted for both temperature and magnetic fields. These simulations tracked how the magnetic spins of iron atoms align under different conditions. When the north and south poles of an iron atom line up, the atom becomes ferromagnetic and strongly magnetized. When they do not align, the atom is paramagnetic and only weakly magnetized.

The results showed that aligned spins raise the energy barrier carbon atoms must overcome to move between cages. As magnetic order increases, carbon diffusion slows down, providing a clear physical explanation for the long-observed effect.

"It takes an extremely strong field to switch magnetic moments," Trinkle said. "If you're near the Curie temperature, the magnetic field has a strong effect… When the spins are more random, the octahedron (cage) actually gets more isotropic: the whole thing kind of opens up and has more space to move."

Implications for Cleaner and Smarter Steelmaking

Trinkle believes the findings could help reduce the energy needed to process steel, lowering production costs and cutting CO₂ emissions. Beyond steel, the same principles could be applied to other materials, allowing scientists to quantitatively predict how magnetic fields influence atomic diffusion more broadly.

"We wanted to be able to do real calculations; to show not just qualitatively but quantitatively the effective field and temperature. Now that we have this information, we can start thinking more about engineering alloys. It may be choosing alloys that already exist or even thinking about alloy chemistries that we're not yet using that could be extremely advantageous."

Dallas Trinkle is a professor in the Department of Materials Science and Engineering at Illinois Grainger Engineering and is affiliated with the Materials Research Laboratory. He holds the Ivan Racheff Professorship appointment.