Unraveling the Mystery of Folded Steel in Japanese Katanas

[tehfrr]

From a materials science perspective, what is the point to folded steel? How does the folding process affect the properties of the metal? The place I see this extensively is in Japanese swords, such as Katanas. They will say "hand folded X times" with X usually being anywhere from 150-750, with the upper end being vastly more expensive.

I’ve taken an upper division course in basic materials science, and I can't figure out what could be accomplished by folding the steel. Id think that most of the austenite to alpha iron and Fe3C type properties can be controlled by cooling rates - resulting in the martensite, spherodite, perlite, etc. What happens during the folding process on the micro scale that makes this desirable?

 

[jawn]

"High carbon steel is very high quality steel; however folded steel is the strongest. Actually it’s not that the folded steel is a different type of steel, but how the blade is forged.

A folded steel blade is typically made from high carbon steel. The difference is that a folded steel blade is just like it says; the steel is folded over and over again until the smith believes that it is adequate.

Some people say that a good high carbon steel blade can be just as strong as a folded steel blade. The smiths for the Thaitsuki Nihonto Swords claim to have mastered a form of forging high carbon steel blades that is just as strong if not stronger than many of the folded steel blades."
http://ezinearticles.com/?Japanese-Samurai-Swords-Buying-Guide&id=10064"

I would assume it would have been done to make the blade lighter. But I'm not sure, I only know how to use them not make them.

 

[Astronuc]

The folded steel is simply a way of successively working (hot or cold) the steel, which basically should produced a fine grain microstructure. Ostensibly, one would achieve a more uniform microstructure.

 

[PerennialII]

Toughness wise microstructural anisotropy of folding type can be used to improve the respective fracture properties of steel. For example when the forging 'flow lines' are oriented parallel to primary stress trajectories and normal to expected crack paths (for example, other orientations can work as well)(sometimes hear people refer to related effects as crack arresters and dividers). Think the most quoted example is the very same samurai sword. Naturally doesn't work always and anisotropy can truly cause mechanical property anisotropy (like increase of toughness somewhere leads to a decrease elsewhere (orientation wise)) and promote some fracture modes while resistance to others is increased.

 

[FredGarvin]

Also note that in terms of properties, Japanese swords were not made of one piece of material. They were laminations to help incorporate the best characteristics of the steel at the part that needed it. The chacteristic hamon on a katana is where you can see changes in microstructure.

 

[tehfrr]

Interesting, thanks for the replies

 

[Anthony Shore]

Also might want to note that the Japanese forge folding process usually involved only 15-20 actual folds however this would result in thousands of "layers" (it's a binary thing...) The primary purpose was was to evenly distribute and silmutaneously control the amount of carbon going into the outer blade. This resulted in ensuring their were no "pockets" of impurities in the blade which would cause a weak spot which might cause the blade to break.

 

[Anthony Shore]

folded steel

"High carbon steel is very high quality steel; however folded steel is the strongest. Actually it’s not that the folded steel is a different type of steel, but how the blade is forged.

A folded steel blade is typically made from high carbon steel. The difference is that a folded steel blade is just like it says; the steel is folded over and over again until the smith believes that it is adequate.

Some people say that a good high carbon steel blade can be just as strong as a folded steel blade. The smiths for the Thaitsuki Nihonto Swords claim to have mastered a form of forging high carbon steel blades that is just as strong if not stronger than many of the folded steel blades."
http://ezinearticles.com/?Japanese-Samurai-Swords-Buying-Guide&id=10064"

I would assume it would have been done to make the blade lighter. But I'm not sure, I only know how to use them not make them.

Well...to be honest...without the carbon, it would not be steel at all as steel is the joining of Carbon and Iron. The carbon content is what makes steel "harder" than Iron however, too much carbon can make the steel brittle, too little will make the steel soft...what is the point? a brittle blade will break easily when struck, a soft blade will not keep a sharp edge. As far as the weight of the blade, the average Katana weighed in between 2.5 and 3.5 lbs which is actually heavier than it's European counterparts of the time period.

The carbon content does not denote the quality of the blade although is a factor. Folding does not make a superior blade, it's just a method that was very good for the blades that were made to do the job the Japanese designed them to do. The Vikings were working with Pattern welded steel a full 200 years prior to the Japanese folding their blades. There were dozens of techniques for forging a sword and many of them were as good if not superior to fold forging

The basic construction of the Japanese blade is a rigid, high carbon content "outer blade" wrapped around a low grade steel or high grade iron for a soft core which provides resiliency. Then, a very high carbon content edge is heat welded to the body of the blade, the blade is then sharpened and polished with a varitey of stones of successive fineness and finally the blade is burnished to prevent rust. The entire process can take as little as a few days to forge the blade and then about a week to finish off the sharpening and polishing process before the blade is mounted with it's fittings.

 

[Lost Angel]

Dear Anthony,
your "basic construction" procedure won't work technically if you fold the steel a number of times - there won't be an inner and an outer blade theoretically, since different metals would be redistributed through folding.

To all, so what are the better methods to produce a sword that cuts better, less suspect to breaking and stays sharp for longer? Since you are mentioning folding to be not the best of them.

 

[mgb_phys]

Both the Japanese and earlier Viking swords were a way to work with very poor and variable quality iron. Japan has very little natural materials and the vikings were working with lumps of bog iron.
European swords of the period are much cheaper and easier to make because they use a single piece of much better quality steel, which means perhaps less artistry but does give your army a larger number of more reliable weapons.