The core structure of a Japanese knife consists of multiple precise parts. Its blade is typically forged from a combination of two types of steel: high-carbon steel (with a carbon content of 0.6% to 0.7%) is covered with a low-carbon steel core. Through more than 15 folding and forging processes, approximately 30,000 layers of microstructure are formed, achieving a hardness of HRC 60-62, while the core maintains a toughness of around HRC 40. According to the research of the Tokyo National Museum on Muromachi period swords, the typical blade length is 60 to 73 centimeters, the curvature is 1.5 to 2.0 centimeters, and the weight is controlled within the range of 900 to 1200 grams. In 2021, Nagoya Institute of Technology discovered through X-ray diffraction analysis that genuine knife makers create a temperature difference of approximately 200 ° C between the blade and the back of the knife during quenching, forming a visible boiling line (hamon). This heat treatment process causes precise phase transformation in the metal crystal structure.
The Tsuba, as a balancing and counterweight component, usually has a diameter of 7 to 8 centimeters and a thickness of about 0.5 centimeters. The weight of the copper alloy material accounts for 15% of the entire sword. A trace element analysis of the Edo period tsuba conducted by Kyoto University of Arts in 2023 revealed that the red copper alloy contains 78% copper, 0.3% gold, and 4.5% silver. This ratio ensures an antioxidant life of over 300 years. The handle roll (Tsukamaki) is wrapped in shark skin up to 240 centimeters long, with the winding Angle strictly maintained at a 45-degree Angle. Each eye nail needs to withstand a tensile test of 20 kilograms. According to the regulations of the Japan Sword Preservation Association formulated in Showa 42, the density of the handle wire winding should reach 5 to 6 cross nodes per centimeter.
The geometric structure of the blade has been precisely calculated. The radius of the blade curve of the Kissaki part is only 0.2 millimeters, and the Angle of the blade tip is 20 to 25 degrees. In 2020, the Japanese Society for Materials Science confirmed through high-speed photography that the contact time between the blade and the object when high-quality katana is chopped is only 0.003 seconds, and the impact pressure distribution reaches 280MPa. The works of the renowned swordsmith Horikawa Kuniyoshi during the New Sword period show that the hardness gradient of the cutting edge area changes by HRC 15/mm. This gradient structure enabled the knife to successfully penetrate the iron armor of the Korean army at that time during the Battle of Munroku in 1592.
The blade sheath (Saya) is made of tochigi or beech wood. The grinding accuracy error of the inner wall does not exceed 0.1 millimeters, and it maintains a uniform gap of 0.3 millimeters with the blade. According to the 2022 Preservation Science Report of the Agency for Cultural Affairs of Japan, the humidity inside the sheath needs to be constantly maintained at 45%-50%RH, and the temperature fluctuation range should be controlled within ±2℃. Modern high-end replicas are made using CNC five-axis machining centers. Each tool sheath requires 18 hours of fine processing, and the cost accounts for 30% of the entire tool. The ancient knives preserved in the Shosoin from the early 8th century show that the scabies with a paint layer thickness of 0.5 millimeters can remain crack-free for 800 years in a constant temperature environment.