STAINLESS STEEL SURFACE FINISHES
The surface of Stainless Steel is actually an extremely thin but stable and passive Chromium rich oxide film, on which Stainless Steel relies for its excellent corrosion resistance. The surface finish on Stainless Steel should therefore be developed and maintained to ensure this vital property, and also for the secondary reason of the pleasing aesthetic appearance of Stainless Steel.
The Standard Mill Surface Finishes are laid down in Specifications (BS 1449, Part 4, and the Committee of Stainless Steel Producers, American Iron & Steel Institute).
The finishes are designated by a system of numbers, and these are broadly described hereunder relative to the finishing operations employed. It should be remembered that different grades of Stainless Steel can result in variation of visual appearance for the same finishing operation. The thickness can also have an effect, generally the thinner the material the smoother the surface finish.
The thicker gauge sizes of Stainless Steel are Hot Rolled. This is done at high temperatures and will always result in a scaled surface.
Stainless Steel Flat Product is supplied in the annealed, i.e. softened, condition. This is also a high temperature operation, and unless carried out in a very closely controlled inert atmosphere, will result in oxidation (scaling) of the surface. The scale is usually removed by a pickling process (the removal of the scale by use of suitable acids), and passivated by the use of Nitric Acid.
Stainless Steel Mill Surface Finishes
|Hot rolled, annealed and pickled
|Dull cold rolled, annealed and pickled
|Bright cold rolled, annealed and pickled
|Grained 100-120 grit
|Grained 150-180 grit
|Bright annealed (highly reflective)
No 1 Finish
Plate is hot rolled, annealed, pickled and passivated. This results in a dull, slightly rough surface; quite suitable for industrial applications which generally involve the range of plate thicknesses. Grinding marks may be visible in isolated areas.
Some of the thinner thicknesses within the plate range are Cold Rolled; but Sheet, Coil and Strip gauges are produced by Cold Rolling, i.e. rolled without and heating of the material. Cold Rolling hardens the material, and the thinner sizes may have to be subjected to an intermediate anneal and pickle, or bright annealed, during the reduction of thickness to final gauge.
The starting material for Cold Rolling always has a No. 1 finish. Cold Rolled material is supplied with the following standard mill finishes.
No 2D Finish
A No. 1 Finish after being Cold Rolled, Annealed, Pickled and Passivated. This results in a uniform dull matt finish, superior to a No. 1 Finish.
Suitable for industrial applications, and eminently suitable for severe deep drawing as the dull surface (which may be polished after fabrication), retains the lubricant during the drawing operation.
No 2B Finish
A 2D Finish is given a subsequent light skin pass cold rolling operation between polished rolls.
This is the most common finish produced and called for on sheet material. It is brighter than 2D and is semi-reflective. It is commonly used for most deep drawing operations, and is more easily polished to the final finished required than is a 2D finish.
No. 3 Finish
This is a ground, unidirectional and uniform finish obtained with 80 – 100 grit abrasive.
It is a good intermediate or starting surface finish for use in such instances where the surface will require further polishing operations to a finer finish after subsequent fabrication or forming.
No. 4 Finish
This is a ground unidirectional finish obtained with 150 grit abrasive. It is not highly reflective, but is a good general purpose finish on components which will suffer from fairly rough handling in service (i.e. restaurant equipment). The final surface is blemish free with a high degree of image clarity, and is the true mirror finish.
Note: The finer polished finishes (No. 4, or greater) are generally only produced on one side of the sheet, the reverse side being either a 2B or No. 3 Finish.)