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.

STANDARD MILL FINISHED – FLAT ROLLED PRODUCTS

The Standard Mill Surface Finished 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 herein relative to the finishing operations employed. It should be remembered that different grades of Stainless Steel can result in a 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, ie fully 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, that is the removal of the scale by use of suitable acids, and then passivated by the use of Nitric Acid.

No. 0 Finish

Also referred to as Hot Rolled Annealed (HRA). The plate is hot rolled to required thickness, and then annealed. No pickling or passivation operations are effected, resulting in a scaled black finish.

This does not develop the fully corrosion resistant film on the Stainless Steel, and except for certain high temperature heat resisting applications, this finish is unsuitable for general end uses.

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, ie rolled without 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 application, 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 2BA Finish

This is more commonly referred to as a BRIGHT ANNEALED (BA) FINISH. Material with a No. 1 finish is Cold Rolled using highly polished rolls in contact with the steel surface. This smooths and brightens the surface.

The smoothness and reflectiveness of the surface improves as the material is rolled to thinner and thinner sizes. Any annealing which needs to be done in order to effect the required reduction in gauge, and the final anneal, is effected in a very closely controlled inert atmosphere. No oxidation or scaling of the surface therefore occurs and there is no need for additional pickling and passivating.

The final surface developed can have “MIRROR” type finish similar in appearance to the highly polished No. 7 and No. 8 Finishes.

Note
Much of the 2B Finish sheet imported is not a true 2B Finish. Mills which operate bright annealing facilities will often carry out all the annealing operations of Cold Rolled material in such facilities. This leads to a superior “2B” finish as no oxidation or scaling takes place during the annealing operation, even though the actual rolling may be effected on polished rolls as for normal 2B Finish, but not highly polished as would be needed to produce a BA finish.

The following finishes are all mechanically produced polished finishes. As well as being standard mill finishes, they are also applied to stainless steel articles and components to meet the required aesthetic criteria. It should be appreciated that factors such as hand polishing vs. mechanical polishing; polishing a flat product as against a component of complex shape; thickness and composition of material can affect the visual appearance of the final surface.

No. 3 Finish

This is a ground unidirectional 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 (eg outdoor handrails).

No. 6 Finish

These finishes are produced using rotating cloth mops (Tampico fibre, muslin or linen) which are loaded with abrasive paste.

The finish depends on how fine and abrasive is used, the uniformity and finish of the original surface.

The finish has a non-directional texture of varying reflectiveness. “Satin Blend” is an example of such a finish.

No. 7 Finish

This is a buffed finish having a high degree of reflectiveness.

It is produced by progressively using finer and finer abrasives and finishing with Buffing compounds. Some fine scratches (grit lines) may remain from the original starting surface.

No. 8 Finish

This is produced in an equivalent manner to a No. 7 Finish, the final operations being done with extremely fine buffing compounds.

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, No. 6, No. 7 and No. eight) are generally only produced one side of the sheet, the reverse side being either a 2B or No. 3 Finish.

The specific alloy of stainless steel should be chosen to match the intended use of the metal. For simplicity sake, I would say there are roughly eleven standard architectural stainless steel alloys out of the close to sixty alloys recognized by the American Iron and Steel institute. Of these eleven alloys, consisting of the “200 Series”, “300 Series” and “400 Series”, the “300 Series” make up the majority and are predominatly the ones specified for landscape, site and marine applications.

Narrowed down further, Types 301, 302, 304 and 316 are the alloys commonly used for architectural stainless steel applications; Type 301 being the least corrosive resistant and Type 316 being the most. In general, Type 304 is usually my choice, given its excellent corrosion resistance, good workability, and good for welding – low carbon content. If the stainless steel application happens to be within an exterior urban or coastal environment where chloride and sulfur deposits might predominate making pitting corrosion a concern, then Type 316 is the choice.

A word about low carbon content alloys. If welding of stainless steel is required, a modified alloy type, designated by the letter “L” following the alloy number may be used, such as 316L. A condition is created when chromium carbides form at the site of the weld, stripping the protective chromium layer and allowing localized corrosion to occur. The “L” designation stands for low carbon alloy stainless steel.  Designating a low carbon stainless steel alloy reduces the formation of chromium carbide. The one caveat however is that the lower carbon versions of the 304 and 316 stainless steel alloys have a reduced yield stress, therefore the lower strength trade off must be considered when using the low carbon alloys.

Thanks for reading and stop back in January when my next Specification News topic will address stainless steel finishes, their number system designation, description and uses.

Bedding sand under concrete unit pavers should conform to ASTM C 33 or CSA A23.1. This material is often called concrete sand. Masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust. The bedding sand should have symmetrical particles, generally sharp, washed, with no foreign material. Waste screenings or stone dust should not be used, as they often do not compact uniformly and can inhibit lateral drainage of moisture in the bedding sand.

ICPI Tech Spec 17-Bedding Sand Selection for Interlocking Concrete Pavements in Vehicular Applications provides additional guidance on selecting bedding sand. Bedding sand should be spread and screeded to a nominal 1 in. (25 mm) thickness. Frozen or saturated sand should not be installed. If there is an uneven base (due to inconsistent compaction or improper grading), the bedding sand should not be used to compensate for it. Over time, unevenness in the bedding sand will reflect through to the surface. Uneven areas on the base surface must be made level prior to placing the bedding sand. Once the base is complete, screed pipes or rails are placed on it and the bedding sand spread over them. The sand is screeded or smoothed across the pipes with a straight and true strike board. Screed pipes are removed and the resulting void filled with bedding sand. After the sand is screeded it should not be disturbed. Sufficient sand is placed and screeded to stay ahead of the placed pavers. Powered screeding machines that roll on rails and asphalt spreading machines adapted for screeding sand have been successfully used on larger installations to increase productivity.