Most people think that stainless steel will not rust, in fact, it is like many other metals, hand contact or contact with the air affected by the environment will be oxidized rust, in addition, the stainless steel surface workpiece after mechanical processing such as welding, stamping, curling, heat treatment and other processes to produce a layer of black oxidation metamorphic hardening layer, At the same time, it has formed a lot of small burrs that are visible and invisible to the naked eye and acute Angle burrs, etc., which are generally difficult to remove by mechanical methods, affecting the play of the characteristics of the workpiece material itself. In order to solve these problems, people use various methods to grind and polish metal surfaces such as stainless steel. In order to solve these problems, people use various methods to grind and polish metal surfaces such as stainless steel.

Stainless steel surface finishing – polishing classification

1.1 Mechanical polishing: Mechanical polishing is by cutting, plastic deformation of the surface of the material to remove the polished convex part and get a smooth polishing method, generally using the whitetstone strip, wool wheel, sandpaper, etc., mainly manual operation, special parts such as the surface of the rotary body, can use auxiliary tools such as the rotary table, surface quality requirements can be used super polishing method. Super finishing polishing is the use of special grinding tools, in the polishing liquid containing abrasive, tightly pressed on the workpiece to be machined surface, for high-speed rotation. The surface roughness of Ra0.008μm can be achieved by using this technology, which is the highest among various polishing methods. This method is often used in optical lens molds.

1.2 Chemical polishing: Chemical polishing is to dissolve the micro-convex part of the surface of the material in the chemical medium preferentially than the concave part, so as to obtain a smooth surface. The main advantage of this method is that it does not require complex equipment, can polish the workpiece with complex shape, and can polish many workpieces at the same time, with high efficiency. The core problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is generally 10μm.

1.3 Electrolytic polishing: The basic principle of electrolytic polishing is the same as chemical polishing, that is, by selectively dissolving small protruding parts on the surface of the material to make the surface smooth. Compared with chemical polishing, the effect of cathode reaction can be eliminated and the effect is better.

The electrochemical polishing process is divided into two steps: (1) the macro leveling dissolved product diffuses into the electrolyte, the geometric roughness of the material surface decreases, and Ra > 1μm. (2) Low light level anodic polarization, surface brightness increased, Ra < 1μm.

1.4 Ultrasonic polishing: The workpiece is put into the abrasive suspension and placed together in the ultrasonic field, and the abrasive is ground and polished on the surface of the workpiece by relying on the oscillation of the ultrasonic wave. Ultrasonic processing macroscopic force is small, will not cause workpiece deformation, but tooling production and installation is more difficult. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, so that the dissolved products on the surface of the workpiece are separated, and the corrosion or electrolyte near the surface is uniform. Ultrasonic cavitation in the liquid can also inhibit the corrosion process, which is conducive to surface brightness.

1.5 Fluid polishing: Fluid polishing relies on the high-speed flow of liquid and its particles to wash the surface of the workpiece to achieve the purpose of polishing. Common methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding and so on. Hydrodynamic grinding is driven by hydraulic pressure to make the liquid medium carrying abrasive particles flow through the surface of the workpiece at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flow under low pressure and mixed with abrasives, which can be silicon carbide powder.

1.6 Magnetic grinding polishing: Magnetic grinding polishing is the use of magnetic abrasives under the action of magnetic fields to form an abrasive brush to grind the workpiece. This method has the advantages of high processing efficiency, good quality, easy control of processing conditions and good working conditions. With suitable abrasive, the surface roughness can reach Ra0.1μm.

Factors affecting the polishing quality of the die Because the mechanical polishing is mainly completed by manual, polishing technology is still the main reason for affecting the polishing quality. In addition, it is also related to the mold material, the surface condition before polishing, and the heat treatment process. High-quality steel is a prerequisite for good polishing quality, if the steel surface hardness is uneven or there are differences in characteristics, often produce polishing difficulties. Various inclusions and pores in steel are not conducive to polishing.

3.1 Influence of different hardness on polishing process The increase of hardness increases the difficulty of grinding, but the roughness after polishing decreases. Due to the increase in hardness, the polishing time required to achieve lower roughness increases correspondingly. At the same time, the hardness increases, and the possibility of excessive polishing is correspondingly reduced.

3.2 Influence of workpiece surface condition on polishing process In the crushing process of steel cutting machinery, the surface will be damaged by heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment, so the EDM should be fine-calibrated EDM dressing before the end of the EDM, otherwise the surface will form a hardened thin layer. If the EDM refinement criteria are not properly selected, the depth of the heat affected layer can reach a maximum of 0.4mm. The hardness of the hardened thin layer is higher than that of the substrate and must be removed. Therefore, it is best to add a rough grinding process, completely remove the damaged surface layer, and form an average rough metal surface, providing a good basis for polishing.

Chemical polishing for polishing requirements are not high occasions, need to heat the use of current technology, if it is able to use the above three polishing do not use chemical polishing, because the chemical polishing effect is poor, the comprehensive cost is higher.

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