Mill Finishes

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 in this section.

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). Further details may be obtained from ASSDA or its members.

Plate, Sheet, Strip, Coil

Flat products (plate, sheet, strip, coil) form the largest product sector of the stainless industry. In many applications the designations listed in Tables 1 and 2 are adequate, but frequently purchasers wish to specify the surface finish in more objective terms. Because there is potential for confusion, the factors influencing production of the standard mill finishes available and their appearance are examined in Tables 1 and 2.

Table 1 Standard Plate Finishes

Table 2 Standard Sheet, Coil and Strip Finishes

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 (R_a), 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 R_a value is calculated electronically. The calculation is made using a characteristic cut-off wavelength, usually 0.8 millimetres. The choice of cut-off wavelength is important as it affects the R_a measured on a surface. A typical surface roughness trace from a 2B surface is shown in Figure 1. The very high magnification (about 25,000 x) in the vertical direction shows the microtopography of the surface very clearly.

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 (>1000 degrees Celsius) 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 No. 1, or HRAP or S&D (for softened and descaled). Hot rolled coils are also treated to produce this finish before being cold rolled and small amounts of five and six-millimetre-thick material are used in this condition, where smoothness of finish is not important. Surface roughnesses are generally in the range two to six micrometres. 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. Roughness of a typical coil is shown in Figure 1. This curve is fundamental for understanding how 2B and BA surfaces are generated.

The bulk of stainless produced is cold rolled to reduce the thickness by more than 50 percent, and then re-annealed (at over 1000 degrees Celsius) 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 by the cold rolling reduction, although many other factors affect the surface roughness (Table 3). As is obvious from Figure 1, it is not possible to achieve matching surface finish at different steel thicknesses. The roughness is largely determined by the total amount of cold rolling and lies in the range 0.1 to one micrometre. 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 to three 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 0.8 millimetres.

The most reflective mill finish is achieved by Bright Annealing in a reducing dry nitrogen/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 needed 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 (silicon carbide gives a softer appearance and less torn surface than aluminium oxide of equivalent abrasive mesh size), the number of belts used and their condition, the head pressure and travel speed of the stock and whether the grinding process is lubricated or dry, all affect the appearance of the finish. It is unlikely that No. 4 finishes 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 treatment 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 0.2 to 0.6 micrometre. A typical surface trace is shown in Figure 1. For most practical purposes standard mill finish designations are adequate, but very critical applications may require discussions with the steel manufacturer or polisher before placement of the order, often using typical samples retained as reference pieces. The use of such samples should include an agreement on the viewing conditions - sample size, light level, viewing distance, angle of inspection - and must recognise that the manufacture of stainless steel is complex and some variation in surface appearance is inevitable.

Tubular Products: Stainless Steel Tube and Pipe

Tubular products may also be polished to obtain alternative surface finishes, however instead of typical finish designations, the polished finishes for tubing are usually indicated by the grit size, such as 80 grit, 120 grit, 180 grit, 320 grit and 600 grit.

Stainless steel seamless tubing is available in certain sizes with the internal surface electropolished. This process is designed for applications requiring the ultimate in cleanliness and corrosion resistance, eg the dairy, food and pharmaceutical industries. To complement the tubing, a range of internally electropolished fittings is also available.

Tubing may be specified polished on outside, inside, or on both surfaces. Welded and cold finished seamless tubing is suitable for grinding or polishing and is produced in sizes up to 168 millimetres outer diameter (OD) with polished outside surfaces. It is not practical to polish interior surfaces or tubes with an inner diameter (ID) less than 19 millimetres.

Typical applications

Tubular products are widely used in key process industries; for architecture (hand rails, canopies, sunshades); transportation (seating, luggage racks); marine applications (boat rails, fittings); etc.

Pipe Finishes

Stainless steel pipe and fittings are generally supplied in the annealed and pickled condition to facilitate fabrication. Pipe can be externally grit polished if required; this finds some applications in structural architectural work.

Typical applications

• Food Processing - Heat exchanger shells, process piping

• Industrial - Furnace muffle tubes, burners, quench headers

• Petroleum and Petrochemical - Refinery and chemical process lines and equipment; reactor vessels

• Pulp and Paper - Condensate lines, bleach liquor plant piping, process piping

• Other key industries include oil and gas, power generation, water and sewerage treatment, etc.