Quenching and Martensitic Transformation—The Science of Steel Microstructure Changes That Harden Japanese Swords

What is Tempering — The Final Process that Transforms a Sword into "Living Steel"

Tempering (yaki-ire) is the most dramatic process in Japanese sword manufacturing. A sword blade that has been coated with clay (tsuchi-oki) is heated in a furnace, and at the moment it reaches optimal temperature, it is plunged into a water tank all at once. Within these few seconds, intense structural changes occur inside the steel, transforming the previously soft steel into a hard edge. At the same time, a beautiful edge pattern (hamon) emerges, and the sword's "face" is completed.

Tempering is also a process of irreversible final decision. If the temperature, timing, or water temperature deviates even slightly, the sword becomes a "failure (dame)." The forms of failure are as follows:

• The thin blade edge cannot withstand the thermal stress of rapid cooling and cracks
• The process ends in insufficient hardness called "under-tempering (yaki-busoku)"

After being refined through over a hundred processes, a sword blade could become waste in this single moment—this is the scene where the swordsmith's skill and mental fortitude are tested most severely.

From Austenite to Martensite — How Steel Becomes Hard

The reason steel becomes hard through tempering is due to changes in the crystal structure of iron. At room temperature, carbon steel is a mixed microstructure of body-centered cubic (BCC) ferrite and cementite (iron carbide: Fe₃C), but when heated to the A1 transformation point (eutectoid temperature: approximately 727°C), transformation to austenite begins. In hypo-eutectoid steel with lower carbon content, it must be heated to the higher A3 point to achieve complete austenite (or above the Acm point in hyper-eutectoid steel). Austenite has a large carbon solid solubility (up to approximately 2.14%), bringing all the carbon that had precipitated at the ferrite stage into a dissolved state.

When rapidly cooled from this austenite state, carbon has no time to escape, and the transformation from FCC structure to BCC structure is frozen mid-process, forming a "frozen" structure. This is the steel microstructure called "martensite," which has a body-centered tetragonal (BCT) lattice structure. The distortion of this lattice is the source of hardness; because dissolved carbon distorts the lattice, the material resists deformation, and hardness can reach 60 or higher on the Rockwell hardness (HRC) scale.

In the case of Japanese swords, the edge portion (blade steel: hagane) uses high-carbon steel containing 0.6–1.5% carbon, and tempering forms high-hardness martensite. Meanwhile, the spine side protected by clay coating and the core steel with medium carbon content cool more slowly, transforming into softer and tougher microstructures called "pearlite" or "bainite." This dual hardness structure is the fundamental reason why Japanese swords achieve the seemingly contradictory performance of being "hard yet unbreakable."

Determining Tempering Temperature — How Swordsmiths Read the Flame's Color

The heating temperature suitable for tempering varies depending on the carbon content of the steel, but the optimal range for general Japanese sword carbon steel is typically 760–820°C. Below this temperature, the austenite transformation becomes incomplete, resulting in insufficient hardness. Conversely, if the temperature is too high, the grain coarsens and decarburization progresses, increasing the risk of quench cracks (yaki-ware).

In modern industrial tempering, precise thermometers and furnace temperature controls are used, but traditional Japanese sword tempering is still performed almost entirely by "visual judgment." The color of the light emitted by the sword blade in the furnace functions as an accurate thermometer for the swordsmith's eye.

The approximate correspondence between steel color and temperature is as follows:

Because this judgment is greatly influenced by the work environment (natural light, lighting), many swordsmiths perform tempering in the dim early morning before sunrise. This is accumulated wisdom that eliminates visual disturbances.

Some master swordsmiths use as an auxiliary method holding the heated blade near a magnet to confirm the "moment it loses magnetism." Iron loses ferromagnetism above the A2 transformation point (Curie temperature: approximately 770°C), so the temperature at which the magnet no longer adheres becomes a guide for the tempering temperature range.

Ms Point and Mf Point — The Beginning and End of Martensite Transformation

Martensite transformation has a distinct starting temperature and ending temperature. The "Ms point (martensite transformation start point)" is the temperature at which martensite begins to form during cooling, and the "Mf point (martensite transformation finish point)" is the temperature at which transformation is complete.

The higher the carbon content, the lower the Ms and Mf points shift. In steel with 1% carbon, the Ms point drops to approximately 230°C, and the Mf point can fall below room temperature. This means that even when high-carbon steel is cooled to room temperature, transformation may not be completely finished, and untransformed austenite (retained austenite) may remain.

The structural design of Japanese sword forging, which combines high-carbon blade steel (hagane) with medium-carbon core steel through "composition building," cleverly utilizes these differences in Ms and Mf points. The high-carbon steel blade edge forms high-hardness martensite through tempering, while the medium-carbon core steel, combined with slower cooling speeds, forms tough pearlite and bainite microstructures. Although traditional swordsmiths did not scientifically understand this phenomenon, the fact that they established optimal material design and processes through over a thousand years of trial and error continues to astonish modern materials scientists.

Tempering (Reheating) — Balancing Hardness and Toughness

A sword blade immediately after tempering has high hardness but also high brittleness. Because the lattice distortion of martensite is too great, even minor impacts can easily create internal cracks. The process used to solve this problem is "tempering (yaki-modoshi, also called reheating or tempering)."

Tempering involves reheating the tempered sword blade at a low temperature (typically 150–250°C). At this temperature range, some of the carbon in the martensite precipitates to form carbides, and the lattice distortion is relieved. As a result, hardness decreases slightly, but toughness (resistance to impact) improves significantly.

In traditional Japanese sword manufacturing, the operation of air-cooling the blade for a short time after it is removed from the water following tempering corresponds to a practical tempering treatment. As internal heat generated by tempering transfers from the sword's core to the surface, the surface is appropriately reheated. This effect of "self-tempering (jiko-yaki-modoshi)" is thought to contribute to maintaining the toughness of Japanese swords and represents knowledge that connects to modern high-speed steel tempering techniques.

Hamon — The Face of the Japanese Sword Created by Tempering

The visual product of the tempering process is the "hamon" (edge pattern). This brightly glowing pattern that appears after polishing is the boundary between the martensite microstructure (hardened region) and the pearlite and bainite microstructure (softened region), reflected as a difference in light reflection. Martensite appears white because it diffuses light, while pearlite appears gray to black because it reflects light uniformly.

The shape of the hamon is determined by the shape of the clay coating, and each school creates its own unique patterns (straight edge, undulating, clove-bud shaped, alternating patterns, and so on). In other words, the hamon is not merely decorative but is a scientific proof of successful tempering, a "steel fingerprint" that records the swordsmith's technique and individuality. The ultimate criterion for judging whether tempering was successful is whether the hamon appears as beautifully intended, and it is here that the swordsmith finally learns the success or failure of a single blade.