Technical Information about Stainless Steel - Mill Forms

Standard industry designations for stainless steel finishes are classified by mill form, there being separate finishes (or conditions) for plate, sheet and strip, bar, rod, wire and tubing. Standard finishes applicable to each mill form are described below.

It is worth noting that there is also a wide range of non standard product finishes suitable for a variety of applications. Stainless steels may be supplied with coloured finishes, textured finishes or combinations of both, they may be electropolished for smoothness or mechanically polished to a fine mirror finish. Finishes can also be applied by fabricators or manufacturers (such as glass bead blasting).

Plate, Sheet, Strip, Coil

Flat products (plate, sheet, strip, coil) form the largest product sector of the stainless industry.

For most practical purposes it is not possible to measure the surface appearance exactly. Samples which the human eye sees as different can be measured as the same, and vice versa.

The easiest thing to measure about the surface appearance is the surface roughness. This is defined as the average deviation of the height of the surface from the centre line of the surface (Ra), and is usually measured with an instrument which drags a diamond stylus across the surface. The movement of the stylus is converted into an electrical signal, from which the Ra value is calculated electronically. The calculation is made using a characteristic cut-off wavelength, usually 0.8mm. The choice of cut-off wavelength is important as it affects the Ra measured on a surface.

Manufacture of Stainless Steel Flat Products

Today almost all stainless steel is melted in an Electric Arc Furnace (EAF) or Basic Oxygen Furnace (BOF), the chemistry is refined in the ARgon Oxygen Decarburising process (AOD), and slabs made by Continuous Casting. These slabs may be partially or fully surface ground before being charged to a reheating furnace, then hot rolled to plate or coil.

After hot rolling plates are annealed by heating at high temperature (>1000C to give a more consistent internal structure than can be achieved by hot rolling. The resulting surface is designated "O" or "HRA" - hot rolled and softened but not descaled. Infrequently, plate in this condition may be used for economy where it would scale in high temperature service anyway, but there is a chromium depleted layer under the oxide and the steel surface is less corrosion resistant than after pickling. The surface appears black and will undergo superficial corrosion in wet conditions, although the corrosion rate will generally be much slower than for carbon steel.

More typically, plates are pickled to remove the high temperature oxide and associated chromium depleted layer. This may include shot blasting before pickling to crack the heavy black surface oxide to speed the removal of the oxide and steel surface layer by the pickling acid (mixture of nitric and hydrofluoric acids). This finish is known as "S & D" (for softened and descaled), or "No 1", or "HRAP". Hot rolled coils are also treated to produce this "S & D" finish before being cold rolled and small amounts of 5 and 6 mm thick material are used in this condition, where smoothness of finish is not iportant. Surface roughnesses are generally in the range 2 - 6 micrometers. Grades respond differently to the pickling acid, which may make them look different..

Plates are commonly conditioned by the producer for the removal of surface imperfections on either or both surfaces by hand grinding after pickling provided the ground area is well merged.

Cold rolling smoothes the steel surface with each succeeding pass through the rolling mill.

The bulk of stainless produced is cold rolled to reduce the thickness by more than 50% and then reannealed in a furnace with an oxidising atmosphere which oxidises the steel surface. This oxidation, and subsequent pickling, roughens the cold rolled surface. The coils are then given a final light skin pass on highly polished rolls of large diameter, which reduces the roughness only slightly but improves the lustre, as well as the flatness of the coil. The resulting finish is "2B", the most common finish on stainless steels.

The appearance of the finish is dominated bythe cold rolling reduction, although many other factors affect the surface roughness. It is not possibleto achieve matching surface finish at different steel thickness. The roughness is largely determined by the total amount of cold rolling and lies in the range 0.1 - 1.0 micrometers. Where a rougher surface is needed for properties such as anti galling, paint adhesion etc, the surface is termed "2D". This finish may be achieved by supplying a surface in the "as-pickled" condition, or by doing some of the cold rolling on roughened rolls. The finish may be skin passed. "2D" surface roughness is in the 0.5 - 3.0 micrometre range, although when roughened surface rolls are used the wavelength of the surface undulations is too long to be measured with the normal cut-off wavelength of 0.8mm.

The most reflective mill finish is achieved by Bright Annealing in a reducing dry nigtrogen/hydrogen atmosphere - "BA" finish. The smooth surface developed by cold rolling is retained by the annealing atmosphere which prevents oxidation of the surface. The high reflectivity requred may require a smoother starting surface prepared by surface grinding the coil before rolling. The surface is too smooth to be measurable by common surface roughness instruments and gloss measurement is the best available technique.

"No. 4" finish is produced by polishing the surface with abrasives. A series of abrasive belts is used which give a decorative polished finish widely used for restaurant and kitchen equipment, stoves, refrigerators etc. No. 4 is generally finished with abrasives of approximately 120 to 150 mesh, after initial grinding with coarser abrasives. The coil may then be tension levelled or skin passed, increasing the lustre of the surface.

The grade and type of abrasive used, the number of belts used and their condition, the head pressure and travel speed of the stock and whetere the grinding process is lubricated or dry, all affect the appearance of the finish. It is unlikely that No. 4 finish from different suppliers will match, since there is no standardisation between suppliers. In-market processors will omit the tension levelling or skin passing step, giving a surface of lower lustre.

Different grades also respond to a standard polishing streatment in different ways, so grades will not match exactly. Of course, the variation of finish with thickness seen with 2B does not apply to No. 4: different thicknesses of the same grade should match. The roughness of No. 4 typically ranges from about 0.4 to 1.0 micrometers.