In the post-processing of CNC-machined stainless steel impellers, surface polishing is a crucial step in improving their performance and service life. As a core component of fluid machinery, the impeller's surface quality directly affects fluid dynamics efficiency, corrosion resistance, and operational stability. Polishing not only removes machining marks and reduces surface roughness but also reduces fluid resistance, suppresses cavitation, and enhances fatigue resistance. Therefore, it is necessary to select appropriate polishing processes and procedures based on the impeller's complex curved surface structure and the characteristics of stainless steel.
Mechanical polishing is a traditional method for impeller surface treatment, using tools such as grinding wheels and wool wheels in conjunction with polishing paste for progressive grinding. Given the curved surface characteristics of the impeller blades, flexible polishing wheels or CNC polishing machines should be used to ensure the polishing trajectory closely follows the surface profile, avoiding over-polishing or localized residue. In the rough polishing stage, hard grinding wheels are used to remove tool marks; in the intermediate polishing stage, fiber wheels are used to refine the surface; and in the fine polishing stage, diamond polishing paste is used in conjunction with wool wheels to achieve a mirror finish. This method offers high controllability, but complex internal cavities or thin-walled structures are prone to deformation due to stress concentration, requiring strict control of polishing pressure and feed rate. Chemical polishing selectively dissolves microscopic protrusions on a surface using an acidic solution, making it suitable for areas difficult to reach with mechanical polishing, such as impeller bores or intersecting holes. Its advantage lies in the ability to process multiple workpieces simultaneously without the need for complex fixtures. However, the polishing solution composition must be precisely formulated according to the stainless steel grade to avoid excessive corrosion of the substrate by highly corrosive components such as hydrofluoric acid. Thorough neutralization and rinsing are necessary after treatment to prevent pitting corrosion caused by residual acid. Chemical polishing results in high surface uniformity, but the roughness control precision is slightly inferior to mechanical polishing, and it is often used as a pretreatment or auxiliary process.
Electrochemical polishing utilizes electrochemical principles, using the impeller as the anode to dissolve micro-protrusions on the surface in an electrolyte, forming a dense passivation layer. This process can significantly reduce surface roughness, making it particularly suitable for fields with extremely high corrosion resistance requirements, such as medical devices and food machinery. Electrochemical polishing can eliminate the stress layer generated during machining, improve surface hardness and fatigue resistance, and leaves almost no loss in impeller profile accuracy after treatment. However, this process involves high equipment investment, requires specialized operation to avoid excessive corrosion, and is sensitive to parameters such as workpiece conductivity and electrolyte temperature, necessitating strict process control.
Fluid polishing uses a high-pressure pump to deliver abrasive fluid (such as silicon carbide powder + polymer media) to scour the impeller surface, achieving micro-cutting through hydrodynamic pressure. This process has unique advantages for complex internal cavities and micro-aperture structures, uniformly removing burrs and reducing roughness. Fluid polishing leaves no residual mechanical stress and simultaneously cleans oil and particles, making it suitable for the final treatment of high-precision impellers. However, abrasive recovery and wastewater treatment are costly, and strict equipment sealing is required to prevent media leakage and environmental pollution.
After polishing, the impeller needs passivation to further improve corrosion resistance. Passivation forms a chromium oxide protective film on the surface through chemical or electrochemical methods, isolating it from corrosive media. For electropolished impellers, passivation fills microscopic defects and enhances the density of the passivation film; for mechanically polished surfaces, passivation removes residual ferrite and reduces the tendency for electrochemical corrosion. The composition and treatment time of the passivation solution need to be adjusted according to the stainless steel material to avoid excessive oxidation that could lead to surface darkening or dimensional changes.
Surface quality inspection is a crucial step in post-polishing treatment. Surface roughness, scratch depth, and profile accuracy can be assessed through visual inspection, optical microscopy, or laser confocal microscopy sampling. For impellers used in food machinery or aerospace, salt spray testing or corrosive medium immersion testing is also required to verify corrosion resistance. After passing the tests, the impeller must be immediately cleaned and dried, and packaged with rust-preventive oil or vapor phase rust inhibitors to prevent secondary oxidation during transportation and storage.
Post-processing of CNC-machined stainless steel impellers requires a combination of mechanical, chemical, and electrochemical processes. The appropriate solution must be selected based on the impeller's structural complexity, material properties, and application scenario. By optimizing polishing parameters, implementing strict process control, and conducting multi-level quality inspections, both the surface quality and functional performance of the impeller can be improved, meeting the stringent requirements of high-end equipment for core components of fluid machinery.
