Properties and Applications of Molybdenum

Molybdenum alloys have an exceptional combination of properties, including high thermal & electrical conductivity, high strength at elevated temperatures, and low thermal expansion. Molybdenum metal and its alloys are the preferred choice in many demanding and specialized applications.

Initially molybdenum was perplexed with lead and graphite ore, until it was prepared in 1781 by Hjelm in the impure state. Molybdenum does not emerge native, and is extracted mostly from MoS₂ (molybdenite). Additional minor commercial ores of molybdenum are wulfenite (PbMoO₄) and powellite (Ca (MoW) O4). It can also be produced from tungsten and copper operations as a by-product.

Molybdenum metal is manufactured from the powder made by the hydrogen reduction of purified ammonium molybdate or molybdic trioxide. This metal is very hard and silvery-white in nature. However, it is more ductile and softer than tungsten and is voluntarily worked or drawn into very fine wire. It cannot be toughened by heat treatment, only by working. It displays a very high melting point and high elastic modulus. A molybdenum metal form an oxide at temperature above 750ºC, that evaporates the moment it is formed and its corrosion resistance is very high. Its heat conductivity is twice as that of iron and it has a low thermal expansion. It is one of the few metals that have a little resistance to hydrofluoric acid.

Major Properties of Molybdenum:

• It is a refractory metal usually used in high temperature applications.
• It has low co-efficient of thermal expansion (5.2x10-6 m/m/°C) which is about half that of the steels.
• Good electrical conductivity.
• High density (10.2 g/cm3)
• Good firmness and rigidity, greater than that of steel (Young's Modulus 317MPa).
• High melting point (2617°C).
• Good thermal conductivity.
• Good hot strength.
• Good ductility and strength at room temperature.

Molybdenum products have the great ability to withstand high temperatures and maintain strength. These conditions are responsible for the verity that molybdenum finds most of its application at elevated temperatures. It can also work at temperatures above 1200°C (within non oxidizing conditions), which is higher than nickel and steels based super alloys.

Rapid oxidation takes place when molybdenum metal is exposed to high temperatures, above 750°C in air. In these conditions, the base metal is attacked and the oxide layer sublimes. Therefore, machining molybdenum can be best performed in inert of vacuum environments.

Applications of Molybdenum:

Molybdenum metal is used in:

• Alloying agent contributing toughness, hardenability to tempered/quenched steels. It also enhances the strength of steels at high temperatures.
• In stainless steels and nickel based alloys it imparts corrosion resistance and heat resistance to chemical solution.
• Nuclear energy applications, as aircraft parts and missile (where high-temperature resistance is very important).
• Electrodes for forehearths and electrically heated glass furnaces.
• As a filament material in electrical/electronic applications.
• As a support members in light bulbs and radio.
• Arc resistant electric contacts.
• In thermocouple sheaths.
• As a catalyst in the refining of petroleum.
• Flame and corrosion-resistant coatings for other metals

Molybdenum Compounds

Molybdenum and its compounds are used in:

• Its selenites and sulphide are used as a high temperature lubricant in favour to petroleum based oils, because of its greater high temperature resistance.
• Na2MoO4 (Sodium molybdate) in the anhydrous form and is used as a dry powdered fertilizer.
• CaMoO4 (Calcium molybdate), molybdic oxide, Molyte, molybdenum-chromium are used as sources of molybdenum for steels.