Sure thing, in simple terms you are heating steel of different composition to a temperature hot enough to allow them to fuse (weld) together under pressure.
Example:
Here a stack of two different high carbon steels 1084 and 15n20
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The layers are stacked together and then heated to a welding temperature....
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Note that when welding the surfaces each piece of steel needs to be protected from oxidation because that prevents the steel from welding. The most common way is the use of flux (aka borax), it turns into a corrosive fluid when heated and coats the surface, which prevents oxygen from getting to the surface of the steel and it also "melts" any oxidation that is there....
On alternate to flux is sealing it from the open air, ie welding the pieces together and this keeps the surface from seeing any additional oxygen other than what is in the little space that is between the layers. This oxygen isn't enough to keep if from welding.... IE the billet (stack of steel) above was welded all around like this.
the third way is to hold the stack together and soak it in kerosene and then put it in a oxygen deficient forge, ie this can be risky but many smiths use it very successfully, ie my forge is oxygen deficient, ie you can actually get a slight "propane" smell of un-burned propane being pushed out of the chamber by what does burn.
Now, once the stacked steel is hot enough to weld it is put under pressure to facilitate the welding. The pressure can be a hammer or a press of some kind (see the flat plates on the press below)
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The billet is worked, ie "drawn out" with rounded plates on the press, anvil horn and hammer, or a rounded hammer face. Once it is all drawn out it is shapped, cut, or ground to shape and then the knife bevels are ground in. The steel is then "etched" in acid and lightly sanded to reveal the individual layers from the billet. Note that the different steel composition causes a different color "etch" from the acid.
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Note that the darker lines are the 1084 steel and the shinier are the 15n20 steel. The shininess is cause by a higher nickel content in the 15n20 composition the brighter lines are the edges are cause by the acid raising up the edge so when sanded (2000 grit) it polishes the 15n20 edge more.
In the can above rather than using "plate" steel, pieces (ball bears, fish hooks, etc.) are placed in a non-hardenable (mild or low carbon steel) steel can, along with powdered steel to fill in all the gaps. The can keeps it all together, prevents oxygen to allow a weld to happen inside the can. Once the can is drawn out the mild steel is ground off revealing the pattern the parts a pieces created. It should be noted that a medium (usually something with Titanium oxide, ie without, or white paint) can be used to line the can which will prevent the can from welding to the contents inside, this allows the can to be peeled off from the high carbon steel and then you work on the billet from there. Its just another variation of how to keep the materials from oxidizing and separated from the mild steel.
Note that in the can above, there are 3 pieces of plate steel in the middle, ie a 1084 core with 15n20 on either side, this should create a dark cutting edge with a shiny wavy line/band above the edge. Next the bearings which are 52100 steel and the fish hooks which are 1084a steel. The 1084 steel with 4% nickel is basically 15n20 steel in a powered form. However, the composition of them all is different enough that when etched it will create a random pattern of darks and lights, ie creates a unique piece that can never be duplicated again.
jcam222
Here is an example of just 3 layers (san mai), 1084 core with 15n20 outer.
