Originally molybdenum was confused with graphite and lead ore, and was not prepared till 1782 by Hjelm in the impure state. Molybdenum does not occur native, and is obtained mainly from molybdenite (MoS2). Other minor commercial ores of molybdenum are powellite (Ca(MoW)O4) and wulfenite (PbMoO4). It may also be recovered from copper and tungsten operations as a by-product.
The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. Molybdenum the metal is silvery-white, and very hard. However, it is softer and more ductile than tungsten and is readily worked or drawn into very fine wire. It cannot be hardened by heat treatment, only by working. It exhibits a high elastic modulus and a very high melting point. Above temperatures of 760°C (1400°F) molybdenum the metal forms an oxide that evaporates as it is formed and its resistance to corrosion is high. It has a low thermal expansion and its heat conductivity is twice that of iron. It is one of the few metals that has some resistance to hydrofluoric acid.
Data sheet
- Composition
Mo 99,95 % min - Melting point
2620 °C - Density
10,2 g/cm3 - Compressive strength
590-780 MPa - Hardness
200-360 HV10 - Enervation
0,2% 520-620 Mpa - Elasticity
4-18% - Thermal conductivity
a 20°C 142 w/m°C
a 1000°C 105 w/m°C
a 1500°C 88 w/m°C
Molybdenum is a metal with a melting temperature of 2620°C. It is mainly used in applications that utilize its exceptional stability at high temperatures that can reach 1900°C.
Molybdenum however starts recrystallizing between 800 and 1200°C depending on the degree of thermomechanical deformation during the product processing (rolling, forging, swaging etc.).
The recrystallization at high temperatures produces a reduction of its mechanical properties such as UTS, hardness and toughness. This reduction of the properties of pure Molybdenum can be reduced by small additions of elements such as Zirconium, Titanium, Boron and rare earth which increase the recrystallization temperatures and therefore the mechanical properties of such Molybdenum alloys.
Key Properties
Molybdenum is a refractory metal typically used in high temperature applications. Key properties include:
- Low co-efficient of thermal expansion (5.1×10-6 m/m/°C) which is about half that of most steels
- Good thermal conductivity
- Good electrical conductivity
- Good stiffness, greater then that of steel (Young’s Modulus 317MPa)
- High melting point (2615°C)
- Good hot strength
- Good strength and ductility at room temperature
- High density (10.2 g/cm3)
Its ability to withstand high temperatures and maintain strength under these conditions are responsible for the fact that molybdenum finds most of its application at elevated temperatures. In fact, it can work at temperatures above 1100°C (in non-oxidising conditions), which is higher than steels and nickel-based superalloys.
When exposed to temperatures in excess of 760°C in air rapid oxidation can result. Under these conditions, the oxide layer sublimes and the base metal is attacked. Thus, molybdenum performs best in inert of vacuum environments.
Applications
- Alloying agent – contributing hardenability, toughness to quenched/tempered steels. It also improves the strength of steels at high temperatures (red-hardness).
- In nickel-based alloys (such as Hastelloys®) and stainless steels it imparts heat-resistance and corrosion-resistance to chemical solutions.
- Electrodes for electrically heated glass furnaces and forehearths.
- Nuclear energy applications, as missile and aircraft parts (where high temperature resistance is vital).
- As a catalyst in the refining of petroleum.
- As a filament material in electronic/electrical applications.
- As a support members in radio and light bulbs.
- Arc resistant electric contacts.
- Thermocouple sheaths
- Flame- and corrosion-resistant coatings for other metals (generally arc deposited for metallising).
TZM Alloy
Introduction
Molybdenum-titanium-zirconium (TZM) alloys contain small amounts of titanium and zirconium doped with small amounts of very fine carbides. This alloy functions most efficiently in temperature ranges of 700-1400°C (1292-2552°F). TZM alloy is much stronger than pure molybdenum. The crystallization temperature and creep resistance of this alloy is also much higher than pure molybdenum.
Chemical Composition
The chemical composition of TZM alloy is given in the following table.
| Element | Content (%) |
| Molybdenum, Mo | 99.38-99.41 |
| Titanium, Ti | 0.5 |
| Zirconium, Zr | 0.08 |
| Carbon, C | 0.010-0.040 |
Data sheet
- Composition
Mo 99,3% – Ti 0,5% – Zr 0,08% - Melting point
2620 °C - Density
10,15 g/cm3 - Compressive strength
590-790 MPa - Hardness
200-320 DPH10 - Enervation
0,2% 515 – 690 MPa - Elongation
5-18% - Thermal conductivity
a 20°C 126 w/m°C
a 1000°C 98 w/m°C
a 1500°C 86 w/m°C - Coefficient of expansion
a 20°C 5,3 m/m°C
a 1000°C 5,8 m/m°C
a 1500°C 6,5 m/m°C
Probably the most important Molybdenum alloy is TZM which contains small amounts of Titanium and Zirconium. This Molybdenum alloy because of its higher mechanical properties is used in the production of structural parts, which work at high temperatures. In these applications often the users take advantage also of the better weldability of TZM with respect to pure Molybdenum.
APPLICATIONS OF MOLYBDENUM AND ITS ALLOYS
Because of its good electrical properties Molybdenum and its alloys is often used in electronic tubes and in the production of anodes or cathode supports.
For its elevated temperature strength and its good thermal conductivity, TZM is used for the production of inserts for Aluminum and Brass casting dies to protect the dies against the erosion of the liquid metal.
TZM dies are used in the extrusion of Copper alloys.
Molybdenum is widely used for the production of electrodes for glass melting furnaces
An alloy 70%Mo-30%W is used to produce pump shafts and thermocouple protection tubes for application in the very corrosive pure liquid Zinc.
- High temperature applications with heavy mechanical load
- Forging tools
- Rotating anodes in X-ray tubes.
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