Cutting curves in samurai swords.

Cutting curves in Samurai swords

The graceful and fearsome swords of Samurai warriors embody metallurgical genius. Even as far back as the 8th century, Japanese swordsmiths were transforming straight steel strips into lightweight weapons with an unprecedented hardness and toughness that helped elevate the Samurai to legendary status.

Researchers have known for years that these swordsmiths forged two types of steel into blades with interiors, sides and edges respectively containing increasing amounts of carbon, most often picked up from charcoal burning in the forge. Although the high-carbon steel in the edges accounts for some of the swords' superior qualities, research metallurgist William N. Weins of the University of Nebraska in Lincoln suggests that the method of quenching, or cooling, the metal played an equally important role in toughening the edge while automatically producing the blade's graceful arch.

The swordmakers surrounded all but the cutting edge of a partly worked blade with clay. This allowed a heated edge to heat and coal faster than other parts of the blade. During quenching, the crystalline structure of steel undergoes a transformation that results in expansion. For example, Weins calculaes that the exposed edge of an otherwise clay-encased blade spanning 3 feet would expand 3/4 of an inch more than the blade's slower-cooling back. Not only could this differential expansion account for the famous sweeping curve of samurai swords, but it would also leave the finished edge under huge, permanent compressive forces.

"The atoms in the edges are always being pushed together," Weins says. His measurements of strain in sections of 16th- and 17th-century Samurai swords support this picture. Without the compression, the edges would still have become extremely hard but would be so brittle and ridden with microcracks that the blades "would have snapped just like a piece of glass the first time anyone tried to use them," Weins surmises.