Forging and Construction
The authentic Japanese sword is made from a specialized Japanese steel called "Tamahagane" which consist of combinations of hard, high carbon steel and tough, low carbon steel. There are benefits and limitations to each type of steel. High-carbon steel is harder and able to hold a sharper edge than low-carbon steel but it is more brittle and may break in combat. Having a small amount of carbon will allow the steel to be more malleable, making it able to absorb impacts without breaking but becoming blunt in the process. The makers of a katana take advantage of the best attributes of both kinds of steel. The maker begins by folding and welding pieces of high and low carbon steel several times to work out most of the impurities. The high carbon steel is then formed into a U-shape and a billet of soft steel is placed in its center. The resulting block of steel is then drawn out to form a rough blank of the sword. At this stage it is only slightly curved or may have no curve at all. The gentle curvature of a katana is attained by a process of quenching: the sword maker coats the blade with several layers of a wet clay slurry which is a special concoction unique to each sword maker, but generally composed of clay, water, and sometimes ash, grinding stone powder and/or rust. The edge of the blade is coated with a thinner layer than the sides and spine of the sword, then heated and then quenched in water (some sword makers use oil to quench the blade). The clay slurry provides heat insulation so that only the blade's edge will be hardened with quenching, and also causes the blade to curve due to reduced lattice strain along the spine. This process also creates the distinct swerving line down the sides of the blade called the hamon, which can only be seen after polishing; each hamon is distinct and serves as a katana forger's signature.
When steel with a carbon content of 0.7 percent is heated beyond 750 °C it enters the austenite phase. When austenite is cooled very suddenly by quenching in water the structure changes into martensite, which is an extremely hard form of steel. When austenite is allowed to cool slowly its structure changes into a mixture of ferrite and pearlite which is softer than martensite. By carefully controlling the heating and cooling of the blade being forged Japanese swordsmiths were able to produce a blade that had a softer body and a hard edge creating a superior weapon, a process called differential hardening or differential quenching. The reason for the formation of the curve in a properly hardened Japanese blade is that iron carbide, formed during heating and retained through quenching, has a lesser density than its root materials have separately.
After the blade is forged it is then sent to be polished. The polishing takes between one and three weeks. The polisher uses finer and finer grains of polishing stones until the blade has a mirror finish in a process called glazing. This makes the blade extremely sharp and reduces drag, making it easier to cut with. The blade curvature also adds to the cutting power.
Read more about this topic: Katana
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