Welded Stainless Tubing

Welded tubing is produced by rolling strips of stainless steel into a tube and then welding along its full length. After welding, the weld seam, or bead, can be refined by cold rolling and forging methods or left as is.

Cold forming results in smoother finishes and tighter tolerances. Welded tubing can even be drawn like seamless tubing to offer a better weld seam, and surface finish. Welded tubing can also be produced with thinner walls on larger diameter tubes, compared to seamless tubing. Because welded tubing requires less processing, it is typically quicker to produce as well as lower in cost than seamless tubing.

The down side of welded tubing comes in the form of high pressure and heavy wall applications. The long welded seam causes a stress concentration point, reducing pressure ratings to 80% of comparable seamless tubing. Also, the stainless material used to create welded tubing must be thin enough to roll effectively. Therefore, heavy wall tubing is less feasible as welded tubing.

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Upside

• Better lead times
• Tighter tolerances
• Lower cost
• Thinner wall, large diameter applications
• Available in longer lengths
• More consistent concentricity

Downside

• Heavy wall applications
• Slight impurities
• Less corrosion resistant
• Stress concentration

Seamless Stainless Tubing

         To make seamless tubing, the first step is to form a seamless pipe known as a hollow/mother tube. Starting with solid round stock, the billet is first drilled through then heated and forced through a narrow die. The center of the hollow tube is supported during this process by a long rod known as a mandrel. Next, the mother tube is put through a pilger mill.  This machine uses a pair of rolling dies and a second mandrel to elongate and reduce the tube to size.

This process can quickly reduce the tube cross section by as much as 90% in a single run. However, pilgering is not well suited for smaller diameter tubing.

For small diameter tubing, drawing is a better option. In this process, the mother tube is crimped for grasping on one end, then pulled through a narrowing die. By doing so, the tube is narrowed and elongated.

This process may need to be repeated several times to achieve the size reduction needed.

Seamless tubing can contain higher pressure because it does not have the large stress concentration point that welded tubing has. Additionally, seamless tubing often has a cleaner surface finish meaning it is less susceptible to pitting and corrosion. However, there are many more steps needed to manufacture seamless tubing. Because of this, seamless takes much more time to produce and can be more expensive than its welded counterpart.

Upside

• Superior corrosion resistance
• Higher pressure rating
• 내구성
• Higher Purity
• No additional testing
• Ideal for critical applications

Downside

• Higher costs
• Not ideal for thin wall large diameter tubes
• Higher difficulty to manufacture
• Longer lead times