The Number of Folding Cycles and Steel Microstructure: Theory of Over-Forging-Induced Brittleness and Optimal Forging Cycles

What is Folding and Forging — The Heart of the Japanese Sword

In the manufacturing process of the Japanese sword, folding and forging is one of the most fundamental techniques. By repeatedly heating tamahagane forged from tatara smelting at high temperatures and folding and beating it repeatedly with forging rods (tagane rods), the internal structure of the steel becomes homogeneous, impurities are pressed out, and the distinctive jihada of the Japanese sword is formed.

With each folding, the number of layers doubles. Folding ten times theoretically creates 2^10 = 1024 layers, and fifteen times creates 32,768 layers. This layered structure creates patterns called 'jihada' with wood-grain and wave-like designs, which is one of the most important elements in appreciating the Japanese sword.

Number of Folds and Changes in the Steel's Layered Structure

As the number of folds increases, the following changes occur within the steel.

Homogenization of Carbon Distribution

Tamahagane has uneven carbon distribution internally when it is produced. High-carbon zones (0.7–1.5%) and low-carbon zones are mixed, and if the sword is forged as-is, it becomes an unstable blade with varying hardness in different areas.

By repeating the folding and forging, high-carbon and low-carbon zones are alternately stacked, and carbon diffuses uniformly throughout. Through this homogenization, the state where "stable hardness is achieved regardless of where quenching is applied" is realized.

Removal of Inclusions

Tamahagane contains slag (inclusions: iron oxide, silica, etc.) that have been mixed in during the smelting process. During the repeated process of beating and extending at high temperature, these inclusions are gradually pressed out of the steel.

If inclusions remain, that area becomes a stress concentration point and causes breaks or cracks. Folding and forging is not merely a shaping process but also a "purification process."

Refinement of Crystal Grain

Iron crystals (ferrite and austenite) grow at high temperatures, but plastic deformation from striking and folding breaks and refines the crystal grains. The finer the crystal grains, the greater the steel's toughness, resulting in a blade less prone to breaking.

Brittleness from Overforging — The Existence of an Optimal Range

Is it better to fold as many times as possible? The answer is no. This is where the most important "optimal value problem" in Japanese sword forging technique exists.

Excessive Carbon Consumption

During repeated folding and forging, carbon in the steel is gradually lost through oxidation (decarburization). As the carbon content decreases, the hardness necessary for a blade (hardness after quenching) cannot be achieved.

Specifically, tamahagane initially has an average carbon content of around 0.8–1.2%, but it gradually decreases with repeated forging. If excessive folding of 20 times or more is performed, the carbon content drops to 0.3–0.4% or less, resulting in properties close to soft "pure iron."

Excessive Crystal Grain Refinement and Work Hardening

When overforging causes excessive refinement and deformation of crystal grains, conversely, work hardening progresses and the steel becomes brittle. In metallurgical terms, this is called "deformation-induced transformation" or "strain hardening saturation."

Forging, which should make the blade less prone to breaking, becomes excessive and produces the opposite effect of "accumulation of strain from deformation → brittleness."

Signs of Overforging Appearing as "Roughened Grain"

Bladesmiths empirically judge overforging by observing the surface of the jihada. As overforging progresses, the jihada becomes "roughened" — the originally fine-grained wood-grain surface becomes coarse and the boundaries between layers become chaotic. This is a sign that heterogeneous carbon distribution has reappeared internally and the layered structure has collapsed.

Theory of Optimal Forging Cycles — Secrets of the Schools

The "appropriate number of folds" for Japanese sword folding and forging is a secret technique of each school (Yamashiro, Soshu, Bizen, Yamato, and Mino traditions). According to modern research and reconstruction experiments, generally 8–15 folds is considered the optimal range.

Trends by School

• Soshu tradition (Masamune, Sadamune, and others) is known for strong toughness and intense hamon, which relates to the use of "wild steel" that is quenched with relatively few folds while retaining coarse ohiame jihada.
• Bizen tradition (Osafune school and others) is known for tightly-packed small ihame and ko-mokume jihada, with a tendency toward more folds emphasizing homogenization.
• Yamato tradition (Senshuji, Taima, and others) is characterized by flowing masame jihada, with the distinctive technique of maintaining consistent folding direction.

Differences in Optimal Folding by Steel Classification

The structure of a blade is divided into "kawagane" (outer steel) and "shingane" (inner steel).

• Kawagane (outer skin, high-carbon steel): covers the edge and surface. Requires hardness, so it is finished while retaining carbon. The target number of folds is around 8–12.
• Shingane (inner core, low-carbon steel): requires flexibility and toughness, so carbon may be intentionally reduced through more folding.

Warikomi-gane, which combines these two types of steel to create a blade, is the fundamental structure of the Japanese sword, and the management of fold count has different optimal values for kawagane and shingane.

Mechanism of "Grain" Formation in Jihada — Beauty Created by Folding and Forging

What cannot be overlooked as a scientific consequence of folding and forging is the formation of "jihada," which is the essence of the Japanese sword's beauty.

The interfaces (boundaries) of steel stacked in layers are first visualized when polished out by a togishi using finishing whetstones. Jihada appears as wood-grain, flow, and patterns as the slight difference in carbon concentration between layers changes the reflection of light.

• Ihame: As layers with irregularly mixed carbon distribution stack, irregular patterns like wood grain appear.
• Masame: When folding direction is unified in a single vertical direction, linear patterns like flowing growth rings appear.
• Mokume: Spiral or knot-like patterns that appear as a result of locally remaining carbon distribution.

These jihada are not merely aesthetic concerns but are "maps" of internal carbon, layers, and crystal structure. Experienced appraisers read the number of fold tendencies, carbon distribution, and forging methods from the jihada to estimate school, period, and maker.

Conclusion — The Experiential Knowledge of Bladesmiths Proven by Science

The optimal number of folds and the risks of overforging are being elucidated in the form of modern metallurgy (microstructure of carbon steel, work hardening, decarburization theory) "proving after the fact" the wisdom that traditional bladesmiths empirically acquired.

Rejecting the naive assumption that "more forging makes a better blade," the precise technical judgment of "drawing out the finest steel within an optimal forging cycle" is the essence of Japanese sword forging and manifests as the beauty of jihada. Those who wish to actually compare the differences in jihada are encouraged to also read explanations of forging styles by school from the column list.