Grain reduction in Japanese laminated blades

T

Tenebr0s

Joined
Jun 3, 2012
Messages
397
Hi, first post here.

For the past six months I've been learning and practicing Japanese knife smithing here in Kyoto. I'm fortunate to have a couple smiths teaching me, and I've set up a small forge and workshop so I can practice on my own, but I'm a total beginner!

I've literally spent my entire Saturday going nuts reading here and other forums on the subject of grain reduction. Most western smiths seem to be employing some kind of thermocycling to get their grain size down. One ABS master smith whose workshop I attended last summer had us oil-quench every few heats during the forging process. I also see a lot of people doing normalizing cycles prior to quench, or triple quenching, etc.

So what boggles my mind is that none of the Japanese smiths seem to be doing this. :confused: Since almost all Japanese blades are forge-welded, I've got to imagine there's some grain growth going on at those high temps. But the basic process here is: 1.forge weld the billet 2. forge to shape 3. anneal over night in a bucket of ashes 4. rough grind, 5. cold forge, 6. heat and quench clay-coated blade one time only 7. temper 8. finishing, sharpening, etc. So what is reducing the grain size? I asked one of my teachers about whether it's ok to heat and quench several times, but he said he didn't recommend it. I don't know if that's because the yaki-ire process is particularly harsh and would build up too much stress to repeat? Or maybe it's the kind of steel (the Yasuki steels I'm using are over 1.2 % carbon). At any rate, these guys do make great blades; I'm just not sure what I'm missing.

I just read one of the prior threads on cold forging (http://www.bladeforums.com/forums/showthread.php/986418-Cold-Forging), and am wondering whether this could be what's reducing the grain size? Before you guys jump on me about edge-packing, holy water, and other myths, that's totally not what I'm saying here! :)

But the gist of that last thread left me confused. It seemed to go from "cold forging is totally worthless in blades and all effects are erased during HT," to: cold forging prior to HT can in fact reduce final grain size, but it runs the risk of cracking/over-stressing the blade, and modern thermocycling methods (with salt pots, etc) are more reliable. So I'm just interested in hearing people's thoughts, and perhaps some advice on how I can keep my grain size down... in the privacy of my own shop (I do want my teachers to keep teaching me!). Also, if cold forging prior to HT can in fact reduce the final grain size, I would be interested in learning it, whereas until now I've basically never asked to be shown it, since I didn't believe it was backed by any science.

btw, I have a very simple coke-burning forge. No fancy salt pots or anything, although I did recently invest in some tempilstiks, which I'm hoping can help me keep more consistent quenching (and tempering) temperatures.

Thanks for taking the time to read this,

Alexander
 
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One of the last sentences .." if practiced cold forging ...." I assume you mean cold forging + HT , rather than just cold forging . A more complete understanding might be found if you read some metallurgy texts about ''recrystallization and grain growth''.
 
mete,

Thank you for your reply. Yes, I meant cold forging followed by heat treat, which is what the bladesmiths here are doing. I'm wondering if the cold forging is giving them a smaller grain size after HT than they'd have without cold forging. Otherwise, I'm not sure how they're keeping grain size down, unless annealing in ashes is actually closer to normalizing than true annealing.
 
The reason that Japanese smiths get consistent results from their procedures is that the procedures do not vary....ever. They learn one task till it is perfected to the degree it can be done in their sleep. Then they learn the next task. This makes the procedures produce a very repeatable and reliable product. This years yanagi-ba will be just like last years, and next years.

The steel used is also very reliable, as the sources and method of manufacture are very well regulated. The steels purity and lack of unwanted alloy ingredients is also quite different from western steels. The forging procedures, with reducing heats and cold forging also help with the grain reduction.
Is it different from western metallurgy -YES.
Do both produce a good knife - YES.
Will either one be accepted as the be all-end all....NO.
Is one "better" that the other - NO. ( they are just different)

Their methods may not produce the same blade as a western metallurgical procedure will, but their culture and customers are happy with the product they produce.

As mete suggested, doing some serious study on the metallurgy of grain growth, reduction, re-crystallization, and the associated forces that drive these things will help you understand what different procedures will do to change the final grain size.

Now as to whether you should do a thermal cycle on your blades before the final quench...only you will know what the difference could be. Study the thermal cycling methods posted as standard advice and try it on a test blade you have done all the standard Japanese production procedures on. Make an identical blade at the same time with all the normal procedures as the sensei has taught you. After Heat Treatment, test them to see if there is any perceptible difference. Be sure to mark the blades to tell them apart, as they will look identical from the outside. Put an edge on both and do some cutting tests. Chop on some wooden doweling or a 2X4 They probably will cut the same. Slice some rice paper. One may be chippier on the edge, buit I suspect they will both perform well. Break the blades in the middle and carefully inspect the break with good magnification to see if one has a finer grain that the other. This testing will tell you if there is a difference and if it is significant enough to worry about.

My last advice is to study under your sensei and learn all he can teach you. His job is to pass on the craft as he was taught it,....and someday it may be your task, too. Whether you vary that knowledge by adding some metallurgical tweak is up to you, but fully understanding the process from his viewpoint is very important.
 
Bladsmth,

Thanks so much for the reply. I will definitely do some testing and see what I can come up with. With my very basic forging setup, is there a simple thermal cycling I can do that will reduce grain size? A lot of the normalizing cycles I've seen posted seem to require salt pots and more sophisticated machinery. For example, if I can't do progressively lower temp normalizations, will just normalizing several times at the same temp have reasonable results? Or would I be better off doing something like oil-quenching several times during the forging process?

Thanks again,

Alexander
 
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