JAPANESE SWORDS: Modern Steels for Japanese-Style Swords

What is the ideal modern steel to use for a Japanese style katana? Each steel's metallurgical or alloy mix dictates how that steel responds to thermal treatment. Heat treatment is the process of subjecting steel to heat and final quenching in order to achieve the desired crystalline states and thereby achieve the performance characteristics associated with those crystalline structures. However, if the heat treatment is inferior, the sword is inferior no matter how "superior" the manufacturer claims the steel is. It is also important to note that different smiths may heat treat the same kind of steel differently, hence one must avoid "name association"; just because one sword maker uses a certain steel exceptionally well does not mean that another maker will be able to deliver the same results. (Also, claims to deliver "performance" can be deceiving as the maker lacking in heat treatment skills can make a sword thicker or make the edge thicker, hence "outperform" the competition.)

This article examines the properties of various metallurgies of steel from the perspective of forging (or stock removal) of steel for Japanese-style swords.

The goal of the traditional Japanese sword smith was to first make a functional weapon, and second to make a beautiful work of art. The objective for the sword maker is to create the best balance possible between toughness (for impact absorption) and hardness (for the cutting edge - after all, you can only cut through things that are relatively softer than your edge). These properties - hard and soft - are inversely proportional! In terms of aesthetics, one of the most visible aspects of the Japanese sword is the yak ire, which is the cloud-like hardened-edge along the sword.

Japanese swords receive a partial clay coat prior to the final heat treatment and quench in water. This allows the heated sword to cool at different rates, thus achieving a harder edge and softer body.

Many thanks to Randal Graham of Dragon Fist Forge for providing his input and experience with the following steels:

AISI 1050

1050 steel makes excellent swords if you keep within the 57-58 Rockwell hardness range for the edge. Capable of fantastic hamon (temper-line) patterns, AISI 1050 is great steel for teaching and practice (it is a forgiving steel to starting-out students), and can result in super-tough blades. Its carbon content is considered medium-level (0.5%) and may not be as high as advanced-level blade smiths and heat-treaters would like. Blades made from 1050 can sport fair edge-holding capabilities. In maintaining a 60 Rockwell edge, the steel may have a tendency to be brittle. Countering the brittleness, anything more than a light stress-relief will dip the hardness to low.

AISI 1060

This can be considered a good baseline steel for katanas. It allows for a great hamon. Differentially hardened, this steel is superior for a katana whose main requirement is toughness.

AISI 1070 and 1084

In the case of these steels, the hamon will be a little more defined though not as flashy. These steels can be just as tough as 1060 but must be heat-treated very carefully. Due to its exacting and strict requirements to achieve good results in a heat treat, 1070 and 1084 are not the best recommendations for katanas. However, with the use of salt baths to aid in the heat-treating process, they can outperform 1060. However, it is also very easy to the hardening process. AISI 1070 and 1084 can be unforgiving steels!

AISI 1095

For practical reasons, AISI 1095 can be treated the same as 1084, but it may have a higher tendency of cracking during the quenching process. At 0.9% carbon, this steel has more carbon content than the idea 0.8% maximum or level that allows for the steel to harden fully. AISI 1095, thus, may mean added expense due to so many potential points of failure.

AISI 5160

A great steel all around, but not very "hamon-friendly" in appearance, as this steel does not react the same way to a traditional clay coat as the simpler AISI 10XX series steels will. In testing, it was common for a small crack to start and for the whole edge to break away from the body! This steel seemed to like through-hardening much better, and in that situation it is pretty hard to beat.

L-6

L6 is a difficult steel to work with, with many trade-offs, however. Although it responds better to differential hardening than 5160, it has a fairly under-whelming hamon, is prone to rusting, and needs more control in the hardening than the previously listed steels.

D-2

If stainless properties and low maintenance are both required, then D2 may be an answer. However, the edge hardness has to be reduced to around Rc 55. The steel must be treated properly. In comparison with A2 tool steel, I would personally choose D2 over it. Due to its high alloy content, there is no proper way of differentially hardening steels like A2 or D2 in a way that provides for high quality results. (Editor's note: D2 contains chromium, but not enough to qualify it as a stainless steel - considered unsuitable for swords altogether. The chromium level, at higher contents, weakens the molecular bonds in steel, so it's good for short lengths only. The chromium content is countered by the presence of molybdenum.)

A-2 (Air Hardening)

A-2 is a tool steel that is commonly used by some makers of Westernised interpretations of the Japanese Katana. The logic is that a steel used to cut through other steels is probably a good sword steel (bear in mind that the form in which A-2 is used to cut steels is large dies, so the cutting block is immediately many times thicker than a sword edge - this statement can be construed as merely marketing) However, the requirements for Japanese swords is that it must be differentially hardened, and the only way to do so with air-hardening A-2 and give it a harder edge is an inelegant process of running a torch along the edge to make it hot - often quipped as "zone hardening" in marketing these swords. This is pushing the limits of good heat-treating techniques. Given that A-2 is high alloy steel with stringent requirements, this "zone hardening" is partial hardening at best. Because A-2 is air hardening it would cool too quickly, rendering a traditional clay-application useless. The high alloy content makes it extremely difficult to make a real temper line to form.

Forge-welded Cable

Cable-welded and folded steel cable verified at 0.9 % carbon is fantastic for a traditional looking sword, with performance parameters on par with that of traditionally made katanas. Carbon loss is figured into the equation during the welding/forging process. A final content of about .75 or thereabouts is preferable. Of course, it's more labour to fold and weld (the price goes up) and such a sword should have a good polish performed on it to bring out all aesthetic details. Folding the billet 3-4 times will produce a "lizard-skin" pattern on the blade surface. Folding it 7-8 times will produce something more traditional-looking.