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Stainless Solutions

IMOA’s »Stainless Solutions« e-newsletter covers a different stainless steel issue each month, with tips on design and specification, and links to technical resources.

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Specifying Structural Sections

Stainless steel is an ideal choice for both visible and hidden structural sections in corrosive environments because of its corrosion resistance. The superior stiffness retention of austenitic stainless steels in fire relative to carbon steel means that fire protection may not be required in non-industrial buildings with lower fire temperatures. This combination of characteristics makes it possible to use uncoated stainless steel for many aesthetic structural applications.

There are, however, other reasons for its specification.  Austenitic stainless steels in particular have much better impact resistance then carbon steel at ambient temperatures and still perform well at very low temperatures when carbon steel becomes quite brittle. These properties and its blast resistance make stainless steel an ideal choice for safety and security oriented applications. Please see IMOA’s Structural Design page for more resources.

Common stainless steel industry terminology can be confusing. Stainless steel suppliers commonly use “grade” to mean “alloy”. From a structural design perspective, “grade” means strength level and suppliers may be unaware of this terminology difference, which can cause confusion. Using specific yield and tensile strength levels and alloy names can avoid misunderstanding.

Finding Suppliers & Available Sizes
Stainless steel is available in the same product forms as carbon steel and aluminum but the range of stocked sizes is more limited because the market is much smaller. Any size and shape can be custom manufactured but that is generally more expensive and requires longer lead times. It is best to contact suppliers to determine commonly stocked sizes before starting design. This information can often be found on their websites and also for North America, via the links below:

Metals Service Center Institute Directory
SSINA Directories for Structural Service Centers and Manufacturers


Specifications & Guidelines
Always use the appropriate codes, specifications and design guides for stainless steel, including the stainless steel structural welding codes. We have a downloadable list for guidance.

Tube versus Pipe
It is not uncommon for project specifications to mistakenly use pipe or seamless tubing for structural applications. Pipe is specified based on inside diameter and intended for transporting fluids, often under pressure. It is generally not intended for structural applications.  Pressure rated tubing can also dramatically increase cost because of the added testing that must be done for certification.

Welded mechanical tubing is suitable for most common structural applications and is available in small and large diameters and light to heavy walls. Round, square and rectangular shapes are available, and are less expensive than seamless or pressure rated tubing or pipe.

 

Type 316 was specified for these security bollards because of deicing salt exposure.

Credit C. Houska, TMR Consulting

Selecting the Right Stainless Steel
Structural sections often have rougher finishes, such as abrasive blasted, because they are less expensive.  Rougher finishes retain more corrosive deposits and that can lead to a corrosion staining problem that might not occur with a smoother finish. IMOA’s Stainless Steel Selection System should be used for alloy selection guidance, if staining cannot be tolerated.   

If the application is in a more corrosive environment, such as sea or brackish water, please see our guidance for these environments.

Fasteners
Information on fastener specifications can be found in our Specifications & Guidelines list.  The most important rule in fastener specification, other than achieving the desired mechanical properties, is that they should be at least as corrosion resistant as the most corrosion resistant of the materials joined. If not, rapid galvanic corrosion of the fastener can occur if carbon steel fasteners are used with stainless.  Fasteners of all levels of corrosion resistance are available. Obtain assistance from a stainless steel industry association if you can not find suppliers or are unsure about the appropriateness of a stainless steel.

These Type 316 structural sections are exterior and in an environment with coastal and deicing salt exposure. They are connected to painted carbon steel. Paint is a temporary barrier, especially when parts are intended to move and abrasion will occur.  The staining in these pictures is carbon steel corrosion due to paint loss.  Direct stainless steel and carbon steel contact will lead to accelerated corrosion of the carbon steel.  An all-stainless design would have been preferable, at least in sections with moving parts.

Credit C. Houska, TMR Consulting