ASTM B366 Wrought Nickel-Alloy Buttweld Fittings

What ASTM B366 covers

ASTM B366/B366M applies to factory-made wrought nickel and nickel-alloy fittings used in pressure piping and pressure vessel service. The fittings are produced from plate, pipe, forgings or bar and then formed into the finished shape. The specification sets the chemical composition of each covered alloy, the required heat treatment, mechanical properties, nondestructive examination and marking. It does not fix the laying dimensions, those come from the matching dimensional standard, most commonly ASME B16.9 for factory-made buttweld fittings. Socket-weld and threaded versions of the same alloys are dimensioned to ASME B16.11.

A buttweld fitting is welded directly into the run so its bore matches the connecting pipe, giving a smooth full-penetration joint with low turbulence and good fatigue life. B366 is the nickel-alloy counterpart to the stainless specification ASTM A403 and the carbon and alloy-steel specification ASTM A234. You reach for B366 when the service corrosion or temperature pushes the grade above standard stainless into the nickel alloys explained in our nickel alloys overview.

Grades covered by B366

B366 identifies each grade by its underlying alloy, carried over from the parent flat product or forging. The wrought fitting keeps the metallurgy of that parent, so an Incoloy 825 fitting shares the chemistry specified for Incoloy 825 plate under ASTM B424 and a Hastelloy C276 fitting matches the B575 plate. The table below lists the nickel alloys Nandini Steel supplies to B366 and why each is chosen.

The "WP" prefix marks a wrought product, the same convention used across the wrought-fitting specifications. B366 states each alloy's heat chemistry limits on the mill test certificate, so the alloy supplied matches what the piping code and the corrosion case assume. For the fuller designation and equivalents, Incoloy 825 carries UNS N08825 and Werkstoff 2.4858, as set out on the Incoloy 825 grade page.

B366 manufacturing classes

Like the other wrought-fitting specifications, B366 separates fittings by how they are made and examined. The class suffix tells you whether the fitting is seamless or welded and how any weld was inspected, which in turn sets the weld joint factor used in pressure design.

When full line efficiency matters, specify WP-S, WP-WX or WP-WU rather than WP-W. State the class on the purchase order alongside the alloy, so the fitting is supplied and examined in the condition your code expects.

Heat treatment and testing

B366 nickel-alloy fittings are supplied in the heat-treated condition required for the alloy, usually solution annealing, which dissolves carbides and other phases back into the matrix and restores corrosion resistance after forming and welding. For Incoloy 825 and similar Ni-Fe-Cr grades, stabilizing elements help resist intergranular attack at elevated temperature. Where sensitization is a concern, the order can call for corrosion testing to ASTM A262 to confirm the material is not susceptible.

For sour oil and gas duty, cite NACE MR0175 / ISO 15156 on the order so the heat is supplied in the qualified condition. Incoloy 825 is a common choice here, and the mechanism is explained on our sour service and NACE MR0175 page. Every order from Nandini Steel ships with an EN 10204 3.1 or 3.2 mill test certificate recording heat chemistry, mechanical results and heat treatment. See how to read a mill test certificate for what each section means.

Welding and fabrication

Because the fitting is welded in place, filler and procedure matter as much as the base alloy. Incoloy 825 is welded with ERNiFeCr-1 (filler metal 65) to keep the weld metal's acid and sour-service resistance, covered step by step in how to weld Incoloy 825. Over-alloyed nickel-base fillers are used on the higher Ni-Cr-Mo grades to avoid dilution and preserve corrosion resistance in the weld. Where a super-austenitic sits in the same line, see how to weld 904L for the companion approach.

Related standards

ASTM B366 sits in a family of nickel-alloy product and fitting specifications. The table below places it alongside the standards most often quoted on the same order.

The parent flat and long products for other covered alloys include Inconel 625 plate under ASTM B443, Inconel 625 bar under ASTM B446 and Incoloy 800 under ASTM B409.

Selecting the alloy for your service

Alloy choice follows the corrosion duty and temperature. For sour oil and gas and mixed reducing and oxidizing acids, Incoloy 825 is the common pick, and its combined resistance is why it is chosen over 316L or 904L in those conditions. For severe wet-chloride and reducing-acid service, step up to Hastelloy C276. For sulfuric-acid process plant, Alloy 20 and 825 both earn their place. Inconel 625 adds high strength where temperature and corrosion combine. If you are weighing two of the most common choices, compare them on Incoloy 825 vs Inconel 625, and read the chemistry case on sulfuric-acid resistance.

B366 fittings feed directly into our Inconel pipe fittings and Hastelloy pipe fittings ranges, and serve duties across oil and gas, chemical processing, petrochemical and fertilizer and acid-regeneration plant.

Stock and supply

Nandini Steel is an ISO 9001:2015 certified stockist and supplier established in 2003, with B366 nickel-alloy buttweld fittings held ex-stock for immediate dispatch. As a certified specialist in 904L, Alloy 28, Incoloy 800 and Hastelloy, we supply Incoloy 825, Inconel 625, Incoloy 800, Hastelloy C276 and Alloy 20 fittings with full heat traceability. A common buyer error is writing "Inconel 825" on the purchase order when the alloy is actually Incoloy 825 (UNS N08825); getting the designation right on the PO and the MTC avoids receiving-inspection disputes.

FAQ

Request a quote

For live pricing and availability on ASTM B366 nickel-alloy buttweld fittings in any covered alloy, send your sizes, class and schedule and our team will respond with an ex-stock quotation. Contact Nandini Steel or message us on WhatsApp at +91 99202 09184.