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Compressor impeller vacuum heat treatment

The impeller, also known as the working wheel, is the most important component on the compressor rotor. The impeller rotates at high speed with the main shaft, and does work on the gas. Under the action of the impeller blades, the gas rotates at a high speed with the impeller. Under the action of the rotating centrifugal force and the diffusion flow in the impeller, the pressure, speed and temperature of the gas are increased when it flows out of the impeller.

The impeller of the propylene compressor not only bears the centrifugal force generated by high-speed rotation and the vibration force generated by the rotation, but also is affected by the low temperature environment. As a key component of a centrifugal compressor, the impeller should not only have good comprehensive mechanical properties, but also have certain low-temperature mechanical properties according to its service conditions.

25Cr2Ni3Mo steel is developed according to the alloying principle of steel and considering the manufacturing characteristics of compressor rotors, on the basis of foreign power station equipment steel ASTMA-1100No8, etc., and appropriately adjusting the content of main elements such as C, Cr, Mo and Ni. , especially the ratio of alloying elements is the decisive factor for the low temperature performance of the material. 25Cr2Ni3Mo steel has been widely used in compressors. At present, vacuum heat treatment of this material is still relatively rare.

Vacuum heat treatment has the advantages of no oxidation, no decarburization, and the heat treatment effect of keeping the surface of the parts bright. At the same time, it also has the advantages of small distortion of the parts, energy saving, no pollution to the environment, and easy automatic control. Therefore, it has been widely used in high-performance equipment parts. Quenched. For 25Cr2Ni3Mo steel, the forging process adopts electroslag remelting smelting and ingot forging. The subsequent heat treatment can use impeller heating in a vacuum furnace, followed by oil quenching or gas quenching process, to ensure its strength and surface properties, reduce processing errors, and increase production efficiency.

Vacuum degree and quenching temperature:

25Cr2Ni3Mo steel impeller does not have very high requirements for vacuum degree, it only needs to prevent surface oxidation, so the vacuum degree selected in the test is 1.33×10-2～1.33×10-3Pa. The quenching temperature of 25Cr2Ni3Mo steel is selected as 830～870℃.

Process flow:

After the impeller is installed in the furnace, close the furnace door to evacuate. After reaching the ultimate vacuum degree, start heating, and use a lower heating rate (2°C/min) to heat up to reduce the temperature difference between the furnace and the workpiece and prevent workpiece distortion and cracking. Increasing the preheating temperature can reduce the temperature difference between the inside and outside of the workpiece and shorten the preheating time, while the holding time at the final quenching temperature should be appropriately extended to fully dissolve the carbides in the steel. The preheating temperature of the impeller is 750℃, and the holding time is calculated according to the thickness of the impeller, and the holding time is more than 30 minutes. Then slowly heat to the quenching temperature, and the holding time is calculated according to the thickness of the impeller, which is more than 20 minutes.

In order to make the impeller have the best comprehensive performance, the cooling rate of quenching should be strictly controlled. The analysis shows that the cooling rate of the gas is one of the main factors that determine the effect of gas quenching. After the distance from the nozzle to the workpiece exceeds a certain value, the gas quenching rate will decrease sharply. Only by ensuring a certain cooling rate can the martensite structure be obtained. Therefore, the impeller is quenched at a cooling rate of 5 × 10⁵Pa, cooled to 50°C, and air-cooled. The specific process is shown in the figure.

In addition to the gas quenching process, the impeller can also be oil quenched, and the specific process is also shown in the figure below.

Tempering process:

According to the technical requirements, the impeller is heated and quenched in vacuum and then tempered. Since the quenched 25Cr2Ni3Mo steel has high hardness and a large amount of internal stress, it must be tempered at high temperature to eliminate the stress and reduce the hardness of the material. , to improve the overall performance of the material. The 25Cr2Ni3Mo steel impeller is tempered at 610°C, and after tempering, it is cooled to 50°C for air cooling.

The difference between oil quenching and gas quenching:

25Cr2Ni3Mo steel is generally oil quenched, and the cooling rate during the quenching process is very important, which directly affects the mechanical properties after quenching and tempering. In order to obtain good mechanical properties and surface structure, the quenching and cooling rate of steel must be increased. For gas quenching, the cooling rate of gas is one of the main factors affecting the effect of gas quenching. The mechanical properties of the 5×10⁵Pa air-cooled impeller are basically the same as those of the oil-quenched impeller, and there is no obvious difference, but the oil-quenched impeller is slightly gray.

Selection of vacuum heat treatment equipment: VOGQ and RVGQ series vacuum heat treatment furnaces produced by SIMUWU are high-quality products for the vacuum heat treatment process of steel impeller parts. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture 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 air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.