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Pollution causes of precision alloy vacuum furnace

Vacuum furnaces are mostly used for ferrous metal heat treatment. Due to the easy access to the required surface quality, the pollution of vacuum furnaces has not attracted much attention. Vacuum furnace manufacturers have not made great efforts to solve this problem, especially for heat treatment furnaces for precision alloys.

The heat treatment of precision alloys has its particularity, such as being extremely sensitive to carbon, requiring high surface quality, and having many thin strips. Through heat treatment, it is not only necessary to obtain general mechanical properties, but also different physical properties. Any slight pollution of elements at high temperatures can change the material and destroy the original properties of materials. Therefore, the “cleanliness” requirements for vacuum furnaces cannot be the same as those for general furnaces.

1.Severity of vacuum heat treatment pollution

Vacuum degree and “cleanliness” degree are two different concepts, especially when they are higher than 1 × Under the condition of 10-1Pa, some uncleanness may not be obviously reflected on the vacuum gauge.

The pollution of the vacuum furnace mainly comes from hydrocarbons, water and metals that are easy to evaporate. In addition to the oxidation of the surface of the heat treated parts, defects such as carburization, metallization, decarburization and dilution may also occur. As there are many precision alloy thin strips, once there is slight pollution, the material performance may be damaged. For example, it seriously affects the initial permeability of 1J50 and 1J79 soft magnetic alloys, making the coercive force larger. In the polluted furnace, the element infiltration continues under the action of high temperature. Although the indication of the vacuum gauge is normal, this kind of pollution becomes more serious with the extension of the annealing time. After 3J53 is polluted, its hardness value will be 30~60HB lower than that of normal materials. A unit treated 4J29 iron nickel cobalt glass sealed alloy with a vacuum furnace for common ferrous metal treatment. Because of contamination and deterioration, hundreds of kilograms of materials lost their applied thermal expansion properties. In the application of vacuum heat treatment, it is necessary to recognize the characteristics of the alloy to be treated, because most precision alloys require high cleanliness of the vacuum heat treatment environment.

2.Pollution type and treatment method of vacuum heat treatment

The pollution of vacuum furnace is divided into external pollution and self pollution. External pollution includes atmospheric humidity, surface carrying of parts and volatile elements. Self pollution includes decomposition, volatilization, adsorption and oil return of structural materials in the furnace cavity.

(1) External pollution

The external pollution of the vacuum furnace is mainly caused by the dirt brought in with the parts and volatile elements in the part materials, except for the air leakage of the furnace body. This requires that the surface of the parts should be carefully cleaned before they are put into the furnace. Try not to use them in one furnace for multiple purposes. In particular, there are many volatile elements in the non-ferrous metals, so treatment should be prohibited.

(2) Internal pollution

The oil return evaporation is a kind of pollution caused by the oil vapor entering the vacuum chamber during the working process of the diffusion pump. The quality of diffusion pump oil is a direct factor. In addition, the structure and performance of the cold trap have great influence. The oil vapor is very easy to enter the vacuum chamber through the gap. If the structural performance can be improved, and the labyrinth baffle is added in the pipe connecting the vacuum chamber, the oil vapor can be further prevented from diffusing into the vacuum chamber.

Any pollutant can be adsorbed in the furnace. Some can be adsorbed on the cold wall, and some can be adsorbed and deposited on the hot wall at a very high temperature. Under the effect of temperature, the properties of structural materials in the furnace are constantly changed. When reheating, there will be no reverse evaporation and decomposition, which will affect the surface quality of parts. Therefore, the furnace liner structure should be designed as a part that can be easily taken out, so that the adsorbates in the furnace can be manually cleaned frequently. If the first layer of reflective screen is polluted, it is better to replace it directly.

The refractory brick type should not be used for the furnace structure, because the refractory brick structure can not be cleaned, which can only make the pollution more and more serious. At most, the high temperature and long time baking furnace can be used for purification, but the effect is not significant. In addition, the components in the furnace bricks are impure. During the heating process, the oxides and impurities in the bricks continuously decompose, polluting the vacuum environment from the beginning. Only repeated heating can purify. In addition, the brick microporous air absorption is very strong, and the vacuum degree cannot be guaranteed theoretically. At least, the vacuum pumping time is much longer than that of the metal structure.

Soft magnetic alloys and expansion alloys cannot be heated in the carburizing medium, especially permalloy which is very sensitive to carbon and cannot use graphite as the heating medium. Generally, aluminum oxide powder should be packed in a special box (box) for the furnace. Before use, it should be baked for a long time under high temperature and put into a drying bottle for standby. The aluminum oxide powder that has been used for many times should be updated in time, because the aluminum oxide powder is also a material with strong adsorption. For the furnace that has not been used for a long time, a high-temperature vacuum purification shall be carried out before the heat treatment of soft magnetic alloy to fully eliminate the moisture in the furnace.

Relatively speaking, elastic materials require less “cleanliness” of vacuum. Because in most cases, the surface chemical treatment can be carried out after the heat treatment of parts. The most common method is electric polishing. But for parts that are not allowed to change the size, especially some important high-precision sensor elements, it is necessary to ensure “cleanliness”. The change of part material will directly affect the product characteristics, such as frequency, temperature coefficient, quality factor Q value, etc.

It is an objective fact that the vacuum furnace is easy to be polluted and difficult to be cleaned. Attention must be paid to the heat treatment of precision alloys at all times.

Heat treatment equipment selection: The RVGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for precision alloy vacuum heat treatment process. Good temperature control accuracy and uniformity ensure the effective implementation of vacuum heat treatment process. SIMUWU focuses on the manufacturing of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.