Surface Finishes
Surface finish is an important element in any specification or purchase order for stainless steel regardless of the intended end use. For those applications in which appearance is important, finish is a design element and must be specified. For those applications where finish is not so important, it should still be specified to avoid receiving a finish that is inappropriate for the application. Surface finishing techniques may be mechanical, electrical, thermal or chemical or a combination. The finish depends on grit size, pressure and product selection.
Variety of surface finishes
| Bright Polish | Electropolishing | Brushed finish | Sandblast finish | Row Casting |
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- Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. Stainless steels are often heat treated; the nature of this treatment depends on the type of stainless steel and the reason for the treatment. These treatments, which include annealing, hardening and stress relieving, restore desirable properties such as corrosion resistance and ductility to metal altered by prior fabrication operations or produce hard structures able to withstand high stresses or abrasion in service. Heat treatment is often performed in controlled atmospheres to prevent surface scaling, or less commonly carburisation or decarburisation.
- Chemical treatments correctly performed can clean the surface and ensure the best possible corrosion performance
- Pickling - acids that remove impurities (including high temperature scale from welding or heat treatment) and etch the steel surface. 'Pickling' means some of the stainless steel surface is removed.
- Passivation - oxidizing acids or chemicals which remove impurities. Passivation works by dissolving any carbon steel contamination from the surface of the stainless steel, and by dissolving out sulphide inclusions breaking the surface. Nitric acid may also enrich the proportion of chromium at the surface.
- Chelating agents are chemicals that can remove surface contaminants.
- Electropolishing - electrochemical treatment is a method of imparting brilliance to its surface by removal of a thin surface layer. Studies have also shown that electropolishing has a beneficial effect on corrosion resistance. The work to be polished is the anode in a cell containing a suitable electrolyte. The process may be considered to be the opposite of electroplating. It has taken its place as an important production tool in the fabrication of the stainless steels along with mechanical polishing processes. Generally speaking, the method supplements the mechanical polishing methods in that it provides an economical means of brightening many shapes or forms that cannot readily be finished by those means. Electrolytic methods should not be employed to remove surface blemishes such as scratches, burrs, pits, scale patterns, forging marks and the like; although they will do so if such defects are very shallow. It is important to realize that defects initially present on surfaces may be greatly accentuated. Surface condition before electropolishing governs appearance as finished. These processes are also applicable to cast stainless steels. However, the resulting surfaces will not be as smooth as those on wrought materials unless they are mechanically prepared beforehand.
Mechanical treatments primary manufacturing processes such as casting produces a surface finish which may be adequate for the end function. If not, there are secondary processes such as machining, cutting, grinding, lapping or mass finishing using tumbling techniques or barrel finishing.
- Sandblasting is often to be used in the process of smoothing, shaping and cleaning a hard surface by forcing solid particles across that surface at high speeds and created a unique surface style; the effect is similar to that of using sandpaper, but provides a more even finish with no problems at corners or crannies. However, this method may affect the corrosion resistance of the steel.
- Mirror polished surfaces, as the name implies, have a bright reflective surface which give a mirror like image. They are the most labour intensive mechanically finished surface with an obvious impact on cost and time of production. As flat product, mirror finishes are produced by post mill abrasive polishing of either hot rolled or cold rolled sheet or strip. Although mirror finishes are only defined in standards for flat products, it is also possible to generate a mirror polish on surfaces where there is sufficient access for the grinders and polishing mops. The exception to this is materials with large or variable microstructures such as some castings or heavily cold worked items or welds. In these cases, it may not be possible to obtain a mirror surface with a clear image or a uniform lustre.
Source:ASSDA
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