Vacuum heat treatment is a metal heat treatment process that heats a metal workpiece under 1 atmosphere pressure (that is, under negative pressure).
In the late 1920s, with the development of electric vacuum technology, a vacuum heat treatment process appeared, which was only used for annealing and degassing at that time. Due to equipment limitations, this process has not made great progress for a long time. From the 1960s to the 1970s, air-cooled vacuum heat treatment furnaces, cold-wall vacuum oil quenching furnaces and vacuum heating high-pressure gas quenching furnaces were successively developed, which enabled new developments in vacuum heat treatment technology. Carburizing in a vacuum, the technological progress of carburizing, nitriding or infiltration of other elements under the action of a plasma field in a vacuum has further expanded the scope of application of vacuum heat treatment.
Characteristics of vacuum heat treatment technology
The heat treatment of metal parts in vacuum can prevent oxidative decarburization and has a degassing effect, but the metal elements may evaporate.
• Prevent the heating chamber of the vacuum heat treatment furnace from oxidative decarburization in a near vacuum state during operation. Only trace amounts of carbon monoxide and hydrogen are present. They are reductive to the heated metal and do not undergo oxidative decarburization reactions; Reduce the formed oxide film, so the surface of the metal workpiece after heating can maintain the original metallic luster and good surface properties.
•Degassing effect When metal parts are heated in a vacuum environment, harmful gases in the metal, such as hydrogen and oxygen in titanium alloys, will escape at high temperatures, which helps to improve the mechanical properties of the metal.
•Evaporation of metal elements. Various elements have their own vapor pressure. If the pressure in the environment is lower than the vapor pressure of a certain element, this element will evaporate. During vacuum heat treatment, the vacuum degree or temperature during heating should be selected according to the vapor pressure of the alloy elements contained in the steel to avoid evaporation of the alloy elements.
Vacuum heat treatment process
Vacuum heat treatment can be used for annealing, degassing, solution heat treatment, quenching, tempering and precipitation hardening processes. After passing the appropriate medium, it can also be used for chemical heat treatment.
There are two types of quenching in vacuum: gas quenching and liquid quenching. Gas quenching is to fill the cooling chamber with high-purity neutral gas (such as nitrogen) for cooling after the workpiece is heated in vacuum. Suitable for gas quenching are high-speed steel and high-carbon high-chromium steel and other materials with low critical cooling rate of martensite. Liquid quenching is to heat the workpiece in the heating chamber, then move it to the cooling chamber to fill with high-purity nitrogen and immediately send it to the quenching oil tank for rapid cooling. If high surface quality is required, the tempering and precipitation hardening after vacuum quenching and solution heat treatment of the workpiece should still be carried out in a vacuum furnace.