Detailed casting technology of Japanese knife

Simply put, the main material of Japanese Dao is Tamahagane. However, the materials and methods used in the sword during the "ancient road" period have been lost. With the present technology, we can analyze the chemical composition of the finished sword, but we can't accurately calculate the material composition before fire treatment, the temperature, time, times and firing mode of fire treatment. The earliest knife-making method that can be handed down mainly comes from the records of the Edo period. Different knife schools have different methods of making knives in different times. The following can only briefly list the typical knife-making steps:

Step one. "Mizuho Poetry"

Also known as "water reduction". That is, "Jade Steel" is heated and hammered into thin slices with a thickness of about 5 mm ... This sounds like a very simple process, but it is not the case ... In order to control the carbon content of steel (retention/loss of carbon content), the number of heating times is strictly limited; Moreover, the hardness of "Jade Steel" will change with gradual cooling. Only experienced knife makers can accurately grasp the change of hammering force and forge "Jade Steel" into thin slices with uniform thickness under limited heating times. After the steel sheet is formed, the knife maker will quickly cool it with water. The part with enough carbon content will naturally decompose and be used as the material for making knives. Knife makers must master the temperature and water consumption of steel plates very accurately in order to collect materials with appropriate carbon content. The rest will be used by the knife maker in the future. From the perspective of modern materials science, this step can be regarded as a method for tool makers to control the carbon content of steel.

Step two. "Kovari"

Break the steel into small pieces of 2-3 cm. The unbreakable part is the low carbon content, which some swordsmen will use as the "core iron" of their swords.

Step three. Make a burning platform.

The combustion platform will become a part of the blade, so it must be made of high-quality "Jade Steel". Fuel rod is not a part of the blade and can be made of any steel. )

Step four. Tsusaka

The broken steel blocks obtained by the "small cutting" process are welded on the combustion platform layer by layer, so that the heat is evenly conducted. The thermal viscosity of steel block has a decisive influence on the welding effect, and the thermal viscosity depends on the purity and carbon content of steel, so we should choose "Jade Steel" and carry out the first step of "water sinking" process. Different knife factions have different welding methods ... parallel welding is called "short volume forging", cross welding is called "beating wood forging" and cross welding is called "wood leaf forging" or "cross forging". Take forging a "knife" as an example, you need to accumulate about 2 to 3 kilograms of steel.

Step five. Tsukuba bridge

Put the materials after the "weight accumulation" process back into the furnace to ensure that the steel can be completely welded. In order to ensure that the steel is completely isolated from the air (to prevent the carbon content in the steel from being consumed by fire) and allow slow and uniform heat treatment, the knife maker will tightly wrap the steel with paper covered with mud juice and straw ash before putting it into the fire. Knife makers must carefully control the temperature and heating time of the fire.

Step six. Return exercise

Japan has never had the opportunity to develop high-temperature smelting technology in ancient times, so it is difficult to forge homogeneous swords. In order to overcome this problem, we can only use the skill of "turn-back exercise". Fold the steel after the "boiling" process and weld it again. Only need to repeat 10 times, you can make 1024 layer (2 10 power) steel. The more layers, the more uniform the carbon and various components in the steel, the finer the iron crystals and the higher the strength of the finished product. (Note: However, if there are too many layers, it means that if the steel stays in the furnace for too long, the carbon content in the steel will also be lost too much, which will affect the hardness of the finished product and limit the sharpness. Generally speaking, Japanese knives don't go through more than 15 turn-back exercises. In the "turn-back exercise", constant hammering will make a large part of impurities in steel turn into sparks and fly away. Impurities are the "strength weakness" of steel, and the damage often starts from the "strength weakness" and gradually reaches the whole material, becoming a comprehensive damage. The less the number of "weakness of strength", the less the chance of gradual destruction. Therefore, the purer the steel, the higher its strength and toughness. Swords made by high-temperature smelting furnaces all over the world will have the problem of iron crystal hypertrophy after molding. According to the explanation of thermodynamics, in the process of high-temperature refining, in order to reduce the number of fine iron crystals (reduce the total surface area), they will combine with each other and recombine into larger and smaller iron crystals. In this way, the strength of steel will be affected. Therefore, the sword made by high-temperature smelting furnace must be put back into low-temperature fire for several hours after the fire is simmered (that is, the white-hot steel is thrown into water/oil for cooling), so that the tiny iron crystals can be recrystallized between the original crystals and the strength and toughness can be restored. However, long-term fire training will cause excessive loss of carbon content and affect the surface hardness and sharpness of finished products. Compared with western swords, Japanese swords refined in a low-temperature smelting furnace (below 1000℃) can always keep the iron crystal in a fine state, so there is no need for tempering after tempering, which further reduces the loss of carbon and maintains the hardness, strength and toughness. In addition, the sword after "turning back practice" will have a loose wood grain-like surface texture ("ground rib"), which is extremely beautiful. (Note: There are advantages and disadvantages. Steel in high temperature smelting furnace is soft and easy to make and shape; The steel in the low-temperature smelting furnace is hard and difficult to make, even beyond personal strength. If the folded layer cannot be completely welded, it will become a potential crack and defect in the finished product. Therefore, in the general construction process, the knife maker will hold the steel drill tightly and give an order, and two or three strong disciples will beat it with a sledgehammer from the side. In other words, making Japanese knives is a labor-intensive job and a great art of exchanging sweat for quality. )

Step seven. "edge manufacturing"

Japanese knives are sharp and not easy to bend, that is, they are "rigid".

At the same time, Japanese knives are not easy to break, that is, they are "flexible". How to make a Japanese knife with both rigidity and softness? Unlike swords in other countries, Japanese swords are not made of a piece of steel, but are made of a hard layer of "Chuantie" wrapped in another layer of flexible "Xintie" and welded. The leather iron is made of jade steel with high carbon content by 10 to 15 times, and the core iron is made of "Ken Ding Tie" with low carbon content (or low carbon pig iron or jade steel with low carbon content) by 5 to 6 times. With this combination, Japanese knives can be both rigid and flexible. Different knife factions adopt different steel distribution methods, and some even add "Hatetsu" with higher hardness and "Munetetsu" with lower hardness, or adopt double-layer "core iron" which is folded but not welded.

Step eight. Su Ting.

The mixed steel prepared in the process of "edge-making" is made into a long strip and becomes the basic shape of the blade.

Step nine. Create "cut first"

In order to ensure that the "cutting head" has the same distribution of mixed steel as the blade body and obtain a smooth surface texture, the cutter will obliquely cut off a section of the blade tip (the sharp corner is at the flange position), and then use a small hammer to make the sharp corner into a backward curved arc, which is called "cutting head". When the knife maker forged "first cut", the steel was in a high temperature state. After the fire (that is, the blade is cooled in water), the part of the pick shrinks when it is cold, but the crystal of the blade expands, so the blade will shrink in the direction of the building. Making "cutting the head" is the most exquisite manual mule, so the finished "cutting the head" can better show the swordsman's own skill.

Step 10. "fire"

Trim and correct all parts of the blade with a small hammer.

Step eleven. "aging"

The final fire forging process. Knife makers use clay, charcoal powder and millstone powder to make Yakibatsuchi (different schools have different compositions and manufacturing methods), and then package the molded blade with Yakibatsuchi. The scope of "blade" is finer, and the scope of "picking the ground" and "building" is thicker. Basically, the distribution of "burning soil" can be seen from the "edge text" of the finished product. The mud-sealed blades will be put into a fire at 750 to 760 degrees Celsius. The knife maker confirms the temperature by the color of the fire. If the temperature is higher than 800 degrees Celsius, the finished product will appear the phenomenon of iron structure hypertrophy, which will affect the strength. After a certain heating time, the knife maker will move the blade away from the fire, and then put it in water to cool quickly, that is, "fire." (The water temperature, water source, techniques and additives of fire are regarded as the biggest secrets by knife workers of all factions. Due to the different thickness of burning soil, the cooling speed of blades is much faster than that of pickaxes and buildings, so the hardness of blades is much higher than that of pickaxes and buildings. Because of sudden cooling, the iron crystal of the "blade" will change and become larger, so the blade will bend back further after the fire. This situation is different from the problem of "crystal hypertrophy" mentioned above. After firing, the number of "edge" iron crystals remains unchanged, but the volume of each crystal increases. The situation is the same as the density of clear water becomes smaller and the volume becomes larger after freezing.

Step twelve. others

After the initial polishing, nail hole opening, nail hole filing and carving, the responsibility of the knife maker can be said to end here. Generally speaking, other people are responsible for the "polishing", scarring, gold and silver decoration, winding handle and other processes of Japanese knives, which are not within the scope of work of the knife maker.