What's a Hamon line?

[Blacksmith Jim]

The title pretty much says it all. I've seen the term used a lot, and I think I understand visually what part it is, but what's it from? What's it for? What's the propper definition?

[Rantalin]

If I understand correctly, the hamon is the visible line between the hardened and unhardened metal in a knife. The effect is created primarily by putting clay on the spine of the blade, up to about 1/4 inch from the edge. The piece is then heated and cooled (can someone else describe the cooling process, I am unaware of whether this is done by air, oil, water, or if it depends on the steel) When the blade is polished, the hamon line appears where the darker metal meets lighter metal. I'm not sure if there is always a color difference.

[Don A]

Looks like this:

(not my sword; just a good picture)

Edited April 9, 2009 by steve sells
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[oakwoodforge]

Correct me If I'm wrong here guys, (where is Quenchcrack, JPH or Mr Powers when you need 'em ). Oh and please excuse the long winded post.

First some basic carbon steel metalurgy as I think I understand it.

Un hardened carbon steel at room temperature is formed of a chrystline structure called Pearlite, which is a blanced alternating band like structure of Cementite and Ferrite. In this form it is very malleable and easilly machined, filed, drilled or bent but will not hold a cutting edge for any period of time.

Ferrite is the basic Iron crystal and is what makes steel magnetic it carries very little carbon, Cementite is the structure that carries the carbon, and is actually the iron carbide Fe3C, much more complex and brittle. At "Critical" temperature - non magnetinc- some where around 1350 -2000 degrees F (depending on the alloy) Pearlite transforms into Austenite. Steel while Austenitic is a solid, fully non magnetic solution of Iron & Carbon or Cementite & Pearlite, if cooled slowly in a controlled environment (anealing) Austenite reverts back to pearlite. If Austenite is cooled quickly (Quenched ) It goes through a process called martensitic transformation becoming a structure of unstable Austenite and Martensite. Martensite is a iron based crystal that has "captured" some carbon and is hard and brittle. To make the most use of this as quenched Austenite and Martensite structure it must be tempered.

Tempering is a process there the blade is soaked for a long period of time (an Hour or so at least) at a sub critical temperature (350 -550 degrees F for most simple blade steels)

This converts the Martensite & retained Austenite into a happy mix of Martensite and pearlite, or possibly Bainite ( if you are super lucky) a very finely banded structure of Ferrite and Cementite, making a blade that is capable of holding an incredible edge while not being overly brittle. For more on Bainite see Phase Transformations and Complex Properties Research Group

A Hamon line is created by controlling the way a blade is cooled when quenched, basicly preventing part of the blade from forming any Martensite. This is achieved by using clay as a heatsink or by simply edge-quenching. When the blade is polished you see the definite line between the Martensitic and the Ferrite & Cementite structures. The actual shape of the crystals will reflect light differently and thus appear as either "light" or "dark" depending on the polishing & etching methods as well as ambient lighting.

Hope this helps

Jens

Oh and if your having trouble sleeping at night just try re -reading this a few times before bed


PS: The steels that seem to show a hamon easily are simple carbon steels like W 1, 1084 & 1095.

[Nolano]

Yeah, oakwoods got it. It basically keeps the edge super had, and the back softer.

[oakwoodforge]

The piece is then heated and cooled (can someone else describe the cooling process, I am unaware of whether this is done by air, oil, water, or if it depends on the steel)

I Don't know of any Air hardening steels or Stainless steels that will show a hamon. But most shallow hardening simple carbon steels of the oil or water quenching varities work quite well. Some deep hardening steels like 5160 can bennefit from the differential quenching process but will fail to show a well defined hamon line.

So when your looking at a $59.95 Katana that says Stainless China on the blade you KNOW without a doubt that the "Hamon" is of the fake etched on variety.

Jens

[Nolano]

Definately. I used to have many junk swords, just for decoration, the hamon on one was perfectly the same enough that at first I though the blade was serrated.

[HWooldridge]

In Japanese sword construction, the design of the hamon was an outgrowth of an attempt to create a very hard edge capable of severe use before resharpening and providing an edge that if chipped, would not propagate over a large area. In other words, swords hit each other and other hard stuff during combat. If the edge chipped for several inches, it more or less rendered the weapon useless. The wavy hamons were a method to limit the chip to the length of the "scallop". Over time, they also became a basic feature contributing to the beauty of Japanese blades. I have seen pictures of a few of the National Treasure blades, some of which have hamons that are works of art on their own merit.

[job]

A very simple method of describing the hamon line is that it is the line formed when clay heat treating a blade with the result of getting two different grain structires showing on the surface denoting where the soft spine meets the hard edge.
Sometimes it is visible just thru the polishing process and at other times it must be lightly etched top show up.
xxxxx good illustration using that katana in the pic
Most of my using blades are treated in this manner That way I know they will stand up to the work these xxxxxx Aussie bushman put them thru.
JOB from Oz

Edited April 9, 2009 by steve sells
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[kogatana]

Jens,

as a non-metallurgist, you still have a more than decent scientific knowledge;) ! You mention the clay acts as a heat sink. I would have thought it is more an isolant, it doesn't easily conduct heat, does it? Maybe what makes the whole difference is the water content... With enough water, clay will absorb the excess of heat. A nice thermodynamic exercise.

Is there other ways (than clay or mud) to prevent the part of a blade to heat?

Ludo

[oakwoodforge]

When I'm using clay to heat treat I let the clay dry fully in the sun or force dry it in a toaster oven. Wet clay just spalls off when you put it in the fire, the water in the clay turns to steam and blows the clay off. Your not trying to protect the back of a blade from heat with the clay, your trying to retain the heat. By retaining heat, the back of the blade dosen't go through martensitic transformation ( HARDENING ) like the edge does when you quench the entire blade.
Quenching just the edge of a blade also works in the same way except you are letting the spine of the blade air cool to sub critical while the edge is in the quench medium.

Jens

[klickitat]

I personally have never used the clay method. I have always just heated my blades from the spine only and let teh heat travel toward the edge. this is tricky though and you have to be quick as you watch the color moving to the edge. There have been times when I had to do a blade 2 or 3 times because I wasn't quick enough to the slack tub or I was to agressive with the heat.

[B. Norris]

Forming a hamon is the provinence of low-alloy, shallow hardening steels. The clay on the back is only effective in preventing the spine from hardening on steels that have a short amount of time to get below the nose of the curve on a TTT diagram. If the steel is shallow hardening and has to go from Austenitic to below 800 degrees Fahrenheit in less than 2 seconds in order to form martensite, it will not harden under the clay on the spine. A steel like 5160 allows at least 1 minute to lose the same amount of heat and form martensite, this steel is not suited to forming hamon because the clay cannot keep the back above 800 degrees for long enough to prevent martensite from forming.