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Monel 400 (UNS N04400 / W.Nr. 2.4360 / GB NS111) is a nickel-copper solid-solution alloy containing approximately 63–70% nickel, 28–34% copper, and up to 2.5% iron. It is the oldest and most widely used member of the Monel family, introduced in the early 1900s. Unlike precipitation-hardenable alloys (such as Monel K-500, Inconel 718, or A-286), Monel 400 is strengthened only by solid-solution hardening and cold working. Its defining attributes are excellent resistance to seawater and chloride stress corrosion cracking (SCC), tolerance to hydrofluoric acid (HF) and moderate alkalis, and good mechanical properties from cryogenic to ~538°C (1000°F). This guide provides technically accurate composition, properties, and application data for design and specification purposes.
1. Chemical Composition (ASTM B164 / B127 Requirement)
|
Element |
Specification (wt%) |
Typical Commercial Heat |
|---|---|---|
|
Nickel (Ni + Co) |
63.0 – 70.0 |
66.0 – 67.0 |
|
Copper (Cu) |
28.0 – 34.0 (Balance after Ni/Fe) |
31.0 – 32.0 |
|
Iron (Fe) |
≤ 2.50 |
1.0 – 1.5 |
|
Manganese (Mn) |
≤ 2.00 |
0.8 – 1.2 |
|
Carbon (C) |
≤ 0.30 (commercially ≤ 0.15) |
≤ 0.10 |
|
Silicon (Si) |
≤ 0.50 |
≤ 0.25 |
|
Sulfur (S) |
≤ 0.024 |
≤ 0.010 |
|
Cobalt (Co) |
≤ 1.0 (counted within Ni) |
≤ 0.5 |
The high Ni:Cu ratio (~2:1) places the alloy in the complete FCC γ-solid solution region of the Ni–Cu binary system—no second-phase formation under equilibrium conditions.
2. Physical Properties (Annealed Condition, 20°C Unless Noted)
|
Property |
Value |
Note |
|---|---|---|
|
Density |
8.80 g/cm³ (0.318 lb/in³) |
— |
|
Melting Range |
1300 – 1350°C (2370 – 2460°F) |
Solidus ~1300°C, Liquidus ~1350°C |
|
Elastic Modulus (E) |
179 – 183 GPa (26 × 10⁶ psi) |
Decreases with T; ~165 GPa @ 300°C |
|
Poisson's Ratio |
0.32 |
— |
|
Specific Heat (cp) |
427 J/(kg·K) |
20°C |
|
Thermal Conductivity (λ) |
21.8 W/(m·K) @ 20°C; ~26 W/(m·K) @ 300°C |
— |
|
Coeff. of Thermal Expansion (CTE) |
13.9 × 10⁻⁶ /K (20–100°C mean) |
— |
|
Electrical Resistivity |
0.511 – 0.547 µΩ·m |
20°C |
|
Curie Temperature (Tc) |
≈ 21 – 49°C (70 – 120°F, lot-dependent) |
May show weak ferroresponse if T < Tcand cold-worked |
|
Magnetic Behavior (Annealed, > Tc) |
Paramagnetic (µr≈ 1.0006 – 1.0012) |
— |
3. Mechanical Properties
Room-Temperature Minimum Requirements (ASTM B164 — Annealed Bar/ Rod):
|
Property |
Minimum (ASTM B164) |
Typical Annealed Range |
|---|---|---|
|
Tensile Strength (Rm) |
≥ 483 MPa (70 ksi) |
480 – 620 MPa |
|
Yield Strength (Rp0.2) |
≥ 193 MPa (28 ksi) |
195 – 240 MPa |
|
Elongation (A50mm) |
≥ 35% |
35 – 50% |
|
Hardness |
— |
60 – 80 HRB (110 – 150 HB) |
Effect of Cold Work (Typical):
|
Condition |
Tensile Rm(MPa) |
Yield Rp0.2(MPa) |
Elongation A (%) |
|---|---|---|---|
|
Annealed (0% CW) |
480 – 550 |
195 – 240 |
35 – 50 |
|
Cold Drawn ~15–20% |
585 – 655 |
380 – 450 |
20 – 30 |
|
Cold Drawn ~37% ("Half Hard") |
690 – 760 |
515 – 585 |
15 – 20 |
|
Cold Drawn ~60% ("Spring Temper") |
825 – 895 |
655 – 725 |
8 – 12 |
Monel 400 exhibits no ductile-to-brittle transition—toughness is retained to –196°C (liquid nitrogen). At –196°C, Rmtypically rises to ~655–725 MPa, Rp0.2~345–415 MPa, elongation remains ≥ 30%.
Elevated-Temperature Trend (Annealed, Typical):
300°C: Rm≈ 450 MPa, Rp0.2≈ 170 MPa
500°C: Rm≈ 310–345 MPa, Rp0.2≈ 140–170 MPa
Long-term load-bearing above 400°C is not recommended (low creep strength); continuous oxidizing service limit = 538°C (1000°F), steam service = ≤ 315–370°C.
4. Corrosion Resistance Characteristics
Seawater / Brine: Uniform corrosion rate < 0.025 mm/a in flowing seawater; immune to chloride-induced SCC (unlike 300-series SS); resistant to pitting/crevice in normal velocity seawater (PREN ≈ 22–25). Widely used for propeller shafts, pump casings, condenser tubes.
Hydrofluoric Acid (HF): Tolerates all concentrations up to boiling point—unique among common structural alloys. Standard material for HF alkylation units, HF storage/distribution.
Alkalis (NaOH / KOH): Resistant to ≤ 70% NaOH up to ~150°C; at higher concentrations/temperatures, Nickel 200/201 preferred.
Reducing Acids (H₂SO₄, HCl, H₃PO₄): Fair in dilute, de-aerated solutions at moderate T; attacked by oxidizing species (Fe³⁺, Cu²⁺, dissolved O₂).
Oxidizing Acids (HNO₃, hot conc. H₂SO₄ with oxidant): Not recommended—rapid attack.
Atmospheric / Steam: Excellent resistance in rural, urban, and marine atmospheres; in high-T steam > 370°C long-term, intergranular O penetration possible.
5. Fabrication, Welding & Heat Treatment
Hot Working: 650 – 1150°C; finish > 870°C; air or water quench.
Cold Working: Readily cold-worked (work-hardening rate between carbon steel and 300 SS). Intermediate anneals at 704–871°C → rapid quench restore ductility.
Solution Anneal: 704 – 982°C, hold per section, rapid water quench or air blast on thin sections. Removes cold-work stress, restores isotropic properties.
Welding: Excellent weldability by GTAW/GMAW/SMAW. Recommended filler: ERNiCu-7 (AWS A5.14) for GTAW/GMAW; ENiCu-7 (AWS A5.11) for SMAW. No preheat or post-weld heat treatment required for corrosion resistance (unlike Ti-stabilized alloys, Monel 400 has no sensitization issue—but PWHT may be used for dimensional stress relief ≤ 400°C).
Machining: Work-hardens; use rigid setup, low speed, high feed, coolant, and carbide inserts to avoid "riding" the hardened surface.
6. Typical Applications by Industry
|
Industry |
Component / Service |
Key Alloy Benefit Used |
|---|---|---|
|
Marine / Offshore |
Propeller shafts, pump impellers/casings, seawater valve stems, desalination heat exchanger tubing, sonar dome fasteners |
Seawater SCC immunity + low uniform corrosion + biofouling resistance |
|
Chemical / Petrochemical |
HF alkylation unit piping/reactors, H₂SO₃ / H₃PO₄ handling (non-oxid.), caustic evaporators (≤70% NaOH, ≤150°C) |
HF tolerance + moderate acid resistance + weldability |
|
Oil & Gas |
Downhole chemical injection lines (corrosive packer fluids), topside process piping in sour/Cl⁻ env. (NACE MR0175 compliant when annealed & ≤ HRC 35) |
Cl⁻ SCC immunity + CO₂ corrosion resistance |
|
Power / Nuclear |
Feedwater heater tubing (Cu-Ni adjacent systems), nuclear fuel reprocessing equipment (HF/HNO₃ mix tolerant) |
Radiation stability + HF tolerance |
|
Cryogenic / LNG |
Valve stems, seals, bellows |
No DBTT; tough at –196°C |
|
Musical / Architectural |
French horn and trumpet valve slides (historical), decorative marine hardware |
Work-hardening + golden hue + seawater patina |
7. Applicable ASTM / ASME / AMS Standards
|
Product Form |
Standard |
|---|---|
|
Plate, Sheet, Strip |
ASTM B127 / ASME SB-127 |
|
Bars, Rods, Wire |
ASTM B164 / ASME SB-164 |
|
Seamless Pipe & Tube (process) |
ASTM B165 / ASME SB-165 |
|
Condenser & Heat Exchanger Tube |
ASTM B163 / ASME SB-163 |
|
Welded Pipe |
ASTM B725 / ASME SB-725 |
|
Forgings (Flanges, Rings) |
ASTM B564 / ASME SB-564 |
|
Fittings |
ASTM B366 / ASME SB-366 |
|
Aerospace (Bar/Forging) |
AMS 4675, AMS 4544 (sheet/strip) |
|
Welding Consumables |
AWS A5.14 ERNiCu-7; AWS A5.11 ENiCu-7 |
8. Summary of Key Benefits
✅ Immune to chloride stress corrosion cracking (unlike 304/316 SS)
✅ Excellent resistance to flowing seawater and brackish water
✅ Only common structural alloy tolerant of all concentrations of hydrofluoric acid to boiling
✅ Good ductility and toughness from –196°C to 538°C service range
✅ Weldable with standard Ni-Cu filler; no PWHT for corrosion
✅ Can be cold-worked to high strength levels (up to ~895 MPa UTS)
Monel 400 is not a high-temperature creep-resistant alloy (select Incoloy 800HT / Inconel 600 series for that) and is not suitable for strong oxidizing acids or hot concentrated nitric acid (select Hastelloy C-276/C-22 or Inconel 600). Within its envelope—seawater, HF, moderate alkalis, and Cl⁻-containing non-oxidizing process streams—it remains the benchmark Ni-Cu alloy worldwide.
