Deep-freezing is used if cooling to room temperature does not produce sufficient hardness, and involves cooling down the knife blades to a temperature in the range -20°C to -150°C (-4 to -238°F) before they are tempered. In this guide Alleima provides hardening programs with -20°C (-4°F) or -70°C (-94°F), respectively.
The simplest way of deep-freezing the knife blades is to place them in a freezer or immerse them in dry ice. The knife blade is then left to 'thaw' to room temperature, and is then tempered in the usual way.
Deep-freezing increases the hardness by 1–3 HRC, but reduces the toughness slightly. For most applications, hardness between 57 and 58 HRC provides a good balance between edge stability, toughness and grindability.