Surface finishes count for more than just aesthetic considerations
When a design engineer or architect chooses stainless steel for a particular purpose, he or she has an extensive number of different grades to select from. There are also various surface finishes to choose from. The decision as to what type of steel is best suited for any given purpose is based mainly on how corrosive the ambient conditions are. However, surface quality or surface finish also affects the material´s sensitivity to corrosion and its cleanability, which is of particular importance in the food industry and health sector. Surface finish characteristics are therefore not merely a question of appearance.
Hot Rolled Plate and Sheet surface finishes
All steel plate and sheet is hot-rolled as a first rolling step, after which it is annealed in order to attain homogeneous material characteristics. Annealing produces an oxide coating on the steel, which is removed by shot blasting and by pickling in an acid bath. As a result of these manufacturing operations, the steel acquires a relatively coarse, matt grey surface with an Ra value in the order of 2-8 µm; the thicker the plate the coarser the surface, and the higher the Ra value. Localised grinding marks may appear on the surface, but have no practical implications for the use of the steel. Hot rolled plate of this finish is mostly used for heavy process equipment and in thickness from around 5 mm and upward.
Colled Rolled Surface Finishes
Annealed and pickled finish - 2D
By means of further rolling of the steel in a cold state, both its surface smoothness and thickness accuracy are improved. Since stainless steel is cold hardened, that is, it is hardened by the cold-working process, it has to be annealed after the rolling so as to regain its softness and working characteristics. This is most often carried out in an ordinary air atmosphere, which results in a new coating of oxide that has to be removed by pickling in an acid bath. Steel sheet processed in this way has a semi-matt silver-grey surface, which is known as finish 2D (D=dull), and has an Ra value in the order of 0.2-1.0 µm. This value increases with greater sheet thicknesses. It is used mostly for industrial equipment and for the manufacture of pipes and tubes for the process industry.
Skin passed finish - 2B
Cold-rolled and annealed sheet as described above may be given a smoother surface by means of a further light rolling between highly polished rollers. The result is a semi-bright grey surface which is termed skinpass-rolled or finish 2B (B=bright) and has an Ra value of between 0.1 and 0.5 µm. Skinpass-rolled steel sheet is often used for process equipment within the food industry when a surface which is easy to keep clean is required.
Bright annealed finish – BA or 2R
If, after cold rolling, the sheet coil is annealed in a protective gas that prevents oxidisation of the surface (for example, hydrogen and nitrogen from cracked ammonia), pickling is not needed since a bright finish is directly acquired. This surface quality is termed bright annealed finish, BA or 2R. Since the surface has not come into contact with pickling acids, it attains a brighter finish than either of the above examples, and an Ra value of 0.03-0.1 µm. It is used for the manufacture of household equipment and appliances (dish washers, washing machines, etc.).
Polished
By polishing the cold rolled sheet steel with a grinding belt of a certain emery grain size (measured by mesh grit), a surface is obtained which has a silver-like metallic shine, which improves the decorative effect. The finer the grain of the grinding bell, the brighter and more even the finish. However, polishing often results in a surface finish that is poorer than that of the original material. Polishing with a coarse grinding bell of 80-100 mesh gives, for example, an Ra value of 1.0-2.0 µm. The roughness of the surface in this case is still not up to that of finish 2B. Using successively finer grit bells, an almost mirror-like surface can be produced. This mirror-finish has an Ra value of 0.02-0.05 µm.
Surface roughness and structure can also be affected by different polishing techniques, even if no differences are detectable with the naked eye. Wet polishing, for example, produces a much finer surface than polishing with a dry belt. The Ra value could be less than 0.4 µm. The mesh size of the grinding bell and the resultant surface roughness of the sheet (Ra), do not, therefore, have any fixed relation. Wet polishing also produces a cleaner surface than dry polishing, which can be compared to the ploughing of furrows in a field. This means that dirt, bacteria and corrosive substances do not attach as easily to wet polished surfaces.
The ground or polished surface has more or less clearly visible scoring, which also makes scratches from daily wear and tear less visible. A polished surface is also easy to re-create, for example after repairs or welding work, compared with a 2B or BA surface. Polished sheet is used for purposes where quality of appearance and hygiene are very important, such as in commercial kitchens (work surfaces, cooking utensils, dishwashers, freezers) -, in house- holds (cooking vessels), and in public places (sanitary facilities, hospital equipment, kick-plates).
Brushed
The use of brushing (with Scotch Brite, for example), usually on a 2B surface, provides a silky-matt finish without grinding scores. A brushed surface also has the advantage that it can easily be re-created. Brushed stainless steel sheet is used in situations where appearance is an important factor, such as in the building industry (sheet metal cladding and lift interiors), the vehicle industry (car hub caps, wall panels for express trains) and the home (sink units). The Ra value for brushed sheet can be around 0.2-0.4 µm.
Patterned
Patterned sheet is produced by means of extra rolling of cold-rolled brushed sheet coils between special imprinted rolls. In this way, a waffle-type pattern can be obtained on both sides of the sheet. At the same time, the rigidity and strength of the material is increased by almost 50%. This means that it is possible to use a thinner sheet, and thus obtain a lower weight.