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Monel K‑500 (UNS N05500 / W.Nr. 2.4375 / AMS 4676) is an age‑hardenable (precipitation hardening) Ni‑Cu alloy based on the Monel 400 (UNS N04400) composition, with added Aluminum 2.30–3.15 % and Titanium 0.35–0.85 % to form γ′ precipitates — Ni₃(Al,Ti) — during aging at 570–605 °C. It retains Monel 400's excellent resistance to seawater, brine, and hydrofluoric acid (HF), plus immunity to chloride stress corrosion cracking (Cl⁻ SCC), while delivering minimum yield strength ≥ 690 MPa (100 ksi) — roughly three times that of annealed Monel 400. It is not a high‑temperature creep alloy like Incoloy 800HT, nor a general acid‑process alloy like Alloy 825; its primary niche is high‑strength, seawater‑resistant, weakly magnetic components for marine, offshore, chemical, and aerospace applications.
Chemical Composition (UNS N05500 per ASTM B865 / AMS 4676)
|
Element |
Weight % |
Function |
|---|---|---|
|
Nickel (Ni) |
63.0 – 70.0 |
Austenite stabilizer; base of Ni‑Cu α‑phase; provides Cl⁻ SCC immunity |
|
Copper (Cu) |
Balance (27–33) |
Forms continuous α‑solid solution with Ni → seawater/HF resistance |
|
Aluminum (Al) |
2.30 – 3.15 |
Key hardener — forms γ′ (Ni₃Al,Ti) during aging |
|
Titanium (Ti) |
0.35 – 0.85 |
Co‑precipitates with Al as γ′; refines strengthening |
|
Iron (Fe) |
≤ 2.0 |
Kept low for optimal corrosion behavior |
|
Carbon (C) |
≤ 0.25 (typ. ≤ 0.18) |
Low to minimize carbide segregation |
|
Manganese (Mn) |
≤ 1.5 |
— |
|
Silicon (Si) |
≤ 0.50 |
— |
|
Sulfur (S) |
≤ 0.010 |
Tightened vs. Monel 400 — important for quality & SSC‑related reviews |
|
Phosphorus (P) |
≤ 0.030 |
— |
Product forms: ASTM B865 (bar/rod/forging), B127 (plate/sheet — K‑500 requires aged property verification), B165/B164 referencing dimensions for bar/tube; AMS 4676 (aerospace bar/forging), AMS 7234 (welding wire — typically Monel 60 or ERNiCu‑7 is used as filler; K‑500 itself is rarely welded in aged condition without post‑weld heat treatment).
Heat Treatment: Solution Anneal + Age Hardening (Essential!)
Unlike Monel 400 (single‑step anneal only), K‑500 must be aged to achieve design strength:
Solution Treat (ST): 980–1040 °C (typical 1010 °C), hold proportional to section, rapid water quench → soft condition (Rm ≈ 690–830 MPa, Rp0.2 ≈ 275–380 MPa), suitable for machining.
Age Hardening (AH): 570–605 °C (typical 580–595 °C) × 8–16 h, air or furnace cool → γ′ (Ni₃(Al,Ti)) precipitates → significant strengthening.
Effect on Magnetism: Solution‑annealed K‑500 is essentially non‑magnetic (like Monel 400); after aging it becomes weakly ferromagnetic (weakly magnetic) due to trace Fe‑rich clusters — notsuitable where strict non‑magnetic behavior is required (use Monel 400 in such cases).
Delivery Conditions:
Solution‑annealed only:for customer final machining + in‑house aging.
Solution‑annealed + Aged (STA):ready for installation (hardness typically 27–35 HRC).
Mechanical Properties (Aged / Precipitation‑Hardened Condition — ASTM B865 Minimum)
|
Property |
ASTM B865 Minimum |
Typical Mill Range |
|---|---|---|
|
Tensile Strength, Rm |
≥ 965 MPa (140 ksi) |
1000–1100 MPa |
|
Yield Strength, Rp0.2 (0.2 % offset) |
≥ 690 MPa (100 ksi) |
720–830 MPa |
|
Elongation, A₅₀ |
≥ 15 % |
18–25 % |
|
Reduction of Area, Z |
≥ 45 % |
50–60 % |
|
Brinell Hardness (HBW) |
≥ 265 |
285–340 HB (≈27–35 HRC) |
|
Charpy V‑Notch @ −73 °C (−100 °F) |
≥ 27 J (20 ft‑lbf) |
Typically 30–50 J |
|
Young's Modulus, E (20 °C) |
~179 GPa |
Lower than carbon steel (206 GPa); account for deflection |
|
Density |
8.44 g/cm³ (use 8.45 kg/dm³ for weight) |
— |
|
Melting Range |
1315–1350 °C (2400–2460 °F) |
— |
Note: Prolonged exposure > ∼ 480 °C (900 °F) causes γ′ over‑aging → gradual strength loss. Long‑term load‑bearingservice is normally limited to ≤ 427–480 °C; short‑term unloaded exposure to ∼540 °C (1000 °F) oxidation is acceptable.
Corrosion Resistance Summary (Generally Equivalent to Monel 400)
|
Environment |
Performance |
Comment |
|---|---|---|
|
Seawater / Brine (full immersion, splash, tidal) |
★★★★★ — immune to Cl⁻ SCC; very low general corrosion (< 0.025 mm/a) |
Classic for pump shafts, valve stems, prop shaft sleeves |
|
Hydrofluoric Acid (HF), all concentrations @ ambient–moderate T |
★★★★★ |
Monel family hallmark |
|
Dilute Sulfuric / Phosphoric Acid (non‑oxidizing, ≤ 60–80 °C) |
★★★☆ |
Fair; not for hot concentrated acids |
|
Caustic (NaOH/KOH) to ∼ 300 °C |
★★★★☆ |
Similar to Monel 400 |
|
Strong Oxidizing Acids (hot HNO₃, conc. H₂SO₄ + oxidizer) |
✗ Not recommended |
Both Monel 400 & K‑500 corrode rapidly |
|
Sour Gas (H₂S/CO₂) — low partial pressure, ambient T |
△ Conditionally acceptable; deep sour wells with high pH₂S → review per NACE MR0175 / ISO 15156 (Incoloy 925 or Inconel 718 often preferred) |
— |
|
Hydrogen Embrittlement Risk (aged > ∼30–35 HRC) |
⚠️ Possible in H‑generating environments (over‑cathodic protection, pickling, wet H₂S) → control hardness ≤ 35 HRC and avoid excessive pickling |
Monel 400 (low strength) not susceptible |
Typical Applications
Marine / Offshore: Centrifugal pump shafts, valve stems, propeller shaft sleeves, rudder stocks, mooring chain fittings, fairlead bushings (splash zone / atmospheric — weak magnetism acceptable).
Oil & Gas (non‑sour / sweet): Progressive cavity pump sucker rods, meter‑prover rods, agitator shafts in brines.
Aerospace / Defense: Non‑magnetic (weakly magnetic) high‑strength fasteners, lock pins, springs (note lower E‑modulus vs. steel → different spring rate).
Chemical / Pulp & Paper: Caustic evaporator agitator shafts, beater bars.
NOT used for: high‑temperature creep service (> 650 °C pressure parts → Incoloy 800HT / Inconel 600), strong acidic process vessels (→ Alloy 825 / C‑276), strict non‑magnetic instrument housings (→ Monel 400 / Titanium).
