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Higher Carbon Steel on the Inside...or on the Outside?


Kevin von

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I'm sorry, but I'm a complete noob to this. I've only been watching tons of Forged in Fire episodes and noticed that in blades that used jacketed steels they always put the higher carbon steel on the "inside" or towards the spine, and the lower carbon steel to the edge. But, in what I read about the Japanese katana it is the opposite. The lower carbon steel at the core, surrounded by the higher carbon steel. As it was explained in the documentary, this allows the blade to keep a very sharp edge, but still be resilient to shock.

 

Screenshot 2021-07-17 193902.png

 

Why is this reversed in the kinds of blades on Forged in Fire? Is it that katana blades are so long and thin they need that softer, tougher spine? Or, is it that the metalurgy of katana blades is much more precise, a precision for that particular combination of hardened edge and softer core? Doesn't the Forged in Fire swap make the edge harder to maintain with the lower carbon?

 

As I said, I'm new to this, so any explanation would be appreciated.

 

 

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It depends on how you grind the final blade. The photo you posted is one of the traditional constructions for a katana-type blade, where the inner billet is low carbon, and the jacket is high carbon. On those, the final shaping and grinding will still leave the high carbon on the edge, while the core will be soft. Ie, very little grinding is done to the high carbon layer, the blade is forged almost to shape.

 

Katana_cross_section_diagram_showing_dif

 

jap_sword_kobuse_forging_real.png

 

What you've probably seen on FIF is the "other" way of doing this, where you put the high carbon in the center and the low carbon on the outside. This is often done with three different layers (rather than 2 where one is folded as above). Unlike the above, quite a bit of the low carbon outer steel is ground off, such that the high carbon core is exposed on the edge:

 

6886933f7794e867dcba311676704cd3.jpg

 

The latter is what people typically call "san mai", though all that means is "three layers". To add to the confusion, the same term (or sometimes "honsanmai", "true three layers"), can refers to yet another construction:

 

jap_sword_x-section_types.png

 

 

Edited by Francis Gastellu
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