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First he bends the blade to remove scale on a wood log probably with a softer hammer. Then he smooth the surface and make it straigth on the anvil. Some say he refine the grain structure by coldforging, bit if it has a lot of effect... who knows with metalurgy...

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With respect to cold forging, I have not seen any evidence that it changes or improves grain structure. Without being absolute about it, I think it's an old blacksmith myth. Subject to more information, of course. Have I qualified that enough :) ?

 

Geoff

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(...)once on a power hammer.

 

I guess maybe you are talking about this video, which came up as a suggestion after viewing the one you linked to:

 

He really lets his knives "have it", and I suppose heavy coldworking would have some effect, rather than bending it a bit hither and thither and tickling it with a hammer.

-But only if there's some truth in the myth that says that a distorted, coldworked grain offers more spawn points for new grains in heat treating. That I don't know.

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This type of discussion happened a while back, and you can read it here.

 

Any time you add stresses you aid in grain refinement. This is a metallurgical fact. Whether you are doing enough to make a measurable difference is a whole different question. Personally I would stick with thermal cycling (normalizing) being the best way to refine grain structure. It is quite effective and safe (not likely to ruin the blade), as well as easy.

 

I knew what I was looking for and couldn't readily find it on-line, so let me share some info (paraphrased) from Structure and Properties of Engineering Alloys (2nd ed.) by William F. Smith (starting on pg 59, section Cold Working).

Cold working increases the strength of iron and steel by increasing the dislocation density and by rearranging the dislocations. Dislocations are imperfections in the stack of atoms creating a cell, such as a missing atom in the grid, or an extra shoved in. We're talking billions of atoms, surely it isn't surprising that they don't all fit together neatly. Recovery is when dislocations group together to form a lower energy state within the grain, thus a sub-grain is formed. All the dislocations gathered together aren't as major as an actual grain boundary, so it is still the original grain. Upon reaching the recrystalization temperature the sub-grain will act as a nucleation site for a new grain to form, thus grain refinement.

 

Refined grain structures are great because grain boundaries inhibit dislocation movement (dislocation movement is how metals deform without breaking). Failure along grain boundaries is also slowed due to the increased bath the failure must take to fail (long windy road vs. straight highway). Sub-grain walls are kind of like grain boundaries in these regards, but not quite as effective.

 

By now I'm sure someone is wanting to ask "What temperature is the recrystalization temperature?" Well that is dependent upon the alloy and stress level. It is important to note that recovery and recrystalization are 2 separate events. Therefore it is best for grain refinement if the steel is brought up to temp relatively slowly. Meaning if you are using a salt bath and wanting the best grain refinement in one step then pre-heat your blade before plunging it into the salt. If it were me (assuming I had the guts and resources to have a high-temp salt bath, which I don't) I would plunge it right in and do several normalizing cycles. For the recrystalization, just soak right above critical for a little bit, like 1 minute or so for 10XX series steels. Again: several normalization cycles are just so easy and effective it is really rather impractical to do anything else.

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Great video's! The blades in question are a mix of soft iron and high carbon steel. When the soft iron is supporting the high carbon steel it will actually bend and take a set when cold forged. This allows the smith to straighten the blade by cold forging. When a fully high carbon blade is struck cold it deforms then returns to its previous shape and does not take a set. I know this does not explain the pre ht cold forging but does explain the benefits of cold forging after ht, especially because the warping that happens when heat treating two different steels in a laminate blade.

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