Forge Cracks in Nihonto: Causes, Prevention, and Swordsmith Countermeasures for Steel Cracking During Tanren

What Are Forge Cracks—The Greatest Flaw Threatening Blade Integrity

"Forge cracks" (kitae-ware) are a collective term for cracks, fractures, and delamination that occur in the interior or surface of the blade during the forging process of a Japanese sword. In English, they are referred to as "forge crack" or "delamination." Forge cracks are not merely a cosmetic defect; they have the potential to fundamentally compromise the blade's strength, making them one of the flaws that swordsmiths are most cautious about.

Forge cracks can be broadly classified into two types. The first is "surface cracks," fine cracks that appear on the blade surface during or after forging. Minor ones may be removed through polishing, but deep ones may necessitate discarding the blade. The second is "internal delamination" (interlayer cracks), where the layers of steel that have been layered through folding and forging separate from one another. Though difficult to detect visually, this is the most dangerous type of flaw, as it becomes apparent under use (bending stress and impact). It is often discovered by polishers during their work and is one of the principal defects noted in official appraisals.

Chemical Causes—Uneven Carbon Distribution and Oxidation

The occurrence of forge cracks involves multiple chemical and physical factors working in combination. From a chemical perspective, when the distribution of carbon (C) contained in tamahagane is uneven, high-carbon and low-carbon areas exist locally adjacent to one another. High-carbon steel has low toughness and high brittleness, making it prone to cracking under repeated impacts during forging.

Oxidation is also a significant concern. When steel is exposed to air (oxygen) during forging, iron oxide (scale) forms on the surface. If this scale is not properly removed, it is forced into the steel's interior and remains as a nonmetallic inclusion. Inclusions become points of stress concentration where cracks propagate easily. When the steel taken from the furnace is struck during the forging sequence, iron sparks scatter. It is important to disperse oxidation scale sufficiently at this point, and swordsmiths employ the technique of intentionally blowing away the scale with the first blow of the hammer.

Physical Causes—Temperature Control Failure and Over-striking

From a physical perspective, failure in managing forging temperature is the primary cause of cracking. When steel is forged at excessively high temperatures (overheating range: 1300°C and above), "over-burning" occurs, causing crystal grains to coarsen and grain boundaries to weaken. Over-burned steel has significantly reduced toughness and cracks easily during folding and drawing out. Conversely, forging at temperatures that are too low results in "cold striking," causing "low-temperature cracks" in steel with reduced plastic deformability.

"Over-striking" is also a contributing factor to cracking. When folding and forging are repeated excessively (generally beyond 15-20 times), carbon content decreases too much, and the internal structure of the steel becomes excessively refined, sometimes paradoxically increasing brittleness. The appropriate number of folds varies depending on the quality of tamahagane and the intended purpose of the sword, but the "forge sense" accumulated by individual swordsmiths includes the ability to discern this proper number, and this is often transmitted to apprentices through oral tradition.

Practical Prevention Measures—From Material Selection to Process Control

Practical measures to prevent forge cracks begin at the material selection stage. The appropriate carbon content of tamahagane is roughly 0.6-0.9% in the outer layer (kawagane) and 0.2-0.4% in the core (shingane), and swordsmiths roughly judge carbon content by tapping the material and observing the sound, feel, and color of the fracture surface. Since tamahagane with low homogeneity (uneven carbon distribution) carries high risk of forge cracks, eliminating high-risk materials at the material selection stage is the first prevention measure.

In the forging process, strict adherence to the appropriate temperature range (900-1200°C) is essential. Modern swordsmiths manage temperature by combining infrared thermometers with visual judgments based on color (yellow-orange color is roughly a guide for 1000-1100°C). In forging, some swordsmiths carefully manage the number and direction of folds, consistently folding from the same direction to give directionality to the layer structure and control the direction of internal delamination.

The most important practical prevention measure is maintaining "forge rhythm." Because steel taken from the furnace cools over time, striking at a constant rhythm in terms of speed, force, and frequency is necessary. When rhythm breaks down, specific areas become over-cooled or over-heated, and uneven stress distribution induces cracking. More experienced swordsmiths carefully observe their apprentices' striking rhythm, maintaining proper rhythm through verbal guidance and demonstration.

Discovery and Management—Decision-making When Cracks Occur

When forge cracks are discovered, the swordsmith's decision depends on the "depth, location, and stage" of the crack. If discovered during the early to mid-forging stages, the swordsmith may attempt repair by reheating the cracked area to high temperature and forge-welding it back together through hammer strikes. However, if inclusions have entered the cracked area, forge-welding becomes difficult, and the swordsmith must decide whether to excise that section and continue forging or to discard the entire material.

If cracks are discovered after hardening, repair is impossible, and the blade must be discarded. When a polisher discovers cracks during the polishing stage, there is an obligation to promptly report to the commissioning party (swordsmith or buyer), and in official appraisals, it is required by industry ethics to honestly declare it as a defect. Blades with forge cracks are significantly downgraded in value as "flawed," causing considerable psychological and economic damage to the swordsmith, which is why preventing such flaws is taken so seriously.