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Alloy 20 / Alloy 24 / Alloy 26 Bar & Wire | ASME SB-473 UNS N08020
Three copper-bearing alloys, one specification
ASME SB-473 covers bar and wire of UNS N08020 (Alloy 20, sold under the trade name Carpenter 20), UNS N08024 and UNS N08026. All three are copper-bearing nickel-iron-chromium-molybdenum alloys, and the copper is the reason they exist: it is what makes t
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Three copper-bearing alloys, one specification
ASME SB-473 covers bar and wire of UNS N08020 (Alloy 20, sold under the trade name Carpenter 20), UNS N08024 and UNS N08026. All three are copper-bearing nickel-iron-chromium-molybdenum alloys, and the copper is the reason they exist: it is what makes these grades resistant to sulphuric acid, where the straight nickel-chromium alloys are not.
They also share a second characteristic that matters in procurement, which is the columbium addition tied to the carbon content. In Alloy 20 the specification requires columbium plus tantalum at 8 times the carbon content minus 1.00, with a maximum of 1.00. This stabilises the alloy against sensitisation and is the reason Alloy 20 holds its corrosion resistance after welding without post-weld heat treatment in most applications.
Grade comparison
| Element, % | Alloy 20 (N08020) | Alloy 24 (N08024) | Alloy 26 (N08026) |
|---|---|---|---|
| Nickel | 32.00 – 38.00 | 35.00 – 40.00 | 33.00 – 37.20 |
| Chromium | 19.00 – 21.00 | 22.50 – 25.00 | 22.00 – 26.00 |
| Molybdenum | 2.00 – 3.00 | 3.50 – 5.00 | 5.00 – 6.70 |
| Copper | 3.00 – 4.00 | 0.50 – 1.50 | 2.00 – 4.00 |
| Columbium (Nb) + tantalum | 8 × carbon – 1.00 | 0.15 – 0.35 | — |
| Nitrogen | — | — | 0.10 – 0.16 |
| Carbon, max | 0.07 | 0.03 | 0.03 |
| Iron | Remainder | Remainder | Remainder |
Alloy 20 is the general purpose grade of the three, with 3 to 4% copper for sulphuric acid and the columbium stabilisation. Alloy 24 carries less copper but more chromium and molybdenum and a deliberate columbium addition of 0.15 to 0.35%, which raises its resistance to oxidising acids and to sulphuric acid under reducing conditions. Alloy 26 has the highest molybdenum at 5.00 to 6.70% with a nitrogen addition, and is the grade to look at when the service is strongly oxidising and the copper content matters. Where the distinction is not clear from the service description, we will recommend a grade rather than default to the one that is easiest to supply.
Product forms and heat treatment
We supply SB-473 as hot finished bar, cold finished bar, centerless ground bar and cold finished wire. Bar is furnished annealed and either hot finished or cold finished, with strain-hardened material available only in the cold finished condition. Wire is furnished only as annealed and cold finished. A typical ordering description in the specification itself is 200 bars, UNS N08020, 1 in (25.4 mm) round by 10 to 14 ft (3.0 to 3.6 m), centerless ground, Specification B473. We also supply the bar as blanks for machined components, and as starting stock for forged flanges and fittings to SB-462 and SB-564.
Heat treatment follows the grade. Alloy 20 is furnished in the stabilised annealed condition, with recommended annealing at 1700 to 1850 degrees F (927 to 1010 degrees C) followed by water quenching or rapid cooling. Alloy 26 is furnished solution annealed at 2050 to 2200 degrees F (1121 to 1204 degrees C). Alloy 24 is furnished annealed at 1925 to 1975 degrees F (1052 to 1079 degrees C).
Certification and inspection
Each order is supplied with an EN 10204 3.1 material test report showing the heat number, the SB-473 Table 1 chemical analysis and the Table 2 mechanical properties. If a product check analysis is performed at your works, the product analysis variations prescribed in ASTM B880 apply and we state the certification level accordingly. On request we add ultrasonic testing, eddy current testing and hardness testing, and we can supply PED 2014/68/EU or UKCA certification of conformity for European and UK delivery. Third-party inspection is arranged and witnessed before despatch.
Ordering information
Please specify the grade (Alloy 20, Alloy 24 or Alloy 26), the product form (hot finished bar, cold finished bar, centerless ground bar or wire), the diameter, the length, the quantity and the delivery date, plus the destination port and any certification you need. If the bar is destined for sulphuric acid service, tell us the acid concentration and the operating temperature so we can confirm the grade and anneal condition. Send us drawings or cut lists and we will quote against them with the certificate package stated for each item.
Related specifications and products. Alloy 20 is also supplied as plate and strip and as fittings and flanges. See our nickel alloy bar range Alloy 20 plate and strip Alloy 20 fittings and flanges for the dimensional and material basis behind this page.
