Manufacturing Process of Japanese Knife

  • To begin with, Japanese knife manufacturing processes are broadly categorized into "forging" and "casting." This page aims to explain these methods in detail and help you understand their differences.

  • Forging involves shaping metal by hammering it to achieve the desired form. The metal is heated and hammered, which aligns and compresses the internal crystal structure. This process results in a more durable and resilient material. The toughness gained from forging is known as "toughness," which refers to the material's resistance to impact and sudden force.

  • On the other hand, casting involves melting the metal and pouring it into a mold, where it solidifies as it cools. While cast products may look similar to forged ones, their internal structure differs. Casting allows for mass production and uniform shapes quickly and cost-effectively, making it suitable for producing large quantities of products.

  • Differences Between Forging and Casting

    • Appearance: Forged and cast products may look similar, but there is a significant difference in their internal quality.
    • Strength: Forging compresses and aligns the metal's crystal structure, resulting in enhanced strength and toughness. Casting, however, has a coarser internal structure, leading to lower strength and toughness.
    • Cost and Production Efficiency: Casting is ideal for mass production and cost reduction, but forging involves more time and effort, resulting in higher quality products.

  • At KIREAJI, we offer Japanese kitchen knives forged by craftsmen in Sakai City. This page will delve deeper into the details of the forging process and highlight its techniques and appeal.

  • 1. Forging

  • Forging is the process of shaping metal by hammering it. By heating the metal to make it plastic, it becomes easier to process and shape into the desired form.

  • Forging Temperatures

    Forging is typically done through Awase (forge welding) or Honyaki methods, each requiring different temperature conditions:

  • Awase (Forge Welding):

    Welding (Forge Welding) is a process that involves heating the metal to a high temperature of 1100 to 1200 degrees Celsius. At these temperatures, the metal becomes soft enough to be joined together by hammering. This high heat ensures that the metal parts can fuse properly.

    Also known as "Wakashitsuke," this technique is a traditional blade forging method used in Sakai City. In this method, soft iron is heated to high temperatures and bonded with steel. The composition and quantity of the clay used for bonding vary by craftsman. The steel is heated to over 1100 degrees Celsius and fully bonded by hammering, resulting in a strong and cohesive metal structure.

  • Honyaki:

    In this method, the metal is heated to approximately 800 degrees Celsius, a lower temperature compared to welding. Because forge welding is not involved, the temperature does not need to be as high. This lower temperature helps to maintain finer internal crystal structures and reduces grain growth. As a result, the quality of the metal is higher, and the finish is often superior.

  • Preliminary Forging (Shaping the Knife):

    Once the base metal and steel are fully bonded, the craftsmen use a belt hammer to gradually shape the knife. If the temperature of the knife gets too high, the essential carbon for sharpness escapes, resulting in a blade that lacks cutting edge quality. The craftsmen continuously focus on precise temperature control, carefully forging each knife with meticulous attention to detail.

  • To stretch a 300mm Yanagiba knife made of Shirogami steel with a hammer, it typically takes about 20 minutes. On the other hand, stretching Aogami steel takes about twice as long. This is because Aogami steel is significantly harder. While you can visibly see Shirogami steel stretching when hammered, Aogami steel feels as if it isn't changing at all at first. This illustrates just how different and time-consuming it is to work with each type of steel.

  • Annealing

  • Annealing is a crucial step in knife production, aimed at removing internal stress after forging and softening the steel structure to improve workability. In the production of Sakai knives, traditional methods are used, with the steel being slowly cooled over a full 24-hour period. This process adjusts the hardness and resilience of the material, enhancing the knife's overall quality and sharpness. The craftsmanship of the artisans involved ensures that this technique maintains the high reputation of Sakai knives.

  • 2. Quenching (Yaki-ire)

  • Quenching is a heat treatment process that involves rapidly cooling metal from a specified high temperature. This process alters the metal's microstructure, resulting in increased hardness.

  • Phase Transformation Points

    Austenite:

    When metal reaches a certain temperature (approximately 800 to 850 degrees Celsius), iron, carbon, and trace elements dissolve and mix uniformly. This state is known as austenite. This phase, also referred to as austenitic transformation, makes the metal softer and easier to work with.

    Martensite:

    By rapidly cooling the austenite, a harder microstructure known as martensite is formed. Rapid cooling is crucial for achieving this structure, which results in a significantly harder material. This phase transformation is known as martensitic transformation, and the greater the amount of carbon in the iron, the harder the material tends to become.

  • Importance of Quenching

    Quenching is essential for dramatically improving the hardness and durability of metal. The transformation from austenite to martensite through this process significantly enhances the metal's performance. As a result, quenching produces a metal that is stronger and more durable.

  • Let's dive into a more intriguing aspect of quenching: the methods used for rapid cooling. There are primarily two methods for quenching: water and oil. The process of quenching with water is called water quenching, and the process using oil is called oil quenching.

  • Water quenching cools the metal extremely rapidly, resulting in a very high quenching rate. In contrast, oil quenching cools the metal more gradually, with a slower quenching rate compared to water quenching.

  • The speed of cooling significantly affects the formation of martensite. Martensitic transformation occurs when the metal transitions from austenite to martensite. The faster the cooling rate, the more robust the martensite structure becomes. In other words, the quicker the quenching process, the more effectively martensite forms. Thus, water quenching, with its rapid cooling capability, tends to produce a superior martensite structure and is generally considered to provide better quality.

  • However, some people prefer oil quenching for its specific advantages. While water quenching is often seen as superior in terms of the microstructure of martensite, oil quenching has its own set of benefits, which can be a matter of personal preference. Therefore, while water quenching is typically favored for its performance in creating a strong martensite structure, oil quenching also has unique advantages that may be preferred by some.

  • 3. Tempering (Yaki-modoshi)

  • Tempering is a crucial process that involves reheating metal after quenching to adjust its hardness. This step is essential for optimizing the performance of a blade.

  • After quenching, the metal becomes very hard, but excessive hardness can lead to reduced toughness, making the blade susceptible to fractures. To prevent this, it is necessary to perform tempering after quenching.

  • During the tempering process, the metal is reheated to adjust its hardness. This reheating is typically done at a temperature of around 500 degrees Celsius, though this can vary depending on the material and the desired properties. Specifically, tempering temperatures can range from as low as 200 degrees Celsius for low-temperature tempering to as high as 600 degrees Celsius for high-temperature tempering.

  • The goal of tempering is to enhance the toughness of the metal. By adjusting the hardness, tempering improves the blade's durability and resistance to breaking. Essentially, tempering transforms a metal that is too hard and brittle into one that is more resilient and long-lasting.

  • Tempering significantly affects the performance of a blade. Different blacksmiths may use various methods and temperatures for tempering, leading to variations in the final quality of the blade. Understanding the tempering process is therefore crucial for evaluating and selecting high-quality blades.

Sub-Zero Processing

KIREAJI's Sub-Zero processing technology rapidly cools steel to sub-zero temperatures, significantly enhancing knife hardness and durability. This process, rooted in the craftsmanship and expertise of skilled artisans, results in knives with superior sharpness and lasting performance.

Sub-Zero Processing

Discover the Allure of Forged Knives

Forged knives represent the pinnacle of craftsmanship, where strength, durability, and beauty converge.

Discover the Allure of Forged Knives

  • 2. Sharpening

  • Starting with a coarse whetstone and gradually moving to finer ones, the blade is carefully sharpened. By sharpening while running cold water, the rise in temperature, which can cause tarnishing and reduce hardness, is prevented. Using various whetstones for rough sharpening, main sharpening, and back sharpening, the entire knife and its blade are meticulously refined. Through this process, a beautiful, distortion-free knife is completed.
Coarse_Sharpening

2-1 Coarse Sharpening

Sharpen the entire blade against a rotating whetstone (en-to) to roughen it. This step reduces the thickness of the blade edge.

Primary_Sharpening

2-2 Primary Sharpening

Sharpen the entire blade against a rotating whetstone (en-to) to roughen it. This step reduces the thickness of the blade edge.

Back_Sharpening

2-3 Back Sharpening

The Urasuki is carved out. This is a critical process for creating a Japanese knife, designed to relieve pressure from the cutting edge. However, excessive removal of the steel can impact the blade's performance.

  • 3. Handle Attachment

  • The part called the "nakago" (tang) is heated and then inserted into the handle. By tapping the end of the handle with a wooden mallet, the tang is firmly secured all the way in. Once any distortions are corrected, ensuring the blade is perfectly straight, the Sakai forged knife is complete.

The Process of Making Japanese Knives

The Process of Making a Knife

The process of making a Japanese knife involves numerous steps and significant artisanal handiwork, taking one to two months per knife. The primary methods in Japan are traditional "forging," where steel is hammered and shaped, and "stamping," where blades are cut from steel sheets. These techniques, rooted in Japanese sword-making history, showcase rare technical prowess and high quality in knife manufacturing.

The Process of Making a Knife

KIREAJI's Three Promises to You

  • japanese_knife_made_in_Sakai

    1. High-quality Japanese Knive

    We offer knives crafted by craftsmen from Sakai City.

  • Honbazuke

    2. Genuine Sharpness

    To ensure you experience sharpness, we provide a free Honbazuke by our skilled craftsmen.

  • 3. Lifetime Knife Use

    Every knife comes with a free Saya(Sheath) for durability. We also provide paid after-sales services with Sakai City Japanese knife workshop.