Yakimodoshi (Tempering) Technique: Post-Quenching Low-Temperature Treatment and Toughness Science

Finishing the Quench—What Is Tempering?

Among all the manufacturing processes of Japanese swords, the "yaki-ire" (quenching/quench-hardening) is the most frequently discussed. Heating a sword blade coated with clay-tempering compound in a furnace and then plunging it into water to rapidly cool it, thereby hardening the edge and creating a beautiful hamon (blade pattern)—this dramatic process is the very symbol of the Japanese sword.

However, quenching has a serious side effect. The "martensitic transformation" resulting from rapid cooling hardens the blade while introducing brittleness (morosa). A blade that is hard but brittle risks breaking under the impact of actual combat or test-cutting.

To mitigate this brittleness, a process called "yakiyamashi (tempering/heat treatment)," also known as "tempering heating," is applied. This is a low-temperature treatment process. In realizing the "three virtues" of the Japanese sword—"does not break, does not bend, cuts well"—tempering is just as important as quenching itself.

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Quenching and Martensite from the Perspective of Metal Science

To understand tempering, we must first grasp what happens during quenching.

From Austenite to Martensite

When steel (with carbon content of approximately 0.3–1.5%) is heated to around 800°C or above, iron atoms take on a face-centered cubic lattice (austenite) atomic arrangement, with carbon atoms uniformly dissolved within the lattice.

When rapidly cooled from this state (quenching), carbon atoms do not have time to be expelled from the lattice and remain "trapped" as the iron lattice transforms to a body-centered tetragonal lattice (martensite). This lattice distortion generates hardness—a Vickers hardness of HV700–900 or higher, which is 2–3 times that of ordinary steel.

The Brittleness of Martensite

The problem is that because of lattice distortion, martensite makes it difficult for dislocations to move, causing a marked decrease in the material's toughness (resilience). In response to impact, brittle fracture (breaking) rather than ductile deformation (bending) becomes more likely.

Immediately after quenching, the blade is in a "hard and brittle" state, which is not practical for actual use as is.

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The Principle of Tempering—Low-Temperature Treatment to Mitigate Brittleness

Tempering is a process in which steel that has been quenched is reheated to a low temperature of 150–300°C. Within this temperature range, no transformation significant enough to soften the steel occurs, but the following changes take place.

1. Fine Precipitation of Carbides

Carbon trapped within the martensite begins to precipitate as "cementite (iron carbide, Fe₃C)" in fine crystalline form through low-temperature heating. This relieves lattice distortion and restores toughness.

2. Stabilization or Decomposition of Retained Austenite

"Retained austenite" that did not completely transform to martensite during rapid cooling either stabilizes or transforms to a different microstructure through low-temperature treatment. Retained austenite carries the danger of sudden transformation to martensite under external stress (transformation-induced plasticity), and relieving this prevents unexpected deformation.

3. Mitigation of Internal Stress

"Residual stress" from thermal unevenness during rapid cooling is partially relieved by low-temperature heating. Since internal stress is a source of warping (sori) changes and fine cracking in the blade, its relief contributes to blade stability.

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Forms of Tempering Implementation in Japanese Swords

In traditional Japanese sword forging, whether the "tempering" process was implemented with the same conscious understanding as in modern metallurgy remains debated due to sparse historical documentation. However, the following perspectives suggest its implementation.

The Sword Smith's "Refinements in Cooling"

The water temperature immediately after quenching, the quenching speed, the manner of holding the blade—all of these were controlled with precision by experienced sword smiths. Holding the blade while still hot in a specific state after being withdrawn from water, a process that may or may not have been intentional, likely brought about a tempering effect through low-temperature heating.

A Process Known as "Sashimodoshi"

Some sword smiths have records of a process called "sashimodoshi" (re-insertion/re-tempering) or "modoshi" (tempering). Different smiths took different approaches: low-temperature heating of the quenched blade in a furnace or residual fire, or maintaining it at a temperature close to body heat by fitting the hilt and holding it.

From the perspective of modern materials science, such processes are interpreted as bringing about a tempering effect at approximately 150–250°C, thereby improving blade toughness.

The Modern Sword Smith's Scientific Approach

Some contemporary sword smiths (particularly those certified by the Agency for Cultural Affairs to practice the craft) actively incorporate materials science knowledge and consciously implement tempering. Electronic furnaces and thermocouple thermometers are used for temperature control, and tempering in electronic furnaces set to 150–200°C is sometimes performed.

However, in traditional forging there is debate about whether to perform "tempering in electronically controlled furnaces," with philosophical disagreement between sword smiths who emphasize "maintenance of traditional processes based on hand-feel and experience" and those who prioritize "quality stabilization through scientific control."

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What Happens When Tempering Is Insufficient or Excessive

Insufficient Tempering (Excess Brittleness)

When tempering is insufficient, the brittleness of martensite remains, and the blade becomes vulnerable to impact. During test-cutting or sparring, the edge becomes prone to chipping (hagane-kobore), and in the worst case, the blade breaks. In particular, insufficient toughness on the spine (mune) side increases the risk of blade fracture.

Excessive Tempering (Hardness Loss)

Conversely, if the tempering temperature is too high or the duration too long, the martensitic hardening is lost, and the blade's cutting ability diminishes. Additionally, the hamon (the boundary of the quenched structure) may become blurred, potentially reducing its value in appraisal.

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The Effect of Tempering on Hamon Appearance

Tempering does not change the shape of the hamon itself, but is said to exert subtle influence on the appearance of "nie" (boiling points—granular luster on the blade pattern boundary) and "nioi" (scent—haze-like gradation at the blade pattern boundary).

• Nie (granular luster on the blade pattern boundary): More prominent as martensite particles are larger, but the luster may change as particles become finer through tempering
• Nioi (haze-like gradation at the blade pattern boundary): Related to the temperature gradient at the boundary and the carbide precipitation state; the appearance changes depending on tempering conditions

Some appraisers possess the skill to judge the degree of tempering from the "standing" of nie particles in the hamon.

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Conclusion—The Science Supporting "Does Not Break, Does Not Bend, Cuts Well"

The three great virtues of the Japanese sword—"does not break, does not bend, cuts well"—cannot be realized by a single process. While quenching provides "cutting ability and blade pattern," tempering plays the role of securing "toughness that does not break."

The mechanisms of martensitic transformation, carbide precipitation, and residual stress relief elucidated by modern materials science validate techniques that Japanese sword smiths have long practiced through experience and intuition. Tempering technology, where tradition and science intersect, stands as a symbol of the depth of Japanese sword forging.