Large-Diameter Stainless Pipe: A358 EFW vs Seamless — Classes, Sizes, Cost

Quick answer: For large-diameter stainless pipe (roughly above NPS 24 / DN600), the practical choice is EFW (electric-fusion-welded) pipe per ASTM A358 or A778, not seamless — seamless mills stop offering big sizes economically, and properly welded and tested EFW meets pressure codes at a fraction of the price. Spec the weld seam with 100 % RT or UT and the argument disappears.

What Are the Route Options for Big Stainless Pipe?

For how welded and seamless compare across normal sizes, see our seamless vs welded guide.

Is Welded Big Pipe Code-Compliant for Pressure?

Yes, with the right class. ASTM A358 defines classes by test rigor:

• Class 1: 100 % RT/UT of the weld + hydro — full pressure service, code work
• Class 2: spot radiography — general service
• Class 3: no RT, hydro only — low-pressure utility

Most refusals to accept EFW trace to "Class 3 got installed where Class 1 was needed." Put the class (or "100 % radiographed") in the PO and EFW passes the same code checks as seamless for most services. Wall selection then follows normal schedule logic.

Where Does Seamless Still Win at Large Sizes?

• Thick-wall small-to-mid sizes where a weld is the weak link (high-pressure manifolds)
• No-weld specs: some customer and legacy specs ban welds regardless of class
• Exchanger tubing and mechanical tube — different product, governed by A269/A213

What Belongs in the RFQ for Large EFW Pipe?

1. Size: OD × wall (or NPS + schedule), A358 Class 1/2/3 stated
2. Grade with UNS, plate origin requirement (some specs demand A240 plate of known mills — see A480 tolerance context)
3. Weld test scope: 100 % RT or UT, hydro per class, weld procedure (WPS/PQR) available
4. Heat treatment: solution annealed (A358 Class 1 typically requires it) vs as-welded (A778)
5. End preparation: plain, beveled for welding (30–37.5°), or faced; lengths (random vs fixed with delivery inspection in mind)

FAQ

Q: Is EFW pipe weaker than seamless? A: A properly tested Class 1 EFW seam meets the same code design allowables as seamless. Statistically, modern welded seams fail less often than seam-free pipe fails from body defects — the weld is inspected, the body isn't (unless you buy UT'd seamless).

Q: Can I get A358 in 316L and duplex? A: Yes — 304L/316L are routine; duplex 2205 and super duplex EFW pipe is standard for desalination and offshore water service. Alloy choice criteria are in our seawater alloy comparison.

Q: Why is quoted EFW wall often thicker than my seamless equivalent? A: EFW carries a weld-joint quality factor in some designs, and plate tolerance differs from pipe tolerance — designers often add 1–2 mm. Confirm the design basis before comparing price per ton.