Vacuum hardening:
It is the process of heating and cooling the work pieces under vacuum conditions. Inert gases like Nitrogen or argon is used for quenching. The gas used should have high degree of purity. This can be guaranteed by the use of a liquid source. The advantage of using inert gas is that it will not react with metal surface. The objective of vacuum is to prevent unwanted gases form contaminating and oxidizing the metal surface. Since oxygen is not present the parts subjected to this treatment can be bright and scale free. By incorporating gas or oil quenching features in vacuum furnace, most of the metals and alloys can be subjected to vacuum hardening. Since the work pieces subjected to this process retains brightness and clean appearance. In some cases even the polishing and sizing can be avoided. There is only a negligible deformation in this process.
Furnace operation in vacuum hardening:
The furnace is loaded with the work and then the air is sucked out and vacuum is created. Then a required quantity of inert gas pumped in to the furnace, before the heat is on. The furnace leak has to be checked before switching the heat on. The load is the preheated to 1200F and when the temperature is balanced, a second preheat temperature of 1500F is employed and maintained until an equilibrium is reached again. Once the equilibrium is reached, the pressure in the furnace is raised to the required degree with nitrogen of high degree of purity from a liquid source. Now the metal is ready to austenitized and temperature is raised to 1850F. The soaking time is decided depending on the cross sectional area. After soaking period, the furnace is allowed to fill with nitrogen and then the cooling fan starts automatically. The load is allowed to cool to the room temperature.
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