Monel alloy is a corrosion-resistant nickel alloy commonly utilized in marine and electronic components. It is significantly more expensive than stainless steel, often costing over ten times as much. This may lead you to ask:
Why choose the pricier Monel alloy over stainless steel, given that both are corrosion-resistant metals?
What impact does the chemical composition of Monel alloys have?
In this article, we will explore the various aspects of Monel alloys in detail.
Monel refers to a variety of nickel-copper alloys. Essentially, Monel alloys are primarily composed of nickel and copper, though the specific composition can vary among different grades. Here are some common Monel grades:
– Monel 400
– Monel 401
– Monel 404
– Monel R-405
– Monel K-500
– Monel 502
Each grade possesses unique performance characteristics, making them suitable for diverse applications. We will examine the differences between these grades in terms of chemical composition, mechanical properties, and physical properties.
Popular Grades of Monel Alloys
Monel 400 Alloy
MONEL® nickel-copper alloy 400 (UNS N04400 / W.Nr. 2.4360 and 2.4361) is a solid-solution alloy that can be hardened only by cold working. It has high strength and toughness over a wide temperature rangeand excellent resistance to many corrosive environments.
Alloy 400 is widely used in many fields, especially marine and chemical processing. Typical applications are valves and pumps; pump and propeller shafts; marine fixtures and fasteners; electrical and electronic components; springs; chemical processing equipment; gasoline and fresh water tanks; crude petroleum stills, process vessels and piping; boiler feedwater heaters andother heat exchangers; and deaerating heaters.
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.
Monel K-500 Alloy
MONEL® alloy K-500 (UNS N05500 / W.Nr. 2.4375) is a nickel-copper alloy which combines the excellent corrosion resistance of MONEL alloy 400 with the added advantages of greater strength and hardness. The increased properties are obtained by adding aluminum and titanium to the nickel-copper base, and by heating under controlled conditions so that submicroscopic particles of Ni3 (Ti, Al) are precipitated throughout the matrix. The thermal processing used to effect precipitation is commonly called age hardening or aging.
Typical applications for MONEL alloy K-500 products are chains and cables, fasteners and springs for marine service; pump and valve components for chemical processing; doctor blades and scrapers for pulp processing in paper production; oil well drill collars and instruments, pump shafts and impellers, non-magnetic housings, safety lifts and valves for oil and gas production; and sensors and other electronic components.
A useful characteristic of the alloy is that it is virtually nonmagnetic, even at quite low temperatures. It is possible, however, to develop a magnetic layer on the surface of the material during processing. Aluminum and copper may be selectively oxidized during heating, leaving a magnetic nickel-rich film on the outside of the piece. The effect is particularly noticeable on thin wire or strip where there is a high ratio of surface to weight. The magnetic film can be removed by pickling or bright dipping in acid, and the nonmagnetic properties of the material will be restored.
The combination of low magnetic permeability, high strength and good corrosion resistance has been used to advantage in a number of applications, notably oil-well surveying equipment and electronic components.
Chemical Composition of Monel Alloys
Monel is a nickel-copper alloy. The nickel element brings good structural stability and balanced mechanical properties to the alloy. Copper plays an excellent role in corrosion resistance, especially in flowing sea water.
| Composition | Content Range | Effect | Typical Alloy |
|---|---|---|---|
| Nickel | 40% ~ 70% | Stabilize the Austenitic Structure | Monel 400 |
| Copper | 22% ~ 56% | Corrosion Resistance, Electric Conductivity | Monel 401 |
| Aluminum | 0% ~ 3.5% | Strength | Monel K-500 |
| Titanium | 0% ~ 0.85% | Strength | Monel K-500 |
Monel Alloys Chemical Compositions Comparison Table:
| Elements (%) | Monel K-500 | Monel R-400 | Monel 400 |
| Nickel | 63 min. | 63 min. | 63 min. |
| Copper | 27-33 | 28-34 | 28-34 |
| Iron | 2 max. | 2.5 max. | 2.5 max. |
| Manganese | 1.5 max. | 2 max. | 2 max. |
| Almunium | 2.3 – 3.15 | 0 | 0 |
| Silicon | 0.5 max. | 0.5 max. | 0.5 max. |
| Sulfur | 0.01 max. | 0.025 – 0.060 | 0.24 max. |
| Titanium | 0.35 – 0.85 | 0 | 0 |
Mechanical Properties of Monel Alloys
Monel alloys can be classified into two broad categories based on their strengthening methods: solid solution strengthened alloys and precipitation strengthened alloys. These categories can be distinguished in two ways:
1. Naming Convention: Alloys with names starting with “4” are solid solution strengthened, such as Monel 400 and Monel R-405. Alloys starting with “5” are precipitation strengthened, such as Monel 502 and Monel K-500.
2. Chemical Composition: Precipitation strengthened alloys contain aluminum and titanium, which are key elements for this type of strengthening. Conversely, alloys without these elements are solid solution strengthened.
Solid solution strengthened Monel alloys offer better plasticity, making them ideal for applications like housings or pipes in marine environments and wires in electronic components. Precipitation strengthened alloys, on the other hand, provide higher strength, making them suitable for components that need to withstand pressure, such as fasteners in marine applications.
| Alloy | Category | Tensile Strength | Yield Strength | Elongation |
|---|---|---|---|---|
| MPa | MPa | % | ||
| Monel 400 | Solid Solution Strengthened Alloy | 480 | 170 | 35 |
| Monel 401 | Solid Solution Strengthened Alloy | 441 | 134 | 51 |
| Monel R-405 | Solid Solution Strengthened Alloy | 480 | 170 | 35 |
| Monel K-500 | Precipitation Strengthened Alloy | 1000 | 760 | 35 |
| Monel 502 | Precipitation Strengthened Alloy | 1158 | 1034 | 15 |
Physical Properties of Monel Alloys
In Monel alloys, density and Curie temperature are the two most critical physical properties.
Density influences the weight of the alloy. Both copper and nickel, key components of Monel alloys, have relatively high densities. Consequently, solid solution strengthened Monel alloys exhibit higher densities. In contrast, precipitation strengthened alloys include lower density metals such as aluminum and titanium, resulting in a lower overall density.
| Physical Properties | Monel 400 | Monel R-405 | Monel K- 500 |
| Density (g/cm3) | 8.8 | 8.8 | 8.44 |
| Curie Temperature | 21 – 49 | 28 – 34 | – |
| Electrical Resistivity @ 100 C (Annealed) (µΩ.m) | 0.537 | 0.537 | 0.618 |
| Thermal Linear Expansion @ 100 C (Annealed) (µm/m.°C) | 14.2 | 14.2 | 13.7 |
| Thermal Conductivity @ 100 C (Annealed) (W/m.°C) | 24 | 24 | 19.4 |
| Specific Heat @ 100 C (Annealed) (J/kg.°C) | 445 | 445 | 448 |
Applications of Monel Alloys
Monel is employed in various applications that demand high corrosion resistance and toughness. As a result, it is frequently utilized in the marine and chemical processing industries for the following equipment:
– Propeller shafts
– Fasteners
– Heat exchangers
– Process vessels
– Valve stems
– Process piping
– Petroleum stills
– Pumps and valves
– Commercial screw machines
– Chains and cables
FAQs
1.Is Monel a superalloy?
By definition, a superalloy is designed for use at high temperatures. Monel alloys, which contain copper, are not suitable for prolonged high-temperature use because copper loses its oxidation resistance at elevated temperatures. Thus, while Monel alloys are classified as nickel alloys, they do not qualify as superalloys.
2.What is the difference between Monel and copper-nickel alloys?
Monel is a nickel-copper alloy with a higher nickel content compared to copper. This makes Monel a nickel alloy. In contrast, copper-nickel alloys have a higher copper content than nickel, categorizing them as copper alloys.
Conclusion
Monel is a corrosion-resistant nickel-copper alloy predominantly used in marine and electronic applications. Its corrosion resistance surpasses that of stainless steel, though it is significantly more expensive.
Despite its cost, Monel remains irreplaceable. The choice of Monel alloy grade should be based on the specific strength requirements and magnetic properties needed for practical applications.
We specialize in producing high-performance Monel alloys. Please feel free to contact us for any of your requirements.
Monel® is a registered trademark of Special Metals Corporation.
