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An Art Worth Learning : Stainless Steel Finishing

As architects and designers request more unique and decorative designs in stainless steel, the need for metal manufacturers to understand and be able to create, match or duplicate these designs becomes increasingly complex.
Compounding this challenge, subtle variations of traditional common finishes make it not only difficult to identify the finish, but also nearly impossible to duplicate it using hand-held methods. Fit, form and finish are now more important to customers, and the time of fabricators producing simply a “commercially acceptable” finish has passed.

Finishing has evolved into an art form, with fabricators struggling to learn how to achieve these modern touches in a cost-effective way.

This article will identify the most common stainless steel finishes recognized by the sheet metal industry – specifically the SSINA (Specialty Steel Industry of North America) – and explain both how they are created by the manufacturer and how to best replicate the finish in the field using hand-held tools.

Commonly used grades of stainless steel include:

300 Series – austenitic chromium-nickel alloys

Types 301 through 309, with 304 being the most frequently used
Type 310 and 310S for high-temperature applications
Type 316 for food and surgical stainless steel uses, as well as marine and nuclear applications

400 Series – ferritic and martensitic chromium alloys
Types 420 and 440 for cutlery applications

No.1 Finish
No. 1 Finish is produced by rolling stainless steel that has been heated prior to rolling (hot-rolling).
This is followed by a heat treatment that produces a uniform microstructure (annealing) and ensures that the stainless steel will meet mechanical property requirements. After these processing steps, the surface has a dark, non-uniform appearance called “scale.”
Surface chromium has been lost during the previous processing steps, and, without removal of the scale, the stainless steel would not provide the expected level of corrosion resistance.
Chemical removal of this scale is called pickling or descaling, and it is the final processing step. A No. 1 finish has rough, dull, and non-uniform appearance.

Since this finish is seldom used in fabricating, there is no way to duplicate it by hand without removing the “scale” and altering the finish appearance. Any attempt to remove the scale using hand tools in the field will change the finish entirely. There is no way to duplicate the No. 1 finish with hand tools.

No.2B Finish
No. 2B Finish is a bright, cold-rolled finish commonly produced in the same manner as No. 2D, except that the final light-cold rolling pass is done using polished rolls.
This produces a more reflective finish that resembles a cloudy mirror. Finish reflectivity can vary from manufacturer-to-manufacturer and coil-to-coil, with some coils looking quite mirror-like and others being fairly dull.
No. 2B is a general-purpose cold-rolled finish commonly used for all but exceptionally difficult deep drawing applications.
It is more readily polished to high luster than a No. 1 or No. 2D finish

The 2B finish is the fabrication and finishing industry’s most frequent starting point for the fabrication of petrochemical and paper mill equipment, sheet metal tanks, and other equipment where a decorative or linear finish is either not needed or wanted.
Duplicating the bright, cold-rolled finish can be difficult because of the variation in finish from one manufacturer to another and because the manufacturer-applied finish is very uniform and free of roller markings.
Any attempt to match this finish mechanically by hand will result in changing the uniformity of the base finish.

No. 4 Finish
No. 4 Finish is characterized by short, parallel polishing lines extending uniformly along the length of the coil. It is obtained by mechanically polishing a No. 3 finish with gradually finer abrasives.
Depending on the requirements of the application, the final finish can be anywhere between 120 and 320 grit. Higher grit numbers produce finer polishing lines and more reflective finishes. The surface roughness is typically Ra 25 micro-inches or less.
This general-purpose finish is widely used for restaurant and kitchen equipment, storefronts and food processing and dairy equipment. If a fabricator needs to blend in welds or do other refinishing, the resulting polishing lines are usually longer than on product polished by a producer or toll-polishing house.

This finish is the most popular finish for appliances, the food and beverage industry, elevators, escalators, hospitals and any work area where there is food contact. It is the favorite finish of architects and is often classified as a sanitary finish.

No. 8 Finish
No. 8 Finish is the most reflective polished finish that is covered by the ASTM standards. It is produced in the same manner as the No. 7 finish, except that the buffing is continued for an additional five to 10 minutes.
In comparison to a No. 7 finish, the grit lines are much less visible, but they can be seen if the finish is examined closely. The resulting finish is mirror-like but not a perfect mirror.

These two finishes are ornamental finishes generally used where there is little contact with outside sources due to the high cost of maintaining the finish in the field. Applications include panels and columns, often in high-end luxury hotels and office buildings. This finish is becoming more and more popular.
One example is “Cloud Gate,” the public sculpture located in Millennium Park in Chicago.
This sculpture, polished to a mirror finish, is viewed by millions yearly and serves as a focal point for tourism in Chicago.

Disclaimer: Many things here represent our opinion. Others are information from the Internet. We can therefore never claim to be correct or complete. And never base a business decision solely on the news you receive from us.