Molybdenum

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Molybdenum is used as an alloying addition in steels, and molybdenum as well as it's alloys are used for electrical and electronic parts, missile and aircraft parts, high-temperature furnace parts, die casting cores, hot-working tools, boring bars, thermocouples, nuclear energy applications, corrosion-resistant equipment, equipment for glass-melting furnaces and metallizing. Molybdenum also finds use as a catalyst in chemical reactions. Molybdenum is not suitable for continued service at temperatures above 500 oC in an oxidizing atmosphere unless protected by an adequate coating.

Typical Characteristics:

Symbol: Mo

Atomic Number: 42

Atomic Weight: 95.94

Density: At 20 oC: 10.22 g/cm3

Compressibility: At 293 oC: 36 micron 2/N

Structure:

Crystal Structure: Body centered cubic, a= 0.31468 nm at 25 oC

Slip Planes: {112} at 20 oC; {110} at 1000 oC

Slip Direction: [111]

Interatomic Distance: 0.27252 nm min

Metallography: Electrolytic polishing is preferred. Etching: (1) 10 g NaOH + 30 g K3Fe (CN)6 + 600 liters water; (3) Murakami's reagent

Thermal Properties:

Melting Point: 2610 oC

Boiling Point: 5560 oC

Specific Heat: At 20 oC: 0.276 kJ/kg x K

Latent Heat of Fusion: 270 kJ/kg (estimated)

Thermal Conductivity: At 20 oC: 142 W/m x K

Heat of Combustion: 7.58 MJ/kg Mo

Recrystallization Temperature: 900 oC min; commercial products normally require higher temperatures.

Electrical Properties:

Electrical Conductivity: At 0 oC: 34% IACS

Electrical Resistivity: At 0 oC: 52 n ohm x m

Thermal Electromotive Force: Versus platinum, 0 to 100 oC: 1.45 mV

Electrochemical Equivalent: Valence 6, 0.1658 mg/C

Hydrogen Overpotential: At 100 A/m2: 0.44 V

Magnetic Properties:

Magnetic Susceptibility: Mass: 1.17 x 10-8 mks at 25 oC; 1.39 x 10-8 mks at 1825 oC

Optical Properties:

Reflectivity: 46% at 500 nm, 93% at 10,000 nm

Color: Silvery white

Mechanical Properties:

The mechanical properties of molybdenum and molybdenum alloys greatly depend on the amount of working performed below the recrystallization temperature and on the ductile-to-brittle transition temperature. The minimum recrystallization temperature for molybdenum is 900 oC (1650 oF).

Chemical Properties:

Molybdenum has particularly good resistance to corrosion by mineral acids, provided oxidizing agents are not present. It is also resistant to many liquid metals and to most molten glasses. In inert atmospheres, it is unaffected up to 1760 oC by refractory oxides. Molybdenum is relatively inert in hydrogen, ammonia, and nitrogen up to about 1100 oC, but a superficial nitride case may be formed in ammonia or nitrogen.

Fabrication Characteristics:

Consolidation:

In most instances, molybdenum is consolidated from powder by compacting under pressure followed by sintering in the range from 1650 to 1900 oC. Some molybdenum is consolidated by a vacuum arc casting method in which a preformed electrode is melted by arc formation in a water-cooled mold.

Hot-working Temperature:

Generally forged between 1180 and 1290 oC down to 930 oC

Annealing Temperature:

Normal stress-related temperature is 870 to 980 oC.

Recrystallization Temperature:

Depends on prior working and condition; 1180 oC for full recrystallization in one hour of a 16mm (5/8 in.) bar reduced 97% by rolling.

Suitable Forming Methods:

Conventional methods.

Precautions in Forming:

Must be heated to the proper temperature relative to its thickness and forming speed.

Heat Treatment:

Not hardenable by heat treatment but only by work hardening.

Suitable Joining Methods:

Can be brazed or joined mechanically, as well as welded by arc, resistance, percussion, flash, and electron beam methods. Arc cast molybdenum is preferred to a powder metallurgy product for welding. Absolute cleanliness of surface is essential. Fusion welding must be carried out in closely controlled inert atmosphere.