Liquid nitrogen is nitrogen in liquid form. Liquid nitrogen can be made by special equipment or as a by-product in the process of making liquid oxygen.

Liquid nitrogen has a very low temperature, its boiling point is -195.8℃, can be used as a cryogenic agent. Liquid nitrogen can also be changed from liquid to gaseous state to nitrogen, which can be used as a protective gas in the heat treatment furnace. Nitrogen makes up the largest proportion of air. Liquid nitrogen is not as flammable and explosive as liquid oxygen, so liquid nitrogen has the advantages of abundant resources and good safety. It has the following main uses in heat treatment of materials.

Liquid nitrogen

Application of liquid nitrogen in vacuum heat treatment

Vacuum heat treatment is a relatively advanced heat treatment method being used at home and abroad. In vacuum heat treatment, nitrogen is often used. For example, when steel parts are heated in a vacuum furnace, if the vacuum degree is too high, in order to prevent the alloy elements in the workpiece material volatilize, it is necessary to fill the furnace with an appropriate amount of nitrogen, in order to lift the pressure of trace gas in the furnace. For another example, in the vacuum quenching process of vacuum heating oil quenching, sometimes in order to ensure or improve the cooling capacity of quenching oil, it is necessary to return enough nitrogen to the vacuum furnace before the workpiece is cooled. During high pressure gas quenching, it is necessary to quickly fill the vacuum furnace with nitrogen at a pressure of more than one atmosphere. In the vacuum tempering of steel parts, after vacuuming, nitrogen should be recharged to realize uniform heating of protective gas circulation in the furnace and rapid cooling after tempering.

Nitrogen for vacuum heat treatment comes from a variety of sources. Because liquid nitrogen is much denser and smaller in volume than nitrogen, it is easier to transport and store, and cheaper to supply in large quantities. Therefore, liquid nitrogen is one of the important sources of nitrogen for vacuum heat treatment.

In foreign countries and Shanghai and other places in China, many manufacturers use larger special storage tanks to store the liquid nitrogen transported by the car of the liquid nitrogen supplier, and the nitrogen formed by the gradual vaporization of liquid nitrogen is used for vacuum heat treatment. They argue that using liquid nitrogen is more economical than using bottled nitrogen or nitrogen production mechanisms.

Of course, whether it is economical to supply nitrogen with liquid nitrogen or with bottled nitrogen or nitrogen making machine depends on the local gas source conditions and the amount of nitrogen used in the heat treatment plant.