Technical Resources

Stainless steel is the general name for the family of iron-based alloys that contain at least 10.5% chromium. When there is sufficient exposure to oxygen, an invisible protective passive chromium-rich oxide film forms on the surface. This invisible film forms automatically as long as the surface is clean and exposed to oxygen. Higher levels of chromium and the addition of other alloying elements such as nitrogen and molybdenum enhance this surface layer and improve the corrosion resistance of the stainless material.

The first number is the amount of chromium that is contained in the stainless, so 18 is 18% chromium. The second number is the amount of nickel, so 8 stands for 8% nickel. So 18/8 means that this stainless steel contains 18% chromium and 8% nickel. 18/10 is 18% chromium and 10% nickel. Both 18/8 and 18/10 are often used to describe Type 304 or austenitic (300-series) stainless steels with similar chromium and nickel levels. The ASTM chemistry requirements for the stainless steel grades that are sometimes identified with these designations vary, but generally, they contain 17 to 20% chromium and 8 to 10.5% nickel. Although a higher nickel content is implied, the term 18/10 is sometimes used strictly for marketing purposes when the stainless steel’s actual chemistry is closer to 18/8. These designations should never be used for buying raw materials.

Both of these stainless steels contain chromium and nickel alloy additions, but Type 316 also has a molybdenum (Mo) alloying addition. Molybdenum improves corrosion resistance and is particularly helpful when there is exposure to chlorides (coastal or deicing salts) or corrosive pollutants. Type 304 contains 18 to 20% chromium and 8 to 10.5% nickel. Type 316 contains 16 to 18% chromium, 10 to 14% nickel and 2 to 3% molybdenum. Corrosion resistance increases with higher alloying additions of molybdenum.

The AISI (American Iron and Steel Institute) was the originator of the 300 and 400 series numbering system (i.e. Type 304 stainless steel). They also published a stainless steel products manual that listed these designations and the typical chemical analysis and mechanical and physical properties of each individual grade. These documents were NOT specifications or material standards. The AISI has not issued numbers for stainless steel for some time. Although AISI numbers are still commonly used to generically identify stainless steels, the UNS (Unified Numbering System) numbering system is preferred internationally to identify specific chemistries. AISI or UNS numbers only indicate the chemistry, so it is important to also use to the appropriate material standards. In North America, the ASTM International standards should be used. See “Specifications for Stainless Steel ” for more information, located in our Library.

An overview of the alloying elements used in each stainless steel family and their role is provided on our website via Product Resources.

The term “duplex” describes the microstructure that is characteristic of this family of stainless steels, which is typically equal parts austenite and ferrite. Duplex stainless steels are characterized by high strength levels and provide a range of corrosion resistance levels. They are used in applications ranging from moderately corrosive architectural applications to corrosive industrial and seawater environments. They have very good resistance the chloride stress corrosion cracking and provide better chloride crevice corrosion resistance than austenitic stainless steels with comparable pitting corrosion resistance.

The SSINA Designer Handbooks provide guidelines that can be helpful in selecting appropriate stainless steel and finish for a particular application, but they are not standards or specifications. It is important to always use legally recognized industry standards such as those published by ASTM International when specifying stainless steel. For more information review “Specifications for Stainless Steel ” located in our library.

The use of the letter L after the grade number (i.e. 304L, 316L) means that the carbon content is restricted to a maximum of 0.03%. In stainless steels that are not low carbon, maximum levels may be set at 0.08 to 0.15%. Low carbon levels should be specified when sections 0.25 inch (6 mm) or heavier will be welded. Specification of low carbon reduces the risk of sensitization and improves the weld’s corrosion resistance.

There is a common misconception that specifying low carbon will improve the corrosion resistance of surfaces outside the weld zone. This is not the case. Corrosion resistance is improved by specifying stainless steels with higher chromium, molybdenum and/or molybdenum levels. Specification of low sulfur (0.005% or less) may also improve corrosion performance.

Stainless steel gauge thicknesses are not defined by ASTM and can vary from producer to producer. Therefore, it is important to order stainless steel products by the specific thickness range, maximum or minimum that is required in either inches or millimeters. Gauge numbers can be referenced, but should not be used exclusively. A cross-reference showing the nominal thicknesses typically associated with stainless steel gauge numbers can be found in our Designer Handbook: Stainless Steel Architectural Facts available in our Library. It is important to note that the typical thicknesses associated with the gauge numbers for stainless steel, carbon steel, and aluminum are all different.

Stainless steel is 100% recyclable. An international study by the ISSF determined that the average recycled content of stainless steels is 60%, but, in areas of the world where there have been more historic use of stainless steel, the recycled content is typically higher. In North America, SSINA has issued a LEED statement indicating that the average recycled content of the 300-series stainless steel sheet, plate and reinforcing bars used in building and construction is between 75 and 85%.

Yes. Stainless steel has excellent properties at both extremes of the temperature scale. Stainless steel can be used down to liquid nitrogen temperatures and up to about 1800°F.

No. The austenitic or 300 series stainless steels are generally non-magnetic but high levels of cold work can make them somewhat magnetic. Castings made out of these stainless steels can also be somewhat magnetic. This slight magnetism is not an indication of a problem with the stainless steel. Other stainless steels families are naturally magnetic (e.g. ferritic 400-series and duplexes).

Yes. The 300 series stainless steel can be “hardened” but only by “work hardening.” This is done by cold working the material. This can be done by cold rolling to lighter thicknesses, cold “drawing” through a die, deep drawing a sink, pressing a pattern into the surface (i.e. embossing) or by the other operation that changes the metal’s shape while it is cold. “Annealing” stainless steel will remove this work hardening effect.

Annealing is a heat treatment for stainless steel. Stainless steel is usually sold in the “annealed” condition.

Stainless steel products (sheets, plates, bars, wire, etc.) are usually supplied in the “annealed” condition. That means that the last operation is to heat the material up to a temperature where the residual stresses of manufacturing can be relieved, and the material will be in the “soft” condition. Most flat-rolled products, however, are made in coils and when a “sheet” is cut from the coil it is usually “flattened” which does add some small amount of stress to the material. Bar products are usually straightened and that adds some small amount of stress as well. The term “dead soft” usually refers to a product where even this small amount of stress is removed, but as a practical matter, this condition is not readily available.

No. Stainless steels do not need to be preheated before welding.

Yes. However, the standard grades of stainless steel are usually “gummy” and will not produce a clean chip when machined or turned. To solve this problem, many companies produce “free-machining” grades of stainless where they add a “chip-breaker” to the matrix.

Yes. Stainless steel is easily welded, but the welding procedure is different than that used with carbon steel. Available in our library, SSINA has a Designer Handbook that provides guidance. The “filler” rod or electrode must be appropriate stainless steel. American Welding Society standards should be used when specifying welding procedures and the fabricator should be able to provide current certifications for the welders. When the welding is structural, AWS D1.6 Structural Welding Code: Stainless Steel should be specified.

Yes. Stainless and carbon steels can be welded together. It is important to use a filler metal that is appropriate for stainless steel. Please see our Designer Handbook Welding of Stainless Steels and Other Joining Methods, available in our library. It is important to note that galvanic corrosion may be a problem if there is an electrolyte (moisture) present on a regular basis that will connect the two metals at this welded joint. Appropriate steps, such as applying a protective coating, should be taken to prevent moisture from bridging the metals and causing selective corrosion at the welded joint. It is important to note that, if the carbon steel has been galvanized, the coating must be completely removed from the surface prior to welding or zinc embrittlement could occur.

Stainless steel surface corrosion can occur. The reasons for this can vary and include inappropriate stainless steel selection for the service environment, surface contamination with iron or carbon steel (i.e. contaminated tools, carbon steel brushes), or cleaning with inappropriate products. Please see our handbooks for specific guidance for different service environments or contact your stainless steel supplier for advice. See “The Care and Cleaning of Stainless Steel ” for more information on cleaning stainless steel. All handbooks and guides are available in our library for your review.

Muriatic (hydrochloric) acid is very corrosive to the stainless steels that are commonly used in architectural applications and should never be used on or near them. There are alternative products for cleaning tile, concrete, and masonry that do not contain this acid. Even if the acid does not touch the stainless steel, the “fumes” in an enclosed space may cause surface corrosion of the stainless. Thoroughly rinsing the acid off the surface and neutralizing it as quickly as possible can limit or prevent corrosion. Surface refinishing or component replacement may be necessary it significant damage has been done.