During the oil quenching heat treatment process using a vacuum hardening furnace, the cooling capacity of the vacuum hardening oil decreases under low vacuum pressure, which may not achieve the hardening purpose for certain steels. To address this, it is necessary to fill the hardening chamber with high-purity neutral or inert gas before using the vacuum hardening furnace for hardening. This creates a certain pressure above the oil surface, enabling full hardening of the steel and achieving a bright surface. This allows for the same hardening hardness as under atmospheric pressure.The liquid surface pressure is referred to as the critical quenching pressure. The critical quenching pressure is related to the properties of the vacuum quenching oil and the hardenability of the steel.

       When using a vacuum quenching furnace to quench steel with poor hardenability, it is recommended to adopt the method of pre-inflating with gas before immersion in oil for quenching. The oil surface pressure should be higher than the critical quenching pressure. Generally, the adjustment isApproximately 5×10^4 Pa, but not below 1.0×10^4 Pa. For steel with moderate hardenability, the quenching process should be carried out by first inflating and then immersing in oil. The pressure of the oil surface can be adjusted to around 1.0×10^4 Pa, but it should not be lower than 5×10^3 Pa. For steel with excellent hardenability, quenching can be done with or without inflation. If inflation is used, it can be done first in the vacuum furnace's heating chamber before immersing in oil, or vice versa. When using the first inflation method, the inflation pressure can be selected within the range of (2.5~5)×10^4 Pa. To prevent and reduce steam entry and contamination of the heating chamber in the vacuum quenching furnace, a higher inflation pressure should be used, and the gas should be a high-purity neutral or inert gas. For the no-inflation quenching method, no neutral or inert gas is introduced into the workpiece before or after immersion in oil.