Polishing Stainless Steel
The process of dull polishing stainless steel sheet and plates involves using an abrasive belt on the surface to change the finish of the metal. Stainless steel sheets and stainless steel coils are normally supplied from the mill in one of two finishes – either 2B or 2R (bright annealed)*. This metal can be polished to achieve a desired finish and the type and direction of polish depends on the abrasive and process used. thyssenkrupp Materials (UK) supplies a wide range of polished stainless steel from stock as well as having in group capabilities to offer many more finishes.
There is a range of stainless steel finishing options available for customisation, depending on the intended use. The finishing of steel can be categorised into the three sections mill finishes, mechanically polished finishes and special finishes. All finishes are to industry standards and the choice of finish should be clearly be specified at point of enquiry.
Polished stainless steel sheet from stock
Either 2B or bright annealed stainless steel that is polished using a 240grit aluminium oxide belt. This is the most common polished finish and gives a directional grained effect.
The typical RA reading is approx. 0.6μm
240 silicon (240s)
Either 2B or bright annealed stainless steel that is polished using a silicon carbide grit that is approx. 240 grit.
This results in a smoother surface than 240 grit and has an approx. RA reading of 0.3μm
Either 2B or bright annealed stainless steel that is polished using circular heads. The overlapping of the heads result in a circular pattern.
Although Ultrabright can be polished from 2B stainless steel, it is recommended to start with a bright annealed substrate because 2B is duller and requires more work to achieve the bright, mirror finish. Ultrabright is a bright, reflective finish, because it had been achieved from a mechanical process and so there may be a number of small, minor grit marks on the surface.
The typical RA reading for this finish is approx. 0.05μm
Bright annealed is not a mechanical finish but is achieve through a softening process at the mill. Because of this, there is no standard for the reflectivity or even RA reading but generally, it is a good, reflective finish.
What is an 'Ra' Reading?
'Ra' (roughness average) is the measure of roughness of a surface and is predominantly measured in micrometers (μm). It is the measure of the miniscule peaks and troughs on the surface as deviating from the average. The higher the reading, the rougher the surface. Although there is no industry standard for an 'Ra' reading on polished stainless steel, there are general accepted norms.
What are Chatter Marks?
Stainless steel can be polished in either sheet or coil form. If the material is polished in coil form, it can be tension levelled after polishing ensuring a smoother surface finish. If the stainless steel is polished in sheet form, the polish is applied after tension levelling so any distortion in the flatness of the stainless steel will result in distortion in the final polish. This distortion occurs because of the inconsistency in the flatness as more or less pressure is applied to the polish and these distortions are called chatter marks. Although chatter marks are not visible in all sheet polished stainless steel, there is no industry tolerance or standard to measure them against and so they are accepted as standard. If the finish of the material is critical, it is recommended to have material that is coil polished which thyssenkrupp Materials UK can supply either from our European Service Centre or from our Vetchberry Coil Processing division.
Through our Vetchberry Coil Processing division or through our European Service Centre we also have the ability to offer coil polished products. This is either in sheet form that has been polished from a coil or as polished coil. This allows thyssenkrupp Materials (UK) to provide dull polished material in non standard sizes that is flat and chatter free.
*On occasion, some mills supply bright rolled material.