Hardness Rockwell (HR) is a unit of measurement used to indicate the degree of hardness of technical materials, including the metal (usually steel) from which knife blades are made.
Hardness is generally defined as the mechanical resistance of a material to the mechanical penetration of another body. The hardness of a material is an important indicator of its wear behaviour, because hard materials wear less than soft ones. At the same time, however, the hardness of a material increases with its brittleness, i.e. the harder a material is, the easier it breaks during plastic deformation. For knife blades, this means that hard blades remain sharp longer, but also break more easily.
To determine the hardness of a material, the Americans Hugh and Stanley Rockwell invented a method at the beginning of the 20th century in which the penetration depth of a so-called test piece into the material is measured. Since different test specimens are also used for different materials, there exist eleven different Rockwell scales which are marked with the letters A to H, K, N and T. The C scale is used for steel knife blades. The hardness value of a material is finally indicated by a two-digit number, whereby higher values stand for higher hardness and lower values for lower hardness. The measurement result is indicated by an abbreviation containing the hardness value (e.g. 60), the test method (hardness according to Rockwell, HR) and the scale used (for knife blades C), which in the example would be 60 HRC.
Steel knife blades have hardnesses between 56 and 66 HRC. German and American knives have hardness values of about 56-62 HRC (although there may be significant upward deviations for special knives), while Japanese knives have hardness values of 64-66 HRC. In German knives, most kitchen and hunting knives have hardness values of 57-60 HRC, while particularly fine, sharp blades have hardness values of 60-65 HRC.