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I need to describe the manufacturing of japanese knives. I struggle with the step when the steel blade is put together with the wooden handle. Here are some pictures from the video https://www.youtube.com/watch?v=j2L_Ku47afo (at 6'42"):

Merging between warm steel and wood

After putting the hot blade inside the handle, you can see that the steel immediately cools down and after hitting the handle with a hammer, the two pieces are merged. I've been told that the welding could not function between two different materials, especially not wood. Could you please explain me how those two materials merge together in this situation please ?

Thanks in advance for your answers !

EDIT:

  1. Sorry if this doesn't seem directly related to chemistry, I was more interested of what happened in the molecular scale than the physic reactions, if you think about a better forum to ask this do not hesitate to tell it to me;
  2. For those who can't see the picture or the video, the blacksmith has made a very long, thin end to the steel blade which he has warmed up until it becomes orange and then he puts it inside the wooden handle (which has a drilled hole). After he did this you can see smoke going out the handle, and finally the blacksmith hits the handle with a hammer (in the direction of the length).
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    $\begingroup$ It looks to me that wood, heated up by steel, undergoes in various extend plastic mechanical deformation and thermal decomposition, releasing space for the steel object. $\endgroup$
    – Poutnik
    Apr 9, 2022 at 9:46
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    $\begingroup$ There is already a drilled hole in the wood when the steel is inserted in it, how do you think those reactions could prevent the blade from getting out this hole ? And why the hammer ? $\endgroup$
    – Emile Couzin
    Apr 9, 2022 at 10:01
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    $\begingroup$ Plasticity goes either way, together with possibility of making wood to stick to steel. $\endgroup$
    – Poutnik
    Apr 9, 2022 at 10:40
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    $\begingroup$ I think it is a "press fit" , mechanical. The pilot hole is smaller than the tang. The hot tang burns a hole that is just the right size. $\endgroup$
    – blacksmith37
    Apr 9, 2022 at 16:13
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    $\begingroup$ As I mention in my comment to blacksmith37's answer, below, there is possbility that the heat creates some melting in the wood, and this causes a bonding to the metal. We know that wood contains lignins and other components that, under the right conditions, can melt to form bonds to itself (see youtube.com/watch?v=X0k04hjdYuQ and link.springer.com/article/10.1007/s00226-006-0097-2). So I'm wondering if the wood might also be able to melt slightly to bond to the metal. $\endgroup$
    – theorist
    Apr 9, 2022 at 22:09

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A common press fit , no chemistry involved. Most commonly used for files (in US). All my files (old) use this same press fit, I believe newer files have plastic handles. The use of heat to custom shape the hole for the tang is a good modification. A smaller pilot hole can be used because it will be enlarged to the right size by burning. This likely gives a tighter fit compared to a larger round hole

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    $\begingroup$ What about the possbility that there is welding involved? Wood contains lignins and other components that, under the right conditions, can melt to form bonds to itself (see youtube.com/watch?v=X0k04hjdYuQ and link.springer.com/article/10.1007/s00226-006-0097-2). So I'm wondering if the wood might also be able to melt slightly to bond to the metal. $\endgroup$
    – theorist
    Apr 9, 2022 at 22:10

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