Marine & naval
Propeller shafts, pump impellers, valve trim, condenser tubes. The Navy's alloy of choice for flowing-seawater service.
Ni-Cu alloy · UNS N04400
Monel 400
Nickel-copper alloy that bridges stainless and copper alloys for marine and chemical service. Resists seawater pitting, hydrofluoric acid, alkalis, and reducing media that destroy stainless. Standard for naval shafts, pump impellers, and chemical-process hardware.
UTS
550 MPa
Density
8.80 g/cc
Modulus
179 GPa
Mach.
35 /100
When to use
Pick Monel for marine and reducing acid service.
Monel 400 occupies the gap between stainless and Hastelloy. It resists seawater corrosion better than 316L (immune to chloride stress-corrosion cracking), handles HF acid and alkalis that destroy nickel-chromium alloys, and costs about a third of Hastelloy C-276. The U.S. Navy specs Monel for shipboard hardware where chloride-induced stress corrosion is the dominant risk.
The trade-off is more modest mechanical strength than precipitation-hardening alloys, and machinability that’s noticeably worse than 316L. For higher-strength variants, look at Monel K-500 (precipitation-hardenable, UTS 1100 MPa). For full chemical-process resistance, step up to Hastelloy C-276.
Strengths
Where it wins.
- Seawater workhorseExcellent in flowing seawater and immune to chloride stress-corrosion cracking; can pit under stagnant, low-velocity conditions. The Navy's go-to for in-water hardware.
- HF acid resistantOne of the few alloys that resists hydrofluoric acid at moderate concentrations.
- Reducing mediaHolds up in caustic, ammonia, and reducing acid streams that wreck nickel-chromium alloys.
- Cryogenic ductilityMaintains ductility down to liquid-helium temperatures.
- WeldableWelds with matching filler (ERNiCu-7) without post-weld heat treatment.
- Magnetic transitionCurie point near room temperature — non-magnetic above ~25 °C, weakly magnetic below.
Trade-offs
Where it lags.
- Lower strengthUTS 550 MPa — 316L territory. For higher strength, go to Monel K-500 (age-hardenable).
- Cost$15-25/lb stock — between stainless and Hastelloy. Justified for the corrosion behavior, not casual use.
- MachinabilityMach 35/100 — work-hardens like stainless but tougher. Cycle times ~2× 316L.
- Oxidizing acid weaknessDon’t use in nitric acid or strong oxidizers. Hastelloy C-276 handles those better.
- Galling vs steelWill gall against itself and steel. Use anti-seize on threads.
- Magnetic permeability shifts with temperatureMonel becomes weakly magnetic below ~25 °C — design around it for cryogenic instrumentation.
Specs
Mechanical, physical & chemical data.
Mechanical
Properties (annealed)
| Ultimate tensile strength | 550 MPa (80 ksi) |
|---|---|
| Yield strength (0.2% offset) | 240 MPa (35 ksi) |
| Elongation at break | 35% |
| Modulus of elasticity | 179 GPa |
| Shear modulus | 66 GPa |
| Hardness (annealed) | 110-150 HB |
| Hardness (cold drawn) | ranges to 250 HB |
| Charpy V-notch impact at 21 °C | 230 J |
Source: Special Metals datasheet. Cold-drawn bar: UTS up to 760 MPa, YS 480 MPa. Monel K-500 (age hardened): UTS 1100 MPa.
Physical & thermal
At room temperature
| Density | 8.80 g/cc (0.318 lb/in³) |
|---|---|
| Melting range | 1300 – 1350 °C |
| Specific heat | 0.427 J/g·°C |
| Thermal conductivity | 21.8 W/m·K |
| CTE (20–100 °C) | 13.9 µm/m·°C |
| Electrical resistivity | 0.547 µΩ·m |
| Curie temperature Magnetic below, non-magnetic above | ~25 °C |
Magnetic transition near room temp can affect instrumentation if the alloy is used near magnetic sensors at low temperatures.
Composition
Chemistry, weight %
| Nickel (Ni) | 63 min |
|---|---|
| Copper (Cu) | 28 – 34 |
| Iron (Fe) | 2.5 max |
| Manganese (Mn) | 2.0 max |
| Carbon (C) | 0.30 max |
| Silicon (Si) | 0.50 max |
| Sulfur (S) | 0.024 max |
The roughly 2:1 Ni:Cu ratio is the alloy's defining feature. Trace iron and manganese improve workability.
Specifications
Common purchase specs.
AMS 4675
Bar, forgings — annealed
AMS 4731
Bar — cold drawn
AMS 4544
Sheet, strip, plate
ASTM B164
Rod, bar, wire
ASTM B127
Sheet, strip, plate
MIL-N-24106
Rod, bar — naval applications
MIL-N-23229
Forgings — naval
DIN 17743 / 2.4360
European equivalent
Machining
How we cut it.
Monel 400 work-hardens like 316L stainless but is tougher. Sharp tools, generous feed, never let it dwell. Use cold-drawn bar where possible — it machines noticeably faster than annealed.
Cutting speed
75-150 SFM with carbide. Slower than stainless — toughness penalizes you.
Feed rate
Aggressive constant feed. Same rule as stainless — no rubbing, no dwell.
Tooling
Coated carbide (TiAlN) for production. Sharp polished edges critical.
Coolant
Flood with sulfurized cutting oil or high-pressure water-soluble. Heat removal matters.
Climb mill
Always. Conventional milling generates heat that work-hardens the next pass.
Threading
Single-point or HSS taps with sulfurized lubricant. Avoid tap breakage by lots of relief.
Cold drawn first
When tolerances allow, buy cold-drawn bar — it cuts noticeably faster than annealed.
Galling watch
Don’t let cutting tools rub Monel against Monel — anti-seize on collets and clamps.
Applications
Where Monel 400 shows up.
Chemical processing
HF acid plants, caustic streams, ammonia handling. Reducing-media equipment where stainless fails by stress corrosion.
Oil & gas
Wellhead components, downhole tools in sour service, tube hangers. Lower cost than Hastelloy when the chemistry doesn't require it.
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