Monel 400 is a nickel-copper alloy (approximately 67 percent nickel, 23 percent copper, plus iron, manganese, and other elements) known for its outstanding corrosion resistance, high strength, and excellent toughness over a wide temperature range. It is one of the most widely used nickel alloys, particularly in marine and chemical processing environments.

Key properties include good weldability and formability, resistance to seawater corrosion and stress corrosion cracking, and excellent performance in hydrofluoric, sulfuric, and hydrochloric acids under reducing conditions. Monel 400 also maintains useful mechanical properties from sub-zero temperatures up to about 480°C (900°F).

Typical mechanical properties (annealed):

- Tensile strength: 480–550 MPa

- Yield strength: 170–240 MPa

- Elongation: 35–50 percent

- Density: 8.8 g/cm³

Common standards: ASTM B127 (plate/sheet), B165 (tube), B164 (bar/forging).

Applications: marine engineering (pump shafts, propeller shafts, seawater piping), chemical plant equipment (valves, pumps, heat exchangers), oil and gas industry, and aerospace components.

Monel 400 is often compared to Monel K-500 (which adds aluminum and titanium for precipitation hardening). For most corrosion-resistant marine and chemical applications, Monel 400 is the standard choice. For higher strength with similar corrosion resistance, Monel K-500 is used.