Fine-tuned Quenching

It is fundamental blacksmith's know-how that to harden steel you heat it and then quench it in water. That basic phenomenon underlies SRG's effort to design stronger steels. By developing sophisticated understanding of what happens at the level of atoms and electrons when heated steel is rapidly cooled, Olson and his colleagues are, in effect, taking quenching to its limits, and they have gained an unprecedented ability to control the properties of the resulting steel.

When steel is cooled rapidly, a particular structural transformation occurs, called a "martensitic transformation." "Instead of atoms moving around individually," explains Olson, "they all move in a coordinated mechanical way." The crystal structure spontaneously changes --technically speaking, it shifts from face-centered-cubic to body-centered-cubic. The result is a lower energy state with a fine grain structure that is more cohesive and harder.

The top diagram represents the "face-centered cubic" (fcc) crystal structure of iron at high temperature. Two cubic "unit cells" are shown (red outline) with an atom at the center of each face. The two fcc cells also contain another cell (blue outline) with a square top and rectangular sides. By imposing a uniform deformation to this structure, it transforms to a perfect cube with an atom at the center of the cell. The shift to a "body-centered cubic" (bcc) structure is illustrated by the two lower diagrams.

By learning to control this structural change --in particular, by using it to create finer spacing between the carbide particles in steel, SRG has made steels 50% stronger (meaning 50% more resistant to permanent deformation) than conventional steels of the same carbon content. SRG's biggest challenge, however, has been to improve the ability of high-strength steel to resist cracking under the stresses of a hostile environment, like the salt water that affects naval aircraft. To meet this challenge, Olson turned to quantum theory and supercomputing.

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