Alloy 20 Buttweld Pipe Fittings
Alloy 20 / Carpenter 20 / UNS N08020 / EN 2.4660 — a niobium-stabilised Ni-Fe-Cr-Mo-Cu alloy developed specifically for sulphuric acid service across virtually all concentrations. Also resists phosphoric and nitric acids and chloride chemical streams. Used in acid plants, pickling, pharmaceutical and chemical process piping. ASME B16.9 and EN 10253-2 Type B. EN 10204 3.1/3.2.
½″–24″
Size Range
3–4% Cu
Acid resistance
551 MPa
Min UTS (Annealed)
EN 10204
3.1 Std / 3.2 On Req.
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Alloy 20 Grade Equivalents
| Trade Name | UNS | EN W.Nr. | EN Name | ASTM (fittings) | ASME P-No. |
|---|---|---|---|---|---|
| Alloy 20 / Carpenter 20 | N08020 | 2.4660 | NiCr20CuMo | ASTM B366 / B464 | 45 |
| 20Cb-3 (generic) | N08020 | 2.4660 | NiCr20CuMo | ASTM B366 UNS N08020 | 45 |
"Carpenter 20" / "20Cb-3" are trademarks of Carpenter Technology. EN designation 2.4660 / NiCr20CuMo. Covered by ASTM B366 for welded fittings and ASTM B462/B464 for forgings and flanges.
Chemical Composition — UNS N08020 / EN 2.4660
| Element | Specification |
|---|---|
| Ni | 32.0–38.0% |
| Cr | 19.0–21.0% |
| Mo | 2.0–3.0% |
| Cu | 3.0–4.0% |
| Fe | Balance |
| Nb (Cb) | 8×C – 1.00% |
| C | ≤0.07% |
Copper + niobium — copper delivers the alloy's hallmark resistance to sulphuric acid, while niobium ties up carbon as NbC to prevent sensitization during welding. No PWHT required.
Mechanical Properties — Alloy 20 (Annealed)
| Min. UTS | Min. YS | Elongation | Hardness | Max Continuous Service |
|---|---|---|---|---|
| 551 MPa | 241 MPa | ≥30% | ≤ 95 HRB | ~425°C |
Supplied in the solution-annealed and stabilised condition for buttweld fittings. Niobium stabilisation maintains intergranular corrosion resistance in the as-welded state.
Welding & Post-Weld Heat Treatment
No PWHT Required
Niobium stabilisation prevents chromium-carbide precipitation, so Alloy 20 keeps its corrosion resistance in the as-welded condition without post-weld heat treatment. Solution annealing at 925–1010°C may be specified after heavy cold work.
Recommended Filler
ER320LR (AWS A5.9) — low-residual matching filler for GTAW and GMAW of N08020, minimising hot-cracking and preserving acid resistance.
Preheat
No preheat required. Clean joints thoroughly to remove oils, grease and sulphur before welding.
Inter-pass Temperature
≤150°C maximum. Use low heat input and stringer beads to preserve corrosion resistance in the weld and HAZ.
ASME P-Number
P-No. 45 (Ni-Fe-Cr alloys). Separate WPS required — cannot be qualified under austenitic SS procedures.
Alloy Selection Guide — Alloy 20 vs Alternatives
| Grade | Base | Key Advantage | Typical Use |
|---|---|---|---|
| 316L / 1.4404 | Fe-Cr-Ni | Cost | General chemical |
| 904L / 1.4539 | Fe-Ni-Cr | Sulphuric (moderate) | Acid, FGD |
| Alloy 20 / N08020 | Ni-Fe-Cr-Cu | Sulphuric acid (all conc.) | Acid plants, pickling |
| Inconel 825 / N08825 | Ni-Fe-Cr | Sulphuric + sour | Acid, sour gas |
Key Application Areas
Sulphuric Acid Plants
The alloy's defining application — resists sulphuric acid across virtually all concentrations and temperatures where 316L and even 904L are inadequate.
Pickling & Metal Finishing
Sulphuric and mixed-acid pickling lines, plating and metal-finishing circuits.
Pharmaceutical & Fine Chemical
Reactor, condenser and piping service in pharmaceutical and specialty chemical plants handling acidic streams.
Plastics & Synthetic Fibre
Viscose, rayon and synthetic fibre processing where sulphuric acid and chlorides are present.
Phosphoric & Nitric Acid
Good resistance to phosphoric acid and nitric acid in fertiliser and chemical production.
Food & Dye Processing
Acidic food, tartaric and citric acid, and dye/pigment manufacturing equipment.
Available Fitting Types — Alloy 20 / UNS N08020
Sizes typically ½″–24″ for nickel alloys due to material cost and availability. Larger sizes available on enquiry with extended lead time.
Elbows — 90° & 45°
Long Radius (1.5D) and Short Radius (1D). ASME B16.9 / EN 10253-2 Type B. ½″–24″.
180° Return Bends
Long Radius. ASME B16.9. ½″–12″.
Equal & Reducing Tees
ASME B16.9 / EN 10253-2 Type B. ½″–24″.
Concentric & Eccentric Reducers
ASME B16.9 / EN 10253-2 Type B. ½″–24″.
Pipe Caps
ASME B16.9 / EN 10253-2 Type B. ½″–24″.
Lap Joint Stub Ends
ASME B16.9 Type A & B. ½″–16″. Paired with backing flanges to reduce cost.
Crosses (4-Way)
ASME B16.9. ½″–8″.
Laterals (45° Y)
ASME B16.9. ½″–8″.
Flanges — All Types
Weld Neck, Slip-On, Blind, Socket Weld, RTJ. ASME B16.5. ASTM B462 UNS N08020. ½″–24″.
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