If you’ve ever sourced steel tubing, you’ve probably come across the terms ERW, DOM, and seamless. These specific callouts are often used interchangeably, but each refers to a different manufacturing process. The process matters because it affects factors such as tolerance, strength, availability, and cost.

Choosing the wrong steel tubing isn’t only a paperwork issue. You could end up paying too much for features you don’t need or selecting a tube that won’t perform when it counts. This guide will help you tell ERW, DOM, and Seamless tubing apart, so you can choose the right one for your next project.

ERW stands for Electric Resistance Welded.

It’s a type of steel tubing made by rolling a flat metal strip into a square, rectangular, or cylindrical shape and fusing the edges longitudinally with high-frequency electric current and pressure, without adding filler metal.  

See our blog on ERW Steel Tubing.

DOM tubing, or Drawn Over Mandrel tubing, refers to a steel tube finishing process.

It starts as electric resistance-welded (ERW) tubing, has its internal weld seam removed, and is cold-drawn over a mandrel to create an incredibly strong, smooth, and dimensionally precise finished product.

A seamless tube is a tubular metal part manufactured without any longitudinal weld or joint.

It’s created by piercing and hot rolling a solid piece of metal (billet) to form a hollow tube. Because it lacks a welded seam, it offers higher load-bearing capacity, smoother surfaces, and superior pressure resistance.

• Uncoiling & Preparation. Large steel coils are unrolled, flattened, and sheared to ensure consistent thickness and straight edges. Alliance Tubular Products, an Alliance Steel Company, sources all slit coil from Alliance Steel at a single integrated campus. [Learn more about our full coil processing capabilities.] The flat strip moves through rollers that slowly shape it, first into a U, then into a full square, rectangle, or circle.

• High-Frequency Welding- A high-frequency current (typically 100–400 kHz) is applied to the tube’s edges. The current heats the edges to forging temperature. Squeeze rollers then press the edges together to form a solid metallurgical bond. This step is the heart of the electric resistance welding process. Welding creates excess molten metal, or weld flash, on both the inside and outside of the seam. Scarfing tools remove this material to produce a smooth surface. The tube passes through sizing rollers to achieve the exact outer diameter and wall thickness required.

• Cutting & Finishing. A flying cutoff saw cuts the tubing to length without stopping the mill.

• Inspection & Testing. Finished tubing is tested for weld integrity and dimensional accuracy using methods like crush testing, ultrasonic testing, hydrostatic testing, and flattening or cone tests.

At Alliance Tubular Products, the entire process takes place on one campus, from receiving coil to shipping finished tube, with no transfers between facilities. This single-site setup gives buyers better process oversight and tighter tolerances than production spread across several locations. 

DOM tubing starts out the same as ERW tubing.

After the ERW process, the flash weld is removed, and the tube is cold drawn over a mandrel. That’s where the name Drawn Over Mandrel comes from. This cold-drawing step reshapes the metal and improves its structure, making the tube longer and stronger. It also grants precise control of the outer and inner diameters, resulting in consistent wall thickness and a high strength-to-weight ratio.

While DOM tubing may look like other smooth-surfaced steel tubes, it still has an internal seam. When manufactured correctly, though, that seam is nearly invisible, which is exactly why it’s so often mistaken for seamless tubing.

A seamless tube is made without any longitudinal welds or joints. It is created by piercing and hot-rolling a solid metal billet to form a hollow tube. With no welded seam, seamless tubing has higher structural strength, smoother internal surfaces, and better pressure resistance. That’s why it is the standard choice for high-pressure and safety-critical uses like hydraulics and oil and gas services.

Choose ERW when cost and lead time matter most, and the application is structural, load-bearing, or general fabrication. This covers the large majority of industrial, construction, and OEM use cases, from building frameworks to truck and trailer chassis to agricultural equipment.

Choose DOM when tight ID/OD tolerance, concentricity, or surface finish is required, for example, precision machining, hydraulic cylinders, or components with close-fit assemblies.

Choose seamless tubing for high-pressure or safety-critical applications where removing any weld-seam risk is worth the extra cost. Examples include oil and gas pipes, high-pressure hydraulics, or parts used near aerospace equipment.

ERW tubing is typically the most available and fastest to source in volume, since it requires the fewest processing steps and is produced by the widest base of domestic mills.

DOM and seamless tubing carry a cost premium due to the extra processing (cold drawing or piercing) and the more limited producer base for each.

•  Assuming “DOM” means seamless. Since DOM has a barely visible seam, it’s often mislabeled as seamless in casual conversation. Make sure your specs and POs use the correct terminology so there’s no ambiguity with suppliers.
• Not specifying ID flash condition or straightness tolerance on ERW orders. These details matter for downstream fabrication and should be explicitly called out.
• Specifying seamless tubing for an application where ERW or HSS tubing would work just as well at a lower cost. Always start with the application’s real pressure, tolerance, and safety needs, not assumptions.

When you partner with Alliance, you're getting more than just steel tubing. You're getting a full-spectrum steel solution, all managed under one roof:

1. Alliance Steel sources and processes flat-rolled coil using state-of-the-art slitting and blanking equipment, delivering high-quality raw materials that meet precise specifications for modular-grade performance.
2. Alliance Tubular Products transforms slit coil into structural steel tubing using a direct-form ERW process that produces square and rectangular tubing with tight tolerances, superior straightness, and excellent load-bearing capacity.
3. Alliance Fabrication and Manufacturing completes the offering with value-added fabrication services, including robotic welding, laser cutting, precision bending, kitting, and assembly—ideal for transforming raw materials into finished, modular frame components.

Together, we offer:

1. A single point of contact for coil, tube, and fabrication
2. Reduced freight costs
3. Faster response times and production turnaround
4. Consistent quality from start to finish

You need a partner who understands the pace and pressure of your project. At Alliance, we're that partner.