Monel Alloy 400

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UNS N04400

Other common names: Alloy 400

Monel 400 is a nickel-copper alloy (about 67% Ni – 23% Cu) that is resistant to sea water and steam at high temperatures as well as to salt and caustic solutions. Alloy 400 is a solid solution alloy that can only be hardened by cold working. This nickel alloy exhibits characteristics like good corrosion resistance, good weldability and high strength. A low corrosion rate in rapidly flowing brackish or seawater combined with excellent resistance to stress-corrosion cracking in most freshwaters, and its resistance to a variety of corrosive conditions led to its wide use in marine applications and other non-oxidizing chloride solutions. This nickel alloy is particularly resistant to hydrochloric and hydrofluoric acids when they are de-aerated. As would be expected from its high copper content, alloy 400 is rapidly attacked by nitric acid and ammonia systems.

Monel 400 has great mechanical properties at subzero temperatures, can be used in temperatures up to 1000° F, and its melting point is 2370-2460° F. However, alloy 400 is low in strength in the annealed condition so, a variety of tempers may be used to increase the strength.

What are the characteristics of Monel 400?

• Resistant to seawater and steam at high temperatures
• Excellent resistance to rapidly flowing brackish water or seawater
• Excellent resistance to stress corrosion cracking in most freshwaters
• Particularly resistant to hydrochloric and hydrofluoric acids when they are de-aerated
• Offers some resistance to hydrochloric and sulfuric acids at modest temperatures and concentrations, but is seldom the material of choice for these acids
• Excellent resistance to neutral and alkaline salt
• Resistance to chloride induced stress corrosion cracking
• Good mechanical properties from sub-zero temperatures up to 1020° F
• High resistance to alkalis

Chemical Composition, %

Corrosion Resistant Monel 400

Alloy 400 is virtually immune to chloride ion stress corrosion cracking in typical environments. Generally, its corrosion resistance is very good in reducing environments, but poor in oxidizing conditions. It is not useful in oxidizing acids, such as nitric acid and nitrous. Nevertheless, it is resistant to most alkalis, salts, waters, food products, organic substances and atmospheric conditions at normal and elevated temperatures.

This nickel alloy is attacked in sulfur-bearing gases above approximately 700° F and molten sulfur attacks the alloy at temperatures over approximately 500° F.

Monel 400 offers about the same corrosion resistance as nickel but with higher maximum working pressures and temperatures and at a lower cost due to its superior ability to be machined.

In what applications is Monel 400 used?

• Marine engineering
• Chemical and hydrocarbon processing equipment
• Gasoline and freshwater tanks
• Crude petroleum stills
• De-aerating heaters
• Boiler feed water heaters and other heat exchangers
• Valves, pumps, shafts, fittings, and fasteners
• Industrial heat exchangers
• Chlorinated solvents
• Crude oil distillation towers

Fabrication with Monel 400

Alloy 400 can easily be welded by gas-tungsten arc, gas metal arc or shielded metal arc processes using appropriate filler metals. There is no need for post weld heat treatment, however, thorough cleaning after welding is critical for optimum corrosion resistance, otherwise there is the risk of contamination and embrittlement.

Finished fabrications can be produced to a wide range of mechanical properties when proper control of the amount of hot or cold working and the selection of appropriate thermal treatments is done.

Like most other nickel alloys, Monel 400 is typically tough to machine and will work harden. However, excellent results can e obtained if you make the correct choices for tooling and machining.

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ASTM Specifications