An example of Cola’s Flash Bainite. |
A
Detroit entrepreneur surprised engineers at Ohio State University
recently when he invented a heat-treatment that makes steel 7 percent
stronger than any steel on record – in less than 10 seconds.
In fact, the steel, now trademarked as Flash Bainite, has tested stronger and more shock-absorbing than the most common titanium alloys used by industry.
Now
the entrepreneur is working with researchers at Ohio State University
to better understand the science behind the new treatment, called flash
processing.
What they’ve discovered may hold the key to making cars and military vehicles lighter, stronger, and more fuel-efficient.
In
the current issue of the journal Materials Science and Technology, the
inventor and his Ohio State partners describe how rapidly heating and
cooling steel sheets changes the microstructure inside the alloy to make
it stronger and less brittle.
The
basic process of heat-treating steel has changed little in the modern
age, and engineer Suresh Babu is one of few researchers worldwide who
still study how to tune the properties of steel in detail. He’s an
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, headquartered at
the university.
“Steel
is what we would call a ‘mature technology.’ We’d like to think we know
most everything about it,” he said. “If someone invented a way to
strengthen the strongest steels even a few percent, that would be a big
deal. But 7 percent? That’s huge.”
Yet, when inventor Gary Cola initially approached him, Babu didn’t know what to think.
“The
process that Gary described – it shouldn’t have worked,” he said. “I
didn’t believe him. So he took my students and me to Detroit.”
Cola
showed them his proprietary lab setup at SFP Works, LLC., where rollers
carried steel sheets through flames as hot as 1100 degrees Celsius and
then into a cooling liquid bath.
Though
the typical temperature and length of time for hardening varies by
industry, most steels are heat-treated at around 900 degrees Celsius for
a few hours. Others are heated at similar temperatures for days.
Cola’s entire process took less than 10 seconds.
He
claimed that the resulting steel was 7 percent stronger than
martensitic advanced high-strength steel. [Martensitic steel is so named
because the internal microstructure is entirely composed of a crystal
form called martensite.] Cola further claimed that his steel could be
drawn – that is, thinned and lengthened – 30 percent more than
martensitic steels without losing its enhanced strength.
If
that were true, then Cola’s steel could enable carmakers to build
frames that are up to 30 percent thinner and lighter without
compromising safety. Or, it could reinforce an armored vehicle without
weighing it down.
“We
asked for a few samples to test, and it turned out that everything he
said was true,” said Ohio State graduate student Tapasvi Lolla. “Then it
was up to us to understand what was happening.”
Cola
is a self-taught metallurgist, and he wanted help from Babu and his
team to reveal the physics behind the process – to understand it in
detail so that he could find ways to adapt it and even improve it.
He
partnered with Ohio State to provide research support for Brian
Hanhold, who was an undergraduate student at the time, and Lolla, who
subsequently earned his master’s degree working out the answer.
Using
an electron microscope, they discovered that Cola’s process did indeed
form martensite microstructure inside the steel. But they also saw
another form called bainite microstructure, scattered with carbon-rich
compounds called carbides.
In
traditional, slow heat treatments, steel’s initial microstructure
always dissolves into a homogeneous phase called austenite at peak
temperature, Babu explained. But as the steel cools rapidly from this
high temperature, all of the austenite normally transforms into
martensite.
“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
pointed out that this unique microstructure boosts ductility — meaning
that the steel can crumple a great deal before breaking – making it a
potential impact-absorber for automotive applications.
Babu,
Lolla, Ohio State research scientist Boian Alexandrov, and Cola
co-authored the paper with Badri Narayanan, a doctoral student in
materials science and engineering.
Now
Hanhold is working to carry over his lessons into welding engineering,
where he hopes to solve the problem of heat-induced weakening during
welding. High-strength steel often weakens just outside the weld joint,
where the alloy has been heated and cooled. Hanhold suspects that
bringing the speed of Cola’s method to welding might minimize the damage
to adjacent areas and reduce the weakening.
If
he succeeds, his discovery will benefit industrial partners of the NSF
Center for Integrative Materials Joining Science for Energy
Applications, which formed earlier this year. Ohio State’s academic
partners on the center include Lehigh University, the University of
Wisconsin-Madison, and the Colorado School of Mines.
