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Everybody has heard about modern replicas of Japanese swords being "folded" or "forge folded" in imitation of the folding techniques used in making traditional Japanese blades.  But frequently people don't really have a clue what this means.

As a practical matter, it means that a blade has a whole lot of layers of steel, leaving a visible pattern on the blade  But how is it done?

The answer...forge welding.

So what is forge welding?  Everybody is familiar with the sort of welding that's done with a torch or an arc welder.  But the kind of welding used in making swords is a totally different animal.  It's called forge welding.

Forge welding is a process by which steel is heated in a forge, then hammered or squeezed together.  At which point the steel bonds and what once was a number of individual pieces of steel is now one solid piece.

If different kinds of steel are used, the original pieces of steel will be visibly differentiable after they are shaped into a knife.  When sufficient number of layers have been forged, the surface of the blade will appear to have a "grain."

A typical "forge-folded" blade has about a thousand layers of steel. (Note: this article doesn't seek to explain traditional Japanese forge-welding techniques.  Traditional Japanese blades are made from a raw form of steel known as tamahagane.  This steel is refined through a forge-welding process.  Once it's been refined to a certain point, it's welded more or less the same way that I weld modern steel.  But it should be clearly understood that I'm not describing the traditional Japanese steel-making process here. I will write another article later, in which I describe strictly traditional Japanese welding techniques.)

So.  Here's how I make high layer forge-folded steel in my shop.

I begin by taking two long bars of 1/8" by 1" high carbon steel, each with slightly different carbon levels.  For Japanese-style blades I'll often use 1050 and 1095 steel.  I then grind all mill scale off the steel, so the clean, bare steel is revealed.  Then I cut the long bars into between ten and twenty pieces of steel, each of which is six or seven inches long.  I then stack them on a 1/4" thick bar of 1095, alternating steel types as I go.

Next, I use a MIG welder to run a small weld bead up each corner of the stack.  I don't want to weld too much or I'll contaminate the billet.  But if I don't weld enough, the billet will expand and break the welds during the initial heat.

After the weld is made, I insert the steel into my forge.  My forge is a cylindrical propane forge, capable of reaching temperatures in the neighborhood of 2300 to 2400 degrees Fahrenheit.  I want the atmosphere to be relatively neutral -- that is, not too heavy on fuel and definitely not too heavy on oxygen.  Too much oxygen, in particular, is fatal to forge welding because it causes iron oxide scale (Fe2O3) to form on the surface.  And scale won't weld.

Once the billet begins to heat up, I pull it out and coat it with powdered anhydrous borax, a fluxing agent, which both protects the surface of the steel from oxygen and dissolves the scale, allowing it to run off the surface of the steel.

In this photo, the billet is pretty hot...but not there yet.  The little bubbles of flux are still sticky looking.  At welding temperature, they'll be much wetter looking, dancing on the surface of the steel.

I continue to heat the steel until it reaches a temperature around 2400 degrees.  At that point it is a bright yellow, verging on white.  The borax flux begins to boil on the surface of the steel.  I then pull the billet out of the forge and quickly squash it in my hydraulic forge press.

A hydraulic forge press, for those who don't know, is a machine for squashing things.  Hard.  My press puts about 50,000 lbs of mojo on the work.  The way my dies are designed, this yields a pressure of roughly 25,000 lbs per square inch on the billet.

I don't smash the billet too hard on the first weld.  Instead, I just give it a quick bump, feed the billet in a little further, bump it again, then a little further and another bump.  This is all that is required to make the weld.  If you press too hard, you're likely to shear the welds.

Here I'm taking the folded billet out of the forge.  I'm about to stick it into my press.  That's the black thing behind me in the picture.

I then draw the billet out a little, reflux and insert it into the forge again.  Generally I'll do a second drawing operation, making sure that the billet is nice and even.  Very small differences in the width of the billet can cause problems later on.

Once the billet has been squared up and drawn out to a length of ten inches or so, I'll use a cutting die and chop it in half, leaving a small web of steel to connect the two pieces.  Then I'll flux up the surface which is about to be welded, stick it in the forge for thirty seconds or so, pull it out, and wire brush the flux off.  This helps clean off impurities that might cause inclusions and other flaws in the weld.

Then I use my hand hammer to fold the pieces over until they come together like two jaws.  I re-flux the surface to be welded, hammer it all the way shut, and stick it back in the forge.  I'm very careful to make them as square and neat as possible.  If the two halves of the billet don't mate up very well, you're likely to get welding flaws.

After this, it's just repetition.  Weld it, forge it, draw it, cut it, weld it again.

So here's the math.  Let's say we start with sixteen pieces.  After the first fold the billet now has 32 layers.  Two folds, 64.  Three folds, 128.  Four folds, 256.  Five folds, 512.  Six folds, 1024.  Seven, 2048.  And that's roughly where I quit most of the time.

At this point, all the welding is complete.  But the billet is still very short and fat.  You might be able to make a hammer out of it...but certainly not a blade.  If it's been folded longitudinally, it's probably around 10 to 12 inches.  If it has been folded lengthwise, it may be as short as 4 or 5 inches.  In either case it's still between one and two inches thick.

Therefore I then forge out the billet to a workable length with my press, drawing the billet out about three or four times its length into a bar.  With a thousand layers in the bar, each "layer" of steel is now roughly one four-thousandth of an inch thick.  I put the word layer in quotes, because the steel is now one single structure, with the layers existing simply as residual evidence of the original components of the billet.

Here's a picture of what it looks like once it's forged into a blade and polished.

The white misty area on the top is the hamon or hardened area of the blade (which has nothing to do with forge-welding, but which is a standard feature of Japanese style blades).  The subtle streaks and swirls of light and dark are the hada -- or grain pattern -- caused by the forge-welded layers.

Unfortunately the terminology of forge welded blades is not very clear.  There are a number of confusions that can come up when talking about welding schemes.  I prefer not to use the term "forge-folded" myself.  I prefer to refer to "high layer forge-welded" steel.  This distinguishes it from conventional damascus steel of the sort used by Western knife-makers, which generally has a somewhat lower layer count.

There are various other welding schemes which add to the confusion.  The traditional Japanese blade contained two separate welding operations.  The first served to produce the steel.  The second forge welding operation was used to produce the billet.  There were a variety of welding schemes including san-mai (three piece), kobuse (a sort of hot dog-style arrangement in which a soft, ductile, low carbon steel is welded into the center of hard, high-carbon jacket) and various others.

These same welding schemes can be used with modern mono-steels steels.  A high carbon piece can be jacketed in two low carbon pieces to form a san-mai blade, for instance. 

Are there other forge welding techniques?  You bet.  Steel cable, of the sort used to haul elevators up and down, can be forge welded to produce a surface pattern that looks like reptile skin.  Or that same steel cable can be welded into billets, drawn out, cut and stacked and then re-forge-welded.  When taken to relatively high layers, this can mimic a traditional Japanese hada quite effectively.

Western smiths have made damascus in cans out of steel powder, producing ridiculously complex patterns.  And then there's wootz steel, a type of steel made in crucibles which develops a pattern that can easily be mistaken for the product of forge-welding.

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