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Nickel alloy based metal parts. Image Credit: Shutterstock.com/Sergey Ryzhov

All About Monel® Alloys: Definition, History, and Applications

Learn more about the history of this material and how it is used in manufacturing today.

Xomety X
By Team Xometry
October 24, 2022
 7 min read
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Monel® is an alloy of nickel and copper that was first developed for commercial use in 1905. It is mainly used for its excellent corrosion and temperature resistance and is especially valued for applications in the marine and chemical processing industries.

There are three widely used Monel alloys, namely 400, R-405, and K-500. Monel 400 and R-405 are similar in mechanical properties, but R-405 is easier to machine because of its higher sulfur content. K-500 is specially formulated with aluminum and titanium additions to increase its mechanical strength. Monel 400 and R-405 can only be work hardened, whereas K-500 can be both work hardened and precipitation hardened. 

This article will explore the history of Monel, the various grades that are available, and the mechanical properties of the different alloys.

What is Monel?

Monel refers to a group of nickel alloys that have copper as the main alloying element. In addition to these two primary metals, Monel alloys may also contain small amounts of iron, manganese, carbon, silicon, sulfur, aluminum, and titanium. The specific composition of a Monel alloy depends on its desired properties and applications. In general, Monel alloys are known for their high strength and resistance to corrosion in both fresh and saltwater while also having excellent high and low-temperature resistance. 

Who Was the Inventor of Monel?

Monel was first developed by Robert Crooks Stanley in 1905 while he was working for the International Nickel Company (Inco). The alloy was patented in 1906 and named after the owner of the company at the time, Ambrose Monell. The name Monel was registered as a trademark in 1921. The trademark is now owned by Special Metals Corporation, a company specializing in nickel and cobalt alloys. 

How Were Monel Alloys First Used?

Monel was first used in the 1920s in a range of applications, most notably in the manufacture of battleship components after WW1, due to its excellent resistance to seawater. Monel was also used as a decorative architectural material. It was often used for kitchen sinks and moving parts in brass instruments and strings for guitars and other string instruments.

What Are the Different Types of Monel?

The three main Monel alloys, each of which has several variations, are listed below:

  1. Monel 400: Monel has excellent resistance to attack by a number of corrosive agents, including seawater and strong acids - even hydrofluoric acid. It is a tough, high-strength alloy that can only be cold worked, not heat treated, for additional increases in strength. Monel 400 is machinable but presents some challenges. The alloy work hardens under the action of the cutting tool and requires close attention to feed and speed settings and tool wear.
  2. Monel R-405: This alloy has the same composition as Monel 400 except for a small increase in the sulfur content (from 0.024 to 0.06 %). The additional sulfur gives R-405 better machinability than the 400 grade by creating nickel-copper sulfide inclusions in the material that act as chip breakers.
  3. Monel K-500: This alloy is a high-strength variant of Monel with small amounts of aluminum and titanium added to the basic formula. It has all the advantages of standard Monel 400, but with increased hardness and strength from precipitation hardening heat treatments. 

What Are Monel's Applications?

Monel is used in a wide range of applications that require high corrosion resistance and toughness. For that reason, it is often used in the marine and chemical processing industries for the following types of equipment:

  1. Propeller shafts
  2. Fasteners
  3. Heat exchangers
  4. Process vessels
  5. Valve stems
  6. Process piping
  7. Petroleum stills
  8. Pumps and valves
  9. Commercial screw machines
  10. Chains and cables

What Are the Advantages of Monel Alloys?

Listed below are some of the key advantages of Monel:

  1. Corrosion Resistance: Highly corrosion resistant to acids and alkalis, surpassing other common corrosion-resistant alloys like AISI 316 stainless steel.
  2. Formability: Easy to form into a wide range of standard stock shapes like rods and pipes.
  3. Weldability: Can be easily welded using standard welding techniques.
  4. Temperature Resistance: Monel maintains its strength over a wide temperature range from sub-zero all the way to 480 °C.

What Are the Limitations of Monel Alloys?

Listed below are some of the key disadvantages of Monel:

  1. Work Hardening: During machining, Monel metal will harden, making subsequent cuts more difficult while also resulting in accelerated tool wear. Monel R-405 was specially formulated to improve machinability.  
  2. Pitting Corrosion: While resistant to fast-flowing sea water, Monel is susceptible to  pitting corrosion during prolonged exposure to seawater at low flow velocities.
  3. Stress Corrosion Cracking: Monel K-500 is susceptible to stress corrosion cracking.
  4. Galvanic Corrosion: Monel readily forms a galvanic cell when bolted to steel, which accelerates corrosion. 
  5. High Cost: Monel is a high-cost alloy and should only be used where cheaper materials will not work. 

What Are the Chemical Components of the Monel Alloys?

Monel alloys are approximately one-third copper (27 to 34%) and two-thirds nickel (63% min). Monel R-405 alloys have increased sulfur content, and Monel K-500 contains added aluminum and titanium, but very low sulfur. Table 1 below shows the chemical compositions of the various alloys:

Table 1: Monel Alloy Chemical Compositions
ElementMonel 400Monel R-405Monel K-500
Element

Nickel (wt %)

Monel 400

63 min.

Monel R-405

63 min.

Monel K-500

63 min.

Element

Copper (wt %)

Monel 400

28 - 34

Monel R-405

28 - 34

Monel K-500

27 - 33

Element

Manganese (wt %t)

Monel 400

2 max.

Monel R-405

2 max.

Monel K-500

1.5 max.

Element

Iron (wt %)

Monel 400

2.5 max.

Monel R-405

2.5 max.

Monel K-500

2 max.

Element

Sulfur (wt %)

Monel 400

0.024 max.

Monel R-405

0.025 to 0.060

Monel K-500

0.01 max.

Element

Silicon (wt %)

Monel 400

0.5 max.

Monel R-405

0.5 max.

Monel K-500

0.5 max.

Element

Aluminum (wt %)

Monel 400

0

Monel R-405

0

Monel K-500

2.3 to 3.15

Element

Titanium (wt %)

Monel 400

0

Monel R-405

0

Monel K-500

0.35 to 0.85

Table Credit: www.specialmetals.com

What Are the Physical Properties of the Monel Alloys?

Monel has a wide range of properties that make it a desirable material in a broad range of applications. The physical properties for the Monel alloys are listed in Table 2 below:

Table 2: Monel Alloy Physical Properties
Physical PropertiesMonel 400Monel R-405Monel K- 500
Physical Properties

Density (g/cm3)

Monel 400

8.8

Monel R-405

8.8

Monel K- 500

8.44

Physical Properties

Curie Temperature

Monel 400

21 - 49

Monel R-405

28 - 34

Monel K- 500

-

Physical Properties

Electrical Resistivity @ 100 C (Annealed) (µΩ.m)

Monel 400

0.537

Monel R-405

0.537

Monel K- 500

0.618

Physical Properties

Thermal Linear Expansion @ 100 C (Annealed) (µm/m.°C)

Monel 400

14.2

Monel R-405

14.2

Monel K- 500

13.7

Physical Properties

Thermal Conductivity @ 100 C (Annealed) (W/m.°C)

Monel 400

24

Monel R-405

24

Monel K- 500

19.4

Physical Properties

Specific Heat @ 100 C (Annealed) (J/kg.°C)

Monel 400

445

Monel R-405

445

Monel K- 500

448

What Are the Mechanical Properties of the Monel Alloys?

The Monel alloys have a wide range of mechanical properties which vary depending on the degree of hardening by cold working or precipitation hardening. The strength properties also differ depending on whether the material is supplied in the form of a plate, rod, or some other standard form. Table 3 below indicates the maximum strength properties of rod and bar stock for easy comparison of the various grades of Monel:

Table 3: Monel Alloy Mechanical Properties
Mechanical PropertiesMonel 400Monel R-405Monel K-500
Mechanical Properties

Hardening Method

Monel 400

Cold drawn & stress relieved

Monel R-405

As cold drawn

Monel K-500

Hot finished & aged

Mechanical Properties

Tensile Strength (MPa)

Monel 400

579 to 827

Monel R-405

586 to 793

Monel K-500

695 to 1310

Mechanical Properties

Yield Strength (0.2 % Offset) (MPa)

Monel 400

379 to 690

Monel R-405

345 to 724

Monel K-500

690 to 1034

Mechanical Properties

Elongation (%)

Monel 400

22 to 40

Monel R-405

15 to 35

Monel K-500

20 to 30

Mechanical Properties

Hardness (Rockwell)

Monel 400

20 to 85 B

Monel R-405

23 to 85 B

Monel K-500

27 to 38 C

Table Credit: www.specialmetals.com

What is the Melting Point of Monel?

The melting point will depend on the specific Monel alloy composition. Table 4 below indicates the melting temperatures of the main alloys:

Table 4: Monel Alloy Melting Points
Monel 400Monel R-405Monel K-500

Melting Temp (°C)

Monel 400

1300 to 1350

Monel R-405

1300 to 1350

Monel K-500

1315 - 1350

Table Credit: www.specialmetals.com

Are Monel Alloys Magnetic?

No, Monel is generally non-magnetic but can have useful magnetic properties under certain conditions. The Curie temperatures of Monel 400 and R-405 are close to ambient temperature, so they are not magnetic at higher temperatures. Furthermore, variations in composition even within the material specification tolerances can cause some lots to be magnetic, while others are not.

Monel K-500 is just different enough that it has a very low Curie temperature, is well below ambient, and can be considered to be reliably non-magnetic using normal material production methods. However, by selectively removing the aluminum and copper content from the surface of Monel K-500 through oxidation, a surface layer enriched with magnetic nickel can be formed. Wire and thin strip manufactured by this special process is used in electronic component applications.

What is the Difference Between Monel and K Monel?

K-500 Monel includes up to 3.15% aluminum and 0.85 % titanium as alloying elements to improve hardness and strength from precipitation hardening heat treatments.

Summary

This article presented Monel alloys, explained what they are, and discussed their history and how they are used in manufacturing. To learn more about Monel alloys, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

  1. Monel® is a registered trademark of Special Metals Corporation. 

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.