Re-tempering Japanese Blades

J

John Frankl

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Oct 16, 2001
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Hello All,

I am just wondering what all of your thoughts are on re-tempering Japanese blades, specifically kitchen knives.

I like how thin and hard my Japanese knives are (kurouchi style, @63 HRC), but they are also quite "chippy." Until now, I and many others sort of thought this was part of the package. But I am now wondering.

Many Japanese makers do little (if anything) in the way of a separate and scientific tempering process. What might 3 two-hour cycles at around 450-475F do to these blades in terms of hardness and resistance to chipping?

All thoughts appreciated.
 
That would depend on the steel used. I would recommend you try it on one blade, start at 350F 2hours x2 and see if that would do what you want. If not work your way up in 25F increments until you reach the performance you are looking for. Then use the heat you found to be best on the rest of the blades. I would do this only for kitchen blades not collector pieces.
 
My concern would be about the knives' construction -- if they're the typical Japanese full chisel grind with a thin sheet of high carbon steel as back/edge welded to an iron body, you may have some interesting adventures in warpage.

Just found a bit of actual HT info: http://www.paragoncode.com/temp/YSS_HCC_spec.pdf (remember temps are in Centigrade).

-- Dwight
 
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Thank you for the correction, Chuck. I meant to list a very low temp in order to preserve hardness. Don't know why I brainfarted and went to the high end.

As for the construction, it is high carbon steel (Hitachi white or blue) forge welded between two layers of mild steel. Also, they are V ground, not chisel ground.
 
John, if you are just talking about retempering them, not rehardening and tempering them (some people use the word "tempering" to include the whole heat treating process) than you shouldn't have anything to worry about. I would start however at 400, then give them a try, then 450, etc.
 
I don't see a problem tempering the blades as suggested. However, the other concern is the handle material and construction. If it's a take down design, you'll obviously want to remove the handles. If it's a permanent construction, you may have some unwanted effects depending on the handle construct. Also, if epoxy was used in construction (I doubt it with traditional style knives but you never know), it will break its bond at those temps.

--nathan
 
more often than not, the reason for chipping on japanese style knives is the end user, they take a different technique than western knives.
You might also want to micro bevel the edge to give it more "meat".
 
Thanks for all the replies.

I am talking about tempering, not hardening. They are plenty hard, but likely have undergone only a single, often brief, tempering.

The blades are held in the handles with friction. I will remove them before tempering.

While I agree that the hard, thin blades do require a different style of use, I also think they could be improved with a better, more thorough tempering.
 
John Frankl said:
Thanks for all the replies.

I am talking about tempering, not hardening. They are plenty hard, but likely have undergone only a single, often brief, tempering.

The blades are held in the handles with friction. I will remove them before tempering.

While I agree that the hard, thin blades do require a different style of use, I also think they could be improved with a better, more thorough tempering.
Click to expand...

no, they are made to be that hard. Some "honyaki" made blades are up around 64HRC
Here's a suggestion from the korin website:
http://korin.com/Levels
Whether you are a beginning chef or a professional chef, Korin has a knife most suitable for your needs. For beginners, we suggest knives that are easier to sharpen with steel that is not as brittle. These knives, therefore, are easier to use and maintain. For professionals, we suggest knives that are forged out of the hardest steel which hold a razor sharp edge for a longer duration than steel that is not as hard. These knives, therefore, are incredibly sharp and efficient. We will gladly match a knife to your skill level so that you get the most out of your knife, as well as enjoy the experience of using it.

that said, what steel is the blade?
 
SShepherd,

Perhaps I am not being clear enough. I am looking to temper at a temperature that will leave the blades up around 62-3, but also relieve a bit of the brittleness. That's why I am talking about 350F, as opposed to 400 or 425, which would likely bring the hardness down to 60 or under.

My point is that just because traditional Japanese blades are both hard and brittle does not mean that we cannot use a bit of early modern technology to maintain the hardness while alleviating some of the associated problems.
 
John Frankl said:
SShepherd,

Perhaps I am not being clear enough. I am looking to temper at a temperature that will leave the blades up around 62-3, but also relieve a bit of the brittleness. That's why I am talking about 350F, as opposed to 400 or 425, which would likely bring the hardness down to 60 or under.

My point is that just because traditional Japanese blades are both hard and brittle does not mean that we cannot use a bit of early modern technology to maintain the hardness while alleviating some of the associated problems.
Click to expand...

whats the steel? White? blue? super blue? stainless? it's going to matter if you want a correct temp.
 
John,
I am not trying to be argumentative, but the temperature you are quoting is too low. You should see no improvement of chipping at that temp. Start with 400F and do a two hour temper. Test the edge and if it is good, do a second one hour temper at 400F. If it still chips, go up 15-25 degrees.
Stacy
 
Hi John,
going off the hitachi chart it looks like your temp of 350 is a good place to start since you don't know exactly what steel you are dealing with. The good thing is that most of the white and blue steel suggest tempering temps of 160-240 C = 320-464 F. but it dosen't give a specific temp for a specific hardness. the link posted above is pretty much the same information.
 
It is made by Ittosai. These are inexpensive knives, but they seem to be quite well made. They are kurouchi (simple high carbon steel sandwiched between mild steel). I am guessing it is White steel, or something similar.

The people at Hida Tool in Berkeley, where I bought it, were very honest in comparing it to the many more expensive knives they sell. They told me that their founder and former owner liked these knives best for performance, but said that many people wanted something shinier, or, yikes, even stainless.

Bill, Thank you. I am at 325 right now, but will likely step it up to 350.

Stacy, I don't want a knife that will pass the ABS test. I want to keep it hard; I just don't think it has been ideally tempered.

To give you an idea of what I guess the tempering process is, have a look at the following from Murray Carter's website:

"The knives should come in around HRC 63-64 after I heat them in a pine charcoal fire, quench them in water, then temper them over the open flames of the forge. I use no modern measuring device for temperature, etc. It is all done by eye and experience (and a silent prayer never hurts, either).

I can only guess as to the HRC of each blade as they are all heat treated one at a time, by hand, without the aid of any modern devices."

Don't get me wrong. Murray's knives are great. I bought one for my brother many years ago, and he loves it. But as good as his eye may be, this is hardly an ideal tempering process if we apply what we know about sufficient time and accurate, controllable temperature, multiple tempering cycles due to retained austenite and new untempered martensite, etc.
 
What? I guess you don't hold with the olde tyme "by guess and by gum" techniques, huh? :p I've got a couple of Murray's knives myself - and yes, he does seem to know what he's doing. But since it's something you actually need a lot of experience and skill to master, I'd be rather leery of the tempering quality of the lower end traditional Japanese knives myself.
 
Thank you, zerogee. But I think even the methods of many masters could be improved. I am pretty sure the people making the Ittosai knives know what they are doing, and are using very similar methods to what I posted from Murray above.

What I am talking about (and have now done) is adding a degree of time and temperature control that is wholly absent from the traditional Japanese process.

From "Steel Heat Treatment: Metallurgy and Technologies" George E. Totten, ed., 2007:

"At tempering, the assigned level of properties is achieved by changing the structure of the quenched steel. Heating a quenched steel during tempering to 150–2008C (300–390F) causes the precipitation of small e-carbide plates from martensite and reduces the carbon concentration. Such tempering only slightly impairs the steel’s hardness but significantly improves its strength and viscosity." (p. 219).

There you have it. I am trying to keep the characteristic high hardness of many Japanese kitchen blades. But I also believe that a bit of science can be added. High hardness does not have to mean high brittleness.

I also find this whole thing interesting as I think about the ABS. I can make and have made several blades that pass the ABS tests. And I think they are good all purpose blades. But I think the real value of the test, or the process of learning that allows one to pass it, is to be able to build a certain knife to a certain task. In this case, the goal is not 2x4s and rope, but the same knowledge allows me to use different heat treating processes and temperatures, different edge thicknesses and configurations, etc. in order to come up with the right blade for my purposes.
 
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