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Alloy 31 (UNS N08031) Butt Weld Fittings
Alloy 31 UNS N08031 butt weld fittings to ASME SB-366 and B16.9. Elbows, tees, reducers, caps in WP3127 and CR3127 grades. seawater and chemical service.
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Alloy 31 is a 6%-molybdenum super austenitic stainless grade supplied to UNS N08031. It fills the gap between 6Mo superaustenitic grades and the higher-nickel alloys: it carries roughly twice the molybdenum of 316L while keeping nickel content in the 30–32% range, which puts it in a different price bracket from both 316L and full nickel alloys. Buyers usually arrive at N08031 after 316L has failed in seawater or hot sulphuric acid, and it is now common in desalination intake systems, marine transfer piping, flue-gas scrubbing and pulp-plant bleaching equipment.
Product range and dimensions
We supply Alloy 31 butt weld fittings as 45° and 90° elbows (long and short radius), 180° return bends, equal and reducing tees, concentric and eccentric reducers, caps and lap-joint stub ends, all produced to ASME B16.9 dimensional standards. Forged proportions for high-pressure or wide-flange branches can be supplied to MSS SP-43. Socket weld and threaded configurations to ASME B16.11 are quoted separately. Standard nominal sizes run NPS 1/2 to NPS 24, with larger sizes and special wall thicknesses subject to mill review. Wall thickness is ordered against the matching pipe schedule in ANSI B36.19M for stainless and nickel-alloy pipe, and every fitting is supplied with a matching nominal wall and end preparation for butt welding.
Material specifications
| ASME specification | Form | Grade designation | UNS |
|---|---|---|---|
| SB-366 / SB-366M | Wrought butt weld fittings | WP3127 / CR3127 | N08031 |
| SB-462 | Forged or rolled flanges, forged fittings, valves and parts | Forgings, flanges | N08031 |
| SB-625 | Plate, sheet and strip | Annealed | N08031 |
| SB-622 | Seamless pipe and tube | — | N08031 |
| SB-619 | Welded pipe | — | N08031 |
Grade comparison
| Property | Alloy 31 (N08031) | 316L (UNS S31603) | Alloy 20 (N08020) |
|---|---|---|---|
| Nickel | 30.0–32.0 % | 10.0–14.0 % | 32.0–38.0 % |
| Chromium | 26.0–28.0 % | 16.0–18.0 % | 19.0–21.0 % |
| Molybdenum | 6.0–7.0 % | 2.0–2.5 % | 2.0–3.0 % |
| Copper | 1.0–1.4 % | — | 3.0–4.0 % |
| Nitrogen | 0.15–0.25 % | — | — |
| Carbon, max | 0.015 % | 0.030 % | 0.07 % |
| Chromium + Mo + N (PREN basis) | High | Moderate | Moderate |
Composition per ASME SB-625 Table 1. Copper and nitrogen are the additions that distinguish N08031 from plain 6Mo grades such as N08925.
Heat treatment and mechanical properties
SB-366 requires solution annealing at 2175 °F [1189 °C] minimum, followed by rapid air or water cooling; all forming and welding must be completed before the final heat treatment. For forgings to SB-462 the minimum requirements are:
| Property | Minimum | Specification |
|---|---|---|
| Tensile strength | 94 ksi (650 MPa) | SB-462 Table 2 |
| Yield strength | 40 ksi (276 MPa) | SB-462 Table 2 |
| Elongation in 2 in. (50 mm) | 40.0 % | SB-462 Table 2 |
For plate, sheet and strip to SB-625 Table 2, minimum tensile strength is 94 ksi (650 MPa) in all three product forms.
Applications
Marine and offshore seawater piping, desalination and water-treatment plants, flue-gas desulphurisation scrubbers, pulp and paper bleaching and wash plants, sulphuric and phosphoric acid equipment, and chemical injection systems.
Certification and inspection
EN 10204 3.1 and 3.2 material certification, full PMI verification on each heat, hydrostatic testing where required, and supplementary PMI, ferrite count and intergranular corrosion testing to ASTM A262 Practice A on request. NACE MR0175 / ISO 15156 sour-service qualification and PED 2014/68/EU conformity for pressure-retaining parts can be supplied on request.
Ordering information
Please include nominal size, wall thickness or schedule, grade (WP3127 or CR3127), fitting type and angle or branch size, quantity, and destination port for freight terms. If the application is seawater or sulphuric acid, please state the concentration and temperature so we can confirm the grade is appropriate. A dimensional drawing or an ASME B16.9 table reference is the fastest way to get an accurate quotation.
