Specifying mechanically polished, brushed and buffed stainless steel finishes and their applications

Introduction

Mechanical finishes for stainless steels are covered as finish codes G, J, K and P in BSEN 10088-2. Terms grinding, polishing, brushing and buffing even when used along with these codes are not sufficient to accurately define the finish. There can often be confusion about what these terms mean.
To precisely specify a finish also requires agreed samples. Contamination rust staining on mechanically polished stainless steel surfaces is often the result of using contaminated finishing media, often via hand tools which have previously been used on carbon steel.

See Euroinox publication The Mechanical Finishing of Decorative Stainless Steel Surfaces for more information.

Polishing

Both grinding and polishing involve the deliberate removal of metal from the surface using an abrasive. The resulting surface will have some directional marks, partially dependent on the grit size of abrasive used. In the case of the very fine abrasives used in polishing it should only be possible to see any 'directional marks' under a microscope. Viewed normally 1P/2P finishes should appear non-directional. Satin finishes (2K of BSEN 10088-2) are an intermediate between dull polished (2J of BSEN 10088-2) and bright polished (2P of BSEN 10088-2). These finishes can be enhanced by a final brushing operation.Mill finishes '1' and '2' are described in the article Specifying finishes for stainless steel flat products (sheet and plate)

There is no accepted definition of an abrasive grain or grit size that differentiates grinding from polishing. As a guide, but not a definition, grit sizes of 80 and coarser would be associated with grinding, whereas grit sizes of 120 and finer are used in preparing polished finishes. Like polishing, which often involves using successively finer abrasive grit sizes to obtain the desired final finish, grinding can also involve more than one abrasion stage.

The final grit size used in both grinding and polishing does not fully define the finish and must not be used in an attempt to specify a ground or polished finish on stainless steel. Other parameters such as abrasive pressure, contact time, material feed rate and whether the operations are done dry or wet all affect the character of the finish produced.

Mechanical finishes merely described as 'satin', 'polished', 'dull', 'bright' or 'mirror' can vary quite significantly between mechanical finishing contractors.

The correct choice of steel grade is also important when considering bright polished finishes.Grades such as 1.4541 and 1.4571, which contain small amounts of titanium to improve their "intercystalline" corrosion resistance, cannot be expected to be capable of completely defect-free mirror polished finishes. "Flaky" surface defects are likely to be left after polishing as the hard titanium carbide particles are dislodged from the softer surrounding steel surface. When 2P finishes are required the alternative 1.4307 or 1.4306 and 1.4404 or 1.4432 should be selected

Brushing

Although brushing normally involves the use of a fine abrasive action on the surface of the metal, in contrast to grinding and polishing there is no deliberate attempt to remove a surface layer. Rather it is modified by the action of bristles or a nylon fabric medium (Scotch-Brite) that may have some fine abrasive or lubricant included.

Although it can be a single stage process, following a suitable polishing preparation stage, brushing can be done in several stages to obtain a particular finish. Brushed finishes have the same special finish code, 2J in BSEN 10088-2 as dull polished.

Buffing

In buffing no attempt is made to remove metal from the surface. Buffing is only intended to smooth and brighten the existing surface. Traditionally buffing uses cotton or felt based media, often with the addition of lubricants applied to the buffing wheel.

Whenever buffing is being considered as the final finishing operation, it is important that the pretreated (or existing) surface is defined and controlled. Buffing cannot be used as a substitute for polishing to obtain finishes such as 1P/2P on 'intermediate' abraded ground or polished surfaces. It will only smooth down the surface and will not impart the same characteristics as if the surface has been abraded with successively finer grit sizes (ie as in polishing).

Buffing cannot be used as shortcut to obtaining a polished finish. If the surface that is to be finished by buffing is too coarse, there is risk that traces of the underlying surface finish will be visible on the finally buffed surface.

Specifying mechanically finished stainless steel surfaces

An excellent source of information for specifying mechanically finished stainless steel surfaces is the BSSA 'Stainless Steel Surface Finishes' manual. This has an extensive range of surface finish swatch samples, literature and contact details of finishing companies.

Ground and polished (satin) finishes

Mechanically ground and polished finishes for stainless steel flat products are specified in BSEN 10088-2 in Table 6 as 'special finishes'. Typical Ra surface roughness measurements in micro-metres are based on manufacturers or surface finishing contractors' data. Normally only one surface is required to meet the agreed standard of finish. Stainless steel long products to BSEN 10088-3 only have two special finishes defined G, centreless ground and P, polished.

Abbreviation Finishing Process Route Notes Typical (Ra) micro-metres
1G or 2G Ground Can be based on either 1or 2 ex-mill finishes. A unidirectional texture, not very reflective. Grade of grit surface roughness can be specified. -
1J or2J Brushed or dull polished Can be based on either 1 or 2 ex-mill finishes. Smoother than G with a unidirectional texture, not very reflective. Grade of brush or polishing belt or surface roughness can be specified. 0.5-1.5
1K or 2K Satin polished Can be based on either 1 or 2 ex-mill finishes. Smoothest of the special non-reflective finishes supporting the appropriate steel types' corrosion resistance for most external applications and marine environments. Transverse Ra should be below 0.5 micro m. with clean cut surfaces. less than 0.5
1P or 2P Bright polished Can be based on either 1 or 2 ex-mill finishes. Mechanically polished non-directional reflective finish with a high degree of image clarity (includes mirror finishes). less than 0.1

Buffed finishes

There is currently no provision for specifying buffed finishes on stainless steel flat products in BSEN 10088-2. BS 1449-2 (1983), which was replaced by BSEN 10088-2, was the previous standard for stainless steel flat products and did define two buffed finishes, 3B, dull buffed and 7, bright buffed. The American standard that defines stainless steel flat product finishes, ASTM A480 also includes a No 7 bright buffed finish. A comparison of buffed, ground and polished finishes is included in the table below. Note there is no No5 finish in ASTM A480.

 

BSEN 10088-2 Description BS1449-2 ASTM (A480)
2G Cold rolled, ground 3A No3
- Dull buffed 3B -
2J Cold rolled, brushed or dull polished 4 No4
2K Cold rolled, satin polished 5 No6
- Bright buffed 7 No7
2P Cold rolled, bright polished 8 No8

BS 1449-2 described taking mill finishes 2B or 2A (bright annealed) and creating the number 7 finish by fibre or cloth mop buffing with the addition of a suitable buffing compound. The 3B finish was less of a buff, rather a single pass grind/polish using a 220 grit abrasive, but was classed as a 'buff' finish.
There were no intermediate successive grind/polish stages were required when producing a BS 1449-2 3B finish.

Applications for mechanically finished stainless steel

Mechanically finished stainless steel is widely used, including both building internal and external applications.The surface appearance, corrosion resistance and dirt retention of mechanically finished stainless steel surfaces can vary widely, depending, in part, upon the nature of the abrasive medium used and the polishing practice. The1K/2K finish gives a fine, clean cut with minimal microcrevices. This helps optimize the corrosion resistance and minimising dirt retention of the surface. These finishes are more suitable for external applications than the 1J/2J finishes, especially where service environments are aggressive. The coarser 1J/2J and 1G/2G finishes, where required for their aesthetic appearance are more suitable for indoor applications.

Brushed striated finishes are susceptible to damage, but scratches can be readily abraded out. These surfaces do not fingerprint easily and therefore can be used successfully in areas of high contact such as doors or windows. Atmospheric deposits and other forms of surface soiling are generally washed away most easily if any uni-directional polishing or grinding marks are oriented vertically, in the direction of water run off. Polished reflective surfaces are also susceptible to damage. Remedial polishing is possible but it is more difficult to get satisfactory results than on non-reflective finished surfaces.

Risk of contamination staining or rusting in service

Mechanically produced finishes on stainless steel products, produced by the manufacturing mills, specialist stockholder / service centres and specialist surface finishers are normally free from any contamination that could result in rust staining in service. These specialists often supply polished and brushed products with protective plastic coatings, which can help reduce the risk of damage or iron contamination during downstream storage, fabrication, finishing and installation operations, if left on the steel surfaces.

Contamination rust staining is very often caused when contaminated finishing media have been used. This is a particular risk in multi-metal fabrication shops unless special precautions are not taken to avoid the spread of contamination.

When choosing finishing contractors it is worth bearing these points in mind. The Buyers Guide section of the BSSA web site and the 'Stainless Steel Surface Finishes' manual can be used for selecting specialist stainless steel mechanical finishers.

See http://www.euro-inox.org/fla_240_EN.html for examples of finishes.

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