Preventing Decarburization and Scorching: The Science of Surface Oxidation During Forging and Its Impact on Nihontō Quality

In nihontō forging, decarburization (dattan) and scorching (yake) are among the problems that demand the most careful attention from the tōshō. Both are surface-deterioration phenomena that occur in steel under high-temperature conditions and directly affect the quality of the finished blade. Technical discussions of tanren tend to focus on the beauty of the hamon and jigane, but the prerequisite for those qualities is mastery of surface oxidation control. This article explains the mechanisms behind decarburization and scorching, and the preventive techniques practiced by the tōshō.

What Is Decarburization: The Phenomenon of Steel "Wasting Away"

Decarburization is the phenomenon in which carbon near the surface of the steel reacts with oxygen in a high-temperature environment and escapes as CO₂ or CO. Carbon content is a determining factor in the hardness and toughness of steel; as decarburization advances, the carbon concentration at the surface decreases and the hardness of that area is lost.

Tamahagane is a high-carbon steel with a carbon content of approximately 0.6–1.5%, but repeated heating during forging gradually depletes the surface carbon. When yaki-ire (quenching) is applied, martensite transformation is less likely to occur in the surface layer where carbon is deficient, causing the hamon to appear inconsistently or the hardness to become uneven.

The rate of decarburization is proportional to the heating temperature and the intensity of the oxidizing atmosphere. Higher temperatures accelerate it, and decarburization is further sped up in an "oxidizing flame" (sankaen) environment rich in oxygen. Conversely, decarburization can be suppressed under "reducing flame" (kangenen) conditions, in which the oxygen level inside the furnace is restricted. This is one reason why the tōshō carefully adjusts the air volume supplied to the furnace (the airflow from the bellows).

What Is Scorching: Surface Damage Caused by Overheating

Scorching (yake) is a nihontō forging term for a condition in which the surface of overheated steel becomes covered with an oxide film (scale) and the steel microstructure coarsens and embrittles due to excessive heat. In English it is described as "overheating" or "burning," and in metallurgical terms it represents a state in which "burning" caused by excessive heating has occurred.

The surface of a scorched area turns black with a rough, crumbly texture. Because it produces a dull sound when struck—different from normal steel—an experienced tōshō can also detect scorching by ear. Attempting to forcibly continue tanren on a scorched area causes that section to crack or spall. Scorching also coarsens the steel microstructure and is a cause of the loss of fineness in the hadame (surface grain pattern) of the jigane.

Scorching and decarburization often progress simultaneously. Excessive heating increases the rate of decarburization and can lead all the way to scorching. To avoid exceeding the temperature "upper limit," the tōshō constantly works while reading the fire color (hiiro) by eye.

Preventive Techniques Practiced by the Tōshō

The practical techniques for preventing decarburization and scorching can be organized around three main axes: "temperature management," "minimizing heating time," and "use of protective agents."

• Temperature management is the most fundamental countermeasure. The appropriate forging temperature for tamahagane is said to be approximately 900–1,200°C, but the precise figure varies with the carbon content of the steel and is judged not by instruments but by the color of the flame (hiiro) and the glow of the steel. When the brightness exceeds the proper range of yellow-orange to white-hot and surpasses a pure-white glow, the steel has entered the danger zone. This judgment is a sensory skill acquired through years of training, and it is an area where a significant gap exists between veterans and younger smiths.
• Minimizing heating time is equally important. Even at the same temperature, the longer the steel remains in the furnace, the greater the amount of decarburization. The tōshō cycles rapidly through the process of "heating for only the minimum necessary time, removing immediately, and striking." Especially in finishing work on fine details, even small differences in the way the flame contacts the steel affect surface quality, so the rhythm of the work is critically important.
• Use of protective agents: in the tansetsu (forge-welding) process, warabai (straw ash) is used as a solvent (flux). Warabai acts to dissolve and remove the oxide scale during heating, exposing a fresh steel surface. This chemical action locally prevents oxidation during forge-welding and enables uniform bonding. Deliberately creating a "carbon atmosphere" using charcoal can also produce a carburization-protection effect that guards against decarburization.

Removal of scorched areas is another countermeasure. If scorching does occur, the affected area is actively hammered out and removed during the tanren process. Since scorching concentrates at the surface, the scale is hammered off while only the sound steel at depth is preserved. If this work is not done properly, scorched material becomes incorporated into the interior of the blade and leads to a degradation in the quality of the finished sword.

Post-Treatment of Decarburization and Scorching: The Relationship with Togi

When a completed sword has minor decarburization or scorching, the affected layer may be removed during the togi (polishing) process. The togishi progressively changes whetstones from rough to medium to finishing grade while grinding down the blade surface; in this process the decarburized surface layer is removed and the steel surface with its original carbon concentration is exposed.

However, when the decarburized layer is deep, togi alone cannot address it, and it may be necessary to "remake" the sword itself. This is why it is important to minimize decarburization and scorching during forging. Post-treatment after completion is only an option in minor cases.

Decarburization and scorching are one facet of the "chemistry of steel" that underlies the beauty of the nihontō. The tōshō's work of controlling these four variables—carbon, oxygen, temperature, and time—through all five senses forms the foundation that gives rise to the beauty of the jigane and the brilliance of the hamon in the completed sword.