With the rapid development of industry, stir mixing has been widely used in petroleum, chemical, medicine, pesticide, daily chemicals, cosmetics, paper making and wastewater treatment industries. In these industrial production, it cannot do without mechanical stirring equipments, and stainless steel mixing impeller for agitator is the key component of mechanical stirring equipment. Agitator blade impeller is a complex casting, which not only has complex shape, but also needs to be symmetrical and balanced, and some have complex curved surface. At present, the agitator blade impeller blank is mainly produced by stainless steel investment casting.
Stainless Steel Investment Casting Process
Wax Models Making
Due to the complex casting structure and high dimensional accuracy requirements, to obtain high quality wax models, stainless steel casting manufacturer should pay more attention on the cooling of mould and wax models, which can prevent wax models shrinkage. It is also important to note that quartz tubes should be placed in the mold before closing. In order to make the wax model can be easily removed from the whole mold, several live blocks are cleverly used in the mold design. The live blocks must be placed correctly, otherwise the impeller blades will be in the opposite direction, resulting in a wax mold with incorrect structure, which is difficult to detect during inspection. Before the combination of wax models, the appearance and size of wax models should be tested.
Shell Making
The shell shall meet a series of performance requirements, including strength, permeability, thermal conductivity, linear variation, thermal shock stability, thermochemical stability and shucking ability.The process of silica sol shell includes preparation of surface layer and back layer. The total number of shell layers is 5 and a half. The surface layer uses silica sol binder +320 mesh zirconia powder, and the back layer uses silica sol binder +200 mesh mullite powder.
Gating System Design
The pouring system is not only the channel for molten steel filling, but also can effectively feed during casting solidification. From the structural characteristics of blade impeller casting for agitator , the casting blade impeller structure is complex and the wall thickness is relatively thin .In order to prevent casting defects such as insufficient casting, shrinkage cavity, shrinkage cavity, cold separation and deformation in the process of agitator blade impeller casting, the top gating system type is adopted in the design of gating system.The analysis shows that this type gating system is not only fast and stable, but also can form solidification sequence, and has good feeding effect.
Smelting & Pouring
The stainless steel material is prepaid according to table 1 and smelted in medium frequency induction furnace. When the material is melted, the sample is analyzed by direct reading spectroscopy. When the stainless steel compositions meet the requirements, they rapidly rise to the appropriate temperature, and the mass fraction is 0.1% manganese metal and 0.07% ferrosilicon was predeoxidized. Then it is reduced to a smaller power to allow the molten steel to stand still for floating impurities and slag removal for 2-3 times. Adjust the temperature to 1690℃, add the mass fraction 0. 05% aluminum for final deoxidation, and then add 0.06% rare earth silicon for refining. After power failure, slag was scraped and direct pouring should be done from the furnace. Before pouring, the shell was roasted to 1 100℃ and kept warm for 15 min. After pouring, insulation agent was covered on the gate cup.The higher casting temperature and shell temperature can reduce the temperature difference between molten steel and shell, reduce the cooling speed of molten steel, and improve the filling capacity of molten steel. Because the agitator blade impeller is too thin, if casting temperature and shell temperature are lower, the impeller blade will appear serious cold separation and scrap. The results show that the casting quality and yield are very high when the shell temperature is 1,050 ~ 1,100 ℃ and the pouring temperature is 1,670 ~ 1,700 ℃.
Cleaning and Reparing of Castings
Because there are quartz tubes embedded in the castings, hydrofluoric acid is used for soaking during cleaning. Because the wall thickness of blade impeller is too thin, in order to prevent deformation, the shot blasting machine should not be used in the cleaning process. After cleaning, passivation and sandblasting should be carried out. Due to the high casting temperature, shrinkage cavity defects appeared in some castings at the junction between transverse and longitudinal blades, so some blade impeller castings needed to be simply repaired by welding.
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