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

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

In the manufacturing process of Japanese swords, folding forging is one of the most essential techniques. By repeatedly heating tamahagane (sword steel) to high temperatures and folding, pounding, and drawing it out with a hammer, the internal structure of the steel becomes homogenized, impurities are expelled, and the distinctive steel body characteristic of Japanese swords is formed.

Each fold doubles the number of layers. Folding 10 times theoretically yields 2^10 = 1,024 layers, and 15 times yields 32,768 layers. This laminated structure creates the pattern called "skin"—a wood-grain or wavy appearance—which is one of the most important elements in appreciating Japanese swords.

Changes in Layer Structure as Folding Cycles Increase

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

Homogenization of Carbon Distribution

Tamahagane has uneven carbon distribution at the point of production. High-carbon zones (0.7–1.5%) and low-carbon zones are mixed together, and if a blade is forged from it as-is, it becomes unstable with varying hardness in different areas.

By repeatedly folding, high-carbon and low-carbon zones are layered alternately, allowing carbon to diffuse evenly throughout. This homogenization achieves a state where "stable hardness is obtained regardless of where quenching is applied."

Expulsion of Inclusions

Tamahagane contains slag (inclusions: iron oxide, silica, etc.) that entered during iron smelting. Through repeated heating and hammering at high temperatures, these inclusions are gradually expelled from the steel.

If inclusions remain, they become stress concentration points and cause breakage or cracking. Folding forging is thus also a "purification process."

Refinement of Crystal Grains

Iron crystals (ferrite and austenite) grow at high temperatures, but plastic deformation through hammering and folding breaks and refines the crystal grains. The finer the crystal grains, the higher the toughness of the steel, and the less likely the blade is to break.

Brittleness Induced by Over-Forging—The Existence of an Optimal Range

So, is it always better to fold more times? The answer is no. Here lies the most important "optimization problem" in Japanese sword forging technology.

Excessive Carbon Consumption

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

Specifically, the initial carbon content of tamahagane is typically 0.8–1.2%, but it gradually decreases with each folding. If excessive folding of 20 times or more is performed, the carbon content drops to 0.3–0.4% or below, resulting in properties approaching soft "wrought iron."

Excessive Grain Refinement and Work Hardening

When over-forging causes excessive grain refinement and deformation, work hardening progresses and the steel becomes brittle. This is metallurgically known as "deformation-induced transformation" or "saturation of strain hardening."

Forging intended to prevent breakage, when taken to excess, paradoxically causes "accumulation of deformation-induced strain → brittleness."

Signs of Over-Forging Appearing as "Rough Skin"

Bladesmiths empirically judge over-forging by observing the steel surface. As over-forging progresses, the steel skin becomes "rough"—the originally fine-grained wood-grain texture becomes coarse, and the boundaries between layers become disrupted. This indicates that carbon distribution inhomogeneity is reoccurring internally and the layered structure is breaking down.

Theory of Optimal Forging Cycles—School Traditions

The "appropriate number" of folds for Japanese swords is proprietary knowledge of each school (Yamashiro, Soshu, Bizen, Yamato, and Mino traditions). According to modern research and restoration experiments, 8–15 folds are generally considered the optimal range.

Trends by School

The Soshu school (Masamune, Sadamune, and others) is known for strong toughness and vigorous blade patterns, which relates to the use of "spirited steel"—deliberately retaining a coarse large-grain surface pattern by quenching after relatively fewer folds.

The Bizen school (Osafune lineage and others) is characterized by densely packed small-grain patterns and tends to emphasize homogenization through relatively more folds.

The Yamato school (Sensh'uin, Taima, and others) is characterized by flowing straight-grain patterns and distinctive technique of maintaining consistent fold direction.

Variation in Optimal Cycles by Steel Classification

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

Outer steel (surface, high-carbon steel): covers the edge and outer surface. Because hardness is needed, it is finished while retaining carbon. Approximately 8–12 folds is the guideline.

Core steel (inner core, low-carbon steel): because flexibility and toughness are needed, carbon may be deliberately reduced through more folds.

"Differential forging"—combining these two types of steel to create a blade—is the fundamental structure of Japanese swords, and the management of fold cycles has different optimal values for outer and core steel.

The Mechanism of Steel "Skin" Formation—Beauty Born from Folding Forging

Among the scientific consequences of folding forging, one cannot overlook the formation of "skin"—the essence of Japanese sword beauty.

The interface of the layered steel becomes visible through polishing and buffing. The slight differences in carbon concentration between layers change the reflection of light, creating the visual appearance as grain, flow, and patterns.

Itame (wood-grain pattern): when layers with irregularly mixed carbon distributions are folded together, an irregular pattern resembling wood grain appears.

Masame (straight-grain pattern): when the fold direction is consistently kept vertical, flowing linear patterns resembling tree rings appear.

Mokume (swirl pattern): spiral or knot-like patterns that appear as a result of localized carbon distribution retention.

These steel skin patterns are not merely aesthetic but a "map" of the internal carbon, layers, and crystal structure. Trained appraisers read the skin patterns to determine trends in fold cycles, carbon distribution, forging methods, and deduce the school, era, and smith.

Conclusion—Scientific Proof of Bladesmiths' Empirical Knowledge

The optimal fold cycle and risks of over-forging have been clarified through modern metallurgy providing "retrospective proof" of the empirical knowledge that past bladesmiths developed through experience.

Rejecting the naive assumption that "more forging makes a better blade," the essence of Japanese sword forging lies in refined technical judgment—"extracting the finest steel within optimal forging cycles"—and this is crystallized in the beauty of the steel skin.