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Monel K‑500 (UNS N05500) and Monel 400 (UNS N04400) share the same Ni‑Cu base (Ni 63–70 %, Cu balance) and therefore nearly identical general corrosion resistance in seawater, brine, and hydrofluoric acid, plus immunity to chloride stress corrosion cracking (Cl⁻ SCC). The fundamental difference is that K‑500 contains added Al (2.30–3.15 %) and Ti (0.35–0.85 %) and is precipitation‑hardened by aging to develop γ′ (Ni₃(Al,Ti)), achieving ≥ 690 MPa (100 ksi) yield — roughly three times that of annealed Monel 400. Monel 400 is a solid‑solution alloy, annealed only, with yield ≈ 172–207 MPa min (typical 240–290 MPa) and is fully non‑magnetic, whereas K‑500 in the aged condition is weakly magnetic. Below is a rigorous, side‑by‑side comparison to guide selection.
1. Chemical Composition & Metallurgical Difference
|
Element |
Monel 400 (N04400) |
Monel K‑500 (N05500) |
Significance |
|---|---|---|---|
|
Ni |
63.0 – 70.0 % |
63.0 – 70.0 % |
Same base |
|
Cu |
Balance (~28–34 %) |
Balance (~27–33 %) |
Same Ni‑Cu α‑phase |
|
Al |
≤ 0.50 % (tramp/impurity) |
2.30 – 3.15 % |
★ Forms γ′ — K‑500 exclusive |
|
Ti |
≤ 0.50 % (tramp/impurity) |
0.35 – 0.85 % |
★ Co‑precipitates γ′ with Al |
|
Fe |
≤ 2.5 % |
≤ 2.0 % |
Slightly tighter in K‑500 |
|
C |
≤ 0.30 % (typ ≤0.15) |
≤ 0.25 % (typ ≤0.18) |
Comparable |
|
S |
≤ 0.024 % |
≤ 0.010 % |
K‑500 typically cleaner |
|
Heat Treatment |
Anneal 700–900 °C → WQ (single step) |
① Solution 980–1040 °C → WQ |
K‑500 requires aging for strength |
|
Magnetic (as supplied) |
Non‑magnetic (paramagnetic) |
Solution: near non‑mag |
Critical for non‑mag instrument apps |
2. Mechanical Properties — Room Temperature (Typical / ASTM Minima)
|
Property |
Monel 400 (Annealed) ASTM B164 min |
Monel K‑500 (Aged) ASTM B865 min |
Typical K‑500 (Aged) |
|---|---|---|---|
|
Tensile Strength, Rm |
≥ 483 MPa (70 ksi) |
≥ 965 MPa (140 ksi) |
1000–1100 MPa |
|
Yield Strength, Rp0.2 |
≥ 172 MPa (25 ksi) — typ 240–290 MPa |
≥ 690 MPa (100 ksi) |
720–830 MPa |
|
Elongation, A₅₀ |
≥ 35 % |
≥ 15 % |
18–25 % |
|
Hardness (HBW) |
≤ 110–140 HB (≈60–80 HRB) |
≥ 265 HB (≈27–35 HRC) |
285–330 HB |
|
Young's Modulus, E |
~179 GPa |
~179 GPa |
Same |
|
Charpy V‑Notch @ −73 °C |
— |
≥ 27 J |
Typ > 30 J |
|
Solid‑Solution (unaged) K‑500 ref. |
— |
Rp0.2 ≈ 275–380 MPa / Rm ≈ 690–830 MPa |
For machined‑before‑aging stage |
Cold‑worked Monel 400 can reach ~550–620 MPa tensile but with directional properties and no thermal stability; K‑500 strength is isotropic (within grain) and stable until over‑aged > ~480 °C.
3. Corrosion Resistance — Are They Different?
|
Environment |
Monel 400 |
Monel K‑500 |
Practical Verdict |
|---|---|---|---|
|
Seawater full immersion / splash / tidal (Cl⁻ SCC) |
★★★★★ immune |
★★★★★ immune (identical uniform corrosion rate < 0.025 mm/a) |
Interchangeable on corrosion grounds |
|
Hydrofluoric Acid (HF), all conc. @ ≤ mod. T |
★★★★★ |
★★★★★ |
Identical — Monel family hallmark |
|
Dilute H₂SO₄ / H₃PO₄ (non‑oxidizing, ≤ 60–80 °C) |
★★★☆ |
★★★☆ |
Essentially same |
|
Caustic NaOH/KOH ≤ ~300 °C |
★★★★☆ |
★★★★☆ |
Same |
|
Strong Oxidizing Acids (hot conc. HNO₃ etc.) |
✗ Not recommended |
✗ Not recommended |
Neither suitable |
|
Hydrogen Embrittlement (high‑strength aged K‑500) |
Not applicable (low strength) |
⚠️ Aged > ~30–35 HRC in H‑generating environs (over‑cathodic protect., pickling, wet H₂S trace) → control hardness ≤ 35 HRC |
400 has no HE risk |
|
Sour Gas (H₂S/CO₂ — low p_H₂S, ambient T) |
△ Limited accept. |
△ Limited accept. — deep sour usually Incoloy 925 / Inconel 718 per ISO 15156 |
Not primary sour‑well choice |
Conclusion: Corrosion media suitability is effectively the same. Choose K‑500 when you need ≥100 ksi yield + seawater/Cl⁻ SCC resistance; choose 400 when you need full non‑magnetism, lower cost, or only moderate strength.
4. Temperature Capability & Limitations
|
Aspect |
Monel 400 |
Monel K‑500 |
|---|---|---|
|
Recommended continuous stressed service |
≤ 427–480 °C (strength stable; no aging effect) |
≤ 427–480 °C — γ′ over‑ages > ~480 °C → strength decays; do NOT design for high‑temp creep > 650 °C |
|
Oxidation limit (air, unloaded) |
≈ 540 °C continuous |
≈ 540 °C continuous (Ni‑Cu oxide scale) |
|
Low‑Temp toughness |
Excellent to −196 °C (no DBTT) |
Excellent to −196 °C (no DBTT) |
|
Magnetic note |
Fully non‑magnetic at all temps (annealed) |
Weakly magnetic after aging — notfor strict non‑mag apps |
5. Cost & When to Specify Which Alloy
Relative Base Cost Index (Ni‑price dependent): Monel K‑500 ≈ 115–130 % of Monel 400 (extra processing: solution + aging, tighter chemistry, UT). Actual spread varies.
✅ Specify Monel K‑500 when:
Pump shafts, valve stems, agitator shafts in seawater/brine requiring YS ≥ 690 MPa + Cl⁻ SCC immunity.
Mooring hardware, tie‑rods in splash zone where high strength + corrosion matter.
Aerospace/defense non‑critical non‑mag (weak mag OK) high‑strength fasteners.
You willperform the aging heat treatment (or buy ST+Aged).
✅ Specify Monel 400 when:
Heat exchangers, condenser tubes, process vessel linings, HF storage tanks — no high yield needed.
Strict non‑magnetic requirement (compass proximity, magnetic sensor housings).
Cost sensitivity and strength requirement met by annealed or light cold‑work condition.
❌ Don't substitute blindly:
K‑500 → strict non‑mag instruments (use 400 or Ti).
400 → high‑load pump shaft in seawater (yield too low → over‑size or use K‑500).
Either → high‑temp creep service > 650 °C (use Incoloy 800HT / Inconel 600+), strong oxidizing acids (use C‑276 or Ti).
