THE KATANA - STILL SHARP OVER 1,000 YEARS ON...

From Feudal Japan to zombie apocalipse - 'The Katana'. Read on for a description of the metallurgy and engineering behind the world's best blade.

Introduction:

The katana is the result of many years of refinement in steel and smelting technology its architects over many years sought to produce a blade that combined the two seemingly opposite properties of hardness and flexibility. This was achieved through the formation of layers of high, medium and low carbon steel.

The Anatomy of The Blade

The Cutting Edge:

also known as the 'Yakiba' in all shown examples the cutting edge of the blade is formed from hardened high carbon steel (Hagane). This type of steel is brittle and does not bend under stress properties which allow it to be sharpened to a very high degree making it ideal for cutting. These properties are dependant on 'dislocations'. Scroll the bottom of the page to read more on this...

The Core:

In most of the examples the core is formed from softer low carbon steel (shigane). This steel is 'softer' than high carbon steel but also 'tougher'. The main difference between the two is the way in which they undergo failure. Hagane (~0.7% Carbon) shatters under stress whilst the softer shigane (~0%) deforms first elastically moving to plastically as the stress on it is increased. In this way hagane is superior for the majority of the sword allowing it to bend and absorb energy reducing the chance of the sword failing during combat.

Dislocations:

dislocations are irregularities in the crystal structure of a metal. They are much like the extra line of grains found in sweetcorn as shown (top left). These dislocations create compressive and tensile stresses in the metal which allow some movement in the layers. These dislocations appear as lines on the surface of the metal as shown in the TEM (transmission electron microscopy) image (top right). More dislocations allows more movement and a more flexible metal.

Carbon Content

This steel is however hard and inflexible, this is achieved through the addition of carbon. Thereby pinning the dislocations in place by disturbing the regular lines of atoms thus preventing movement. The amount of carbon is proportional to the amount of flexibility of the resulting steel. High carbon steel is approximately 1% carbon; medium carbon is 0.3% to 0.6% while low carbon steel contains 0.05% to 0.15% carbon.

Hardening:

hardening of steel can be achieved in many different ways all of which seek to create crystal lattice defects which prevent the movement of dislocations. The method used in the creation of the cutting edge is a process called 'tempering' or 'quenching'. This is known as a 'marstenitic' transformation so named after the type of crystalline structure the material is formed into.

stainless steels fall into 3 categories: marstenitic, austenitic and ferritic. In tempering steel is heated to a near melting point at which it forms a crystalline structure known as austenite. Austenite is known as a solid solution - the body of material remains solid however the atoms have enough energy to move to some degree. In this phase more carbon can be absorbed into the structure. The steel is then plunged into very cold water to cause rapid cooling. This transforms the cubic crystals to 'tetragonal crystals' (cuboids).

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