With the rapid development of industry, mixers are widely used in petroleum, chemical, pharmaceutical, pesticide, chemical, cosmetics, paper, and wastewater treatment industries. In these industries, work cannot be properly done without mechanical stirrers; on the other hand, stainless steel mixer blades are considered a key part of the stirrer. Casting the mixer blade is a complex process that not only has a complex shape but must also be symmetrical and balanced; additionally, some of them have curved surfaces. Currently, mixer blades are mainly produced through the precision casting method.
Stainless Steel Precision Casting Process
Wax Pattern Making
Due to the complex structure of the part and high dimensional accuracy, to obtain high-quality wax models, the manufacturer must pay special attention to the cooling process of wax patterns to prevent shrinkage. It is also important to ensure that quartz tubes are placed into the mold before closing so that the wax pattern can be easily removed from the entire mold. The parts must be positioned correctly; otherwise, the mixer blades will be positioned oppositely, resulting in a wax pattern with an incorrect structure, which is difficult to detect during inspection. Before assembling wax patterns, the appearance and size of wax models must be tested and inspected.
Shell Building
The shell must meet a series of requirements and features, including strength, permeability, thermal conductivity, linear changes, thermal shock stability, thermochemical stability, and ease of mold removal. The total number of shell layers is 5 to 6. In the surface layer, a silica sol binder is used along with zirconia powder with a 320+ mesh number, and in the backing layer, a silica sol binder is used with mullite powder of 200+ mesh.
Runner System Design
The runner system is not only a channel for filling molten steel but can also effectively feed during solidification. The structural feature of the mixer blade is its complex shape and relatively thin wall thickness. To prevent casting defects such as incomplete system filling, shrinkage cavities, segregation, and deformation during the casting process, the runner system inlet is placed at the top. Analysis shows that this type of runner system is not only fast and stable but also results in orderly solidification and proper feeding.
Pouring Process
The prepared stainless steel alloy is melted in a medium-frequency induction furnace. When the materials melt, the sample is analyzed by direct spectroscopy. When the stainless steel composition meets the requirements, it quickly reaches the desired temperature. Then, the power is reduced so that impurities float to the surface, and slag is skimmed off for the second and third time. The temperature is set at approximately 1690°C. Aluminum with a mass fraction of 0.05% is added for final deoxidation, followed by 0.06% silicon for purification. After power is cut, slag is collected. Before pouring, the shell is heated to 1100°C and held for 15 minutes. After pouring, the runner inlet is covered with an insulating material. Higher casting and shell temperatures can reduce the temperature difference between the molten steel and the shell, improve cooling rates, and enhance the filling process. Since the mixer blade is very thin, if the casting and shell temperatures are lower, the blade may develop defects. Results show that casting quality and yield are high when the shell temperature is between 1050-1100°C and the pouring temperature is 1670-1700°C.
Cleaning and Repairing Castings
Due to the presence of quartz tubes, the mixer blades are immersed in hydrofluoric acid solution for cleaning. Also, since the thickness of the blade is very thin, shot blasting machines should not be used in the cleaning process to avoid deformation. After the cleaning process, passivation and sandblasting must be performed. Due to the high casting temperature, shrinkage cavity defects appear in some cast parts, so some blades need to be simply repaired by welding.
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