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Thread: Differential Heat Treat in a Kitchen Knife. Why?

  1. #11

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    Quote Originally Posted by GlassEye View Post
    I thought the concept was to make the cutting edge even harder than normal while still maintaining some durability to the blade as a whole.
    Well if you go back to sword making you might have an edge at around 60 HRc that quickly drops to around 40 HRc back to the spine. Not really harder than "normal."

  2. #12

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    Quote Originally Posted by DevinT View Post
    Because of the soft back, it is easier to keep straight while grinding and polishing.

    Hoss
    I don't understand that Hoss, why?

    -AJ

  3. #13
    Quote Originally Posted by ajhuff View Post
    So people will pay extra for these knives just because they value the aesthetics?

    -AJ
    It's a little more than aesthetics. I mean, it IS a method of heat treating developed for swords. Having a knife that is made the same way as a legendary traditional weapon is sort of an x-factor that goes beyond looks.

    I think most people will agree that a standard Space Shuttle is usually less visually pleasing than, say, a Ferrari. Yet which would you pay more to own?

  4. #14
    Quote Originally Posted by DevinT View Post
    Because of the soft back, it is easier to keep straight while grinding and polishing.

    Hoss
    yup... this is it. When heat treating, its almost impossible to keep the blade straight while doing the quenching and it cant be fixed without breaking the knife if its all solid hard steel. The hamon is a by-product and a sign that this process has been done well (on shallow hardening steels).

    FWIW, this info can be found in a number of japanese books on knives/sharpening.

    also, for zenkou baldes (mono-steel), they are not hand forged, so keeping it straight in the heat treatment is a lot easier. Honyaki hand forged knives require a bit more to keep them straight and tend to be harder across the board than zenkou knives.

  5. #15
    Senior Member Justin0505's Avatar
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    I have not read anything on destructive testing of Japanese knives where differential HT was compared to even temper / mono HT mono-steel, but I know that Ed Fowler did quite a bit of testing when he developed his method of HT for 52100 (Bill Burke also uses this method). The blades with his unique triple quench hamon where much, much more durable and reliable than the mono method.

    The advantage/necessity of diff HT is clear when your life depends on a blade (like a sword, "tactical", or rescue knife) not breaking. Is it overkill for the kitchen? Maybe. But couldn't you say that much of what we knuts are into is overkill? So, if a knife with a differential HT / softer spine is less likely to break than a mono HT, and if what you want in a kitchen is a functional, reliable, intact blade, isn't that then a reason / advantage?

    Yeah, most people that would be buying a honyaki today tend to own multiple knives and have backups and also don't beat on their knives anywhere hard enough make blade fracture a worry, But if that cool looking wavy line makes the difference between a repairable broken tip or chip and a cracked, dead blade when some accidental happens to the knife, then I think the $900 with fancy diff HT starts to look better than the $500 mono for more reasons than just the wavy line.

    Here a series of videos from a talk that Ed Fowler gave on diff HT:
    http://www.youtube.com/playlist?list=PLEA51A301FBB7BC3D

  6. #16

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    Quote Originally Posted by JBroida View Post
    yup... this is it. When heat treating, its almost impossible to keep the blade straight while doing the quenching and it cant be fixed without breaking the knife if its all solid hard steel. The hamon is a by-product and a sign that this process has been done well (on shallow hardening steels).

    FWIW, this info can be found in a number of japanese books on knives/sharpening.

    also, for zenkou baldes (mono-steel), they are not hand forged, so keeping it straight in the heat treatment is a lot easier. Honyaki hand forged knives require a bit more to keep them straight and tend to be harder across the board than zenkou knives.
    Thanks Jon, so it sounds like it is strictly a default manufacturing step, that's just the way it's made. They can't be made otherwise?

    Also, I'm inferring from your comment that only forged knives are differentially heat treated. Knives cut from blanks are not?


    -AJ

  7. #17
    usually not, but they can be

    with regard to the honyaki knives, it seems it is a way to allow for a harder blade (better edge retention and ability to hold acute angle at cost of brittleness) in hand forged knives.

  8. #18
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    I bought a Takagi 240 drop nose gyuto fr. Japanwoodworker 3 yrs. ago.I got this because I wanted a Honyaki blade that did not cost a fortune.It was less than half the price at EE.Had a D handle that I changed out for a Stephen octagon Ebony.It has a low spot on the rt. side,but does not affect cutting at all.

    It is not a lazor,more of a beasty gyuto,but a great workhorse knife.You can put a sharp edge on the blue steel & it holds that edge better than other blue & white carbons I have.Also better than any stainless I have used.This leads me to believe that forging & HT make a difference.

    Do special clay mixtures, repeated heating in a charcoal forge,quenching in water make for better steel in a kitchen knife?Even stamped higher quality blades have pretty good HT.Still yet my less than perfect beasty drop nose Takagi will keep an edge longer.

  9. #19
    Senior Member Crothcipt's Avatar
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    I read some were that if the blade was solid hardness at about 60ish hrc, and you drop it, it will shatter. I'm sure there is many factors that goes into that, and I am paraphrasing. But that is how I understand it.

  10. #20
    Quote Originally Posted by ajhuff View Post
    I don't understand that Hoss, why?

    -AJ
    Most steels grow after hardening and tempering brings them back some, but not completely. There is always stress being induced or relieved while grinding or polishing. The soft back allows you to straighten the blade without braking it.

    Hamons have become a type of art. If you ever get a chance to hold/use one, I think that you will stop asking what the big deal is. They have a certain organic beauty to them and at least for me, they bring out some emotion. When I hold one it makes me want to cut with it.

    If you ever make one, you will learn the amount of skill required to have it turn out just right. A good hamon changes an ordinary knife into something spectacular.
    There are some mechanical advantages to having a hard edge and a soft back, but the best reasons for having a hamon can't be explained using words.

    Love and respect

    Hoss

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