knifeopedia
Hardening by Heat Treatment
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Principle of heat treatment
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Using the best steel alone won’t guarantee a great knife - proper heat treatment is essential. When steel arrives from the mill, it’s too soft for knife production. Only through heat treatment does it transform into knife-grade steel. With excellent heat treatment, even average steel can become a high-performing blade. But with poor heat treatment, even the most expensive powder steel can result in a knife that chips, rusts, and underperforms.
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Heat treatment works by heating the steel, causing the atoms to rearrange and alter the metal's properties. This is a delicate process, as even small variations in temperature or timing can drastically affect the steel's characteristics. That's why it’s crucial for hardening to be performed with professional, precision equipment in a controlled environment.
The hardness range for kitchen knives varies from HRC 45 (hardness on Rockwell scale) for the lowest end knives to HRC 70 for knives with high end powder steel. It’s important to understand that the Rockwell scale is exponential, not linear. Therefore a jump from HRC 50 to 51 means a much smaller increase in factual hardness than one from HRC 64 to 65.
Phases in heat treatment
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Steel is heated to temperatures > 1000°C, during which the material is transformed to Austenite (austenization). Austenite is a phase of steel that only exists at high temperature.
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Steel is then rapidly cooled to room temperature (a process called quenching), in air, water or oil, during which the transformation to Martensite occurs. Martensite is very hard in a needle-like structure, but very brittle, too brittle for a knife.
To address this, the steel is reheated to temperatures > 150°C (tempering) and finally transformed into Ferrite and Cementite, which is a slightly softer, but tougher and stable form.
Austenite
Martensite
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Cementite
Ferrite
Cryogenic treatment
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Achieving a complete transformation from Austenite to Martensite through quenching alone is challenging. When the process is incomplete, the steel retains a portion of Austenite, known as retained Austenite. This is undesirable as it reduces the blade’s hardness, toughness, and corrosion resistance.
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Cryogenic treatment, also known as ice hardening, at temperatures below -70°C helps eliminate retained Austenite, forcing its transformation into Martensite. This process significantly improves the overall material properties. Ice hardening is performed in cooling chambers at -70°C or with liquid nitrogen at -196°C for maximum effectiveness.
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While heat treating steel is a complex and scientific process, this is a simplified explanation. For a deeper dive, visit www.knifesteelnerds.com.