Mechanical finishes of stainless steel Appearance influences many characteristics. How do you choose the best surface for every use among the many different finishes?

Stainless steel, unlike other metals, contains a certain percentage of chromium. This alloying element creates a film of protective oxide, a few atoms thick. Despite being very thin it is very resistant and prevents corrosion. Also it regenerates itself: if it is removed or damaged it instantly reforms. For this reasons, stainless steel can also be used without being painted. There are many finishes that fit the natural aesthetics of steel to the most different contexts and uses.

Finishes can be divided between mechanical, electrical, electrochemical and chemical. Below we are going to list only the mechanical ones. The categories with the number 1 are hot rolled, while those with the digit 2 are cold rolled. They are categorized in accordance with EN 10088-2. These are the most common mechanical finishing.

Standard mill finishes

1U Hot rolled sheet, not annealed. Mill scale is not removed. Mainly used as a by-product. 1C Hot-rolled sheet, heat treated (annealed). Mill scale is not removed. Mainly used as a by-product.

1D Hot-rolled sheet, heat treated and pickled. It's the most common finish on high thicknesses. It's mostly used on hidden parts because of its opaque and rough surface. The D means dull.

2D Cold-rolled, heat treated like 1D, and pickled. It's more refined than 1D. The dull appearance limits its usage to non-decorative uses.

1E Hot-rolled and heat treated by annealing. Mill scale has been removed by mechanical means at the discretion of the supplier.

2B This finish is obtained from 2D with a further rolling with highly polished rolls. Final appearance is grey, and can be used as a starting point for further polishing or brushing. It's the most commonly used finish.

2R Cold-rolled with polished rolls and heat treated in controlled oxygen-free athmosphere. Material is not damaged so pickling is not needed. It can also found under the obsolete label BA or 2BA (Bright Annealed). It's largely used without further processing because of its appearance and because it's easy to clean. It's also used as basis for mirror-polishing or for colouring.

2H The H stands for Work (H)ardened, because a specific cold rolling increases the sheet metal strength. Surface appearance is bright. 2C Cold-rolled and heat treated. Mill scale is not removed. 2E Cold-rolled sheet, heat treated. Mainly used on steels that have a hard mill scale that in this finish has to be mechanically removed. 2Q Cold rolled and heat treated by tempering. This finish is mostly done on martensitic steels.

Special finishes 1G o 2G Ground sheet metal, with a unidirectional and rough surface. It's often possible to specify the grit grade or the desired roughness. Usually only one face of the sheet is processed.

1J e 2J Brushed sheet metal, smoother than 1G/2G. Also here it's possible to specify the grit grade or surface roughness. Mostly used in interior architecture.

A typical example is the Scotch Brite finish, named after the fine-grit abrasive material. This finish is used in interior architecture.

1K e 2K Polished satin matte finish, similar to 1J/2J but smoother, with a maximum roughness of 0.5 microns. Usually silicon carbide polishing belts are used, because alumina (aluminium oxide) weaken corrosion resistance. This type of sheet metal is mainly used on exterior architecture and marine/seaside areas. 2F Heat treated by annealing, pickled and then rolled with bead-blasted rolls to get a dull finish.

1M e 2M Uniform surface can cause unpleasant deformed reflections. To avoid this effect, sheet metal with 1M/2M finish are decorated with a textured pattern on one side, by stamping or rolling. The other side is smooth. Textured sheets also have an increased rigidity that allows for reduced thicknesses. Many decorations, textures and patterns are available.

1W e 2W Like 1M/2M categories, but sheet metal is textured on both sides. Also this category is widely used in architecture. In some cases, the surface helps making scratches and dents less visible. 1P/2P Polished bright stainless steel, non directional, mirror-like. It's the best choice where corrosion resistance is fundamental. Surface roughness can be specified. This finish is obtained with polishing felts and specific chemical products.

Bead blasting This finish is not included in EN 10088-2 regulation, but it's widely used. It's similar to sandbasting in its principle, the difference being that beads are bigger. They can be made of different materials like stainless steel, glass, ceramics, nut shells, aluminum oxide, according to the desired effect. Bead shape can be spherical or irregular. Austenitic steels can see their surface hardness increase with heavy beads blasting. In some cases, both surfaces must be treated to balance internal stresses. Mild steel beads, or other materials containing iron, must not be used to avoid stainless steel contamination.

Engine turning This unmistakable effect is obtained by grinding the sheet metal with rotating abrasive pads, with a constant offset, usually 60°. Complicated and custom patterns and decorations can also be acheived.

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