Tamahagane - Japanese sword steel

[nozh2002]

By Esav request.

All what I learn is from here and from here:

http://www.ksky.ne.jp/~sumie99/steel.html
http://www.hitachi-metals.co.jp/e/tatara/index.htm

From what I read in short it is:

Steel melted under low temperature and so impurities did not get dissolved and became part of steel composition, only Iron and Carbon in composition. There is no air pumping during melting, mix of iron sand and charcoal burn for 3 days in 30 tons mix until completely burn out for itself.

Result of this process is piece of raw steel with variable carbon content and a lot of dirt and air bubbles in it. But steel itself is chemically very pure.

This pieces got sorted by skilled craftsmen and best ones with high carbon content goes to produce swords - those one called tamahagane. Other pieces with lower carbon content used for knives, axes, shovels...

Exceptional chemical purity is what differs tamahagane even from modern production steels, it does not have Sulfur, Silicon, Phosphor and other impurities, because under low temperature in TATAR method they did not get dissolved into steel.

Being chemically clean, tamahagane still contain air bubbles and pieces of dirt inside (with all this undissolved Sulfur and Phosphor etc). Those need to be removed from steel and they do this by folding and forging - and this is only reason to fold and forge - to clean steel from bubbles and dirt. Result of this preparation is clean steel bar, which then can be used to forge sword, or two swords.

However sword forging is different production step and at that point nobody does folding and forging but instead focused on shape and heat treatment of the blade. Damascus pattern is what left from previous stage - steel preparation, getting clean steel bar (which now with modern steel manufacturing can be bought from the factory).

This is in my words what I read here:

http://www.ksky.ne.jp/~sumie99/steel.html

Thanks, Vassili.

 

[amroc]

If I'm not mistaken, the low carbon steel is also used for the core of the blade. Often times, the hard steel would be folded around the low carbon. think almost like a hot dog. Then the clay tempering makes the edge VERY hard, while the back/core remains soft. That's the beauty behind this style of steel. Hard, to soft, to protect from cracking.

 

[nozh2002]

If I'm not mistaken, the low carbon steel is also used for the core of the blade. Often times, the hard steel would be folded around the low carbon. think almost like a hot dog. Then the clay tempering makes the edge VERY hard, while the back/core remains soft. That's the beauty behind this style of steel. Hard, to soft, to protect from cracking.

Correct they may use different steel in many different ways. Us I understand they may also reduce carbon content by folding and forging more then needed for cleaning. Because every time it get heated in the forge - it lose carbon (until they use some trick to actually add more putting it into reach carbon part of the flame - heard something about this too).

Master directly urge to be careful and do not over layer steel, because it loosing carbon during this process, so more layers - less carbon, according to him. Again major point in folding and forging - clean steel not make layers.

For Japanese layered pattern may identify sword making school and period, but for blade quality they are looking to steel color depth and intensity indicating grain structure on the edge, on the hamon and on the hamon shadow. This show quality of heat treatment, not how many layers it has.

Thanks, Vassili.