Metal vacuum heat treatment
A metal heat treatment process in which a metal workpiece is heated below 1 atmosphere (ie, under a negative pressure). With the development of electric vacuum technology, a vacuum heat treatment process appeared, and only annealing and degassing were used at that time. Due to the limitations of the equipment, this process has not made great progress for a long time. In the 1960s and 1970s, successively developed air-cooled vacuum heat treatment furnaces, cold-wall vacuum oil quenching furnaces and vacuum-heated high-pressure gas quenching furnaces, which made the vacuum heat treatment process a new development. Carburizing in a vacuum, the progress of carburizing, nitriding or infiltrating other elements under the action of a vacuum medium ion field, and further enlarging the application range by vacuum heat treatment.
Heat treatment characteristics
The heat treatment of metal parts in vacuum prevents oxidative decarburization and has a degassing effect, but metal elements may evaporate.
Prevent oxidative decarburization
The heating chamber of the vacuum heat treatment furnace is in a near vacuum state during operation, and only traces of carbon monoxide and hydrogen are present, which are reductive to the heated metal, do not undergo oxidative decarburization reaction, and can also form formed oxidation. The film is reduced, so that the surface of the heated metal workpiece can maintain the original metallic luster and good surface properties.
When metal parts are heated in a vacuum environment, harmful gases in the metal, such as hydrogen and oxygen in the titanium alloy, will escape at high temperatures, which is beneficial to improve the mechanical properties of the metal.
Evaporation of metal elements
Each element has its own vapor pressure, which evaporates if the pressure in the environment is below the vapor pressure of an element. In the vacuum heat treatment, the degree of vacuum or temperature during heating should be selected according to the vapor pressure of the alloying elements contained in the steel to avoid evaporation of the alloying elements.
Vacuum heat treatment process
Vacuum heat treatment can be used for annealing, degassing, solution heat treatment, quenching, tempering and precipitation hardening. It can also be used for chemical heat treatment after passing through a suitable medium.
Annealing, degassing and solution treatment in vacuum are mainly used for workpieces with high purity or high surface quality, such as softening and de-stressing of refractory metals, solution treatment of stainless steel and nickel-based alloys, and removal of titanium and titanium alloys. Gas treatment, soft magnetic alloys to improve magnetic permeability and coercivity annealing, and bright annealing of bright carbon steel, low alloy steel and copper. Quenching in vacuum is both gas quenching and liquid quenching. The gas quenching is to cool the workpiece by heating the vacuum chamber with a high-purity neutral gas such as nitrogen. Suitable for gas quenching materials such as high-speed steel and high-carbon high-chromium steel with low critical martensite cooling rate. The liquid quenching is to heat the workpiece in the heating chamber, then move it to the cooling chamber and fill it with high-purity nitrogen gas and immediately send it into the quenching oil tank for rapid cooling. If high surface quality is required, the tempering and precipitation hardening of the workpiece after vacuum quenching and solution heat treatment should still be carried out in a vacuum furnace.
Vacuum carburizing is to put the workpiece into the vacuum furnace, vacuum and heat it to purify the furnace. After the carburizing temperature is reached, the hydrocarbon (such as propane) is introduced to carburize. After a certain time, the carburizing agent is cut off. Vacuum is applied for diffusion. This method can achieve high temperature carburization (1040 ° C) and shorten the carburizing time. There is no internal oxidation in the infiltrated layer, and there is no problem that the carbon content on the surface of the carburized layer is lower than that in the sublayer, and the carburized by vacuum can be used to obtain a uniform carburized layer in the blind hole and the small hole.
Vacuum heat treatment application
After vacuum heat treatment of the parts, the distortion is small, the quality is high, and the process itself is flexible and free from pollution. Therefore, vacuum heat treatment is not only a necessary means for heat treatment of some special alloys, but also applied in the heat treatment of general engineering steels, especially tools, molds and precision couplings. After vacuum heat treatment, the service life is larger than that of general heat treatment. Improvement. For example, after vacuum heat treatment of some molds, the service life is 40 to 400% higher than that of the original salt bath treatment, and the life of many tools can be increased by about 3 to 4 times.
In addition, the vacuum furnace can operate at higher temperatures and the workpiece can maintain a clean surface, thus accelerating the adsorption and reaction processes of the chemical heat treatment. Therefore, certain chemical heat treatments such as carburizing, nitriding, chromizing, boronizing, and multi-commutation can achieve faster and better results.