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View Full Version : Differential Heat Treat in a Kitchen Knife. Why?



ajhuff
06-11-2012, 02:44 PM
I want to break this out of the thread I started on Hamons.

What advantage is there to differentially heat treating a kitchen knife? I like Son's answer:


Remember most of these guys came from a long line of sword makers, when the Samurai were banned there were thousands of unemployed sword makers and nothing for them to do with there skills. Kitchen knives were made by the village blacksmith and were rudimentary at best. Making farming implements and kitchen knives was looked at as beneath them, consequently they began to starve. Someone swallowed his pride and set up shop and started making knives using the techniques he used to make swords, because that is the only way he knew how to do it. He made better kitchen knives then anyone else and people started to come from all over to get the knives with the Hamon, because that was a sign of quality, That's how you knew you got the real deal and the knife was made properly. I mean didn't this guy make swords for the Shogun or something, he must make the best knives. I think it probably started like that and then stayed on as a traditional sign of quality more than anything else. It is done that way, because it has been done that way for 17 generations or more. Not so much a trick, but marketing, it serves a real purpose.

But that's not really a functional advantage. I agree that it is cool that a smith can do it and it tells how good they are at their technique but how does that make that knife better than a non-differentially heat treated knife?

It made sense for swords that needed the ductility in the spine to survive impact and flex, but when in the kitchen do you need that? Spike suggested:

How about breaking down a big ass tuna?
But I've never broken down a big ass tuna so I don't know the application.

Thanks!

-AJ

Eamon Burke
06-11-2012, 02:47 PM
I've never noticed a difference in any aspect of use between a differential heat treat and a regular monosteel knife.

It looks different, but it performs exactly the same way.

sachem allison
06-11-2012, 03:11 PM
I want to break this out of the thread I started on Hamons.

What advantage is there to differentially heat treating a kitchen knife? I like Son's answer:



But that's not really a functional advantage. I agree that it is cool that a smith can do it and it tells how good they are at their technique but how does that make that knife better than a non-differentially heat treated knife?

It made sense for swords that needed the ductility in the spine to survive impact and flex, but when in the kitchen do you need that? Spike suggested:

But I've never broken down a big ass tuna so I don't know the application.

Thanks!

-AJ

I really don't think it really has anything these day's to do with function, it is done that way, because that's the way it's always been done. Traditions are incredibly hard to break in a society based on tradition.

ajhuff
06-11-2012, 03:22 PM
So people will pay extra for these knives just because they value the aesthetics?

-AJ

Andrew H
06-11-2012, 03:22 PM
So people will pay extra for these knives just because they value the aesthetics?

-AJ

Shocker, huh?

ajhuff
06-11-2012, 03:26 PM
Shocker, huh?

In some respect, yes. I guess I am so much or a function guy rather than form.

-AJ

Andrew H
06-11-2012, 03:29 PM
Just look at all the damascus and custom handles around here, of course aesthetics are important to some people.

DevinT
06-11-2012, 03:38 PM
Because of the soft back, it is easier to keep straight while grinding and polishing.

Hoss

sachem allison
06-11-2012, 03:41 PM
Because of the soft back, it is easier to keep straight while grinding and polishing.

Hoss

now we have an answer

GlassEye
06-11-2012, 03:42 PM
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.

Edit: Just saw Devin's post, sounds like a good reason. Will the blade tend to stay straighter over time due to the less hard area, as well?

ajhuff
06-11-2012, 03:47 PM
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."

ajhuff
06-11-2012, 03:48 PM
Because of the soft back, it is easier to keep straight while grinding and polishing.

Hoss

I don't understand that Hoss, why?

-AJ

Eamon Burke
06-11-2012, 03:48 PM
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?

JBroida
06-11-2012, 03:53 PM
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.

Justin0505
06-11-2012, 04:01 PM
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

ajhuff
06-11-2012, 04:14 PM
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

JBroida
06-11-2012, 04:29 PM
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.

keithsaltydog
06-11-2012, 06:25 PM
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.

Crothcipt
06-11-2012, 06:43 PM
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.

DevinT
06-11-2012, 06:59 PM
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

ajhuff
06-11-2012, 08:09 PM
Thanks for the replies. I'll stop asking for more information now.

-AJ

WillC
06-11-2012, 08:33 PM
I would love to learn the art of hamon, but even without all that clay, I relieve all mine with the edge clamped between two plates, then gas torch the tang/spine and take it back to around 50 hrc on the spine. No-one told me to do this, It came about through necessity of taking out any gentle warps after ht and temper and as a safety measure Against ending up getting a blade back in several pieces. I also like to mix my steels up and use a cladding dammy mix which is softer at a given temper than the core, so it already has a differential temper. Nothing new there, it comes from sword construction, although they would be composite constructions in European swords anyway.

NO ChoP!
06-11-2012, 10:37 PM
I've said it before; people who know watches know a Rolex from a mile away.... A knife knut can spot a honyaki from a mile away. It's a status symbol, above all; something we (I) covet....

a.lber.to
06-12-2012, 04:09 PM
Not to highjack the thread, but IMOHO, in the watch world a Rolex is pretty much the equivalent of a Shun knife - a first step one could take into serious watches. But to continue your analogy, the real connaisseurs can spot a mile away watches like a Greubel-Forsay (think Bob Kramer), a Kari Voutilainen (maybe a beautiful Devin Thomas damascus), a Philippe Dufour (think Takeda) or an F.P.Journe (perhaps a Hattori KD)... :D