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Molybdenum brief introduction
Probably the most versatile of the refractory metals, molybdenum is also a natural resource of the United States. It is an excellent structural material for applications requiring high strength and rigidity at temperatures to 3,000'F where it can operate in vacuum or under inert or reducing atmospheres.
Unalloyed molybdenum and its principal alloy, TZM, are produced by powder-metallurgy methods and by vacuum-arc melting. Both are commercially available in ordinary mill product forms: forging billets, bars, rods, wire, seamless tubing, plate, strip, and thin foil. Compared to unalloyed molybdenum, the TZM alloy (Mo-0.5%Ti-0.1%Zr) develops higher strength at room temperature and much higher stress-rupture and creep properties at all elevated temperatures. At 1,800 to 2,000'F, TZM can sustain a 30,000-psi stress for over 100 hr, three times that for unalloyed molybdenum.
Molybdenum and TZM are readily machined with conventional tools. Sheet can be processed by punching, stamping, spinning, and deep drawing. Some parts can be forged to shape. Molybdenum wire and powder can be flame sprayed onto steel substrates to salvage worn parts or to produce long-wearing, low-friction surfaces for tools.
In nonoxidizing environments, the metal resists attack by hydrochloric, hydrofluoric, sulfuric, and phosphoric acids. Molybdenum oxidizes at high temperatures to produce volatile, nontoxic, molybdenum trioxide; however, parts such as gimbled nozzles have been used successfully in rocket and missile-guidance systems when exposure time to the very-high temperatures of ballistic gases was brief.
Molybdenum parts can be welded by inertia, resistance, and spot methods in air; by TIG and MIG welding under inert atmospheres; and by electron-beam welding in vacuum. The best welds are produced by inertia (friction) welding and electron-beam welding; welds produced by the other techniques are less ductile. Generally, arc-cast metal develops better welds than do powder-metallurgy products. Heavy sections of molybdenum should be preheated and postheated when they are welded to reduce thermal stresses.
Because molybdenum has a modulus of elasticity of 47 * 10^6 psi at room temperature, it is used for boring bars and the quills for high-speed internal grinders to avoid vibration and chatter. Its relatively high electrical conductivity makes unalloyed molybdenum useful for electrical and electronic applications. It is used in the manufacture of incandescent lamps, as substrates in solid-state electronic devices, as electrodes for EDM equipment and for melting glass, and as heating elements and reflectors or radiation shields for high-temperature vacuum furnaces.
Because it retains usable strength at elevated temperatures, has a low coefficient of thermal expansion, and resists erosion by molten metals, the TZM alloy is used for cores in die casting of aluminum, and for die cavities in casting of brass, bronze, and even stainless steel. Dies of the TZM alloy weighing several thousand pounds are used for isothermal forging of superalloy components for aircraft gas turbines, and die inserts made of TZM have been used for extruding steel shapes. Piercer points of TZM are used to produce stainless-steel seamless tubing.
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