Selection of oil surface pressure in heat treatment of vacuum quenching furnace

In the process of oil quenching heat treatment with vacuum quenching furnace, the cooling capacity of vacuum quenching oil decreases under low vacuum pressure, which may not achieve the purpose of quenching for some steels. Therefore, before using the vacuum quenching furnace to quench, it is necessary to fill a high purity neutral or inert gas into the quenching chamber to cause a certain pressure on the oil surface, so that the steel can be fully quenched and a bright surface can be obtained. The lowest surface pressure which can obtain the same quenching hardness as that under atmospheric pressure is called critical quenching pressure. The critical quenching pressure is related to the properties of vacuum quenching oil and the hardenability of steel.

When using the vacuum quenching furnace to quench the steel with poor hardenability, the way of filling gas first and then adding oil should be used to quench. The oil level pressure should be higher than the critical quenching pressure. Generally, the adjustment is about 5 × 104pa, but it should not be lower than 1.0 × 104pa. For medium hardenable steel, the method of filling gas before oil should be adopted. The pressure of the oil surface can be adjusted at about 1.0 × 104pa, but it can not be lower than 5 × 103pa. The steel with good hardenability can be quenched by means of air charging or non air charging. The way of air charging can be adopted. The heating chamber of the vacuum furnace can be filled with air first and then oil, or oil first and then gas. When adopting the way of inflation first, the inflation pressure can be selected within the range of (2.5 ~ 5) × 104pa. In order to prevent and reduce the steam entering and polluting the heating chamber of the vacuum quenching furnace, a higher inflation pressure should be selected, and the gas used should be high-purity neutral or inert gas. The method of post inflation can be used to fill the gas with lower purity and lower inflation pressure. Adopt the way of not filling air for quenching, that is, the workpiece is not filled with neutral or inert gas before and after filling oil.