The Science of Folding and Forging — How Repetitions, Temperature, and Layer Count Transform Steel Structure

What is Folding and Forging—The Fundamental Process that Transforms Japanese Sword Steel

Folding and forging (orikaeshi tanren) is one of the most important processes in Japanese sword manufacturing, involving the repetitive operation of heating steel, hammering it out, folding it, and hammering again. The purpose of this process is not simple "strengthening," but rather to homogenize the internal structure of the steel, remove impurities, and ultimately create a material with optimal sharpness and strength.

The raw block of tamahagane obtained from tatara iron-smelting is, on the surface, a chunk of metal, but harbors various problems within. Local variations in carbon content, residual slag (ore dross), oxidation contamination, and coarse crystal grain growth—if these are not eliminated before hammering the steel into the shape of a sword, they become causes of insufficient strength and brittle fracture. Folding and forging is the core technology of the manufacturing process that resolves these problems.

Folding Count and Layer Formation—The Exponentially Increasing Interface

The number of folding operations and the resulting number of steel layers have an exponential relationship. The relationship where n foldings produce 2ⁿ layers is shown below.

In typical Japanese swords, folding is performed approximately 8 to 15 times, forming layered structures of extremely thin steel.

The formation of these innumerable interfaces—boundaries between steel layers—produces the essential effects of folding and forging. Each interface becomes a site for the redistribution of carbon, carbides, and residual slag, and through repeated heat and pressure, local compositional irregularities are gradually equalized. This phenomenon's principle is what modern materials science calls "diffusion enhancement through plastic deformation."

However, more folding is not necessarily better. When exceeding 20 folds, the layer thickness reaches nanometer order, and it has been reported that the interface energy becomes excessive, paradoxically making the steel brittle. Additionally, carbon becomes depleted through oxidation, causing the carbon content to drop excessively. The traditional sword smith's empirical rule of "limiting folding to 12 to 15 times" is consistent with modern materials science knowledge.

Temperature Control—The Optimal Heating Temperature Range for Forging

The heating temperature in folding and forging is the most critical variable directly affecting changes in steel microstructure. The temperature range suitable for processing carbon steel is called the "forging temperature range," which varies depending on steel composition, but for the medium-to-high carbon steel used in Japanese swords (carbon content 0.5–1.5%), approximately 900–1200°C is considered appropriate.

Within this temperature range, the steel assumes a face-centered cubic lattice structure called austenite, becoming a flexible state amenable to plastic deformation by hammering. Stepping outside this range results in the following problems:

• Below 900°C, ferrite—with its body-centered cubic lattice—begins to form, deformation resistance increases sharply, and the steel becomes "hard and brittle"
• Above 1200°C, the steel approaches its melting point, oxidation accelerates sharply, and "overheating (burning)" occurs, degrading the steel structure

Traditional sword smiths do not use thermometers, but judge temperature from the flame of the forge and the color of the steel. Reading temperature from color requires several years of training, and the correspondence between color and temperature is roughly as follows:

• Dark red (approximately 750°C)
• Red (approximately 850°C)
• Orange (approximately 950°C)
• Bright orange (approximately 1100°C)
• Yellow (approximately 1200°C)

Determining the lower limit of forging temperature (around 900°C) is particularly difficult, and continuing to hammer steel that has cooled too much risks generating "cold cracks" internally.

The Mechanism of Impurity Removal—How Slag and Oxidation Products Are Expelled

Another important effect of folding and forging is the removal of impurities from the steel. Tatara tamahagane retains trace amounts of slag (silicate and oxide complexes), which causes steel strength reduction and brittle fracture.

When heated steel is hammered, internal slag deforms and is pushed toward the surface. On the surface of hot steel, slag mixes with the oxide layer (scale) and scatters as a spray called "oroshi." By repeating this process, the slag content inside the steel drops rapidly.

Simultaneously, the distribution of carbon also becomes homogenized. In the raw block of tatara tamahagane, regions of high carbon content (white-surfaced "white iron" or hakutetsu) and low carbon content (dark-surfaced "black iron" or kurotetsu) coexist. Through folding and forging, this carbon segregation is gradually resolved, and steel with uniform carbon distribution throughout is formed. This homogenization allows the hamon to appear beautifully uniform after quenching.

Microstructure Refinement—How Grain Size Control Creates Sharpness

The effect of folding and forging on steel's crystal grain size is also important. Through repeated heating, cooling, and deformation, steel's crystal grains gradually refine through cycles of recrystallization and grain growth. Steel with fine crystal grains exhibits superior properties in strength, toughness, and uniformity compared to steel with coarse grains (Hall-Petch relationship).

The fineness of the martensite structure formed after quenching also depends on the crystal grain size before forging. By pre-refining the crystal grains through folding and forging, the martensite after quenching forms uniformly and finely, achieving a steel structure that combines high hardness with toughness. The surface finish of the jigane that appears after polishing—diverse patterns such as mokume, itame, and ayasugi—is the manifestation of this fine crystal structure on the polished surface, and differences in the quality of folding and forging are directly reflected in the beauty of the jigane.

Degradation from Over-folding—Carbon Depletion and Brittleness Problems

The most serious drawback of excessive folding is carbon depletion. When steel is exposed to air at high temperature, surface carbon oxidizes and escapes as carbon dioxide—a phenomenon called "decarburization." The more folding occurs, the more heating cycles the steel undergoes, and carbon loss from decarburization accumulates.

Steel with reduced carbon content loses hardness after quenching, becoming a "sword that won't cut." Conversely, using excessively carbon-rich raw material to compensate for carbon results in excess carbon even after folding, making the steel brittle. Traditional sword smiths empirically understand the "carbon balance during forging" and maintain optimal carbon content by selecting appropriate folding counts and heating times.

This "carbon management" is one of the highest techniques of the sword smith's craft. Modern materials science analysis of cross-sections of famous swords from the Edo period reveals remarkably uniform carbon distribution, and one cannot help but marvel at how the sword makers of that era achieved this advanced materials management through experience and intuition alone, without theoretical knowledge. TOUKENZA's online catalog features swords that convey this precise forging technique.