Heat-Treating a custom knife blade is a process to harden a blade by super heating the steel to change the molecular makeup of the metal.
In technical terms the typical molecular alignment of knife blade steel is a crystal structure consisting of atoms that are grouped in a way called a lattice. Upon applying great heat to the metal, this molecular order will rearrange itself based upon conditions like temperature and pressure.
What this means is that during the blade shaping process the knife steel is in a "Softened" state, and once the blade is shaped, and before the final sharpening is undertaken, the knife blade is heat treated in order to harden the knife to its final hardening level.
All various types of steels have different requirements of temperature when they are heat treated. Good custom knife makers know the technical needs of the steel and heat treat accordingly, or send the blades off to specialty "Heat Treat Facilities".
The actual process of heat treating involves "Annealing" or quickly super heating, then quickly cooling the blade, usually in an oil of some sort in order to quickly align the steel molecules properly.
Custom knife makers realize the importance of the heat treating of their blades, and spend a lot of time becoming experts in the subjects. Check out http://www.JustCustomKnives.com to see a great selection of finished custom knives.
The pictured custom knife above is made by "The Knife" who has a shop on JustCustomKnives.com.