While carbon steel and copper alloys are often welded by high frequency welding, stainless steels require a fusion (full melting of the metal) welding to ensure a high-quality weld.

Coinciding with the Industrial Revolution and the increased need to process and distribute petroleum early in the 19th century, an increased demand developed for tubes to transmit gas. One of the first methods of manufacturing these tubes included butt-welding of heated, curved strip was developed in Britain then in 1832 in the United States. Similar processes are still being used to produce seamed tubing. An improvement on the hot-pressure butt-weld was developed in the United States about 1921 when the seam was joined by electric-resistance. Steel tubes are still produced this way by roll forming a coil of continuous strip into the shape of a tube and then welding the longitudinal seam. To improve the quality and meet the needs for various applications, the processing of the welded tubes has since evolved and includes annealing, straightening, cold-drawing and various testing based on the customer requirements. While carbon steel and copper alloys are often welded by high-frequency welding, stainless steels require a fusion (full melting of the metal) welding to ensure a high-quality weld.

Welded stainless steel tubing is generally selected for its lower cost relative to seamless, as well as lead times which are typically less than seamless. Welded steel tubing generally allows thinner walls at larger diameters and is available in longer lengths. With greater available alloys from coil, welded stainless steel tubing offers flexibility and reduced lead times in applications most typically used for mechanical purposes, heat transfer, and hydraulic systems.

Seamless stainless steel tubing is commonly seen in applications like instrumentation and aerospace products.  Seamless tubing has 15% higher ASME working pressures compared to welded tubing of the same material and size because it doesn’t have a seam weld. When seamless tubing is specified, a welded tube often cannot be substituted.  However, seamless tubing usually has significantly more eccentricity than a welded product as it is very difficult to control placing the hole in the hollow made from the initial bar.  As a welded product is made from a strip with a very carefully controlled thickness, once the tube is formed, it is much more concentric.

Stainless Steel Pipe is often used to convey various types of fluids, such as water, oil, gas, propane, paint, beer, etc. from one location to another. In some circumstances, Pipe specifications can also be met using tubing manufacturing processes. On a per lb basis, tubing is usually more expensive than pipe due to tighter manufacturing tolerances and enhanced performance criteria.  Tubes can be manufactured in different shapes, such as square and rectangular, based on product requirements, and can be stronger than pipe.

When cold worked, tubing offers the highest concentricity tubular product and ideal when looking to achieve tighter

Understanding the process of how your material is made is important when specifying a new requirement. When welding a tube to size, most stainless tube manufacturing mills solely form the tube and weld it and perform little or no additional operations on the weld.  Many applications dictate the importance of welding a tube with some reinforcement and then cold work that weld using both OD and ID tooling, which provides multiple functions.  For starters, it blends the weld geometry to be smooth and flush with the base metal to assist with future operations on the tube. When cold worked, tubing offers the highest concentricity tubular product and ideal when looking to achieve tighter dimensional tolerances, elevated mechanical properties, and improved surface finishes on tubes. The drawing process can also be used to achieve tighter tolerances, unusual sizes, special mechanical properties, and improved surface finishes. Cold drawn tubes are seen in applications that require tubular products to go beyond standard commercial requirements for consistent tolerances and mechanical properties, such as motor and pump shafts, pneumatic cylinders, and aircraft hydraulic tubing.

A final function of cold working with ID and OD tolling is it assists in break-up the as-cast structure of the weld and creates critical cold work needed to homogenize the weld during the following annealing operation to provide optimum mechanical properties and weld corrosion resistance following the annealing operation. Depending upon the alloy and application, the choice of the type of annealing operation is important.  For alloys containing 7% or more nickel, the time to fully anneal the weld may take a few minutes as the nickel significantly reduces the atoms diffusion rate.  Higher nickel stainless steels like the 6% Mo alloys may even require times twice as long as 300 series stainless.  To fully homogenize these stainless steels, an off-line continuous furnace anneal is required.

Those alloys that have low nickel, like 439 and SEA-CURE® high-performance stainless steel have very high diffusion rates and can be effectively annealed using in-line induction annealing.  The high diffusion rates also can have detrimental results.  Secondary phases that can embrittle or lower corrosion resistance can very quickly form in low nickel HP stainless steels.  Those alloys need to be quenched very quickly to prevent this from happening.

A tubing manufacturer has many alternatives for manufacturing and testing stainless steel tubing for various applications including feedwater heaters, condensers, heat exchangers, industrial water and a variety of aerospace end uses

A tubing manufacturer has many alternatives for manufacturing and testing stainless steel tubing for various applications including feedwater heaters, condensers, heat exchangers, industrial water and a variety of aerospace end uses.  In general, ASTM/ASME specifications have minimum requirements that may not be sufficient for a specific application. No single NDE test is sensitive to all defect orientations. For a minimal additional cost, many applications can justify a combination of tests. In general, an eddy current test and an air-under-water test combination are cost-effective for finding most defects that would cause future problems in service and the addition of ultrasonic testing is often added for critical applications.

It’s important to know the stainless steel tubing supplier as there are no ASTM and ASME requirements for third party review and inspection of the manufacturing process to ensure that the tube complies with the specification.  Additionally, without this review to reduce costs, a manufacturer may stretch interpretations of what is required in a specification to satisfy compliance with their end user’s application requirements. With global competition, these issues have become significantly more apparent. The purchaser’s expectations may be significantly different than what the supplier is providing.

This article scratches the surface on only a few aspects of not only stainless steel material but the associated manufacturing processes. It’s a best practice for material specifiers to understand the process of how the material is produced and tested. This will lead to making better materials decisions and holding suppliers to a higher standard of manufacturing and product quality.  To learn more about processes Plymouth Tube uses to manufacture tubing or if you’d like to arrange a meeting with our technical team to discuss your operation, click here!