The Japanese katana, or "Samurai sword," actually only one of two blades carried by the Samurai but the one that everyone in most Western cultures associates with them, was made by case-hardening a steel bar, beating it flat, folding it, beating it flat, and repeating this process until the high-carbon layer on the outside (created by the case-hardening process) had been layered many thousands of times with the low-carbon writhen iron in the middle. Most swords were folded twelve times or less, and twenty is theoretically sufficient to homogenize the steel to a level of less than molecular width.
Thisis not really accurate; and needs several corrections.
1) The Japanese samurai carried a number of different weapons. The katana is the most well-known, and most common. Swords of different lengths and styles were also carried: the tachi or no-dachi, which was a much longer sword; the chakuto, which was a straight-bladed sword; and the wakizashi, which was a shorter version of the katana. The "two sword" style was a later development, and many samurai carried only one. Samurai also typically carried a polearm-type weapon known as a "naginata"; which had a sword-like blade very similar to the katana in style and construction, and which was their primary weapon of battle (the sword being used more in close-quarters and often ritualised single-combat conflicts).
2) Japanese swords were typically made with at least two different types of steel (different alloys containing different concentrations of carbon and other trace contaminants), and some times as many as four, sandwiched and welded together. Some of these steels were folded in specific patterns, as mentioned, but others were not; and the folding technique was not applied to the entire blade the way it was with Toledo swords. The strength and characteristics of the sword came from both the use of multiple steels; as well as a highly refined tempering process that controlled the martensite/austenite transformation very precisely; and is generally considered to be the epitome of sword-making technique. It is, however, far slower and more labour-intensive than more common European techniques; and is more suited to an iron-poor culture seeking to maximize quality without the concern for large-scale production.
Case-hardening was not involved in the process; and is generally undesirable in a sword steel, as it results in brittleness.
A few other points of interest.
According to a recent study published in the Journal of Metallurgy, true Damascus steel is a variant of the super-high-carbon wootz steel:
http://people.howstuffworks.com/fra...rg/pubs/journals/JOM/9809/Verhoeven-9809.html According to the article, the reason Damascus steel was so hard to reproduce is that it requires the use of a specific type of steel alloy with a high vanadium content. Since the technology to create such alloys did not exist, swordmakers were dependent upon ores which contained the necessary mineral impurities; and when these were exhausted, the technique was no longer effective, even when similar tempering patterns were used. Lower quality Damascus blades were also produced with ores that contained high levels of certain other, similar mineral impurities.
The Toledo (popularly and incorrectly known as "damascus" steel, because of the similarity in appearance) process is a pattern-welding technique; and generally involved high-carbon steel layered with wrought iron more often than with low-carbon steel (as well as a less precise tempering process). The "twisted" pattern was only one of several that were commonly found, each smith having his own signature pattern. The problem with this process is that it was very easy to introduce small, hidden flaws during the folding and welding, and so took a very skilled hand and slow, labour-intensive forging to produce a blade that didn't run the risk of breaking. Again, not a technique suited to the large-scale production typically needed for European conflicts.
