Having been around for a few centuries, steel is what is known as a "mature technology" and the basic process of heat-treating has remained largely unchanged in the modern age. So when self-taught metallurgist Gary Cola approached engineers at Ohio State University claiming to have found a way to increase the strength of steel by seven percent, they were justifiably skeptical. However, after the engineers tested steel produced using the new method, Cole's claims were borne out and the engineers set about understanding what was happening.
Although the temperature and length of time for hardening steel will vary by industry, with some treatments taking days, most steels are heat-treated at around 900 °C (1,652 °F) for a few hours. Cola's process, which is run at his proprietary lab setup at SFP Works, LLC., in Detroit, involves steel sheets being carried by rollers through flames as hot as 1,100 degrees Celsius (2,012 °F), before they are deposited into a cooling liquid bath. Cola's entire process takes less than 10 seconds.
Cola said his steel, which he has trademarked as Flash Bainite, is seven percent stronger than martensitic advanced high-strength steel. Additionally, Cola claimed his steel could be thinned and lengthened 30 percent more than martensitic steels without losing its enhanced strength.
After testing a few samples and discovering that everything that Cola said was true, Ohio State University researchers set about revealing the physics behind Cola's process.
Brian Hanhold, who was an undergraduate student at the time, and Tapasvi Lolla, who subsequently earned his master's degree working out the answer, found that although Cola's process formed a martensite microstructure inside the steel, it also formed a bainite microstructure, scattered with carbon-rich carbide compounds.
In traditional, slow heat treatments, steel's initial microstructure always dissolves into a homogeneous phase called austenite at peak temperature, explains associate professor of materials science and engineering at Ohio State and Director of the National Science Foundation (NSF) Center for Integrative Materials Joining for Energy Applications, Suresh Babu.
"We think that, because this new process is so fast with rapid heating and cooling, the carbides don't get a chance to dissolve completely within austenite at high temperature, so they remain in the steel and make this unique microstructure containing bainite, martensite and carbides," Babu said.
Lolla says this unique microstructure boosts the metal's ductility, meaning it can crumple a lot more before breaking. This, coupled with the steel's ability to be thinned and lengthened without losing its strength, make it an ideal process for the automotive industry, say the researchers. Carmakers would be able to build frames that are up to 30 percent thinner and lighter without compromising safety, with the steel also acting as an impact-absorber. Alternatively, the steel could be used to reinforce an armored vehicle without weighing it down.
Cola also says his process is also environmentally friendly as it consumes less energy per kilogram of steel processed compared to traditional methods and uses water instead of oils or molten salt.
Babu, Lolla, Ohio State research scientist Boian Alexandrov, and Cola co-authored the paper detailing the process with Badri Narayanan, a doctoral student in materials science and engineering. The paper appears in the current issue of the journal Materials Science and Technology.