Welded Tubes Vs. Seamless Tubes – What’s the Difference?

Seamless tubes are made from a solid billet with no weld seam — uniform wall throughout. Welded tubes (ERW, LSAW, SSAW) roll flat steel into a cylinder and fuse the edges. Seamless tubes handle higher pressures and are mandatory for heat exchangers, boilers, and critical service. Welded tubes cost 15–30% less and suit structural, large-diameter, and low-pressure applications.

For procurement engineers and project managers choosing between seamless and welded tubes, the decision is not simply about which is “stronger” — it is about which manufacturing method matches the service conditions, inspection requirements, and code obligations of the application. This guide covers the manufacturing differences, mechanical property implications, welded tube types (ERW, LSAW, SSAW), relevant ASTM and API standards, and a practical framework for choosing correctly.

How Seamless Tubes Are Made

A seamless tube starts as a solid steel billet — round bar of the specified grade, typically 100–400 mm in diameter. The billet is heated to around 1,200°C and pierced on a rotary piercing mill: a mandrel is forced through the centre while the billet rotates against angled rolls, forming a thick-walled hollow shell. This hollow is then elongated through a series of rolling passes — pilger mill, mandrel mill, or push bench — until it reaches near-target dimensions.

For finished tubes requiring tight tolerances, the hot-rolled hollow undergoes cold drawing: pulled through a hardened die at room temperature over an internal mandrel. This eliminates mill scale and reduces tolerances to ±0.4 mm OD and ±10% wall.

Cold drawing also increases tensile strength by 10–20% through work hardening. Cold drawn seamless tubes conforming to ASTM A179 (heat exchanger grade) or ASTM A213 T11/T22 (boiler grade) are produced this way.

The defining characteristic of seamless construction is what it eliminates: there is no weld seam, no heat-affected zone alongside a weld, and no zone of potential chemical segregation at a fusion line. The tube’s mechanical properties are isotropic — the same in all directions — which matters in applications involving pressure cycling, thermal cycling, and bending loads.

How Welded Tubes Are Made — ERW, LSAW, and SSAW

All welded tubes share the same basic process: flat steel — either strip from a coil or plate cut to size — is formed into a cylinder and the abutting edges are joined by welding. The three main welding methods produce tubes with different size ranges, weld quality, and cost structures.

ERW — Electric Resistance Welded

ERW is the most common welded tube process for smaller diameters. Steel strip from a hot-rolled coil is fed through forming rolls that progressively shape it into a cylinder. As the edges meet, a high-frequency current (typically 100–400 kHz) heats them to forging temperature — the edges are squeezed together under roll pressure without filler metal. It is a solid-state forge weld.

The seam bead is trimmed externally after welding and, for most tube applications, the internal bead is also removed. ERW tubes are available from NPS ½” to approximately NPS 24″ and are widely specified for structural hollow sections, general mechanical tubing, and low-to-medium pressure distribution piping.

ERW tubes are available from NPS ½” up to approximately NPS 24″ and are widely specified for structural hollow sections, general mechanical tubing, and low-to-medium pressure distribution piping. The weld seam is narrow and, in modern ERW facilities with full ultrasonic seam inspection, fully traceable.

LSAW — Longitudinal Submerged Arc Welded

LSAW is used for large-diameter, heavy-wall pipe — typically NPS 16″ to NPS 60″ — where ERW cannot maintain adequate weld quality at the required wall. Steel plate is pressed into a “U” then “O” shape (UOE process), and the seam is welded by submerged arc welding using a flux-shielded wire electrode with filler metal.

LSAW pipe is the standard for long-distance oil and gas trunk lines and offshore pipelines requiring large-diameter, high-pressure, third-party inspected pipe to API 5L PSL2.

SSAW — Spiral Submerged Arc Welded

SSAW (also called HSAW — helical seam) feeds steel strip at an angle through a spiral forming machine, creating a seam that runs helically along the pipe. The seam is SAW-welded and SSAW can produce diameters up to 140″ not feasible with LSAW.

The helical seam means that under axial loading, the weld sees combined tensile and shear stress rather than pure tension — a structural consideration in pipeline design. SSAW is used in water transmission mains, piling, and lower-pressure gas distribution where the helical seam geometry is acceptable.

Seamless vs Welded Tubes: Direct Comparison

When to Specify Seamless — and When Welded Is Acceptable

In most EIL-contracted refinery and petrochemical projects in India, seamless is the default unless the specification explicitly permits welded. The service conditions — high-temperature, high-pressure, and corrosive process environments — genuinely favour seamless construction.

For a significant range of applications, however, ERW or LSAW is technically adequate and economically sensible. The decision comes down to matching the manufacturing method to service conditions and code requirements.

Specify seamless when:

• Operating pressure exceeds the ERW seam pressure rating for the wall/grade — typically above 70 bar for small-diameter lines in critical service
• The application involves thermal cycling, pressure cycling, or fatigue loading where seam integrity under cyclic stress is a concern
• Service medium is corrosive, sour (H₂S-containing), or requires a uniform corrosion-resistant surface without a weld heat-affected zone
• The specification is ASTM A179, A213, or equivalent — these are inherently seamless standards
• Indian Boiler Regulations (IBR) apply — IBR mandates seamless construction for pressure-bearing boiler circuits above specified pressure ratings
• The project specification explicitly states SMLS (seamless) — as most EIL, HPCL, IOCL, and BHEL project documents do for heat exchanger and boiler tube service

ERW or LSAW welded pipe is technically acceptable when:

• The application is structural — hollow sections, scaffolding, general mechanical support — where seam integrity under static load is adequate
• The service is low-pressure water, utility gas distribution, or cooling water below 20 bar where pressure cycling is not a factor
• Diameter requirements exceed the practical seamless limit (above NPS 16″, LSAW is the standard)
• The governing standard is ASTM A53 (which covers both ERW and seamless) and the service conditions permit either
• Cost is the primary driver and the project engineer confirms the service conditions do not require seamless

What the ASTM Standards Say About Seamless vs Welded

Several ASTM standards cover both seamless and welded construction — but they are not interchangeable within a specification unless the project document explicitly permits substitution. ASTM A53 covers both seamless and ERW. ASTM A106 covers only seamless. ASTM A179 and A213 cover only seamless cold drawn tubes.

API 5L covers seamless (SMLS), ERW, LSAW, and SSAW in the same document, with different testing requirements for each method.

When a specification calls for ASTM A179 or A213, seamless is the only conforming option — not one choice among several. Supplying ERW tube against an A179 order, even with matching chemistry and mechanical properties, is a non-conformance. “Seamless cold drawn” is a stated delivery condition in the standard, not a preference.

Seamless Tubes from Anand Seamless

At Anand Seamless, cold drawn seamless tube manufacturing is our core process. We produce carbon steel seamless tubes, alloy steel seamless tubes, and stainless steel seamless tubes from our two Gujarat facilities — Unit 1 at Kadi, Mehsana and Unit 2 at Changodar, Sanand — covering the grades specified on Indian and export oil and gas, power, fertilizer, and petrochemical projects.

We hold ISO 9001:2015 certification, IBR Well Known Maker status, and EIL vendor approval. Our seamless tubes are supplied with EN 10204 Type 3.1 or 3.2 Mill Test Reports, IBR Form III-C where required, and third-party inspection coordination through Bureau Veritas, SGS, TUV, or your nominated agency. For a practical guide on verifying the documentation that should accompany any seamless tube order, see our MTR reading guide for procurement engineers.

Contact us at +91 90999 96853 or +91 99099 68550, or email inquiry@anandseamless.com with your grade, OD, wall thickness, standard, and project documentation requirements.

Frequently Asked Questions

Q: What is the main difference between seamless and welded tubes?

A: Seamless tubes come from a solid billet with no weld seam — continuous, uniform wall throughout. Welded tubes roll flat steel into a cylinder and fuse the edges by ERW, LSAW, or SSAW. Seamless handles higher pressures, is mandatory for heat exchangers and boilers under ASTM A179/A213 and IBR, and performs better under cyclic stress. Welded tubes cost 15–30% less and come in larger diameters.

Q: What is ERW tube and how is it different from seamless?

A: ERW tube is formed by rolling hot-rolled steel strip into a cylinder and fusing the edges with high-frequency resistance heating — no filler metal, a solid-state forge weld. It covers NPS ½” to ~NPS 24″ for structural, mechanical, and low-to-medium pressure service. Unlike seamless, ERW has a longitudinal seam requiring separate NDE and which can corrode preferentially in aggressive environments.

Q: Can ERW tube be substituted for seamless tube on an ASTM A179 order?

A: No. ASTM A179 specifies cold drawn seamless — no ERW alternative exists. Supplying ERW against an A179 order is a non-conformance regardless of matching chemistry and properties. Same for A213. ASTM A53 differs: it covers both seamless and ERW, and substitution is permitted when the project specification does not restrict the method.

Q: Which is stronger — seamless or welded tube?

A: Seamless tubes perform better under fatigue and pressure cycling — no seam as a stress concentration. Under burst testing, seamless fails in parent metal, not at a seam. Modern ERW with full seam NDE can match parent-metal tensile strength, but the seam remains a weak point under cyclic loads and in corrosive environments where the weld HAZ shows preferential attack.

Q: When is LSAW pipe used instead of seamless or ERW?

A: LSAW (Longitudinal Submerged Arc Welded) is the standard manufacturing method for large-diameter, heavy-wall pipe above approximately NPS 16″ — sizes that are not practically achievable with seamless production. LSAW is used for oil and gas trunk pipelines, offshore pipelines, and large-diameter water transmission mains where ERW is not available in the required size and wall, and seamless is impractical. LSAW pipe is manufactured to API 5L and inspected to PSL2 requirements on critical projects.

Q: Does Anand Seamless manufacture welded tubes?

A: Anand Seamless specialises in cold drawn seamless tubes — carbon steel, alloy steel, and stainless steel — for heat exchanger, boiler, hydraulic, and precision mechanical applications. We do not manufacture welded pipe. If your project specifies seamless construction to ASTM A179, A213, A106, or IBR standards, our product range covers the grades most commonly required on Indian oil, gas, power, and fertilizer projects, with EIL vendor approval and IBR Well Known Maker certification.