Machinery's Handbook, 31st Edition
1452 SHEET METAL MILL FINISHES produced by mechanical polishing or rolling. If mechanical polishing is used, the final pass can be between 120 and 320 grit; average surface roughness on sheets or strips is typi- cally up to 25 µ in. or 0.635 µ m Ra. There may be overlap between the surface roughness measurements and grit ranges for the No. 3 and No. 4 finishes. Either finish can be polished wet or dry. Wet polishing produces a smoother, finer finish. A soft, satin, directional finish, No. 6 was used for the columnar features on the exterior walls of the Empire State Building and other early stainless steel projects. It is typically produced by Tampico brushing a No. 4 finish with an oil and abrasive mixture, for a silver- white appearance; however, the high wear rate of Tampico brushes makes it difficult for workers to obtain a uniform appearance, and this finish is known for having fingerprint problems. ASTM A480/A480M allows other polishing methods to be substituted if they produce a finish with the same surface roughness, reflectivity, and an appearance that a stainless steel surface finish expert considers visually identical. Polishing lines should be soft and the resultant finish much less reflective than a No. 4. A finished surface sample that acts as a visual standard is particularly important when specifying this finish, be - cause a wide range of visually different finishes may be called a No. 6. Still smoother and more reflective sheet finishes are obtained by buffing polished sur - faces. The semi-bright No. 7 is polished to a finely ground surface and buffed to a mirror-like finish, with residual grit lines still visible. The highly reflective, mirror-like No. 8 is progres - sively polished and buffed so only faint polishing grit lines show on final products. Surface Finishes of Stainless Steel Strips.— Five mill finishes are defined by ASTM A480/A480M for stainless steel strips. Within each of these finish categories, some varia - tion is expected, due to the characteristics of different alloys and differences in metal thickness and methods of processing. No.1 strip finish is similar to a No. 2D sheet finish, which is produced by cold-rolling to the specified thickness, followed by annealing and pickling. It varies in appearance from dull gray matte to fairly reflective. No. 2 strip finish is produced the same way as a No. 1 finish, except that it is followed by a final cold roll pass, often on highly polished rolls. It is a smoother, more reflective finish than a No. 1 strip finish. If the rolls are highly polished, it is equivalent to a No. 2B sheet finish. BA (bright-annealed), TR (temper-rolled), and No. 3 and No. 4 polished finishes also can be used on stainless steel strips and follow the requirements for sheet finishes. No. 5 is a separate category and may be negotiated between buyer and seller. Surface Films.— When stainless steel arrives from the mill or processor, it generally has a protective passive film on the surface. As long as the surface is clean and exposed to oxygen, the passive film automatically forms when the surface is abraded or machined. If the surface is protected from contamination and scratching during fabrication, transit, and installation, and there is no welding, then chemical passivation of the surface after fabrication is not required. Washing with a mild, chloride-free detergent and water solu- tion or an ammonia and water solution still may be necessary. Protective Plastic Films: These can be applied by the producer, processor, fabricator, or finisher to protect stainless steel surfaces. The use of strippable protective plastic film during forming operations that do not require heat helps protect surfaces from contami- nation by other metals and light scratching and also acts as a lubricant. Film also can help prevent damage from light scratching during shipping, storage, or installation. If an appropriate protective film is specified, it can be left in place until just prior to turnover, eliminating or reducing the need for final cleaning. Commonly used protective films are porous and do not prevent corrosion from saltwa - ter, water containing deicing salt, swimming pool atmospheres, corrosive acids, and other substances that can be corrosive to some stainless steels. Where there is exposure to such elements, a film can even accelerate corrosion by acting as a crevice, so protective strip - pable films should be removed immediately after installation. If film is applied to a component that will be stored or installed in an exterior location, a UV-rated film should be specified. Film life takes into consideration the total time a film is to be used, including potential project delays. Film left in place past its warranted life
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