Analysis And Improvement of Surface Defects in The Inner Cavity of Precision Cast Centrifugal Pump Impeller Castings

Structural analysis and precision investment casting production process of impeller castings

1.1 Analysis of impeller casting structure

The impeller is one of the core components of a centrifugal pump, which functions to directly transfer the mechanical energy of the prime mover to the liquid, in order to increase the static pressure energy and kinetic energy of the liquid [1]. Figure 1 is a two-dimensional plan view of the impeller casting. From Figure 1, it can be seen that the impeller belongs to a closed impeller, consisting of blades and upper and lower cover plates. The enclosed impeller has high working efficiency and is most commonly used in centrifugal pumps. It is suitable for conveying clean liquids with low viscosity, such as clean water, that do not contain particles. The outline dimensions of the impeller are 200mm × 120mm, the inlet diameter is 60mm, the outlet width is 8mm, and there are 6 blades evenly distributed around the circumference. The blades are in a tapered shape, and the thickness of the upper and lower cover plates and blades is 3mm. The diameter of the upper and lower cover plates of the impeller is large, the inlet and outlet sizes are small, the shape of the inner cavity blades is complex, and the surface roughness of the inner cavity flow channel requires Ra of 1.6-3.2 μ m. The cover plate and blades

The wall thickness is very thin, which brings great difficulties to casting production. The impeller material is CF8M.

1.2 Precision casting production process of impeller castings

To meet the requirements of impeller size and inner channel surface roughness, medium temperature wax investment casting process is adopted for production. Due to the enclosed impeller, the inner cavity blades

The shape of the blade is complex, and the inner cavity of the impeller wax mold is formed by combining 6 urea cores. Figure 2 shows the welding process of the impeller wax mold. The cylindrical hole with a diameter of 18mm on the lower cover plate of the impeller is blocked, and a cylindrical top riser is directly placed on this end face. Adopting the silica sol shell making process, the surface layer is made of 80-120 mesh zircon sand, and the 2-4 layers are made of 30-60 mesh mullite sand. The surface layer to the second layer needs to be naturally air dried for 6-8 hours, and after the second layer, each layer needs to be air dried for ≥ 12 hours. Before each layer of coating, an infrared thermometer is used to detect the temperature of the inner and outer surfaces of the shell to ensure sufficient drying of the impeller cavity. Use compressed air to remove floating sand from the inner cavity; After 4 layers, use 60-80 mesh mullite sand to block the inner cavity holes and continue to reinforce until 6

Layer half [3]. Using steam dewaxing kettle for dewaxing and rotary roasting furnace for roasting. Using a 200kg fast medium frequency furnace for melting, the steel material is required to be clean and free of oil stains. The fork shell is poured, and the pouring temperature is controlled at (1600 ± 10) ℃. After pouring, it is placed on a steel frame and the bottom is overhead for heat dissipation and cooling. Using a vibration sheller to remove the shell, sawing and cutting the riser, followed by heat treatment, and then finishing, shot blasting, acid washing and other post-treatment. Figure 2 Welding process of impeller wax mold assembly

Description of surface defects on the inner cavity of impeller castings

The final inspection of the finishing shows that the size of the impeller casting meets the tolerance requirements of the drawing, and the outer surface roughness Ra of the upper and lower cover plates of the impeller meets the requirement of 1.6-3.2 μ m. However, the surface roughness of the inner cavity blades does not meet the standard, with Ra of 6.3 μ m. Due to the large curvature of the inner cavity blades, the blade inner cavity near the impeller outlet cannot be visually detected. After dissection, it was found that there were iron bean defects in this area. Figure 3 shows a photo of the location of the iron bean defect, and Figure 4 is a magnified 20x image of the iron bean defect.

3.2 Elimination of Subcutaneous Pore Defects in Urea Core

Urea will decompose and release ammonia when heated above its melting point. At the production site, urea is melted using a direct heating method with an electric furnace, and the urea is directly heated

Placed in a stainless steel basin, this heating method results in uneven temperature and high urea temperature at the bottom of the basin. According to the infrared thermometer detection, the temperature of urea in the pelvic floor is as high as 145-150 ℃, far exceeding its melting point temperature. Urea undergoes a condensation reaction, producing biuret and releasing a large amount of ammonia [5]. Part of the precipitated ammonia evaporates into ammonia gas and enters and exits the air; Part of it is still dissolved in the molten urea solution. When molten urea solution is poured into the urea core mold, as the temperature decreases, the solubility of ammonia in the urea solution decreases, resulting in a large amount of ammonia precipitation in the urea solution. The thickness of the urea core in the impeller blades is thin, and the solidification and cooling of the contact surface between the urea solution and the mold are fast after pouring. The ammonia in the urea solution is not present.

It can completely float and remain in the urea core of the leaves, forming shallow subcutaneous stomatal defects. To improve the surface quality of the impeller urea core, temperature equalization, temperature control, and prevention of urea condensation reactions are key. Improvements have been made to the core making equipment, replacing direct heating from electric furnaces with thermal oil heating, and equipped with specialized temperature control display instruments to strictly control the temperature below 145 ℃. The defect of subcutaneous pores on the shallow surface of urea core is significantly reduced.

3.3 Surface roughness analysis

By improving the core making equipment, the shallow surface subcutaneous porosity defect of the urea core has been removed, and the iron bean defect in the inner cavity of the impeller casting has been eradicated. However, the use of urea

The roughness of the inner cavity and blade surface of the castings produced by the wax mold of the impeller made from a solid core still does not meet the standard (Ra is 6.3 μ m). There are two reasons for this. Firstly, urea is easily hygroscopic and deliquescent in the air, resulting in a decrease in the surface quality of the urea core. Furthermore, from the surface of the metal mold to the surface of the urea core, and then from the surface of the urea core to form the inner cavity surface of the impeller wax mold, there is an additional surface copying process, and naturally, the inner cavity of the impeller wax mold made from the urea core is not as good as that made directly from the metal mold.

5 Conclusion

(1) The iron bean defect in the inner cavity of precision investment cast impeller castings is formed by subcutaneous pores on the shallow surface of the urea core.

(2) By improving the urea melting device and controlling the temperature of the urea solution within a constant range of ≤ 145 ℃, it is possible to effectively remove subcutaneous porosity defects on the shallow surface of the urea core.

(3) By changing the design of the impeller compression structure and adopting a disconnected bonding form, the wax mold of the impeller inner cavity blades is formed by aluminum core pulling, which can effectively improve the quality

The surface quality of the blade meets the customer's requirements for the roughness of the inner surface of the impeller.