What Is Monel 400? Nickel-Copper Alloy
Chemical Composition and Properties
| Property | Value |
|---|---|
| UNS designation | N04400 |
| Common names | Monel 400, Alloy 400 |
| Nickel | 63.0-70.0% |
| Copper | 28.0-34.0% |
| Iron | 2.50% max |
| Manganese | 2.00% max |
| Carbon | 0.30% max |
| Silicon | 0.50% max |
| Sulfur | 0.024% max |
| Tensile strength (min) | 483 MPa (70 ksi); hot finished |
| Yield strength (min) | 172 MPa (25 ksi); hot finished |
| Elongation (min) | 35% |
| Hardness | Typically 110-160 HBW |
| Density | 8.80 g/cm3 |
| Max service temp | 480°C (900°F); in air |
| Heat treatment | Annealed (870°C, air cool) |
Corrosion Resistance
The high nickel-copper composition gives Monel 400 a unique corrosion resistance profile:
| Environment | Resistance |
|---|---|
| Hydrofluoric acid (all concentrations) | Excellent; standard HF material |
| Seawater (flowing) | Excellent; resists biofouling |
| Brackish water | Excellent |
| Reducing acids (H2SO4, H3PO4) | Good at low concentrations |
| Alkaline solutions | Excellent |
| Fluorine and HF gas | Excellent up to 480°C |
| Steam | Good up to 250°C |
| Oxidizing acids (HNO3) | Poor; avoid |
| Chloride SCC | Resistant |
Monel 400 is vulnerable to oxidizing media. Nitric acid, ferric chloride, and highly aerated acid solutions cause rapid attack. In these environments, Hastelloy C-276 or Inconel 625 is required instead.
ASTM Specifications for Piping
| Product Form | ASTM Specification |
|---|---|
| Forgings (flanges, fittings) | B564 |
| Seamless pipe and tube | B165 |
| Welded pipe | B725 |
| Plate, sheet, strip | B127 |
| Bar and rod | B164 |
| Castings | A494 M-35-1 |
Applications
- HF alkylation: the standard material for all piping, valve bodies, flanges, and fittings in hydrofluoric acid service (API 752)
- Marine systems: seawater intake piping, desalination evaporator tubes, shipboard fire main systems
- Chemical processing: fluorine production equipment, hydrogen fluoride handling
- Oil and gas: sour gas downhole components, production tubing in mildly sour wells
- Power generation: feedwater heater tubes in fossil and nuclear plants
Limitations
Monel 400 has moderate strength (172 MPa yield) compared to Inconel 625 (414 MPa) and is limited to approximately 480°C in air due to scaling. For higher strength applications, Monel K-500 (precipitation-hardened variant) is available. For more aggressive corrosive environments combining acids with oxidizers, Hastelloy C-276 is the better choice.
Testing
Mill test certificates per EN 10204 3.1 must report full chemistry and mechanical properties. NDT includes UT for forgings and hydrostatic testing per the applicable piping or flange standard. For sour service, NACE MR0175 compliance is verified via hardness testing (35 HRC max for annealed condition).
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