SHEET METAL MILL FINISHES Machinery's Handbook, 31st Edition
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Table 19. Standard Mechanical Finishes for Stainless Steel Sheet, ASTM A480/A480M Table 19. (Continued) Standard Mechanical Finishes for
No. 8
Mirror-like finish
A highly reflective, smooth finish typically produced by polishing with successively finer grit abrasives, then buffing. Faint polish lines may be visible on the final product. Blending after part assembly can be done with buffing. Results from the cold-rolling of an annealed and descaled or bright annealed product to obtain mechanical properties higher than that seen in the annealed condition. Appearance will vary, depending on the alloy, starting finish, and amount of cold work.
TR (temper- rolled)
Cold-worked to obtain specified properties
Standard Rolled Mill Finishes: Standard rolled mill finishes are created when a stainless steel coil is passed through a set of rolls, and the finish on the rolls is transferred to the surface of the coil. No. 1 is the standard rolled mill finish and least expensive. Produced by hot-rolling, annealing, and descaling by pickling, it is dull with uneven color and texture. Grinding marks and similar blemishes are common. Heavy sheet and plate for structural applica- tions are supplied with this finish. When components will be visible, it is common to apply a secondary finish, such as polishing or abrasive blasting, for a uniform appearance. No. 2D, No. 2B, and 2BA are standard mill sheet and strip finishes. They start with a cold-rolled coil that is annealed and pickled. No. 2D is a uniform, dull, low-reflectivity sheet and strip finish. The dull appearance is produced by pickling or descaling. This is the preferred finish for deep drawing applica - tions, as it holds lubricants well. No. 2B finish is more reflective, resembling a smooth, cloudy mirror. The final finish is produced by a light pass through polished rolls. Heavier passes can be used if higher ten- sile strengths are required. This is a common starting finish for polishing and embossing. A 2BA finish is a bright, reflective, mirror-like surface, ranging from hazy to clear. It is used as a lower cost alternative to highly polished stainless for architectural trim, panels, and fascia, though it is less uniform. When the final annealing is done in a furnace with a controlled atmosphere, it is referred to as bright annealing (BA); to produce a 2BA finish, the bright-annealed coil is given a light pass through highly polished rolls to flatten the coil and enhance finish uniformity. Buffing can produce a consistent mirror finish; it is not unusual for buffed BA to be specified for aesthetic applications where consistency is important. 2BA also can be the starting finish for embossing, coining, selective polishing, or etching. It is slightly more expensive than a No. 2D or No. 2B finish. For all of these finishes, the rolling direction should be marked on the back of cut panels to facilitate installing them in the same direction. Or a protective strippable film can be applied to the front, with printed arrows indicating the rolling direction. As there can be differences in appearance from one end of the coil to the other, due to roller wear, num- bering panels as they are cut from the coil enables adjacent installation of consecutive panels, optimizing uniformity. If more than one coil is needed for a project, ideally all coils should come from the same producer off the same line. Polished Finishes: These finishes for stainless steel are produced by using successively finer polishing belts or by passing a stainless steel coil through special rollers that simu - late a No. 3 or No. 4 polish. No common stainless steel specification requires that a finish meet specific surface roughness requirements, but surface roughness influences appear - ance and corrosion performance. Therefore, it can be important to specify a range or maximum for surface roughness. No. 7 and No. 8 finishes are produced by polishing with successively finer belts and then buffing, with cost based on the number of operations involved and degree of smoothness desired. These finishes can be applied to sheet, strip, tube, pipe, and any of the other stainless steel shapes. No. 3 and No. 4 polished finishes are produced by belt or wheel polishing with fine grit abrasives or by rolling, and they are characterized by very fine parallel “grit lines.” When a piece is mechanically finished, No. 3 involves abrasives between 80 and 150 grit; ap - plied to sheets or strips, this finish typically produces an average surface roughness of up to 40 µ in. or 1.016 µ m Ra. No. 4 is commonly used as an architectural finish and can be
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