The Properties and Composition of Monel 400

Monel 400 is a nickel-copper alloy that is resistant to corrosion in many environments. It consists of two crystalline solids that form a single new solid.

Monel was the brainchild of Robert Crooks Stanley of the International Nickel Company. Patented in 1906, it was named for the president of the company, Ambrose Monell. The second "L" was removed from the name of the metal because it was not possible to patent a person's name at that time.

Overview

There are multiple variations of Monel alloys, starting with Monel 400, which contains at least 63% nickel, between 29% and 34% copper, between 2% and 2.5% iron, and between 1.5% and 2% manganese. Monel 405 adds no more than 0.5% silicon, and Monel K-500 adds between 2.3% and 3.15% aluminum and between 0.35% and 0.85% titanium. These and other variations all are valued for their resistance to attack by acids and alkalis, as well as for their high mechanical strength and good ductility.

Monel 400 contains the same quantity of nickel and copper as is found in a naturally occurring nickel ore in Ontario, Canada. It has high strength and can be hardened only by cold working. Due to its resistance to deterioration, Monel 400 is most often used in parts found in marine and chemical environments.

While it is a very useful metal, it is cost-prohibitive in most applications. Monel 400 costs five to 10 times as much as ordinary nickel or copper. As a result, it is used rarely—and only when no other metal could do the same job. As an example, Monel 400 is one of the few alloys that maintains its strength in sub-zero temperatures, so it is used in those circumstances.

Fabrication

According to Azom.com, machining techniques used for iron alloys can be used for Monel 400, though it is difficult because it work-hardens during the process. If hardening Monel 400 is the goal, cold-working, using soft die materials, is the only option. Through cold-working, mechanical stress is used instead of heat to change the shape of the metal.

Azom.com recommends gas-arc welding, metal-arc welding, gas-metal-arc welding and submerged-arc welding for Monel 400. When hot-working Monel 400, temperatures should range from 648-1,176 degrees Celsius (1,200-2,150 degrees Fahrenheit). It can be annealed at 926 degrees Celsius (1,700 degrees Fahrenheit).

Applications

Because of its resistance to acids, alkalis, seawater, and more, Monel 400 often is used in applications where corrosion might be a concern. According to Azom.com, this includes marine environments where fixtures, valves, pumps, and piping systems are needed.

Other applications sometimes include chemical plants, including environments using sulfuric acid and hydrofluoric acid.

Another area where Monel 400 is popular is the eyeglass industry. It is among the most popular materials in use for frames, specifically for components along the temples and over the bridge of the nose. According to Eyecare Business, the combination of strength and resistance to corrosion makes it useful for frames. A drawback, however, is that it is difficult to shape, limiting its usefulness for some frames.

Drawbacks

Though valuable in many applications, Monel 400 is not perfect. While resistant to corrosion in many ways, it cannot withstand nitric oxide, nitrous acid, sulfur dioxide, and hypochlorites. So, Monel 400 should not be used in environments where it would be exposed to those elements.

Monel 400 also is susceptible to galvanic corrosion. This means aluminum, zinc, or iron fasteners can quickly corrode if they are used with Monel 400.

Standard Composition of Monel 400

Mostly nickel and copper, the standard composition of Monel 400 includes:

• Nickel (plus cobalt): 63% minimum
• Carbon: 0.3% maximum
• Manganese: 2.0% maximum
• Iron: 2.5% maximum
• Sulfur: 0.024% maximum
• Silicon: 0.5% maximum
• Copper: 29-34%

Properties of Nickel-Copper Alloy Monel 400

The following table describes the properties of Monel 400. Relative to other similar metals, it unusually is strong and corrosion-resistant.

Property	Value (Metric)	Value (Imperial)
Density	8.80*103 kg/m3	549 lb/ft3
Modulus of Elasticity	179 GPa	26,000 ksi
Thermal Expansion (20ºC)	13.9*10-6º C-1	7.7*10-6 in/(in*ºF)
Specific Heat Capacity	427 J/(kg*K)	0.102 BTU/(lb*ºF)
Thermal Conductivity	21.8 W/(m*K)	151 BTU*in/(hr*ft2*ºF)
Electric Resistivity	54.7*10-8 Ohm*m	54.7*10-6 Ohm*cm
Tensile Strength (Annealed)	550 MPa	79,800 psi
Yield Strength (Annealed)	240 MPa	34,800 psi
Elongation	48%	48%
Liquidus Temperature	1,350º C	2,460º F
Solidus Temperature	1,300º C	2,370º F

Sources: www.substech.com, www.specialmetals.com