How Are Welded Tubes Made?

Structural metal tubing is a key component in many metal fabrication projects and applications. Metal tubing is commonly used because it provides a great combination of strength, low weight, and affordability compared to solid metal stock. There are two main types available: welded tubing and seamless tubing. This article will take a deeper look at how welded tubes are made.

What Are Welded Tubes?

Welded tubes are a type of structural metal tubing that has a weld seam along its longitudinal axis. It can be made from essentially any type of metal or metal alloy that can be welded, such as carbon steel, stainless steel, low-alloy steel, and aluminum. Welded metal tube is typically available in the following shapes:

• Round Tube
• Square Tube
• Rectangular Tube

How Are Welded Tubes Made?

Manufacturing welded tubing on a large scale is a complex process that involves several steps.

1. Metal coil is first selected using a thickness that will later match the thickness of the finished tube wall. The coil must also be wide enough to achieve the desired circumference and diameter.
2. The coil is fed through a series of dies and formed into a tube. This brings the two edges of the coil width towards each other until a tube is formed with one edge touching the other.
3. A welding process is used to join these two edges together.
4. Depending on the welded tube manufacturing process, the excess weld reinforcement may be removed at this time. Depending on the desired specifications, the newly formed tubing may also undergo a heat-treatment process.
5. The tube is then cut to the desired length.

What Type of Welds are Used to Make Welded Tube?

Electric Resistance Welding (ERW) – This technique uses the heat generated by electrical resistance to join metal edges, creating a strong weld seam without added filler material. ERW is widely used for carbon steel welded tubes.

High Frequency Welding – High frequency welding utilizes electromagnetic induction to heat the tube edges rapidly before they’re pressed together to form a solid joint. This process creates precise, clean welds ideal for stainless steel welded tubes.

Laser Beam Welding – This advanced method employs a focused laser beam to melt and fuse the metal edges, producing exceptionally narrow, deep welds with minimal heat distortion. Laser welding creates high-quality welded tubes with excellent strength properties.

Gas Tungsten Arc Welding (TIG welding) – TIG welding uses a non-consumable tungsten electrode and inert gas shield to create clean, high-quality welds suitable for stainless steel and specialty alloy tubes.

Plasma Arc Welding – Using a constricted plasma arc, this technique delivers concentrated heat for deep penetration welds with minimal distortion. Plasma arc welding is particularly effective for welded tubes requiring high travel speeds.

Submerged Arc Welding – This process involves a continuously fed electrode covered by a granular flux that shields the weld area, producing deep penetration and high deposition rates. Submerged arc welding is primarily used for larger diameter welded tubes.

Why Are Different Welding Processes Used to Manufacture Welded Tubes?

There are many reasons why one welding process may be chosen over another to manufacture welded tubing. Perhaps the most common method is high-frequency welding. To use high-frequency welding to make a welded tube, a high-frequency welding power source is activated. Once the power source is activated, leads from the power source, which are placed in close proximity to the formed but unwelded tube, begin to emit high-frequency energy. This energy excites the molecules in the tube until a temperature level at which they can be joined is reached. At this point, the two heated edges of the unwelded tubing are forced together through another die, and a weld is made.

One of the reasons high-frequency welding is so popular is that it is an established, repeatable technology, it is cost-effective, and it can handle poor fit-up better, relative to the other tube welding processes. Also, like nearly all large-scale welded tube manufacturing processes, it is performed in a continuous fashion, allowing for high degrees of productivity.

Other welding methods are used in place of high-frequency welding for different reasons. Laser beam welding may be used for metals that are more difficult to weld than mild steel. Examples include titanium, stainless steel, and other non-ferrous alloys. Gas tungsten arc welding and plasma arc welding are sometimes used in place of laser welding because of their reduced equipment cost and complexity; however, they typically provide slower travel speeds and larger heat-affected zones. Resistance welding is occasionally used instead of high-frequency welding for medium-diameter carbon steel tubing. For welded steel tubing with thick walls, submerged arc welding is used.

Seamless Tube vs. Welded Tubes

Seamless tubes are manufactured through a process that creates a hollow tube without any welding seams. These tubes are extruded from a solid billet of metal and pierced to form the hollow section. In contrast, welded tubes are formed from flat metal strips that are shaped and joined along a seam using various welding techniques.

The primary difference between seamless tubes and welded tubes lies in their structural composition. Welded tubes feature a seam where the edges of the metal strip have been joined together, while seamless tubes maintain uniform strength throughout their entire circumference. This distinction affects several performance characteristics:

• Seamless tubes generally offer higher pressure ratings than welded tubes of the same material and dimensions.
• Welded tubes are typically more economical to produce.
• Welded tubes are more widely available in various sizes and can be produced more quickly than seamless varieties.
• Cold-drawn welded tubes undergo additional processing to improve dimensional accuracy and surface finish, narrowing some of the performance gaps between welded and seamless options.

While seamless tubes excel in applications demanding maximum integrity and pressure resistance, modern welding technologies have significantly improved the quality of welded tubes, making them suitable for most industrial applications.