When you compare the price of Monel 400 with the price of stainless steel, you find that the former is much more expensive. (Stainless steel is covered in An Introduction to Stainless Steels.) Have you ever wondered why the Monel alloy is so much more expensive? In order to ascertain the reasons, we must study its composition, characteristics and the critical applications that drive the demand for this expensive alloy.
Monel alloy was first developed by Robert Crooks Stanley at the International Nickel Company and patented in 1906. The name of the president of the company was given to the new invention. In due course, various alloys of the Monel group were developed for different applications.
Composition of Monel Alloys
Monel is the name given to a group of alloys (401, R405, K-500, 400, 404 and 402) containing nickel and copper along with iron, silicon, manganese and titanium in some cases. Although all these alloys are generally distinguished by their good corrosion resistance, each of them has some distinct characteristics and usages.
Monel 404, for example, has a lower nickel content (52%) with aluminum (0.05%) and high copper (47%). Monel R405 has a content very similar to Monel 400, except for the absence of titanium.
Monel K500 contains aluminum (0.3%) and titanium (0.35% to 0.85%). This alloy has a lower coefficient of friction and higher wear resistance compared to other Monel alloys. It also has superior mechanical properties such as strength and hardness up to a temperature of 600°C (1,112°F).
Monel 400 has the same proportion of copper and nickel as naturally found in the nickel ore present in Sudbury mines, in Ontario, Canada. Because this alloy is much more expensive compared to even stainless steel, it is only used in critical applications where inexpensive materials cannot be substituted.
Monel 400 contains copper in the range of 28% to 34% and a minimum of 63% nickel. It also generally contains iron, manganese, silicon, carbon and minute quantities of sulfur and titanium.
Characteristics of Monel 400
Monel alloys are stronger than either nickel or copper alone. Monel 400 contains nickel and copper along with carbon, iron, manganese, silicon and titanium. This composition helps the Monel alloy to retain its superior corrosion resistance along with toughness and mechanical strength over a wide range of temperatures, generally up to 400°C (752°F). This characteristic is a critical requirement for aerospace applications because the operating temperatures can vary widely. Monel 400 is used to make rivets for aircraft with an aluminum outer skin.
At cryogenic temperatures, Monel 400 retains mechanical properties such as hardness and strength, while there is a slight reduction in ductility. (For more about the strength of materials, read An In-Depth Look at Tensile Strength.)
Monel 400 has excellent corrosion resistance in steam at elevated temperatures and in hot saltwater flowing with high velocities. It can prevent stress corrosion cracking (SCC) in freshwater, and its durability in turbulent seawater justifies its cost. Its resistance to hydrofluoric and hydrochloric acids is subject to deaeration. It is normally resistant to acidic food products; however, it does not resist nitric acid, which is oxidizing.
Monel 400 cannot be easily machined because it is work-hardened during machining. Hence, it is preferable to cut and turn the alloy at slower speeds with very low feed rates. This inevitably results in higher machining costs.
The electrical conductivity of Monel 400 is 34% IACS (International Annealed Copper Standard). It retains some ductility even when it is frozen to liquid hydrogen temperatures. It is also weakly magnetic.
Monel 400 has good weldability, and can be welded through the submerged arc, gas metal arc and gas tungsten arc welding processes. Monel 60 filler metal used for welding can ensure that the welded joint has the same hardness, mechanical strength and corrosion protection as the parent metal. (Related reading: Causes and Prevention of Corrosion on Welded Joints.)
Apart from joining by welding, Monel 400 can be joined by soldering or brazing.
Limitations of Monel 400
Monel 400 cannot resist hypochlorites, sulfur dioxide, nitric acid or nitric oxide. It is prone to galvanic corrosion as well.
Applications for Monel
The various grades of Monel alloy are distinguished by their unique uses.
- The addition of sulfur to Monel R405 helps to improve machineability. This alloy grade is used to make valve components and fasteners.
- Monel 404 is generally suitable for electronic applications because it has low permeability. It is amenable to brazing. It contains a higher proportion of copper (47%) with somewhat lower nickel content.
- Monel 401 has a low thermal coefficient of electrical resistance. It is generally suitable for electrical and electronic applications.
- Monel 400 has found applications in petroleum extraction and processing, chemical handling and marine engineering. Pumps and heat exchangers made of Monel 400 are used in many critical plants operating in corrosive environments. This alloy has good corrosion resistance in most alkalis and acids. Valves made of Monel metal include check valves, safety valves, injection valves, flow control valves, plug valves, ball valves, globe valves and piston valves. Casted and welded valves are used in critical marine and petroleum refining plants.
- Monel 400 is used as a sheath (thin covering) for the corrosion prevention of oil rigs and other offshore mild steel structures in splash zones. A thin 3 mm sheath is constructed by welding through the MIG welding process. Monel 400 shows excellent resistance to erosion and corrosion damage in hot turbulent seawater and steam.
- Monel 400 is used in both onshore and offshore petroleum extraction facilities, including in process control systems, control valves and pumps. Good durability in highly corrosive offshore extraction systems makes Monel 400 parts cost-effective and reliable for applications such as gas storage tanks, propeller shafts, valve seatings, stems and the drilling collars of petroleum drilling systems. Historically, this alloy was also used to make dog tags for the military.
- The Monel 400 alloys are also used to manufacture springs and wire products that are used both in acidic and alkaline environments. This material has superior ductility as well as thermal conductivity.
- Monel is also the chosen material for eyeglass frames due to it corrosion resistance and strength.
Current and Future Market Demand for Monel 400
The Asia Pacific metal markets are the dominant buyers of Monel alloys. The marine, automotive, petroleum and chemical industries in the Asia Pacific region drive the growing demand for Monel alloys. The aerospace, automotive, petroleum refining and marine sectors of Indonesia, South Korea, India and China are pushing up the demand, followed by Europe and North America. However, in the near future the demand may be affected due to a looming global recession in the aviation and oil & gas sectors due to the COVID-19 pandemic. In this economic environment, the demand may shift because Monel 400 costs four times as much as pure nickel and six to eight times as much as 304 stainless steel, on average.
Conclusion
Monel 400 contains at least 62% of the expensive metal nickel, and its characteristics make it a unique alloy that cannot be substituted by any cheaper material. It is the first choice of designers in the aerospace, marine and oil & gas industry for a strong, corrosion-resistant material that performs well at high temperatures in highly corrosive environments.