Vacuum heat treatment (shell and tube heat exchanger) is a relatively advanced heat treatment method being adopted at home and abroad. In vacuum heat treatment, the use of nitrogen is often inseparable.

For example, when steel parts are heated in a vacuum furnace, if the vacuum degree is too high, in order to prevent the volatilization of alloy elements in the workpiece material, it is necessary to refill the furnace with an appropriate amount of nitrogen to lift the pressure of the 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 the quenching oil, it is necessary to backfill enough nitrogen into 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 higher than one atmosphere. During vacuum tempering of steel parts (pipe benders are used for bending of steel parts), nitrogen must be backfilled after vacuuming to achieve uniform heating of the protective gas circulation in the furnace and rapid cooling after tempering. 　　

There are various sources of nitrogen required for vacuum heat treatment. Because the density of liquid nitrogen is far greater than that of nitrogen, and the volume is much smaller than that of nitrogen, it is easy to transport and store, and the price is cheaper when it is supplied in large quantities. Therefore, liquid nitrogen is one of the important sources of nitrogen used in vacuum heat treatment. Nitrogen generators are used in the production of nitrogen in industrial production. 　　

In foreign countries and Shanghai and other places in my country, many manufacturers use larger dedicated storage tanks to store liquid nitrogen delivered by special vehicles from liquid nitrogen suppliers, and use the nitrogen gradually vaporized from liquid nitrogen for vacuum heat treatment. They believe that the use of liquid nitrogen is more economical than the use of bottled nitrogen or nitrogen production mechanism nitrogen. 　　

Of course, whether it is cost-effective to use liquid nitrogen or to use bottled nitrogen or nitrogen generators to supply nitrogen is cost-effective. This is related to the local gas source conditions and the amount of nitrogen used by the heat treatment production plant. The price of liquid nitrogen is not only related to its purity, but also to the amount of one purchased. For example, in the spring and summer of 2003, there was an oxygen plant in Xiangtan that produced and sold liquid nitrogen. When buying more than 1,000L at a time, the price was 1 yuan per liter; if it was retailed, it was 4 yuan per liter.