From coil to pipe.

01 · Coil selection

Everything downstream is decided here. The coil is selected by grade, and the grade is selected against the service — the medium, its concentration, the temperature, the chloride level and whether hydrogen sulphide is present. 304L handles general duty. 316L adds molybdenum and with it real pitting resistance. 321 and 347 are stabilised against sensitisation for lines that sit in the 425–815 °C range. 904L and the duplex grades take over where chlorides or acids would defeat 316L.

The coil arrives with its own mill certificate. That certificate is the start of the traceability chain that ends with the Material Test Certificate you receive.

02 · Levelling & slitting

Coil has memory. It has been wound under tension and it wants to stay curved, so it is passed through levelling rollers that work the material back and forth past its yield point until it lies flat. A strip that is not properly levelled will not form a round pipe — it will form a pipe with a straightness problem you cannot correct later.

It is then slit to the developed width: the width of flat strip needed to produce the finished outside diameter. That is simply the circumference:

W = π × OD

A 114.3 mm OD pipe needs a strip 359 mm wide. Slit it too narrow and the edges will not meet under the weld rolls; too wide and the excess has to go somewhere, which means an out-of-round pipe. Slitting tolerance is measured in tenths of a millimetre.

03 · Roll forming

The strip passes through a series of roll stands that bend it progressively — first the edges, then into a U, then closing the U into an O. The sequence matters. The material is bent, never stretched: roll forming changes the shape without significantly changing the thickness, which is why the wall you specified is the wall you get.

Forming too aggressively work-hardens the edges and can start edge cracking, particularly in duplex and the higher alloys. Forming too gently leaves the edges misaligned at the weld box. Between them sits a fairly narrow window, and it is one of the reasons welded pipe from a good mill and welded pipe from a poor one are not the same product.

04 · High-frequency welding

By the time the tube reaches the weld box the two edges are almost touching. A high-frequency current is induced into them, concentrating at the edges and heating them to forging temperature in milliseconds. Squeeze rolls then press the edges together and they forge into a single piece.

No filler metal is added. The parent material becomes the joint. That is the important point for a buyer: in a properly made HFW or TIG-welded seam, the weld is the same alloy as the pipe, with the same corrosion behaviour. The molten metal and any oxide present are squeezed out of the joint as the bead, which is why a bead forms at all.

Weld parameters are recorded and the seam is tested continuously — eddy current on line, and hydrostatic testing afterwards. Pipe made this way is supplied to ASTM A312 or A358 depending on the size and the class.

05 · Scarfing & sizing

The bead squeezed out of the joint is cut away — scarfed — while it is still hot. Outside diameter scarfing is standard. Inside diameter scarfing is a separate operation and needs to be specified: if your service is hygienic, high-purity, or simply cannot tolerate a bead on the bore, say ID scarfed in the enquiry.

The tube then passes through sizing rolls that bring it to the final outside diameter and correct ovality, and through a straightener. This is where the dimensional tolerances on the certificate are actually established.

06 · Finishing & testing

The pipe is cut to length, the ends are prepared — plain, bevelled to ASME B16.25, or threaded — and it goes through solution annealing and pickling where the grade requires it. Annealing restores the corrosion resistance that welding and forming removed from the heat-affected zone; skipping it on austenitic stainless is a false economy that shows up as intergranular attack two years later.

Then testing: hydrostatic, eddy current, and any additional testing the specification calls for. The Material Test Certificate is issued against the heat, and the pipe is marked so it can be traced back to it.

Welded or seamless — which should you specify?

Both are legitimate. The honest comparison:

• Welded (ERW / HFW / EFW) — tighter diameter and wall tolerance, better surface finish, more economical, and available in thin walls that seamless cannot practically reach. The seam is the only discontinuity, and it is tested along its whole length.
• Seamless — no longitudinal weld at all. Preferred where the service is highly cyclic, where the specification simply forbids a seam, and often in sour and high-pressure duty.

For most process and utility service, welded pipe from a good mill with full certification is the correct engineering answer and the correct commercial one. Where seamless is genuinely required, we supply seamless. If you are not sure which your specification demands, call us before you order — it is a five-minute conversation that avoids a rejected consignment.