The degree of vacuum is represented by Pa (1atm =101.331kPa). According to the level of vacuum, it can be divided into four levels: 1.33x10³~1.33Pa is low vacuum, 1.33x10^-1 ~1.33x10^-2Pa is medium vacuum, 1.33x10^-3~1.33x10^-5Pa is high vracuum, Below 1.33 x10^-5Pa is ultra-high vacuum. Vacuum heat treatment refers to the heat treatment process of heating and cooling in an environment lower than 101.325kPa. Vacuum heat treatment includes vacuum annealing, quenching, tempering, carburizing, metalizing, etc. Here only a brief introduction to vacuum quenching.
1. Characteristics of vacuum quenching heating
(1) Surface protection Heating in vacuum, the content of oxidizing gas is extremely low, so the oxidative decarburization is slight.
(2) Purify the surface Under high temperature and vacuum conditions, oxides and dirt on the metal surface will decompose and volatilize, and then be exhausted by the vacuum pump to purify the surface of the workpiece.
(3) Degassing During smelting, liquid metal will absorb H₂, N₂, O₂, and other gases. When the liquid is solidified, due to the fast cooling rate, a part of the gas remains in the solid metal, which is harmful to the properties of the metal. According to Sievers gas partial pressure law, the solubility s of gas in metal is proportional to the square root of its partial pressure p, that is, s=kp^1/2, k is a constant related to temperature.
Due to the low partial pressure of gas in vacuum, the solubility of gas in metal will be significantly reduced. The higher the vacuum, the better the degassing effect.
The process of vacuum degassing is the process in which the gas in the metal diffuses to the surface, escapes from the surface, and is discharged from the vacuum furnace. Among them, the speed of the gas in the metal diffusing to the surface is the main factor affecting the degassing effect, so under the same vacuum degree, increasing the temperature can improve the degassing effect. Among the gases of hydrogen, oxygen and nitrogen, hydrogen diffuses more easily than oxygen and nitrogen, so dehydrogenation is easier during vacuum heat treatment, while deoxygenation and nitrogen are more difficult.
(4) Degreasing The oil on the surface of the workpiece (these oils are carbon, hydrogen, and oxygen compounds) can be decomposed into hydrogen, water vapor, and carbon dioxide when heated in a vacuum, and are discharged by the vacuum pump. Therefore, vacuum heat treatment can degrease and keep the surface of the workpiece smooth.
(5) Evaporation phenomenon When alloy steel is heated in vacuum, some alloy elements with high vapor pressure such as Mn and Cr will evaporate and escape from the surface, resulting in the depletion of surface alloy elements.
(6) Slow heating speed Since the heat transfer in vacuum is only radiation, the heating speed is slow (especially below 600°).
2. Cooling of vacuum quenching
The cooling of vacuum quenching mainly adopts oil cooling or high pressure air cooling. Vacuum quenching oil can be used as the quenching cooling medium for large cross-section workpieces with low hardenability; air cooling can be used for workpieces with high hardenability and small cross-section size, and high-purity nitrogen can be used as the air cooling medium. As mentioned in the first section, the general cooling rate of air cooling ≤ oil.
At present, the vacuum quenching of steel parts is widely used, and its advantages are no oxidation and decarburization, small distortion (the reason for the small distortion is still inconclusive, and there is a consensus that vacuum heating is slow, which is beneficial to reduce distortion), hardness and The wear resistance is high and uniform, the service life of the workpiece is long, the degree of automation is high, and it is environmentally friendly.