Carbon Steel and Alloy Steel Pipe Fittings

Butt-weld fittings are made from pipe or plate to the same wall thickness as the pipe and welded end to end with a full-penetration weld. They are the standard for larger sizes and severe service, and can be fully radiographed. The dimensional standard is ASME B16.9.

Socket-weld fittings are forged. The pipe slips into a socket and is fillet welded. They are quick to fit up on small-bore lines, typically NPS 2 and below, and suit drains, vents, instrument take-offs and utility lines. Screwed (threaded) fittings are also forged and join to threaded pipe with no welding at all, which is useful where hot work is restricted. Both socket-weld and threaded forged fittings follow ASME B16.11.

Socket-weld joints leave a small gap and crevice at the root, so they are avoided in services prone to crevice corrosion or in some cyclic duties. Threaded joints are generally limited by the design code to lower-risk services.

Every pipe specification has a companion specification for fittings with similar chemistry. Wrought butt-weld fittings for carbon and alloy steel are bought to ASTM A234, forged fittings to A105 for carbon steel or A182 for alloy and stainless, and wrought stainless butt-weld fittings to A403. The table below gives the usual pairings for the pipe we supply.

Mixing a carbon steel fitting into a P11 line is a classic shutdown mistake because the two look identical. Ask for the grade to be stamped on every fitting, and check the stamp against the certificate on receipt.

Forged fittings are not ordered by schedule alone. ASME B16.11 rates socket-weld fittings in Class 3000, 6000 and 9000, and threaded fittings in Class 2000, 3000 and 6000. Each class is matched to a pipe wall: Class 3000 socket-weld fittings, for example, are normally paired with Sch 80 pipe. Tell us the pipe schedule and we will suggest the class that matches it.

Compression fittings join tube rather than pipe. A nut drives one or two ferrules onto the tube's outer surface, sealing without welding or threading the tube. They are used on hydraulic lines, instrumentation, sampling systems and lubrication circuits where joints are opened for maintenance. Ferrules grip only on the outside diameter, so the tube must be to the right OD and tolerance; our hydraulic tubes are made for this.

Never mix ferrules, nuts and bodies from different manufacturers. Each brand's components are designed as a set.

Every fitting should be marked with the manufacturer's identification, material specification and grade, size and schedule or class, and a heat code that ties it to the test certificate. Check that the wall of butt-weld fittings matches the pipe schedule, because a Sch 40 elbow welded into a Sch 80 line leaves a thin spot exactly where flow turns. For screwed fittings, check the thread type, usually NPT, matches the pipe threads.

On a steam line covered by the boiler law, fittings need IBR documentation just as the pipe does. Raise this at enquiry, along with the pipe specification, sizes, schedule or class, and quantity. For stainless-only systems, see our stainless steel pipe fittings page; for the pipes that go with these fittings, start from A106 Grade B or ASTM A335.