Military Sword Heat Treatment: Technical Differences Between Traditional Clay-Coat Quenching and Industrial Uniform Quenching

The Basic Principles of Hardening—Steel Structure Changes

The strength of all steel blades, including Japanese swords, is largely determined by a heat treatment process called hardening. Hardening is a technique that rapidly cools heated-to-red steel, changing the internal structure of the steel and dramatically increasing hardness and strength.

The relationship between iron (Fe) and carbon (C), which are the main components of steel, changes dramatically with temperature. When steel is heated to approximately 770°C or higher, the crystal structure of iron changes to a state called the "austenite phase," and carbon atoms are uniformly dispersed within the iron lattice. At these high temperatures, steel is relatively soft and in a state that is easy to work with.

When rapidly cooled from this state, steel transforms to another crystal structure called the "martensite phase." In this state, carbon atoms are fixed in the iron lattice with no place to escape. As a result, abnormally distorted crystal structures form, and the hardness rises dramatically due to this distortion. This is the essence of hardening.

Hardening of Japanese Swords—The Intersection of Tradition and Science

Japanese sword craftsmen understood the principles of hardening in a practical way even in times when they lacked scientific terminology. Through hundreds of years of trial and error, the craftsmen acquired through empirical experience the optimal heating temperature, cooling rate, and cooling medium.

Control of heating temperature was determined by color. As the color of the heated-to-red blade transitions from dark red to bright red and further to yellow, the steel reaches different temperatures. It is believed that sword craftsmen acquired an understanding of this "color observation" through their training and were able to judge temperature with an accuracy of approximately ±10°C.

The choice of cooling medium was also extremely important. Different cooling media such as water, oil, ash, and salt significantly affect the cooling rate. Normally, sword craftsmen used a secret formula liquid called yaki-mizu. This liquid was not merely water but a complex blend of lye, salt, plant ash, and other materials, and it is believed to have precisely controlled cooling rate and cooling unevenness.

Tempering—The 'Balance' of Hardness and Toughness

However, a blade immediately after hardening is more brittle than one might imagine. A blade in the martensite phase is extremely hard, but at the same time extremely prone to cracking and unusable in actual combat. Therefore, an essential process is tempering.

Tempering is a process that reheats a blade after hardening—although not to as high a temperature as during hardening. Typically, it is heated in the range of 150°C to 300°C. At these temperatures, part of the distorted crystal structure of the martensite phase is relaxed, transforming into a more stable "tempered structure." As a result, hardness decreases slightly, but at the same time "toughness (resilience, resistance to cracking)" increases significantly.

The choice of tempering temperature varies depending on the intended use of the blade.

This 'balance of hardness and toughness' is the most critical factor that determines the true value of Japanese swords.

Hardening and Tempering in Practice: Military Swords

The hardening and tempering of Showa-era military swords—particularly the Type 98 military sword (established in 1938)—were designed with 'reliability as a weapon' as the top priority.

The blade of a military sword was often tempered to have higher toughness than typical art swords. This was because it needed to withstand continuous slashing strikes in group combat. In the process of a soldier striking multiple enemies, the blade must not crack or chip. Therefore, the tempering temperature was set higher, and 'tuning that prioritized durability over cutting edge' was performed.

On the other hand, this balance varied greatly depending on the skill of the sword craftsman. Military swords by renowned sword craftsmen realized tuning that 'achieved both hardness and toughness at a high level.' Such swords were highly valued on the battlefield and are said to have been prized by enemy forces as captured weapons.

Microscopy and the Science of Hardening—Modern Verification

Modern metallurgy provides the means to analyze in detail the traces of hardening and tempering on a blade. When the blade is magnified 100 times or more with an optical microscope, the detailed structures of martensitic and tempered structures become visible.

Using hardness tests (Vickers hardness or microhardness tests), the hardness of very small areas of the blade can be measured in 0.1mm increments. When advanced military swords are examined, it can be confirmed that while they fall within a range of HV 550–750 (Vickers hardness) across the entire blade, intentional hardness differences are designed for different parts.

Examples of hardness differences by part are as follows.

• Blade edge: approximately HV 700
• Jihada (body) portion: HV 550–600

From these differences, it can be inferred that sword craftsmen 'deliberately used different cooling and heating conditions for different parts.' This is evidence that they completely understood the 'science of hardening and tempering,' and serves as an indicator of the sword craftsmen's deep technical skill.

Although the phase diagram of iron and carbon in blades was first drawn by chemists in the 19th century, Japanese sword craftsmen were practically controlling this phenomenon hundreds of years before that.