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Hardening by Heat Treatment knifeopedia
Principle of heat treatment
Even using best steel will not result in a good knife, if not heat treated properly. Raw steel as received from a steel mill is soft and not suitable for knife production as is. It will only be converted to knife steel through heat treatment. With an excellent heat treatment, one can produce a good knife from mediocre steel. But a poor heat treatment can ruin the most expensive powder steel and result in a knife that chips and corrodes much more than it should.
Heating causes the atoms in the steel to reorder themselves, which will change the characteristics of the metal. Heat treatment is a very sensitive process, as small variations in the parameters (temperature and holding time) can result in big changes in material properties. Hence it’s imperative that hardening is done with professional 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
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.
Steel is then cooled rapidly to room temperature (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.
Therefore, 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
+
Cementite
Ferrite
Cryogenic treatment
Usually it’s very difficult to achieve a complete transformation from Austenite to Martensite just by quenching. If not completely transformed, the steel retains a certain share of Austenite (retained Austenite). This is undesirable, as it reduces hardness, toughness and corrosion resistance.
By a cryogenic treatment of the material at temperatures lower than -70°C (ice hardening), the transformation of retained Austenite into Martensite can be enforced, measurably improving all material properties.
Ice hardening is either done in cooling boxes at -70°C or with liquid nitrogen at -196°C.
As the heat treatment of steel is a highly scientific affair, the above represents a gross simplification of the process. In-depth information can be found at www.knifesteelnerds.com.