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Alloy 20 (Carpenter 20Cb‑3® / UNS N08020 / W.Nr. 2.4660 / X1NiCrMoCu32‑28‑7), colloquially called "Incoloy 20" in some markets, is a niobium‑stabilized, Cu‑ and Mo‑bearing austenitic Fe–Ni–Cr alloy optimized for sulfuric acid, phosphoric acid, and chloride‑containing mixed acid service — not a high‑temperature creep alloy like Incoloy 800HT. The data below are for the solution‑annealed (SA) condition per ASTM B463 / B473 / B729, representing delivered mill product condition.
Chemical Basis (Context for Property Behaviour)
|
Element |
Wt.% |
Influence on Properties |
|---|---|---|
|
Ni |
32.0 – 38.0 |
Austenite stabilizer; imparts non‑magnetic behaviour & Cl⁻ SCC immunity |
|
Cr |
19.0 – 21.0 |
Oxidation resistance; contributes to PREN (~29–32) |
|
Fe |
≥ 35.0 (Bal.) |
Matrix; lowers density & cost vs. full Ni‑base |
|
Mo |
2.0 – 3.0 |
Raises elastic modulus slightly; improves pitting resistance |
|
Cu |
3.0 – 4.0 |
Solid‑solution strengthens & is key to H₂SO₄ resistance |
|
Nb (Cb) |
0.50 – 0.80% (≥8×C) |
Forms NbC → stabilizes microstructure, no direct effect on physical constants |
|
C |
≤ 0.07 |
Low C preserves IGC resistance |
Room‑Temperature Mechanical Properties (Solution Annealed)
Values comply with ASTM minimums; typical mill ranges shown for design reference.
|
Property |
ASTM Minimum |
Typical Mill Range |
Test Std. |
|---|---|---|---|
|
Tensile Strength, Rm |
≥ 551 MPa (80 ksi) |
600 – 720 MPa |
ASTM E8 |
|
Yield Strength, Rp0.2 |
≥ 241 MPa (35 ksi) |
300 – 370 MPa |
ASTM E8 |
|
Elongation, A₅₀ |
≥ 30% |
35 – 45% |
ASTM E8 |
|
Reduction of Area, Z |
— |
45 – 60% |
ASTM E8 |
|
Brinell Hardness, HBW |
≤ 217 |
170 – 200 HB |
ASTM E10 |
|
Rockwell Hardness, HRB |
— |
75 – 90 HRB |
ASTM E18 |
|
Charpy V‑Notch Impact (RT) |
> 100 J (typical) |
Fully ductile – no DBTT |
ASTM E23 |
Cold working raises yield/tensile but reduces elongation. Pressure‑part or forming designs should use annealed values unless post‑work strength is intentional and verified.
Physical Constants (Solution‑Annealed Condition)
|
Physical Property |
Value / Range |
Note |
|---|---|---|
|
Density (ρ) |
8.08 g/cm³ (use 8.10 kg/dm³ for wt. calc.) |
— |
|
Melting Range |
1370 – 1420°C (2498 – 2570°F) |
Initial melt ~1370°C |
|
Crystal Structure |
Face‑Centered Cubic (FCC), fully austenitic |
Non‑magnetic in SA & cold‑worked condition |
|
Elastic (Young's) Modulus, E (20°C) |
195 – 200 GPa (typ. 197 GPa) |
Decreases with temperature |
|
Shear Modulus, G (20°C) |
~76 GPa |
— |
|
Poisson's Ratio, ν (20°C) |
0.30 – 0.31 |
— |
|
Coefficient of Thermal Expansion, α (20–300°C, mean) |
15.4 × 10⁻⁶ /K |
Close to 300‑series SS & carbon steel |
|
Thermal Conductivity, λ (100°C) |
~12 W/(m·K) |
Rises to ~16 W/(m·K) @ 500°C |
|
Specific Heat, cp (20°C) |
~460 J/(kg·K) |
— |
|
Electrical Resistivity, ρₑ (20°C) |
~1.12 µΩ·m |
— |
Elevated‑Temperature Mechanical Trend (Guidance Only)
Alloy 20 is not listed in ASME Section I/ VIII creep tables for high‑temp pressure design. Approximate softening trend for information:
Yield Strength @ 300°C: ~240–280 MPa
Yield Strength @ 400°C: ~220–260 MPa
Yield Strength @ 500°C: ~200–230 MPa
Creep‑rupture strength is significantly lower than Incoloy 800HT above 600°C. Continuous pressurized service > 400–425°C is not recommended; short‑term unstressed oxidation exposure to ~750–815°C is possible but outside primary design intent.
Microstructural / Metallographic Characteristics Affecting Properties
Fully Austenitic (γ): No δ‑ferrite → immune to σ‑phase embrittlement in normal service, but prolonged exposure 700–900°C can precipitate σ‑phase in high‑Mo heats → avoided by proper solution anneal & quenching.
Nb‑Stabilized: NbC (CbC) finely dispersed in grains → ties up C, prevents Cr‑depleted zones at grain boundaries → excellent intergranular corrosion (IGC) resistance per ASTM A262 Practice E (Strauss test).
Grain Size (SA): Typically ASTM 4–7; finer than creep‑grade Incoloy 800HT (≥ ASTM 5 coarse) — favours formability & IGC resistance rather than creep life.
Typical Corrosion‑Related Property Summary (for Context)
|
Parameter |
Value / Rating |
Implication |
|---|---|---|
|
PREN (Pitting Resist. Eqv. No.) = %Cr + 3.3×%Mo |
≈ 19 + 3.3×2.5 ≈ 27–32 |
Superior to 316L (PREN≈24); inferior to super‑austenitics (≥35) |
|
Chloride SCC Resistance |
Excellent (no failure in boiling 42% MgCl₂) |
Replaces 304/316 SS in SCC‑prone acid services |
|
Max. Recommended H₂SO₄ Temp. (10–40%) |
≤ 60–93°C continuous (typ. design ≤80°C) |
Cu provides resistance; rate rises sharply >93°C |
