Alloy 20 (Carpenter 20) – UNS N08020
Niobium-stabilized Fe-Ni-Cr-Mo-Cu alloy
Austenitic Fe-Ni-Cr alloy with Mo, Cu and Nb, developed for sulfuric acid. The high Ni (32–38%) gives high resistance to chloride stress corrosion cracking; Nb stabilizes against intergranular corrosion. Classified by ASTM as a nickel alloy (B series).
Designations and equivalents
| System | Designation |
|---|---|
| UNS | N08020 |
| W.Nr./EN | 2.4660 (NiCr20CuMo) |
| DIN | NiCr20CuMo |
| Trade names | Carpenter 20Cb-3, Incoloy 020, Nicrofer 3620 Nb |
| Others | Alloy 20 |
Standards by product form
| Form | Standard |
|---|---|
| Bar | ASTM B473 / ASME SB-473 |
| Forging/flange | ASTM B462 |
| Plate | ASTM B463 |
| Tube | ASTM B464 / B468 / B729 |
| Fittings | ASTM B366 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Seamless pipe | 7304.41.90 | 7304.41 | 7304.41.6045 | 7304 41 00 |
| Welded pipe | 7306.40.00 | 7306.40 | 7306.40.5085 | 7306 40 80 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5). Fe-Ni-Cr alloy in which iron predominates by weight: under Section XV Note 5 and Chapter 72 Note 1(e) it is classified as stainless steel (7219, 7222, 7304), not as a Chapter 75 nickel alloy.
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM B473
| Element | Range (%) |
|---|---|
| C | ≤ 0.07 |
| Mn | ≤ 2.00 |
| P | ≤ 0.045 |
| S | ≤ 0.035 |
| Si | ≤ 1.00 |
| Cr | 19.0 – 21.0 |
| Ni | 32.0 – 38.0 |
| Mo | 2.00 – 3.00 |
| Cu | 3.00 – 4.00 |
| Nb (+Ta) | 8×C – 1.00 |
| Fe | balance |
Mechanical properties
Stabilize annealed (ASTM B473)
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 241 MPa |
| Tensile strength | ≥ 551 MPa |
| Elongation | ≥ 30% |
| Reduction of area | ≥ 50% |
| Cold worked (≤ 50.8 mm) | Rm ≥ 620 MPa; Rp0.2 ≥ 414 MPa; A ≥ 15% |
Typical physical properties
| Property | Value |
|---|---|
| Density | 8.08 g/cm³ |
| Melting range | ≈ 1,357 – 1,430 °C |
| Modulus of elasticity | ≈ 193 GPa |
| Thermal conductivity (20 °C) | ≈ 12 W/m·K |
| Thermal expansion (20–100 °C) | ≈ 14.7 µm/m·K |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Corrosion service up to ≈ 425 °C (usual ASME design limit for N08020; confirm the code edition). |
| Heat treatment | Stabilizing anneal 927 – 1,010 °C (1,700 – 1,850 °F), ≈ 30 min per inch, water cooling. Forging between ≈ 1,150 and 1,230 °C, without working below ≈ 980 °C. |
| Welding | Good. Consumable AWS ER320LR / E320LR; for joining to Ni alloys or 316, ERNiCrMo-3. |
| Machining | Machinability lower than carbon steel: it work hardens quickly; use sharp tools, positive rake, constant feed without 'rubbing' and abundant coolant. Behavior similar to nickel alloys. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Sulfuric acid (production, pickling, mixers)
- Fine chemicals, pharmaceuticals and solvents
- Plastics, synthetic rubber and explosives
- Gas scrubbers
- Pumps, valves and shafts in acidic media
- Food and dyes
Alternatives: 904L for dilute sulfuric acid at lower cost; Incoloy 825 with more Ni and Mo; C-276/Hastelloy for mixed acids with chlorides and reducing agents.
Alloy 20 (Carpenter 20): complete guide
What Alloy 20 is
Alloy 20 was born in the 1950s as a super-austenitic stainless steel for sulfuric acid; today ASTM classifies it among the nickel alloys, in the UNS N080xx series, although iron is still the element present in the largest amount. The niobium-stabilized version, 20Cb-3, is the only one manufactured today and is the one covered by ASTM B473 (bar) and B462 (forgings).
Chemical composition and role of the elements
Copper (3 – 4%) is the element that gives resistance to sulfuric acid, a reducing medium in which chromium alone does not protect. The high nickel (32 – 38%) stabilizes the austenite and gives practical immunity to chloride stress corrosion cracking. Molybdenum improves pitting resistance and niobium, in an amount of at least eight times the carbon, ties up the carbon and prevents sensitization in welding.
Corrosion and limits of use
Alloy 20 resists sulfuric acid at all concentrations up to about 60 °C, phosphoric acid, nitric acid and media with moderate chlorides, with a PREN of 26 to 31. It is not a heat-resisting alloy: the usual design limit is about 425 °C. For hydrochloric acid and reducing sulfuric acid, nickel-molybdenum alloys such as Hastelloy B-2 are the choice.
Applications
Pumps, valves, shafts and agitators for sulfuric acid, tanks and heat exchangers for the chemical and fertilizer industries, desulfurization equipment, pharmaceutical and food processing, pulp plant components. For severe chlorides, 904L or 254 SMO.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, solution annealing in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of Alloy 20 (Carpenter 20)?
Per ASTM B473: C ≤ 0.07; Mn ≤ 2.00; P ≤ 0.045; S ≤ 0.035; Si ≤ 1.00; Cr 19.0 – 21.0.
Which standards cover Alloy 20 (Carpenter 20)?
Bar: ASTM B473 / ASME SB-473. Forging/flange: ASTM B462. Plate: ASTM B463. Tube: ASTM B464 / B468 / B729. Fittings: ASTM B366.
Up to what temperature can Alloy 20 (Carpenter 20) be used?
Corrosion service up to ≈ 425 °C (usual ASME design limit for N08020; confirm the code edition).
Which materials can replace Alloy 20 (Carpenter 20)?
904L for dilute sulfuric acid at lower cost; Incoloy 825 with more Ni and Mo; C-276/Hastelloy for mixed acids with chlorides and reducing agents.
Is Alloy 20 a stainless steel?
It started as a super-austenitic stainless for sulfuric acid, but today ASTM classifies Alloy 20 among the nickel alloys (UNS N08020), because it has 32 to 38% nickel. Iron is still the element present in the largest amount.
What is Alloy 20 used for?
Mainly for sulfuric acid equipment: pumps, valves, tanks and piping. It is also used with phosphoric acid, nitric acid and media with moderate chlorides.
Are Carpenter 20 and Alloy 20 the same alloy?
Yes. Carpenter 20 (20Cb-3) is the original trade name; Alloy 20 and UNS N08020 are the generic designations of the same alloy.
