Basic Question Regarding a Hamon and Differential Heat Treatment

R

Rob Garner

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Jan 31, 2021
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I’m a new knife maker who has already caught Hamon fever and have a few questions. I would be greatly indebted for responses to any one of them. Hopefully they can help out some other novice sufferers as well.


My basic understanding:

A hamon is the visual expression of differential heat treatment. This is historically produced by putting clay over the spine of the blade. During heat treat, the clay keeps the spine below non-metallic while the edge reaches critical hardening temperature(s). After quenching, the clay is removed and the blade can be sanded, etched, and polished to reveal beautiful variations between the hardened edge and the softer spine.

Differential heat treatment is the practice of hardening only the edge of the blade through edge quenching (submersing only the edge rather than the whole blade in the quench) or by bringing only the edge of the blade to critical temperature (using, say, a blow torch rather than a forge) before quenching.

Both methods result in the same practical effect: a soft spine and a hard edge. Aesthetically speaking, differential heat treatment can produce a temper line, but since that visual result is unintentional and the process to refine the contrast not followed, its appearance would not be considered a hamon.


Question 1: Is this basic understanding… basically correct?

Question 2: If the clay induces differential heat treatment, is there any functional difference between full quench or edge quenching when going for a hamon?

Question 3: This one kind of depends on Question 2, but if I am just starting out—and therefore unlikely to exhibit exceptional control with either forge or torch—would it not be easier just to clay the blade, heat the edge with a torch, and edge quench?

I should mention I have 1075 steel on hand: it seemed to me almost as user-friendly as 1084 but with better hamon potential.

Bonus Question 4: The idea of sending off a blade to be heat treated sounds very appealing to me. Normalizing and annealing seem less risky and sensitive than the heat treat process. (I have heard chatter that I should normalize the steel I’m using if it won’t be forged. No idea if this is accurate or not—just seems like a process that can’t hurt.) Does anyone know if send-away HT for a blade with baked-on clay is possible? Is this something anyone has experience with? I’m in Maryland if that makes it any easier.


Thanks again.
 
Last edited:
Rob Garner said:
I’m a new knife maker who has already caught Hamon fever and have a few questions. I would be greatly indebted for responses to any one of them. Hopefully they can help out some other novice sufferers as well.


My basic understanding:

A hamon is the visual expression of differential heat treatment. This is historically produced by putting clay over the spine of the blade. During heat treat, the clay keeps the spine below non-metallic while the edge reaches critical hardening temperature(s). After quenching, the clay is removed and the blade can be sanded, etched, and polished to reveal beautiful variations between the hardened edge and the softer spine.

Differential heat treatment is the practice of hardening only the edge of the blade through edge quenching (submersing only the edge rather than the whole blade in the quench) or by bringing only the edge of the blade to critical temperature (using, say, a blow torch rather than a forge) before quenching.

Both methods result in the same practical effect: a soft spine and a hard edge. Aesthetically speaking, differential heat treatment can produce a temper line, but since that visual result is unintentional and the process to refine the contrast not followed, its appearance would not be considered a hamon.


Question 1: Is this basic understanding… basically correct?

Question 2: If the clay induces differential heat treatment, is there any functional difference between full quench or edge quenching when going for a hamon?

Question 3: This one kind of depends on Question 2, but if I am just starting out—and therefore unlikely to exhibit exceptional control with either forge or torch—would it not be easier just to clay the blade, heat the edge with a torch, and edge quench?


Bonus Question 4: The idea of sending off a blade to be heat treated sounds very appealing to me. Normalizing and annealing seem less risky and sensitive than the heat treat process. (I have heard chatter that I should normalize the steel I’m using if it won’t be forged. No idea if this is accurate or not—just seems like a process that can’t hurt.) Does anyone know if send-away HT for a blade with baked-on clay is possible? Is this something anyone has experience with? I’m in Maryland if that makes it any easier.


Thanks again.
Click to expand...
I'm a new maker as well but AFAIK, the clay doesn't stop the blade from reaching critical, it slows the cooling during the quench preventing the clayed area from hardening.

Heating the edge only and quenching with give you a differential hardening line, not a hamon.
 
Ahhh Thanks Nick. That makes sense. I forgot about that small little part about how the quench does the hardening. Goodness gracious.
I'm not sure I understand the difference between a differential hardening line and a hamon, though. Could you expand a bit?
 
Good write up
Why does everyone say temper line? What does it have to do with tempering?
 
Your are close, but not quite right.
The clay slows the quench speed of the covered area. The whole blade reaches the target temperature, but only the exposed area cools fast enough to harden fully. The covered area cools at a rate that just misses the pearlite nose. The transition area (where the hamon forms) is a mixture of different structures. In the Japanese explanation is that it is where the nie and the nioi mix. These are different sizes/types of martensite crystals. There may be floating pockets of these crystals above and below the hamon, too.

The choice of steel is the first concern. Low manganese and low alloy is what is needed. Carbon content close to .75-.85% is desires. Even when using higher carbon content, the HT is such that only about .85% gets used.

The clay application is the next most critical. Enough, but nt too much is what is needed. Err on the side of less. Take a look at the Nuclayer system clays and the associated videos.

HT is very important. It has to reach the critical temperature, but not a lot higher. Heating for a hamon must also be very evenly heated. Use a lower temperature than when quenching an unclayed blade for maximum hardness.
Cleanup is pretty much normal, but getting the most out of a hamon is a skill that takes time and the right polishing techniques and materials.

Start simple and learn slowly.
 
For a blade with a hardened mune, that would be about right.

On a blade with the top half fully covered with clay, the ji (upper bevel) and mune (spine) are mostly pearlite, and be in the upper Rc30's.

If you wipe trhe clay off the mune/spine, as many smiths do, the spine/mune will harden to some depth, giving a hardness from The mid 40's to mid 50's.. This makes the blade a bit tougher and resists a hit with the edge of another sword better.
 
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I didn't explain my answer very well. I was referring to a blade where the mune was wiped off and allowed to harden partially. If it was all clay covered, yes, you should expect upper Rc30s. I edited it to be clearer.
 
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