Something about steel temper

Something about steel temper

When a blade is being forged the knife maker heats and cools the steel repeatedly to achieve the proper crystal structure of the metal. This determines the hardness, durability, and lifetime of the blade.

Steel is quite brittle. The blade must be tempered, which is a process of re-heating and slow cooling of the blade to relieve the tension in the blade. It is done on lower temperature, lower than the process of hardening. 

Using grinders to sharpen your knives can get the steel overheated, which changes and de-tempers the blade. This means it becomes softer and looses its ability to hold an edge like the knife maker intended.