ERW tube, short for Electric Resistance Welded tube, is steel tubing made by rolling a flat strip of steel into a cylindrical shape and fusing the edges together using electrical current. No filler metal or welding rod is added. Instead, the heat generated by the electrical resistance at the seam is enough to bond the two edges into a continuous joint. It’s one of the most common and cost-effective ways to produce steel tubing, used in everything from water pipelines to building frameworks.
How ERW Tubes Are Made
The manufacturing process follows five main stages, each feeding directly into the next on a continuous production line.
It starts with large coils of hot-rolled steel strip, typically weighing several tons. The coil is uncoiled, flattened, and inspected for surface defects like rust, scratches, or scale. Once the strip passes inspection, it enters a series of computer-controlled forming rollers arranged in sequence. Each roller applies incremental pressure, gradually bending the flat strip into a round, open-seam cylinder without cracking or overstressing the steel.
The welding stage is the heart of the process. Electrical current passes between the two open edges of the cylinder, heating them to the point where they can be pressed together and fused. Because no filler material is introduced, the bond relies entirely on the base metal itself. After welding, the excess material pushed out during the joining (called “flash”) is trimmed from both the inside and outside surfaces so the seam sits flush with the rest of the tube wall.
The welded tube then goes through cooling and conditioning, followed by sizing rolls that bring it to its final dimensions. The last step is cutting to length and final inspection.
Available Sizes
ERW tubes come in a broad range of outer diameters and wall thicknesses. Standard production covers outer diameters from roughly 0.120 inches up to 6.625 inches, with wall thicknesses varying across that range. Common specifications include ASTM A513 Type 1 for mechanical tubing and ASTM A500 Grade B or C for structural applications. This size range makes ERW tubing versatile enough for small mechanical parts at the low end and structural columns or pipelines at the upper end.
ERW vs. Seamless Tube
The main alternative to ERW is seamless tube, which is formed by piercing a solid steel billet and stretching it into a hollow cylinder with no weld seam at all. The difference in how they’re made drives most of the practical tradeoffs between the two.
ERW tubes cost less. The manufacturing process is simpler, faster, and uses less raw material, which translates to a lower price per foot. For moderate-pressure applications, that cost advantage is significant. Seamless tubes, on the other hand, are more expensive because of the labor-intensive process required to produce them, but they offer superior strength under pressure. Without a weld seam, there’s no potential weak point, which is why seamless is the standard choice for high-pressure oil and gas pipelines and critical industrial systems.
Seamless tubes also offer a wider range of wall thicknesses and better uniformity of shape, giving engineers more flexibility when designing for extreme conditions. ERW tubes perform well in low to medium pressure service, but the welded seam can become a liability when pressures climb. For most structural, mechanical, and general-purpose piping needs, ERW is more than adequate and significantly easier on the budget.
Quality Testing and Inspection
Because the weld seam is the one area where defects could develop, quality control in ERW manufacturing focuses heavily on that joint. Two primary testing methods are used: hydrostatic testing and ultrasonic inspection.
Hydrostatic testing fills the pipe with water and pressurizes it to verify it can hold its rated pressure without leaking. Under ASTM A53, hydrostatic pressure must be maintained for at least 5 seconds across all pipe sizes. The drawback is that this test requires taking the pipe offline, so it’s typically done at the factory or during scheduled maintenance rather than in continuous service.
Ultrasonic testing uses sound waves to detect flaws in the weld seam without cutting into or pressurizing the tube. Advanced methods can detect cracking as shallow as 0.5 millimeters (about 0.020 inches) in the seam. For pipelines already in service, ultrasonic inspection allows operators to evaluate long stretches of weld seam without shutting down the line. ERW tubes in Grade B under ASTM A53 also require heat treatment of the weld seam after welding, and transverse tensile testing is required for sizes 8 inches and larger.
Industry Standards
ERW tubes are governed by several well-established specifications depending on their intended use. ASTM A53 (and its ASME equivalent, SA53) is one of the most widely referenced. It covers both seamless and welded pipe intended for mechanical and pressure applications, including steam, water, gas, and air lines. A53 pipe comes in Grade A and Grade B, with Grade B offering higher tensile strength. The standard also permits hot-dip galvanizing for corrosion protection.
For structural applications, ASTM A500 covers round, square, and rectangular hollow sections. ASTM A513 covers mechanical tubing used in machinery, automotive parts, and equipment frames. In the oil and gas sector, API 5L is the primary specification for line pipe, covering ERW products used in pipeline transportation systems.
Common Applications
ERW tubes show up across a wide range of industries, almost always in situations where low to medium pressure service or structural support is the primary need.
- Water and irrigation: Municipal water supply lines, industrial water systems, and drainage pipelines are among the highest-volume uses for ERW pipe.
- Oil and gas: Gathering lines, flow lines, and low-pressure transmission pipelines commonly use ERW, though high-pressure trunk lines typically call for seamless.
- Construction and structural: Columns, beams, building frameworks, scaffolding, fencing, and guardrails all rely on ERW sections for their combination of strength and affordability.
- Mechanical and engineering: Machinery components, equipment frames, automotive structures, agricultural equipment, and conveyor systems use ERW tubing as a cost-effective structural element.
- Piling and foundations: Light to medium duty piling and foundation casing, where extreme pressure resistance isn’t required, is another common use.
The general rule is straightforward: when the application involves moderate pressures and standard environmental conditions, ERW is the go-to choice. When pressures are extreme, temperatures are very high, or the consequences of failure are severe, seamless tube is worth the added cost.