The heat treatment of metal parts in vacuum can prevent oxidation, decarburization and degassing effect, but the metal elements may evaporate.
Preventing oxidation and decarbonization
Heating vacuum heat treatment furnace chamber at work in a near vacuum state, only trace amounts of carbon monoxide and hydrogen, they are heated metal is reduced, no oxidation and decarburization reaction; while the reduction of the oxide film, so the heating of the metal surface of the workpiece can keep the original luster and good surface properties.
When metal parts are heated in vacuum, the harmful gases in metals, such as hydrogen and oxygen in titanium alloys, will escape at high temperature, which is beneficial to improve the mechanical properties of metals.
Evaporation of metal elements
All elements have their own vapor pressure, and if the pressure in the environment is lower than the vapor pressure of some element, the element will evaporate. In vacuum heat treatment, the vacuum degree or temperature should be chosen according to the vapor pressure of the alloy element in the steel to avoid alloy element evaporation.
Metal vacuum heat treatment process
Vacuum heat treatment can be used for annealing, degassing, solid solution heat treatment, quenching, tempering and precipitation hardening process. It can also be used in chemical heat treatment after the proper medium is passed in.
In the vacuum degassing, annealing, solid solution treatment is mainly used for pure workpiece degree or high surface quality requirements, such as refractory metal softening and stress, stainless steel and nickel based alloy of titanium and titanium alloy processing, degassing, soft magnetic alloy to improve the permeability and the coercivity of annealing. And the light of carbon steel and low alloy steel and copper annealing. There are two kinds of quenching in vacuum: gas quenching and liquid quenching. The gas quenching is to cool the workpiece into a cooling chamber with high purity neutral gas (such as nitrogen) after vacuum heating. Suitable for gas quenching with high speed steel and high carbon high chromium steel and other critical cooling rate lower material. Liquid quenching is that the workpiece is heated in the heating chamber, then moved into the cooling chamber, filled with high purity nitrogen and immediately sent into the quenching oil tank to cool quickly. If high surface quality is required, the tempering and precipitation hardening of the workpiece after vacuum quenching and solid solution heat treatment should be carried out in a vacuum furnace.
Vacuum carburizing is the workpiece into a vacuum furnace, vacuum furnace and heating, the purification, reach the carburizing temperature after adding hydrocarbons (such as propane) were cut off after a certain period of time after carburizing, carburizing, and vacuum diffusion. This method can realize high temperature carburizing (1040 C) and shorten the carburizing time. There is no internal oxidation in the carburized layer, nor the carbon content in the carburized layer is lower than that in the secondary layer, and the carburizing layer can be obtained by the way of pulse vacuum carburizing to make the blind hole and the small hole.
Application of metal vacuum heat treatment
After vacuum heat treatment, the parts have small distortion and high quality, and the process itself is flexible and harmless. The vacuum heat treatment is not only a special alloy heat treatment of the necessary means, but also in the general engineering with the heat treatment of steel is also used, especially the tools, molds and precision coupling parts, after vacuum heat treatment service life has greatly improved than the general heat treatment. For example, the life of some dies after vacuum heat treatment is 40 – 400% higher than that of the original salt bath, and the life of many tools can be increased by about 3~4 times. In addition, the vacuum furnace can work at a higher temperature, and the workpiece can maintain a clean surface, which can accelerate the adsorption and reaction process of chemical heat treatment. Therefore, some chemical heat treatment, such as carburizing, nitriding, chromizing, boronizing, and multicomponent co permeation, can get faster and better results.