New work improves stainless steel surface

Manufacturers polish, grind, and roll stainless steel to create countless everyday products. Such manipulations can produce, for example, an especially hard material for sharp edges on surgical tools and razor blades. Yet the resulting metal can be more brittle and prone to corrosion than other forms of stainless steel are.

Now, an electrochemical technique seems to improve a stainless steel surface without degrading the rest of the material. The new process is so simple, says Tim Burstein of the University of Cambridge in England, that “you could just about carry it out in a schoolroom.”

Burstein and his colleagues ground and polished 1-centimeter squares made of a steel microstructure called austenite. As expected, X-ray diffraction revealed that their alterations had introduced another crystal microstructure, the brittle martensite, on the squares’ surface.

Then, Burstein’s team placed the steel in a conductive bath of sodium nitrite and water at 80°Celsius. The scientists slowly pulsed electricity back and forth through the solution, alternating between two voltages. After 3 hours, new X-ray observations showed that the martensite had disappeared.

Yet the desirable hardness remained, the team reports in the Oct. 19 Nature. Tests showed a surface even harder than that of the original polished steel square.

For comparison, Burstein’s group also used the conventional method of removing martensite. Treating stainless steel with very high temperatures can eliminate that crystal form, but there’s a tradeoff. The heat can also alter the underlying metal’s desirable properties.

The team showed that heating polished steel samples to 750°C eliminates martensite but simultaneously decreases surface hardness.

The Cambridge team produced their hardest surfaces yet by following an electrochemical treatment with a 550°C treatment. Burstein says any eventual use of the new process will likely include both electrochemistry and heat.

Martensite also decreased during electrochemical processing of cold-rolled steel. Rolling had created the hard, brittle form of stainless steel throughout the samples and not just at the surface. Burstein reports that the electrochemical treatment eliminated the brittle structure even below the topmost atomic layers and reached as deep as 8 micrometers into the metal.

Burstein notes that the mechanisms underlying the electrochemical technique remain unclear. Once researchers understand more of this chemistry, they may be able to use it to harden the surfaces of other metals, he adds.

“It’s not a slam dunk yet,” comments Robert G. Kelly of the University of Virginia in Charlottesville. Still, if further research reveals enough detail about how this new process works, it could have many applications.

Consider razor blades. “You have the martensite at the cutting edge of the blade,” Kelly says. “One of the reasons blades go dull after shaving is that they stay wet, and so they corrode. If you can remove the martensite and therefore slow the corrosion down, those of us who shave…would be much happier.”

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