Discover the latest trends and innovations in the nickel alloy industry. We provide expert insights and in-depth analysis on the newest developments in nickel alloys.
Strictly speaking, "Incoloy 20" is a trade/informal name—the correct designation is Alloy 20 (Carpenter 20Cb-3® / UNS N08020 / W.Nr. 2.4660 / X1NiCrMoCu32-28-7). It is not part of the Incoloy 800 series (N08800/N08811). Alloy 20 is a niobium-stabilized, copper- and molybdenum-bearing austenitic Fe–Ni–Cr alloy developed specifically for excellent resistance to sulfuric acid (H₂SO₄), phosphoric acid, and chloride-containing mixed acids, combined with immunity to chloride stress corrosion cracking (SCC) that plagues 300-series stainless steels. Below is a precise, engineer-focused overview of its chemistry, properties, and typical applications.
Chemical Composition (UNS N08020 per ASTM B463 / B473 / B729)
|
Element |
Content (wt.%) |
Functional Significance |
|---|---|---|
|
Nickel (Ni) |
32.0 – 38.0 |
Austenite stabilizer; prevents Cl⁻ SCC |
|
Chromium (Cr) |
19.0 – 21.0 |
Oxidation resistance; resists nitric / mixed oxidising acids |
|
Iron (Fe) |
≥ 35.0 (Balance) |
Matrix element; reduces cost vs. full Ni-base alloys |
|
Molybdenum (Mo) |
2.0 – 3.0 |
Improves pitting / crevice corrosion resistance (PREN ≈ 29–32) |
|
Copper (Cu) |
3.0 – 4.0 |
Key: inhibits active dissolution in sulfuric & some reducing acids |
|
Niobium (Nb, columbium) |
8×C (min) – 1.0% (typical 0.50–0.80%) |
Stabilizes C as NbC → prevents Cr-carbide precipitation → resists intergranular attack |
|
Carbon (C) |
≤ 0.07 |
Low C minimizes sensitization risk |
|
Manganese (Mn) |
≤ 2.0 |
— |
|
Silicon (Si) |
≤ 1.0 |
— |
|
Sulfur (S) |
≤ 0.035 |
— |
|
Phosphorus (P) |
≤ 0.045 |
— |
Common product standards: ASTM B463 (plate/sheet/strip), B473 (bar), B729 (seamless pipe), B464 (welded pipe), B366 (fittings/forgings).
Chinese grade equivalent: NS1403 (GB/T 15007); German: W.Nr. 2.4660 (DIN EN 10295).
Microstructure & Heat Treatment
Matrix: Fully austenitic (γ) after solution annealing (940–1010°C, rapid quench). No ferrite, no martensitic transformation.
Stabilization: Nb ties up carbon as fine NbC particles within grains → Cr₂₃C₆ does notprecipitate at grain boundaries during slow cooling or welding → excellent resistance to intergranular corrosion (IGC) in the as-welded condition (unlike 316L unless properly low-C or stabilized).
Grain Size: Typically ASTM 4–7 (solution annealed); finer than creep-grade Incoloy 800HT (ASTM ≥5 coarse).
Magnetic: Non-magnetic in annealed condition; remains non-magnetic after cold work.
Mechanical & Physical Properties (Solution Annealed, Room Temp.)
|
Property |
Value (Typical / ASTM Minimum) |
|---|---|
|
Tensile Strength (Rm) |
≥ 551 MPa (80 ksi); typical 600–720 MPa |
|
Yield Strength (Rp0.2) |
≥ 241 MPa (35 ksi); typical 300–370 MPa |
|
Elongation (A₅₀) |
≥ 30%; typical 35–45% |
|
Hardness |
≤ 217 HB (Brinell); typ. 170–200 HB / 75–90 HRB |
|
Density |
8.08 g/cm³ (use 8.1 for weight calc) |
|
Melting Range |
1370–1420°C (2498–2570°F) |
|
Modulus of Elasticity (E) |
~197 GPa |
|
Coefficient of Thermal Expansion (20–300°C) |
~15.4 × 10⁻⁶ /K |
|
Thermal Conductivity (100°C) |
~12 W/(m·K) |
|
Electrical Resistivity (20°C) |
~1.12 µΩ·m |
|
Non-magnetic |
Yes |
Corrosion Resistance – Where Alloy 20 Excels (and Does Not)
|
Environment |
Performance |
Note |
|---|---|---|
|
Sulfuric Acid (H₂SO₄), 10–40%, ≤ 60–93°C |
★★★★★ |
Primary design alloy for H₂SO₄ service (Cu effect); rate ↑ above ~93°C |
|
H₂SO₄ + Chlorides (mixed acid) |
★★★★☆ |
Mo + high Ni resist pitting; far better than 316L |
|
Phosphoric Acid (wet process, with F⁻/SO₄²⁻) |
★★★★ |
Used in phosphate fertilizer plants |
|
Nitric + Sulfuric Mixed Acid (moderate oxid.) |
★★★★ |
Cr provides oxidising-acid resistance |
|
Chloride SCC (aqueous, ≤ 200°C) |
★★★★★ (immune, like 800HT) |
Outperforms 304/316 SS dramatically |
|
Seawater Immersion (long term) |
★★ Moderate |
PREN ≈ 29–32; OK but less cost-effective than 904L or super duplex |
|
Hydrochloric Acid (HCl), pure / strong |
✗ Not recommended |
Use Hastelloy C-276 / C-22 |
|
High-Temp Oxidation (> 800°C creep service) |
✗ Not a high-temp alloy |
Use Incoloy 800HT / Inconel 600+ for that |
Typical Applications
Sulfuric acid plant: heat exchangers, cooler coils, acid storage tank linings, SO₂ scrubber components
Chemical process: reactors handling H₂SO₄/H₃PO₄ + chlorides, dye/intermediate vessels
Wet-process phosphoric acid plants: agitators, pump casings, piping in leach sections
Nuclear fuel reprocessing (HNO₃ + H₂SO₄ mixed acid segments)
Food/pharmaceutical reactors where trace sulfate + organic acids present
Not used for: high-temp furnace tubes, reformer catalyst tubes (that is Incoloy 800HT's role)
Welding, Fabrication & Common Confusions
Welding: GTAW/GMAW using ERNiCrMo-3 (Inconel 625 type filler) or ERNiCrMo-4 (C-276 type) for optimum corrosion matching. Post-weld solution anneal improves creep/uniformity in heavy sections but as-welded condition is usually acceptable for chemical service thanks to Nb stabilization.
Hot Working: 955–1150°C; avoid working in 650–900°C sensitization range.
Cold Working: Similar to 316 but slightly higher work-hardening rate (due to Mo, Nb); intermediate anneals may be needed for deep draws.
⚠️ Important: "Incoloy 20" is notIncoloy 800/800H/800HT. Do not substitute Alloy 20 for high-temp creep service, nor 800HT for sulfuric acid service—each is optimized for a different failure mode.
