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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

The principles of vacuum heat treatment process formulation

Vacuum heat treatment has the advantages of no oxidation, no decarburization, degassing, degreasing, good surface quality, small deformation, excellent comprehensive performance of vacuum heat treatment parts, long service life, no pollution and pollution, clean production can be realized, high degree of equipment automation, alarm system A series of features such as perfection.

The development of vacuum heat treatment process includes the following aspects: determine the heating temperature specification (temperature, heating time and cooling method, etc.); determine the vacuum degree and air pressure adjustment parameters; select the cooling method, cooling medium, etc.

The choice of vacuum furnace type should be determined according to its specific product process requirements. At present, vacuum annealing furnaces, vacuum quenching furnaces, vacuum tempering furnaces, vacuum sintering furnaces, etc. have been applied in batches. The following introduces their applications.

(1) Vacuum annealing

In addition to steel, copper and its alloys, vacuum annealing materials can also be annealed with metals that have affinity with gases such as titanium, electricity, silver, etc. The choice of vacuum annealing vacuum degree should be based on the oxidation characteristics of the metal to avoid the decomposition of compounds in the alloy and desolvation, etc. It should be noted that vacuum annealed parts must be cleaned, degreased and dried, otherwise defects such as oxidation, corrosion, decarburization and carburization will easily occur during the heating process.

The degassing effect of vacuum annealing depends on the process parameters such as heating temperature, time and vacuum degree, and is also related to the physical properties of gases and metal compounds.

(2) Vacuum quenching and tempering

Such process parameters include vacuum degree, heating temperature, holding time, cooling medium, etc., which should be reasonably selected and determined according to the material and performance requirements of the workpiece.

(3) Selection of vacuum degree

To prevent oxidation and decarburization of the surface of the workpiece and the evaporation of alloying elements to reduce the performance of the workpiece and the pollution of the furnace, so the vacuum degree should be selected correctly. Alloying elements such as manganese, aluminum, cobalt, chromium, nickel and other elements have high saturated vapor pressure, so they are easy to evaporate during heating, and the vacuum degree should not be too low. The saturated vapor pressure of titanium is slightly lower than that of chromium and manganese. It is easy to accumulate on the surface under high pressure, and the saturated vapor pressure of tungsten, vanadium, silicon, molybdenum and other elements is low, so it is not easy to evaporate.

①Usually, the quenching heating temperature of steel is divided into two types: high temperature and medium temperature. Medium temperature refers to below 900°C, and high temperature is divided into two types: 900-1100°C and 1100-1300°C. Vacuum degrees in different heating temperature ranges are different. of.

When vacuum heating, the working vacuum degree should be selected according to the workpiece material and heating temperature. First, it must meet the working vacuum degree required for non-oxidation heating, and then comprehensively consider many factors such as surface brightness, degassing and evaporation of alloy elements. Commonly used Recommended vacuum level for tool steels during vacuum heat treatment.

② Selection of quenching heating temperature

Vacuum heating is mainly based on radiation, and the radiation heating efficiency is very low below 700 ° C. The temperature of the workpiece lags behind the temperature of the furnace. Therefore, vacuum heating needs to reduce the hysteresis of the workpiece temperature through multi-stage preheating. Especially for complex and large workpieces, it is particularly important to perform multiple preheating, and the quenching heating temperature generally takes the middle and lower limits of the treatment temperature of the salt bath furnace.

Usually, the quenching heating temperature is taken as the lower limit of the salt bath furnace and the air heating furnace. When the quenching heating temperature is below 900°C, one preheating is used at 500-600°C. When the heating temperature is higher than 1000°C, two preheating is generally used, namely 500°C ~550℃, 800～850℃.

③ Determination of quenching heating and holding time

In the heating of workpieces in periodic vacuum furnaces, there are many factors that affect the time, such as the structural size of the furnace, the amount of furnace, the way of placement, the shape and size of the workpiece, the heating temperature, the heating speed and the preheating process, etc. Generally, An empirical formula is obtained through experimental methods.

Regarding the heating time of vacuum quenching, first look at the characteristic curve of the workpiece when it is heated in vacuum. There are two heating methods: continuous heating and staged heating. The time in the final stage of calculating the workpiece includes the uniform temperature time + holding time. The temperature rise of the parts when heating in vacuum (the temperature of the furnace temperature and the temperature of the surface and center of the heated workpiece), it can be seen that there is a temperature difference between the two.

④ Selection of cooling medium

Vacuum quenching cooling medium is commonly used vacuum quenching oil, high-purity nitrogen, etc., which should be reasonably selected according to specific materials and performance requirements. Generally speaking, when the mold material is added with high-alloy steel and high-speed steel for vacuum quenching, the selected cooling The medium is vacuum quenching oil, which has the characteristics of low saturated vapor pressure, low critical pressure, and good chemical stability, so it has been widely used. The cooling capacity of the vacuum air quenching furnace can increase the convective heat transfer rate proportionally by increasing the density (pressure) and flow rate of the cooling gas, so that the quenching cooling speed can be accelerated.

⑤ Vacuum tempering

Generally speaking, low-temperature tempering below 250°C is done in an air furnace or a low-temperature nitrate bath, and tempering in the range of 260-750°C can be done in a protective atmosphere furnace or a vacuum furnace. During medium tempering, the working chamber should be evacuated to 1.33-0.133Pa, and then filled with high-purity nitrogen for tempering treatment.