The different types of steel used in forging katanas
These are steels composed of a single grade. This is the simplest forging method to forge a blade at a lower cost.
The carbon content determines the future hardness of the steel but also the price:
the higher the carbon content, the more metallurgical work is important and therefore more expensive.
There are generally three nuances in the manufacture of semi-industrial katanas
(hand forged but with industrial heat treatment): 1045, 1060 and 1095.
These three types of steel correspond respectively to 0.45, 0.6 and 0.95% of carbon.
1045 steel is a hard steel with great flexibility, its price is also low, which is why it is found in the first price katanas. A good choice to learn about cutting because it forgives mistakes.
Steels 1060 and 1095 are very hard steels.
1095 steel corresponds to a superior quality in this range of materials, its blade is very hard and sharp, but on the flip side it is not very flexible and brittle during poorly executed cuts.
With two or three different grades of steel, they thus allow the sword to have characteristics that a single type of steel could not achieve.
The principle: a hard steel on the surface, for a sharper edge, and a soft heart for optimal shock absorption.
A soft steel core (Shigane, low carbon rate) is introduced at the time of forging into a hard steel casing (Hagane, higher carbon rate). Forming a real sandwich, the metal thus formed brings considerable added value to the blade.
This also makes it possible to forge very hard and at the same time flexible blades, which is not possible with a very hard 1095 steel type blade for example.
The main composite forging techniques: Kobuse, Honsanmai, Shihozume, Makuri, Waeiha Tetsu, Orikaeshi Sanmai, Gomai, Soshu Kitae.
Masamune, the famous Japanese blacksmith excelled in the Soshu Kitae method.
It is manufactured by combining 2 grades of steel. A hard shade and a soft shade (1095 and 1060 for example).
the steel bar is made up of a stack of layers of these 2 steels.
the blacksmith binds several layers of steel and heat them in the forge hearth to practically the melting temperature then hammers it on the anvil or with a pestle to weld it, stretches it until obtaining half of the starting thickness partially cuts it in the middle and folds it back. He heats it again to practically the melting point, welds it by hammering it, and repeats the operations to have the desired number of layers.
In the end, the blade thus worked is made up of several thousand layers (from 2000 to more than 30,000 layers) depending on the number of folds.
The blade obtained combines the qualities of the two grades, hardness for grade 1095 and flexibility for grade 1060. Folded steels also have an undeniable aesthetic appearance.
It is the same technique but only one shade is used. The gain is just aesthetic.
Tamahagane Steel Blades
It is the traditional Japanese steel, the best steel for a katana.
Tamahagane steel is obtained by heating (for three days) in a furnace between 1200 ° C and 1500 ° C. This process removes impurities in the form of slag. The desired carbon content for high quality blades is between 1.0% and 1.2%
Tamahamagane is heavy, dense, with a silvery shiny appearance and a fine crystalline structure.
Forged blade and manufacturing steps
The forging process defines the Japanese sword. Authentic Samurai swords were hand forged. Many companies offer different types of steels. Most of the time the blades are not forged and therefore do not have the qualities and integrity of forged blades. Forging provides the best and fairest dispersion of carbon in the steel to achieve excellent blade quality that you will never get with modern blades which are stamped or series milled without carbon dispersion.
Step 1: Forge
Forging consists of hammering a carbon steel ball so as to obtain the shape of the blade. The hand forging guarantees a better dispersion of carbon in all the steel in order to achieve an excellent blade.
Step 2: Formatting
The blade, which is still not hardened at this stage, is formed to the required dimensions.
Step 3: Traditional clay coating
To obtain the differential hardening, the blade is covered by hand with a special clay of different thickness near the edge and on the rest of the blade (thicker) so as to obtain a different cooling according to the zones. This results in the harder cutting edge and softer back.
Step 4: Quench
At this point, special know-how is required. The slide is heated to a predetermined temperature and cooled in a water bath.
This step will determine the shape, hamon, sori and straightness of the blade.
The quality depends mainly on the skill of the craftsman.
Step 5: Sori
The curvature (sori) is made according to the point of balance and the point of percussion according to the use either for cutting or for reduction.
Step 6: Finishing and Polishing
The latest hand polishing is done to provide an excellent finish and to get the best visual effects from the hamon line and the different hardened areas.