Monel is a group of alloys of nickel (from 52 to 68%) and copper, with small amounts of iron, manganese, carbon, and silicon. Monel is not a cupronickel alloy because it has less than 60% copper.
Stronger than pure nickel, Monel alloys are resistant to corrosion by many aggressive agents, including rapidly flowing seawater. They can be fabricated readily by hot- and cold-working, machining, and welding.
Monel is a solid-solution binary alloy.As nickel and copper are mutually soluble in all proportions, it is a single-phase alloy. Compared to steel, Monel is very difficult to machine as it work-hardens very quickly. It needs to be turned and worked at slow speeds and low feed rates. It is resistant to corrosion and acids, and some alloys can withstand a fire in pure oxygen. It is commonly used in applications with highly corrosive conditions. Small additions of aluminium and titanium form an alloy (K-500) with the same corrosion resistance but with much greater strength due to gamma prime formation on aging. Monel is typically much more expensive than stainless steel.
Monel alloy 400 has a specific gravity of 8.80,a melting range of 1300–1350 °C, an electrical conductivity of approximately 34% IACS, and (in the annealed state) a hardness of 65 Rockwell B. Monel alloy 400 is notable for its toughness, which is maintained over a considerable range of temperatures.
Monel alloy 400 has excellent mechanical properties at subzero temperatures. Strength and hardness increase with only slight impairment of ductility or impact resistance. The alloy does not undergo a ductile-to-brittle transition even when cooled to the temperature of liquid hydrogen. This is in marked contrast to many ferrous materials which are brittle at low temperatures despite their increased strength.
In addition to Monel's corrosion resistance, it is also known for its good weldability; it is easily welded by most conventional processes, joining it to itself or other alloys. It is very strong-in fact, much stronger than steel. The ability to easily weld Monel is a key factor for its use in applications such as chemical processing equipment, marine components, and more.
Monel is often welded via gas tungsten arc, gas metal arc, and submerged arc welding. In these cases, Monel filler metal 60 is sometimes used, which has the same high strength and corrosion resistance of Monel alloy. As there are different compositions of Monel (which we will cover in the next of this three-part series), specific characteristics and usage of fillers depend on the Monel being used. The most often sought after and required grade of Monel is Monel 400. Monel alloy 400 displays the same proportions of nickel and copper, found naturally in the nickel ore from certain mines.
In addition to welding, Monel can be brazed and soldered. Monel can be used in different forms and shapes, from wires to flat sheets, fittings, and tubing. Interestingly, because of Monel's corrosion resistance and weldability, it has been the metal of choice not only for obvious applications, such as chemical and marine equipment, but for many common, yet surprising products. It was used for military dog tags in World War I and II, as the roofing of New York City's original Pennsylvania Station, and in parts of the Clock of the Long Now, currently under construction and designed to run for 10,000 years
