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To the original topic, there are several properties that are often confused in knife production- toughness, strength and ductility. Strength is resistance to deformation (not bending or soft), toughness is most often referred to as the ability to withstand sudden loading, such as impact, without failure, ductility is the ability of steel to bend or deform easily without brittle type failure and is somewhat the opposite of strength. A fully hardened blade will have the most strength, an edge quenched or clay quenched (differentially hardened in either case) will deal with marginal edge strength and mostly ductility, while a differentially tempered blade, or a blade of certain alloying will deal with toughness.
I must admit that I have a hard time with the idea that 800 year old swords can rival what we can do now, as this is mostly only in Hollywood as opposed to the real world. I regularly work with both modern steels and ancient steels (quite similar to those from a tatara) and much of the heat treat developed for swords both in the east and west was due to the severe limitations of simple iron/carbon alloys compared to modern steels. I am delighted to see one of my best friends in the whole blade making world, Rick Barrett, cited in the conversation, however, since Rick has one of the most down to earth and realistic approaches to the Japanese sword genre that I have encountered, and because of this I believe he is producing some of the best Japanese style blades here in the states.
The whole topic gets VERY complicated if one is not careful to keep the three properties I mentioned at the beginning of this post in perspective. How much force it takes to flex a blade is of course nothing more than a matter of its thickness or cross section regardless of heat treatment, but how much force it will take to bend it depends on the method of heat treatment. Where the bend happens, at 75 foot pounds or 500 foot pounds depends on how you heat treat. The differential hardening will only take the lower end while the differential tempering will take the higher end, one will bend and stretch quite a ways with no added force while one will reach its ultimate strength (snap!) shortly after bending under more force than one mans arm can produce.
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