Hardening is a way of making the knife steel harder. By first heating the knife steel to between 1050 and 1090°C (1922 and 1994°F) and then quickly cooling (quenching) it, the knife steel will become much harder, but also more brittle.
To reduce the brittleness, the material is tempered, usually by heating it to 175–350°C (347–662°F) for 2 hours, which results in a hardness of 53–63 HRC and a good balance between sharpness retention, grindability and toughness.
Tempering should be carried out within a reasonable time after hardening, preferably within an hour or so. It is of vital importance that the blade should be allowed to cool to room temperature before tempering is started. The transformation to martensite will otherwise be interrupted and the hardening results may be impaired.
A higher tempering temperature will yield a somewhat softer material with higher toughness, whereas a lower tempering temperature will produce a harder and somewhat more brittle material, as shown by the figure below.
A camping knife or a survival knife, for example, may be tempered at 350°C (662°F) so that it will be able to withstand rough handling without breaking. On the other hand, if the knife is expected to keep a sharp edge, it can instead be tempered at 175°C (347°F) for maximum hardness.
Tempering temperatures below 175°C (347°F) should be used only in exceptional cases, when extreme demands are made on high hardness, since very low tempering temperatures will result in a very brittle material. Similarly, tempering temperatures above 350°C (662°F) should be avoided, since this could give rise to brittleness and reduced corrosion resistance. Note that if the tempered blade is exposed to temperatures above the tempering temperature (e.g. during grinding), the properties of the knife will be impaired.
Correctly performed hardening will result in a good balance between hardness, toughness and corrosion resistance of the finished knife blade.
Further details of how hardening is carried out