Molybdenum is a metal that is almost always part of an alloy and as such is not like many other metals. The primary reason for its inclusion here is its functional rather than aesthetic appeal. The most notable performance characteristic of molybdenum is that it has one of the highest melting points of all metals and, together with carbon, is one of the most effective hardening elements for steel alloys. Like tungsten, which it is often used to replace, it increases the hardness of steel, but a smaller amount is required to achieve the same hardness.
Molybdenum and vanadium are used in small amounts in steel to increase its tensile strength and toughness. It also increases the fatigue-resistance of steel, and this contributes to the performance of the Global® range of kitchen knives. These ‘designer tools’ borrow from the traditions of Japanese Samurai swords, where each knife is handcrafted to form a seamless construction. The molybdenum/vanadium stainless steel is ice-tempered to produce a hard, razor sharp blade that resists stains and corrosion.
For its application in the knives, it is important to understand its function. In stainless steels, corrosion resistance usually comes from the chromium content, which spontaneously forms a thin, protective inert film on the surface of the steel. Molybdenum enhances this film by making it stronger and helping it to reform quickly if it is damaged and increases the pitting and crevice corrosion resistance of stainless steels.
Image: Global® knife
Key features
•High tensile strength up to high temperatures
•Good corrosion resistance
•High stiffness
•Ductile
•About 34% of electrical conductivity of copper
•High melting temperature of 2610ºC (4753ºF)
Sources
Most molybdenum is produced in the United States, with Norway, China, Chile, Mexico, and Peru also being producers. It is mined as a principal ore, but is also recovered as a by-product of copper and tungsten mining.
Sustainability issues
Estimates by the US Geological Survey in 2012 put known reserves of molybdenum at around 20 million tonnes. They also reported that ‘molybdenum in the form of molybdenum metal or superalloys is recovered, but the amount is small. Although molybdenum is not recovered from scrap steel, recycling of steel alloys is significant, and some molybdenum content is re-utilized. The amount of molybdenum recycled as part of new and old steel and other scrap may be as much as 30% of the apparent supply of molybdenum.’ Compared with other heavy metals it has low toxicity, a reason why it can be used in food processing equipment.
Production
Molybdenum’s main application areas are where it is used as an alloy. In its unalloyed state its softness allows it to be drawn into thin sheets and wire. Molybdenum and its alloys can be formed by all common metalworking practices, such as bending, punching, stamping, drawing and spinning but unlike other metals, it can’t be hardened by heat treatment. A process known as vapour deposition, or thermal spraying, can also be used to apply the metal as protective coating. It can also be formed using metal injection moulding, welded and brazed, but welding is normally used only for applications not subjected to great stress.
Typical applications
One of the most interesting uses of molybdenum is for steel plates in armoured tanks, but one of its main uses is to replace tungsten in highspeed steels. It can be alloyed with steel to improve hardness at high temps, for example, for steel cutting tools; however, by far the biggest application of molybdenum is in structural steel, where it has been used as cladding for skyscrapers.
Derivatives
–Hyten steel containing nickel-chromium and molybdenum
–Duplex stainless steels
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–High tensile strength –Very high temperature resistance –Good stiffness –Food grade –Corrosion resistant |
–Cannot be hardened by heat treatment –Comparatively expensive |