#122 from R&D Innovator Volume 3, Number 10          October 1994

Damascus Steel—A Rediscovery?
by Oleg D. Sherby, Ph.D.

Dr. 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.

Damascus 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 later eras.

Twenty 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). 

Superplasticity 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 image.  Superplastic 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.

In 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. 

Superplastic Steel

That 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.

We 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.

Our 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 steels.

After six months of experimenting with various mechanical working steps on several very high-carbon steels, we achieved the desired microstructure:  ultrafine 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.”

Was This Ancient History?

This 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!” 

“Damascus steel," I responded.  "What’s that, and why do you say that?”

“Well, 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 surface markings.

Sir 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.

I 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 steel swords.

Working 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. 

In 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.

On 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.

My 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 materials.  

Applications

We 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.

Eventually, 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.

Ultrahigh 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. 

My 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.   

1-50  51-100  101-150  151-200  201-250  251-300
301-350  351-400  401-450  451-500 501-550  551-600
601-650

©2006 Winston J. Brill & Associates. All rights reserved.