Katana metallurgy glossary

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Types of steel

Publication date : March 8, 2006

Last update : February 25, 2024

Steel is an alloy of iron and carbon. Japanese tradition uses a special technique to produce steel from a ferruginous sand, Satetsu (砂鉄). A low-temperature furnace (Tatara 鑪) is used to produce a very high-grade steel, Satetsu (砂鉄). Tamahagane (玉鋼).

Most modern katanas (Shinkens 真剣) are made from one or more structural steels. These steels may contain additives such as chromium and molybdenum, for example, to enhance their stainless properties. Carbon content determines hardness (along with heat treatment). In modern katana manufacture, three main grades are used: 1045, 1060 and 1095. These steels contain 0.45, 0.60 and 0.95% carbon respectively. The higher the carbon content, the harder the steel. The European standard designates them XC45 (hard steel), XC60 and XC95 (extra-hard steels). Visually, there's nothing to distinguish these steels. There are, of course, other types of steel, with or without additives.

The forge

Forging is a craft technique that involves heating metal (using a furnace) and hammering it onto an anvil. One or more steels can be used.

Maru-type forge (homogeneous) #Terressens

Homogeneous forging (Maru): this is the simplest form, using a single type of steel, e.g. 1045.
The lower cost of this type of manufacture explains its high proportion in current katana blade production. Although homogeneous, a Maru forge using 1095 steel will produce an excellent quality/price compromise.

Kobuse-type forge, with "soft" steel in the core and hard steel on the surface #Terressens

Kobuse forge (sandwich): two steels are used. At the core, a hard steel (or soft steel, depending on its carbon content, e.g. 0.45% for a 1045) is surrounded by an extra-hard steel (which may be a 1060 or 1095).
By definition, sandwich forging entails a risk at the time of manufacture, as the two steels have to wrap around each other. The weld between the two materials is therefore a point of fragility that must be mastered by an experienced blacksmith.

Soshu-Kitae forge: three steels are used, from "soft" to extra-hard, from the core to the surface of the blade.
Sandwich forging with several steels combines the hardness of an extra-hard steel (1095, for example) with the relative flexibility of a "softer" steel (such as 1045). However, the choice of steel and type of forging can be greatly improved with differential hardening.

Shihomizue forge: three steels are also used, from the "softest" (green) to the extra-hard (gray), from the core to the edge of the blade.
This type of forging produces a hard edge, ideal for penetrating and cutting the target. The softer steel core and sheath provide the suppleness needed for cutting. The clay tempe accentuates this combination of softness and hardness.

Damascus-type forge (damascened or laminated lamae) #Terressens

Damascus forge (damascus or laminated blade): damascus steel refers to two types of forge. The first uses a steel called wootz. Sourced from India and the Middle East, it is renowned for its quality. Unique patterns are present on the surface of the steel. The term also refers to heterogeneous steels made from several grades of steel welded together and forged to produce more or less complex patterns. This material, known as "wrought Damascus steel", borrows the name from the historic metal by misuse of language because of its superficial resemblance to wootz Damascus steel.

There are many other types of forge: Honsanmai, Makuri, Wariha Tetsu, Orikaeshi Sanmai, Gomai... but we have limited the presentation of these for the sake of simplicity.

Heat treatment

A crucial phase in the use of steel, heat treatment involves heating the material and then cooling it rapidly. This operation modifies the steel's crystallographic structure and hardens it to obtain the desired mechanical properties. It is thus possible to work with a relatively soft steel, while imparting a high degree of hardness.

In the manufacture of a katana, the classic (homogeneous) heat treatment consists in cooling the steel uniformly. The only variations in hardness will depend on the depth of the material.

To improve the steel's softness/hardness characteristics, a refractory material such as clay is used to protect the areas we wish to soak less (softer). In this way, the back of the blade can be covered with clay during the heat-treatment operation, resulting in excellent hardness of the blade edge (Ha 刃) while maintaining the flexibility of the back (Mune 棟). This technique creates a characteristic hardening line (Hamon 刃文). This is known as clay or differential quenching.

The temper line will appear at the moment of polishing, as a subtle boundary between the tempered (and hard) area and the area protected by the clay (less tempered and therefore softer). Entry-level or decorative katanas reproduce a false temper line by using a grinding wheel (mechanical surface action).

Sharpening

The polishing and sharpening of a katana blade is a crucial phase in obtaining a quality sword. These two steps can be performed either mechanically or manually. Mechanical sharpening obviously has the advantage of cost, but is not compatible with top-of-the-range katanas, especially those that have benefited from differential clay tempering. All decorative katanas, as well as first-price sharpened katanas, are mechanically sharpened and feature a false temper line produced with a grinding wheel.

Polishing and sharpening by hand confer superior quality for the practice of the art of cutting and, when the tempering is differential, make it possible to obtain the true temper line (Hamon 刃文).

Metallurgical diagram

To make it easier to understand the various factors that influence the quality of a katana blade, we've created this graph in the form of a radar curve. The axes represent the determining elements presented above. The further the curve is from the center, the higher the quality of the blade. As you can see, the curves overlap, as several techniques can be combined to produce a single blade.

On this graph, you can see three curves:
In yellow, a Maru 1045 steel.
Example of a katana: "The Last Samurai" sharpened katana (ref. 0471).

In green, Damascus steel (laminated forge).
Example of katana: sharpened katana Kokuyō (黒夜, black night), damascus blade, black mount (ref. 0480)

In blue, a Maru 1095 (extra-hard) steel with differential clay hardening and hand sharpening.
Example of katana: Sacred Earth katana (聖地, Seichi), sharpened 1095 blade, genuine hamon, brown braiding (ref. 0481)

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侍 #Terressens | Samurai & Katanas | Katana metallurgy lexicon (Practical info) 刀

Samurai & Katanas

Practical info #Terressens

Types of steel

The forge

Heat treatment

Sharpening

Metallurgical diagram

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