Tips to keep in mind when selecting stainless steel tubing include the operating environment, prioritizing tensile strength, ductility and hardness, factoring form and process, customer’s preferences, and managing cost and availability.

Figure 1: Top tips: Choosing the right grade of stainless steel. Source: Eagle Stainless

Stainless steel is supplied to a diverse variety of industries and is a powerful candidate in materials selection. The 300 series austenitic stainless steel is a set of stainless steels that feature iron-based chromium nickel alloys. Type 304 stainless accounts for nearly 50% of stainless steel followed by type 316. It is essentially a low carbon steel that contains chromium at 10% or more by weight. It is the addition of chromium that gives the steel its unique stainless, corrosion-resisting properties. The chromium in the alloy forms a self-healing protective clear oxide layer that forms spontaneously in air. The passive oxide layer means the corrosion resistance remains intact regardless of fabrication operations. However, stainless steel does not provide corrosion resistance below the oxide layer. The passive layer of stainless steel is self-repairing. The chromium in the exposed stainless steel will react with oxygen to form a new chromium oxide if the stainless steel is damaged. The chromium oxide layer will continue to reform and protect the stainless steel surface as long as there is sufficient chromium present.

Stainless steel is commonly used in applications ranging from medical instruments or chemical storage to transport or power generation because of its high corrosion resistance, hygiene and strength. Stainless steel also offers a high strength to weight advantage, ease of fabrication, impact resistance and is 100% recyclable. While there are more than 60 grades of steel, not all stainless steel grades are created equally.

Choose the right metal for your operating environment

The external environment can significantly affect stainless steel performance. Application temperatures and chemicals must be considered. Alloys 304 and 316 retain their strength, toughness and corrosion resistant properties in high temperatures. Stainless steels can be found in applications where high temperature oxidation resistance is necessary, and in other applications where high temperature strength is required. Chromium within stainless steel forms a thin passive layer to the surface. Special high chromium and nickel alloyed grades resist scaling and retain high strength at high temperatures. At higher temperatures, chromium contributes to increased strength; 304 stainless is comprised of 18% chromium.

Stainless steel can corrode when exposed to damaging chemicals, saline, grease and moisture for extended periods of time. Stainless steel is stain-less, not stain-proof. The primary difference between 304 and 316 is the addition of molybdenum in 316 that allows for superior resistance to chlorides. The addition of molybdenum also increases the passivity range. Both are characteristically resistant to oxidation due to their chromium content that passivates the surface with a chromium oxide layer. Knowing where the end product will be used will help decide the type of stainless steel to use. The durability and ability to be recycled make stainless steel one of the most environmentally efficient raw materials.

Prioritize tensile strength, ductility and hardness

• Strength: The stress a metal can withstand before it fractures or deforms
• Ductility: The ability of a material to have its shape changed, such as being drawn out into a wire or thread, without losing strength or breaking
• Toughness: The metal’s ability to deform and absorb energy before fracture

Tensile strength is the resistance of a material breaking under pressure. It is an important consideration when selecting a type of stainless steel. Stainless steels tend to have a higher tensile strength than mild steels. Austenitic steel inherits its toughness and ductility through the addition of nickel; popular grades include 303, 304, 316 and 321. 304 stainless is comprised of 8% nickel, and 316 has 10%. Nickel stabilizes the austenitic structure of the steel at room temperature and below. The ductility is the elongation during tensile strength. High ductility and high work hardening allows austenitic grades to be formed. While hardness of carbon steels can be increased by heat treatment, austenitic stainless is only hardened by cold working operations. Cold forming makes the material harder, stiffer and stronger. All 300 series have a certain degree of work hardening, and 316 grades are easier to harden.

Factor in form and process

Figure 2: Austenitic stainless steel is widely available in bar, wire, tube, pipe, sheet and plate forms. Source: Eagle Stainless

Austenitic stainless steel is widely available in bar, wire, tube, pipe, sheet and plate forms. Most products require additional forming or machining before they can fit into their application.

Stainless steel tubing can be cut to length, bent, coiled, welded, re-drawn, end formed, machined (CNC, EDM), welded or formed into a variety of shapes.

Cutting — When cutting stainless steel to be used in a device, it is important that it is free from burrs. Removing these imperfections creates a smooth, more functional, durable metal part.

Bending — Stainless steel can be bent in multiple directions and angles without excessive distortion, wrinkling or fracturing. Things to keep in mind when bending a tube are the bend radius, yield strength, material thickness and if it is welded or seamless.

Coiled tubing — Used in industries where it is utilized as a heat exchanger or for transporting gas or liquids.

End forming — Developing procedures to ensure repeatability for proper mating of flare and end-reduced tubes. Closed-end tubes are spun closed and welded shut in various end configurations including round, flat or pointe. Some advantages to end forming tubes are enhanced tube strength, connectivity improvements and increased sealing capabilities

CNC machining (computer numerically controlled) — CNC machining can result in improved accuracy, increased production speeds, enhanced safety, increased efficiency and, most of all, cost savings

Though thoughts of tubing may offer circular images, in fact, tubing comes in many shapes, sizes and thicknesses. Round, oval, rectangular or square, stainless steel tubing is versatile and popular. Formed components are ideal for uses in surgical devices, heat exchangers, instrumentation and handling equipment.

Consider your customer preferences

Customers may need to specify to ASTM International Standards. Each ASTM standard fits a series of specifications for customers depending on its chemical composition, heat treatment, temperature, and physical or mechanical attributes. Common ASTM specifications include: A269 for seamless and welded austenitic stainless steel tubing for general service, A213 for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat exchange tubes. A249 is for welded austenitic steel boiler, superheater, heat-exchanger and condenser tube. A908 is for standard specification for stainless steel needle tubing.

Specifications for surface finishes should always be identified if this is an important factor. Many designers choose stainless steel for its aesthetic appearance but stainless steel can sometimes be delivered with a dull mill finish. When material is brought in from the mill, it is either an annealed and pickled finish or a bright annealed finish. Finishes can also be added to improve functionality or for appearance. They can stop corrosion, prevent decay and improve hygiene.

Many customers also require ISO management certifications that ensure a company has solid processes to build a high quality product. ISO 9001 and ISO 13485 are often required in the medical industry. ISO specifications are used to consistently ensure that materials, products and processes follow specific guidelines. These certifications are then audited by an independent third party. ISO series of certifications are recognized internationally, making it more efficient and breaking down barriers for international trade.

Manage material cost and availability

Custom tubing can increase the cost and availability as opposed to using standard sizing. If the standard size tubes are produced in large quantities, then costs will be reduced. Stainless steel tubing is seamless, welded or welded and drawn. Welded tubing starts as long coils of cold-rolled stainless-steel strip that are rolled into tube form. Seamless tube is made from tube hollows that are then drawn down to desired diameters. Seamless tubes can offer improved strength, superior corrosion resistance and higher pressure resistance, but at a price. Heavy walls are more easily achieved with seamless tubing. Welded tubes can be drawn similar to seamless tubing to produce a finer weld seam with better surface finishes and tighter tolerances.

Although high-performing austenitic stainless steels are the most expensive stainless steels upfront, they are well worth the investment. Choosing a corrosion-resistant material well-suited to its application reduces maintenance, downtime and replacement costs. Life-cycle costing methods can quantify current and future costs and create an “apples-to-apples“ comparison of different materials. The low maintenance characteristics also add to stainless steel often being the lowest cost choice. Another way to cut costs is utilizing Kanban -- an inventory and replenishment system that assists in determining what, when and how much to produce. It can help with the ability to efficiently deliver products while continuously improving the process. With stainless steel tubing, the bottom line focus should be on the long-term value of the end product.

Just as not all stainless steel grades are created equal, neither are stainless steel suppliers. An experienced stainless steel supplier can provide guidance about both standard and custom options. Having ISO quality management certifications in place can also ensure reliability to processes and overall value of the products. Quality management systems improve delivery times, reduce customer audit frequency and create an atmosphere of teamwork and customer service. Experts in these organizations can help you prioritize material requirements and guide you toward choosing the perfect stainless steel for an application.

Eagle Stainless is a leading supplier of ultra-high precision, cut-to-length, stainless steel tubing and bar for medical, aerospace and industrial applications. Custom and off-the-shelf sizes and grades are widely available. Eagle Stainless offers cutting, precision machining, bending, CNC machining, end forming, assemblies and more. ISO 9001 and ISO 13485 quality management certifications have been in place since 1998.