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Japanese Cold Forging


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OK-This is not about packing the edge-but I would like to hear some discussion on the Japanese technique of cold forging after annealing and before grinding.

 

  1. Knives are laminated soft steel/hard steel
  2. Forged almost to shape
  3. Normalized (sometimes)
  4. Heated to critical-annealed in ashes (rice straw)
  5. Considerable forging applied when cold (thinning, flattening, straightening etc.)
  6. Heat treated
  7. Ground on a water cooled wheel
  8. Polished and done.

 

  • Murray Carter calls it adding controlled stress (strain)
  • Saw blades are "tensioned" by a similar method
  • Cold rolled 1018 with lower carbon has more tensile strength than hot rolled A36 with more carbon-a function of the cold rolling
  • I watched the Yoshiharas hammer literally for hours on a blade at a black heat. I assumed it was to refine the shape.

So, what is going on here

  1. Only possible on laminated blades?
  2. Is it doing anything for the hard steel or only the soft steel?

We all know the benefits of normalizing to refine grain structure.

But what is this technique doing?

 

The Japanese consider this essential for a superior blade. ????

Edited by Danocon
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Hey... besides the possibility of work hardening the soft iron sides... if we are talking about tamahagane and there is a slag component.. what about compression of grain/voids associated with slag content??

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I would think a lot of it has to do with finish and subtle forging , too subtle for hot work.

I must admit to knowing almost nothing as to grain size reduction in iron or wrought iron other than by forging.

forging soul in to steel

 

owenbush.co.uk

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The only time I have seen this done was on a one-sided laminate like the one in the video, which had a soft iron tang and wrought iron on one side only. The hard steel grew when quenched, warping the blade like a banana, so the smith then meticulously forged the iron only at a tempering heat, say around 400 F, until the blade was once again straight. Then he forged the tang on both sides to work-harden it somewhat. One of those chisel-ground sushi knives in this case, but same idea. I found there to be a high pucker factor in watching a guy hammering a hardened, relatively cold blade in near-finished condition... :ph34r: I don't think grain reduction was happening, but I don't know for sure.

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I would think a lot of it has to do with finish and subtle forging , too subtle for hot work.

I must admit to knowing almost nothing as to grain size reduction in iron or wrought iron other than by forging.

 

This is what I thought. To get a nice finish, and to do a last bit of forging to shape/section.

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Murray Carter demonstrates this in one of his youtube videos.

 

Says its for grain refinement and to smooth the surface.

 

Well that was a good video actually. I'd like to have those hand-shears! But I don't think the cold forging in this case is needed for anything other than shaping. With modern steel I just can't see that there would be any effect on grain structure.

 

They say that 'edge packing' worked because of the grain refinement related to the slag voids in old style steels AND that it was normally done as they were coming down in heat so that they were effectively 'normalizing' without realizing it.

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I had no idea how to even broach this subject and did not know this was an actual technique. having no real technological terms to use, however I forged a very thin (thin for me anyway) 22" blade out of 5160 round bar. I left a tiny bit of work on the flat of the blade, anticipating a nose dive and wanting dead straight or slight positive upwards curve. first time I quenched the blade, it took a good nose dive. I then used a flat face hammer and started working on the flats...its one bevel from cutting edge to spine with very tiny secondary bevel. this worked the blade out of the nose dive, and to just barely positive upwards curve (sori?). this work was done with shop totally dark and I would only heat the blade until I saw a very faint purplish color coming off the blade. this process took HOURS by the way., I did this 3 times, quench, hammer the flats to straighten, and do very final finish so the blade was perfectly flat, and would work the flats until each time it had no nosedive or a very faint upwards curve to the edge....by the 3rd time quenching there was almost no movement in the blade(no nosedive) and no warping. One thing about the hammer technique, I used the rebound of the anvil and mostly my wrist choked up pretty tight on the handle for thousands of fairly direct strikes. I was going in a direction my mind had lead me from commercial welding using thousands of strikes to relieve stress in a weld, and a entire train of thought to go with it that is hard to explain. After reading this subject, tho, I think I did something entirely different..... I was wondering if maybe I had some how bainited the steel since It had been held at a lower heat for probly around 6ish hours at same heat? impossible to say since I did not know what temp it actually was, but this sounds far more like what I did.....but all I know is that blade was worth the effort. its very thin, and I have slammed it so many times into cured oak log , then cut water bottles, whatever, rope, and no edge deformation at all... it retains its edge like nothing I have made so far and is pretty flexible for what it is......I made a sister blade to it at the same time, I managed to get the first one done, but the other one is just so darn hard to hand sand down its still on the shelf ehe... anyway, interesting conversation, glad this came up.

 

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Those vids are good to see. and I like the contrast in that way of working against the common best practice (or versions there of) that seems to be engrained in modern western bladesmiths ( me included ) as an only way of working.

I think there could be a lot to transfer into bloomery blade working from this heritage.

Edited by owen bush

forging soul in to steel

 

owenbush.co.uk

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With all due respect to the Japanese knife-making industry, let's not forget that they have a product to sell, and part of selling that product will involve a certain amount of "patter".

Which is to say, steel does not have special requirements beyond those dictated by its composition just because it was produced in one country or another.

Edited by Dan P.
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Bloomery steel will behave differently than modern steel to state the obvious but still needs to be kept in perspective. I suspect this has more to do with straightening a blade that has been forged very close to finished shape where there is not much meat left for the grinder to correct imperfections. I have done this after heat treat and tempering in blades that were too stubborn to become straight. The first time I did it I thought that the blade will break as I hammered on the tempered martensite but I had already considered the blade a lost given the multiple ways in which it had twisted and warped. To my surprise it survived and so I used the process other times with less fear but still with the acceptance that it might not survive. I don't know anything about how a physical process like hammering affects tempered martensite at the microscopic/structural level. I think this is a simple and practical approach to straightening.

Enjoy life!

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Bloomery steel will behave differently than modern steel to state the obvious but still needs to be kept in perspective. I suspect this has more to do with straightening a blade that has been forged very close to finished shape where there is not much meat left for the grinder to correct imperfections. I have done this after heat treat and tempering in blades that were too stubborn to become straight. The first time I did it I thought that the blade will break as I hammered on the tempered martensite but I had already considered the blade a lost given the multiple ways in which it had twisted and warped. To my surprise it survived and so I used the process other times with less fear but still with the acceptance that it might not survive. I don't know anything about how a physical process like hammering affects tempered martensite at the microscopic/structural level. I think this is a simple and practical approach to straightening.

 

Jesus I don't think it does have an effect on the molecular structure... I think if it did anything it would just be a physical compression of voids in the steel... if it is a steel that DOES have such voids. I think historically it might have had an indirect effect on molecular structure if it was done at low heats as I mentioned above.. i.e. people normalizing without realizing it.

 

I'm glad this thread came up. I was working on a low carbon steel sword guard today.. one in which I wanted it forged as clean an close as possible.. and I spent way more time working it at black heat than usual. Very nice finish on the piece and you are less likely to knock things too far out of whack. And perhaps I improved the quality of the steel somehow? Who knows...

Edited by Scott A. Roush
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Generally cold refinement of grain structure is a larger percentage of reduction then what a blade requires.

I doubt it has any effect metallurgy to the material. Gaining from cold work has to do with dislocations of the crystalline structure and as such can do some measurable things to steel....but generally this is due to larger than 15% cold reduction of the material.

 

It would be easy enough to run lab testing to prove such, but few would be interested in the tests or the results.

 

If done before heat treating then the gains would be negated if austenized again prior to quenching. If done after it may increase strain on the blade, but doubtful to any extent to matter from a functional standpoint....quenching having far far more of an effect then minimal cold work after.

 

I would side on the cold straightening effects of cold hammering and altering the curvature of longer blades.

 

And yes...modern steels do respond differently then more traditional steels. In many ways old steels are more forgiving and much less hardenable.

 

Ric

Richard Furrer

Door County Forgeworks

Sturgeon Bay, WI

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Good question, it sure sets the gears to spinning!

 

So why does mild steel work harden? What mechanism causes the steel to stiffen up when worked cold? After browsing through a bunch of articles on work hardening, this Wiki, Strengthening mechanisms of materials seemed the most relevant. The section titled "What strengthening is" holds forth that whatever method is used, it works by prohibiting the mobility of dislocations within the material. There are 5 mechanisms of doing so listed for metals, 4 major and 1 minor.

 

1) Work Hardening: This seems to work by adding dislocations to the metal but, the increased density of dislocations often work in opposition to one another and act as pinning points to hinder dislocation motion. This method is more applicable to softer metals perhaps because harder metals require more force to move the metal sufficiently to add dislocations and it is difficult to use high amounts of force on harder metals that are inherently brittle.

 

2) Solid Solution Strengthening and Alloying: As blade smiths we are pretty familiar with this one. The atoms of alloying material cause lattice distortions that impede dislocation movement. It is important to point out here that a low-carbon tamahagane steel (i.e. no appreciable alloying elements beside carbon) would gain almost nothing from this mechanism. Unlike modern, Bessemer, steels which contain grain refining alloying elements as a matter of course. It is also important to point out that the low-carbon tamahagane would have no impediment to grain growth at austenitic and above, unlike modern steels. In short, it would grow large grains, rapidly, when heated, and if other methods were applied to control dislocations in the metal there would be benefit.

 

3) Precipitation Hardening: Sort of get this one but, not too sure how it applies to what we do. I think this is the "minor" mechanism.

 

4) Grain Boundary Strengthening: What this comes down to is that smaller grains do not allow dislocations to move as easily. There are indications that work hardening also acts upon the grain size of the material and can be used to promote formation of a smaller grain in low-carbon steels. Ultra Grain Refinement During the Simulated Thermomechanical-processing of Low Carbon Steel.

 

5) Transformation Hardening: We are all familiar with this one. Get it hot and quench it.

 

 

Going back to the original post:

 

1) Only possible on laminated blades?

Laminated blades with low-carbon steels (especially non-modern, bloomery, or non-micro alloyed steels i.e. those with no inherent mechanism to control grain growth at temperature) would benefit the most. Modern, low-carbon, steels could also benefit. This method could also be applied to promote fine grain in any object made from mild steel but, again, would benefit the "traditional" steels far more than modern ones.

 

2) Is it doing anything for the hard steel or only the soft steel?

Alloying and Transformation Hardening rule the roost for martensitic steels. Again, it requires more force to induce work hardening in these metals that are already significantly harder. Doing so with hand tooling (without breaking the piece) would be extremely difficult. One note here, non-modern steels (those with carbon as the only alloying element) could see refinement of grain from work hardening at black heats but, the same could be achieved in a less risky fashion through normalizing.

 

So, in summary: Taking warps and twists out is one reason to do this, achieving a finer surface finish is another, possible smaller grain size of low-carbon components is yet another and especially for low-carbon, bloomery, steel. In theory, this technique would be more effective if employed after hardening instead of prior but, having the hard piece break while doing so would be a major deterrent! Reducing grain size prior to hardening would still result in a finer grain after hardening, as long as the smith was careful not to overheat during hardening.

 

~Bruce~

“All work is empty save when there is love, for work is love made visible.” Kahlil Gibran

"It is easier to fight for one's principles than to live up to them." - Alfred Adler

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I would like to point out that we are not talking about bloom or tamahagane here, just normal steel welded between two layers of mild. Also, all of the arguments over cold working, aka "ausforging" I think have been done to death 10 or more years ago, if I'm not mistaken?

 

Speaking from my own "tradition" there are certainly lots of work practices that are current which are actually throw-backs to earlier times when slightly different materials were current, e.g. wrought iron to mild steel. I still hear a lot of bobbins being talked about the welding or working of mild or wrought, so there's no reason that the same should not be true in Japan; little cultural/practical eddies and currents working themselves out. I can see plenty of good reasons for straightening and finishing a piece cold, but metallurgically?

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I would like to point out that we are not talking about bloom or tamahagane here, just normal steel welded between two layers of mild. Also, all of the arguments over cold working, aka "ausforging" I think have been done to death 10 or more years ago, if I'm not mistaken?

 

Speaking from my own "tradition" there are certainly lots of work practices that are current which are actually throw-backs to earlier times when slightly different materials were current, e.g. wrought iron to mild steel. I still hear a lot of bobbins being talked about the welding or working of mild or wrought, so there's no reason that the same should not be true in Japan; little cultural/practical eddies and currents working themselves out. I can see plenty of good reasons for straightening and finishing a piece cold, but metallurgically?

 

I thought I said that! :lol:

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I would like to point out that we are not talking about bloom or tamahagane here, just normal steel welded between two layers of mild. Also, all of the arguments over cold working, aka "ausforging" I think have been done to death 10 or more years ago, if I'm not mistaken?

 

Speaking from my own "tradition" there are certainly lots of work practices that are current which are actually throw-backs to earlier times when slightly different materials were current, e.g. wrought iron to mild steel. I still hear a lot of bobbins being talked about the welding or working of mild or wrought, so there's no reason that the same should not be true in Japan; little cultural/practical eddies and currents working themselves out. I can see plenty of good reasons for straightening and finishing a piece cold, but metallurgically?

 

When I was researching the Japanese Myochin tongs I read the report of an engineer which stated that the sound qualities were attributed to the cold working of the steel in the tongs. Something about grain reduction and better sound characteristics. So yeah.. It seems like eddies such as this are firmly rooted.

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I think you might be confusing the working of blades cold before heat treatment and intentionally work hardening myochin without further heat-treatment, but not knowing how myochin are heat treated (or not), it is hard to say.

I'm not confused. :-) The tongs are quenched. And hammered cold before heat treat. And most likely afterwards to correct warp (or for whatever reason the smith things it necessary). Nevertheless.. just another example of the technique being used and so firmly rooted that a mechanical engineer used it as an explanation for the 'mystery' of these wondrous tongs.

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