Stainless steel’s familiar appearance, resistance to corrosion and low maintenance make it an ideal material for many applications where the strength of steel is required.
Stainless is rolled into sheets, plates, bars, wire and tubing for use in everything from food handling/processing equipment, medical instruments and appliances to industrial equipment and structural and architectural uses. When used for the handling of food, chemicals and medical equipment, it prevents product contamination and rusting. The same goes for its use as a construction material in roofs, wall panels, entryways and signs.
According to the Specialty Steel Institute of North America (SSINA), stainless steel isn’t a single alloy, but rather a group of iron-based alloys containing a minimum of 10.5 percent chromium. Other elements are added and the chromium content is increased to improve the corrosion resistance and heat-resisting properties, enhance mechanical properties and improve fabricating characteristics. More than 50 stainless steel grades were originally recognized by the American Iron and Steel Institute (AISI).
When looking at the stainless steel classifications, most fall in the 200, 300, 400, 500 and 600 series categories. When selecting a stainless steel that must endure corrosive environments, the high amounts of nickel and chromium in austenitic stainless steels provide outstanding corrosion resistance. Additionally, many austenitic stainless steels are weldable and formable. Two of the more commonly used grades of austenitic stainless steel are grades 304 and 316.
“Grade 304 stainless steel is the basic chromium-nickel austenitic stainless steel and has been found suitable for a range of applications,” the SSINA states in a report titled, “The care and cleaning of stainless steel.” “It is the most readily available in a variety of product forms. This grade is easy to form and fabricate with excellent resistance to corrosion. 304L is the low carbon version of 304. It is sometimes specified where extensive welding will be done.
“Grade 316 stainless steel offers increased corrosion resistance over 304 stainless through the addition of molybdenum,” the report continues. “This grade is desirable where the possibility of severe corrosion exists, such as heavy industrial atmospheres and marine environments. 316L is the low carbon version of 316”.
Because grade 430 is a straight chromium ferritic stainless steel and has a lower corrosion resistance than the 300 series, it’s typically used for interior applications.
Surface finish is an important element in any specification of stainless steel, and a number of stainless steel finishing options are available. An attractive finish enhances the appeal of the end product. And, it can be used to achieve a variety of visual effects. The choice of surface finish is also important for fabrication processes. For instance, rough surface finishes are appropriate when the steel will be ground prior to painting or coating.
The majority of stainless steel finishes can be divided into three categories: mill finishes, mechanically polished finishes and special finishes (see Table 1). Standard mill finishes and mechanically treated surface finishes are performed on hot and cold rolled stainless steels.
A mill finish is the basic supply condition for all stainless steel flat products for subsequent finishing processes. The majority of mill finishes have a dull or matte finish.
Mechanically polished and brushed finishes involve abrasive materials that cut, in effect, the surface of the steel. Brushed or dull polished stainless steel provides a distinctive look as it retains a pattern of very fine lines parallel to the brushing direction. Brushed finishes, however, typically have a detrimental effect on corrosion resistance. The grooves of the finish can accumulate contaminants that enable rust to occur.
Designations for surface finish classifications are given by a combination of a number: 1 for hot rolled and 2 for cold rolled followed by a letter (see Table 1).
Several finishing options are available for stainless steel, but the three most common are the No. 2B matte finish (mill finish), No. 4 brushed finish
(mechanically polished) and No. 8 mirror finish (mechanically polished).
According to the SSINA, No. 2B is a bright, cold-rolled finish resulting in the same manner as No. 2D finish (a dull finish that results from cold rolling followed by annealing and descaling and may perhaps get a light roll pass through unpolished rolls.) However, with the 2B finish, the annealed and descaled sheet receives a final light roll pass through polished rolls. This is the general-purpose, cold-rolled finish that can be used as is or as a preliminary step to polishing.
In regard to the No. 4 polished surface, the SSINA explains that its surface is obtained by finishing with a 120 to 150 mesh abrasive, following initial grinding with coarser abrasives. This is a general-purpose bright finish with a visible ‘grain’ that prevents mirror reflection. The drawback to this finish is reduced corrosion resistance because the grooves of the finish are susceptible to rust.
The SSINA describes the No. 8 surface finish as the most reflective surface, which is achieved by polishing with successively finer abrasives and buffing extensively until all grit lines from preliminary grinding operations are removed to create a mirror-like, highly reflective finish. Because of its highly reflective surface, it’s used as mirrors and reflectors.
When standard mill or polished finishes are not suitable, numerous special finishes for stainless steel are used for various applications (see Table 2). Regardless of the finish, the SSINA reminds that “cleanliness and stainless steel are closely related and, in many applications, each is dependent upon the other. However, stainless steel performs best when clean – cleanliness is essential for maximum resistance to corrosion.”
The SSINA offers a variety of resources and literature about cleaning and finishing stainless. The SSINA is a voluntary trade association representing virtually all the producers of specialty steel in North America. Part of their mission is to promote the expanded use and recognition of stainless steel.