Liquid nitrogen is the liquid state of nitrogen. Liquid nitrogen can be prepared by specialized equipment, or in the process of preparing liquid oxygen as a by-product. Liquid nitrogen has a very low temperature, its boiling point of -195.8 ℃, can be used as a cryogenic agent. Liquid nitrogen can also be changed from liquid to gaseous, nitrogen, can be used as a protective gas into the heat treatment furnace. In the air, the proportion of nitrogen is the largest. Liquid nitrogen is not as flammable as liquid oxygen, explosive, so liquid nitrogen is rich in resources, better security, etc., in the material heat treatment has the main purpose.