from R&D Innovator Volume 3, Number 10
Sherby is professor of materials science and engineering at
Stanford University. He’s a member of the National Academy of Engineering and,
among other important awards, received the Gold Medal from the
American Society of Metals in 1985.
He co-holds seven U.S. patents and has 280 publications.
steel was a legendary steel.
With a keen edge that virtually never dulled, it was good
enough for Alexander the Great, who conquered the known world by
the age of 25, in the third century B.C. And although Alexander's use of Damascus steel may just be
myth, the steel can certainly be found in museum specimens from
years ago, I knew nothing about this legendary material. Then I
became interested in the new field of superplasticity, the
condition in which a crystalline material can be stretched 1000
percent and more (most materials, in contrast, stretch no more
than 50 to 100 percent).
occurs only at high temperature and could revolutionize the
manufacturing industry because superplastic materials can be
formed into complex shapes. They
can flow like taffy into a mold, and take on a perfect mirror
materials could thus reduce or eliminate many of the welding,
cutting, machining, and grinding steps that account for about 30
percent of the cost of making most steel structural products.
the early 1970’s, researchers knew how to make some superplastic
metallic materials, such as zinc-aluminum alloys; but no one
seemed to have considered superplastic steel.
was the challenge I embarked upon with two graduate students in
1973. In selecting to
use steel with an exceptionally high amount of carbon, we
committed a heresy by the standards of the day's metallurgical
knowledge. It was well known that adding carbon to steel increases its
hardness, and that high levels of carbon cause brittleness, so
when the steel is bent at room temperature, it shatters like
glass. This brittleness, found when the carbon content exceeds 0.8
percent, is due to a continuous thick network of iron carbide,
which forms during cooling.
decided to go against the “grain,” and reasoned that by adding
more carbon--from 1.3 to 2.1%--we could create fine grains of iron
carbide which would define the boundaries between the iron grains
and prevent them from growing at high temperature.
If the grains stay small, then we would have lots of
boundaries between iron grains, allowing plenty of sliding, and
superplasticity should be achieved.
technique was to continuously work, by rolling or forging, the
very-high-carbon steel as it cooled from 1200˚C.
The mechanical action broke the iron carbide networks as
they started forming during cooling, preventing the formation of
the thick, brittle networks normally found in ultrahigh carbon
six months of experimenting with various mechanical working steps
on several very high-carbon steels, we achieved the desired
spherical grains of iron carbide embedded in ultrafine spherical
grains of iron. These
materials were superplastic at high temperature and, equally
important, were not brittle at room temperature.
We received a patent for our procedures and compositions,
which are now called “ultrahigh carbon steel.”
This Ancient History?
initial success was followed by a series of surprises.
When I discussed our superplastic steels at a local
metallurgical society, a gentleman in the audience stood up and
said, “Say, Sherby, all I think you did is rediscover how to
make Damascus steel!”
steel," I responded. "What’s
that, and why do you say that?”
the typical composition of carbon in your steels is exactly the
same as in Damascus steel swords of ancient times," he told
me. I thanked him and
promised to investigate further. The next day, poring over my
historical metallurgy books, I discovered that Damascus steel
swords, which were used by Persian warriors, contained about the
same amount of carbon as our steels (the name reflects the fact
that western merchants first encountered the steels in Damascus,
Syria). Not only were
Damascus steel swords renowned for a keen cutting edge, which was
tough and impact-resistant, they also had incomparably beautiful
Walter Scott, in his book, The Talisman, described the volatile encounter between Saladin the
Saracen and King Richard the Lion-hearted in the 13th century.
Saladin’s sword, he said, was a scimitar, a “curved and
narrow blade which glittered not like the swords of the Franks,
but was of a dull blue color, marked with ten millions of
meandering lines.” Saladin
astounded his audience by demonstrating the blade's homicidal
sharpness by effortlessly slicing a cushion in two.
was fascinated by the story, and with my postdoctoral fellow,
Jeffrey Wadsworth, I decided to process our steel to make similar
surface markings, which are seen in museum collections of Damascus
weekends, we finally succeeded, and proposed a specific, detailed
procedure for making the surface markings.
First, the ultrahigh carbon steel bar is heated very hot to
create coarse iron grains. Second,
the bar is cooled very slowly to form a thick continuous network
of iron carbide at the boundaries of the coarse iron grains,
leaving a continuous network visible to the naked eye.
Third, the bar is heated to an intermediate temperature
(about 650 to 750˚C) and mechanically worked to partially
break the network. The
network is now no longer continuous (thus, no longer brittle at
room temperature) and remains visible as a layered structure, very
appealing to the naked eye.
1981, science editor William Sullivan of the New
York Times described our work and hailed us as the
rediscoverers of ancient Damascus steel making—a lost art
developed by skilled blacksmiths over the last two millennia which
disappeared about 300 years ago.
To say the least, we were keenly interested in this
project, and our papers were works of sweat and joy.
a personal note, although I'd seen many Damascus-steel swords in
museums, I could never find any for sale. Then, one of my students told me that several were on display
at an antique store only four blocks from home.
Needless to say, I dashed over and bought their entire
supply: two Persian
swords and two daggers. These
weapons have been a source of great pleasure.
I've displayed them at lectures and have been photographed
with them, in the popular press and on television.
life has changed dramatically as a result of my interest in
ultrahigh carbon steel. I’ve
become involved with technologists and businessmen interested in
promoting the material. I've
also made many new friends among knife-makers, blacksmiths, and
history buffs, and also with scientists interested in superplastic
tested ultrahigh carbon steels with surface markings, and found
them to be tough and sharp. Since
we decided that our ultrahigh carbon steels without surface
markings had better mechanical properties, we went on to optimize
their superplastic properties by making the grains even finer.
We received three new patents, based on the extended
studies, and others are pending.
in 1988, we helped create a consortium led by Caterpillar Inc., a
major steel user, with North Star Steel, a steel producer, and
Ladish Co., a steel manufacturer.
The consortium collaborates with Lawrence Livermore
National Laboratory, which provides the processing and
metallurgical know-how for enhancing the superplastic properties.
Caterpillar has made considerable headway in prototyping
components from superplastic steel.
However, the consortium still cannot form complex
parts with superplastic, ultrahigh carbon steel quickly enough. As a result of a disappointingly low rate of production, we
are now focusing on microstructure changes that should lead to
superior mechanical properties at room temperature.
carbon steels are logical substitutes for the “high strength
steels” (0.8 percent carbon content) now used in wires, cutting
tools, grinding balls, and rails.
Our ultrahigh carbon steels can be formed easily, but are
substantially stronger and harder at room temperature.
instinct says there’s a 50-50 chance that ultrahigh carbon
steels will become important high-production volume materials
within a decade. If
so, this will mark a remarkable reincarnation of a material that
once helped conquer the known world.