Hardening of modern Japanese swords

As promised the next part about Japanese sword hardening on modern made swords. Please make sure you download the FULL SIZE image.

You will experience a highly details image of Japanese steel goodness ;)

Actually I hope that this image will explain why we honestly believe that Kaneie Sword Art currently is the best modern sword forge in the world :D

left) a 'tamahagane' blade by a major Chinese sword company.
center) a Tamahagane blade by Kaneie
right) Antique edo-period wakizashi


As a follower of The Samurai Workshop you should be familiar with this term by now. In short: it gives a sword both extreme sharpness and flexibility as well as shock absorbing feats due to its softer spine.

Now while this may sound great, a freshly differentially hardened sword blade will go through some extreme differential stress. This can cause bending, corkscrewing but also ripping open folding lines and cracks. If a sword survives the quench, it will surely require tempering and geometry correction.

This has everything to do with the way the clay has been place onto the sword blade. The evenness of the clay layer, its symmetry between the ura and omote side of the sword and the consistency and overall distribution of the clay. A fun fact is that martensite (hardened steel) takes up more space than pearlite which explains the curvature of the Japanese sword. The hamon area expands and pushes the sword into a certain curve and not the other way around which is often thought.

The difference in the crystallization of the hard and soft steel cause the 'temper line' or hamon (when speaking of Japanese swords. The line that separates the areas is called the habuchi.

This is the Japanese way of forging a sword in a nutshell.
Even to this day, cracked swords are still very common with traditionally quenched blades.

Please not that we have not yet discussed the steel lamination techniques yet. Differential hardening is one side of the Japanese sword, the combining of steels of different hardness into one sword is a second feature that's found on Japanese swords.

Among modern made swords, many replica swords will be advertised as 'the real deal'. As you can imagine, most of them aren't. However if if they are made properly, they can still be functional. This takes us to the next sword hardening method: Through hardening.

Many cheap swords are through hardened. It means that the entire blade will have a single hardness level. This is the easiest production method since you have a single piece of steel where the steel supplier can exactly inform you how to harden the steel (critical temperature, water temperature, quenching time etc etc). Also since it's not treated with clay, the entire blade will have a similar stress level.

The curve is added while the steel is in its soft state and it won't deform during the quench.
Having a predictable curvature and hardness and no quenching deformation make this process ideal for mass production environments. Using automated machinery to cut out sword blanks, a single saya curvature and easy quenching make it a lot easier to produce a sword. Also water-cooled grinders also allow the sword blade to me shaped fast and efficient without worrying about heating the blade up to a state that it will soften up.

In theory there is absolutely nothing wrong with this as science, industrial machinery and modern steels CAN result in a very reliable product and a low price. With the right knowledge it can become an excellent sword as we have proven with our 2013 Batto shinken collection.

Now since the hamon is created by a difference in steel crystallization / hardness; through hardened blades cannot have a real hamon.
Makes sense right?


This process is used by some of the major Chinese forges. Even brands you would not expect it from. It's a trick which combines several metal working techniques but it doesn't even touch the basics of Japanese sword forging. This process is also known as differential tempering.

What if you do not RAISE the hardness by (differential) quenching but instead LOWER the hardness through tempering? After all, you can purchase industrially hardened steel in almost any hardness.

The process roughly goes as following:
1) Purchase a through hardened piece of steel (maybe even factory folded damascus) of around 62+ HRC (above the ideal hardness of a through hardened sword).
2) CNC / laser cut it into a sword-like shape, including the curvature you want
3) Clay an inverted hamon (thick isolation on the cutting edge, keeping it cool)
4) Heat up the spine of the sword allowing it to quickly become softer
5) The insulated (the cutting edge) part will not heat up as fast as the unprotected part, keeping most of its hardness
6) Enjoy a differentially tempered sword with a 'real' hamon

So now we have a sword WITH a separation line between the soft and hard steel (which is not a true hamon btw), zero deformation, zero cracking chance and a perfectly calculated curvature.
As clearly can be seen in the picture, there will happen some coloration of the steel. The colors will disappear when the sword is polished but it etching the blade should make the difference in hardness visible again.

Sounds ideal for a mass production right? That's correct and this is exactly why Chinese forges use it more often than they should. While the result may seem similar it most certainly is not.

An issue that arises with differential tempering are similar to those with differential hardening.
When adding a clay layer by hand, there are irregularities created in the insulation.

1) the tempering cycle hasn't fully been completed
2) the clay layer is too thick in some spots preventing the tempering temperature in some spots
3) Tempering is done with a torch and some spots aren't tempered properly

These three scenarios will all result in a sword which is left with too hard and brittle spots.
While this may not be an issue with a short hunting knife, on a long katana sword it just snap on impact.
If you even saw coloration on the tang of your Japanese sword it has been made through a tempering process. The quenching process does not create a discoloration of the steel.

Please note that the knives in the picture are of excellent forging quality and that differential tempering is NOT a bad technique. In terms of Japanese swords however, it's just cutting corners and pissing on tradition :)

While the precise details of the effects of the differential tempering and differential hardening are difficult to spot as a basic user, the Japanese sword appreciation community has this all figured out.
Introducing two new terms: nie and nioi.
These terms are used to describe - to the naked eye - visible crystallization of a differentially hardened sword. The crystals will be most visible in the transition zone of the hard to the soft steel.
Differentially tempered sword will never display nie or nioi chrystals as this only happens with a differential hardening, when the unhardened and hardened steel will have to fight for their place on the blade.

The previous picture (on full size) should make clear what we mean with the nie crystals. So for you to remember:
Option A: it has crystallization on the habuchi (dividing line) and it's differentially hardened.
Option B: no crystalization then the hamon is either differentially tempered, through hardened and had a hamon etched.


Well, that was it for now! We hope you enjoyed this series and hope you'll like, share and comment ;)