Heat Treatment is a group of manufacturing techniques used to alter the hardness and toughness of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass.
The techniques include annealing, case hardening, tempering and quenching.
Heat Treatment of swords and knives
Usually a heat hardened item (of iron or its alloys) is too brittle for use until further treated with heat. Depending on the alloy used; it will be evenly heated to 200-500 degrees, held at that temperature (soaked) for an appropriate time (seconds or hours), then cooled slowly over an appropriate duration (minutes or hours). Each cycle of heating and cooling forms crystals within the metal.
The exact heats and times the alloy endures generates specific proportions of certain types of metal crystals. Each type of crystal has a different size and character. Some give hardness, some toughness and flexibility.
Contemporary research finds that all sorts of materials are toughened by cryogenic cooling (commonly soaking in liquid nitrogen) for hours or days. This includes forged metals. Crystal forms in the metals continue to modify when cooled to far below room temperature. Some knife and sword makers already incorporate this new addition to heat treatment options.
Sometimes the entire item is given the same heat treatment, and sometimes different areas of the item are heated and cooled at different rates. This is called differential hardening. It is common in high quality knives and swords. The Japanese katana is the best known for this. However, Chinese swords were traditionally done this way, as were Nepalese Khukuri, and many others.