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Firstly it’s important to look at the BS EN 10088-2 specification with relation to a 1K or 2K surface finish. The 1K refers to a finish applied onto Hot Rolled stainless steel plate, and the 2K refers to a finish produced on either cold rolled plate or sheet – and the sheet can be either 2B or from a 2R (bright annealed) substrate.
The specification and advice from the British Stainless Steel Association reads;“Satin Polish. Similar to 1J / 2J but with maximum specified Ra value of 0.5 microns. Usually achieved with SiC polishing belts. Alumina belts are strongly discouraged for this finish as this will have detrimental effect on corrosion resistance. Recommended for external architectural and coastal environments where bright polish (2P) is not acceptable.”
Having an Ra of less than 0.5 microns (often written as 0.5µm) means that the surface roughness is very fine. The finer the surface finish the less areas there are for contaminates to become trapped and potentially discolour. SiC, or silicon carbide polishing media gives a ‘clean cut’ to the surface finish thus preventing any ‘nooks and crannies’ for debris to sit in. Whilst an alumina (or aluminium oxide) abrasives belts can achieve a surface roughness of less than 0.5µm, they do not give the same ‘clean cut’ finish.
A 1K / 2K finish can be applied to sheet, plate, laser cut profiles and blanks and also long products such as square tube and flat bar.However there will be some visual differences between the finishes produced on different product as, for example, hot rolled material will require a pre-grind to remove surface scale and defects and long products will of course go through a different product line than, for example, a cold rolled sheet at 3000 x 1500 x 2.5mmBy polishing to a 1K / 2K finish not only is the grain itself fine and smooth but there is a natural lustre to the finish. Not only a technically superior polish but also one with exceptional aesthetic appeal.
When measuring and certifying the less than 0.5µm finish it is often termed ‘Ra’. An ‘Ra’ finish is the universally recognized and most used parameter of surface roughness. It is the arithmetic mean of the departure of the profile from a ‘mean’ or ‘centreline’ and is displayed in microns (µm). The finish should be measured and certified to ensure it meets the measurement required of the 1K / 2K specification.