Molybdenum Alloys

Molybdenum Alloys

 

molybdenum-based alloys, used mainly as heat-resistant construction materials. Parts made from molybdenum alloys can perform for long periods in a vacuum at temperatures up to 1800°C, and for short periods (up to 5 min), at 2300°-2500°C in fuel combustion products. The service life of molybdenum-alloy parts with protective coatings at 1200°-2000°C ranges from 500 hr in air to 5 hr in other oxidizing media. Molybdenum alloys are usually produced by smelting in vacuum-arc, electron-beam, and lined furnaces, which provide purity and plasticity of the metal. In the production of molybdenum alloys by methods of powder metallurgy, impurities in the metal significantly decrease the technological properties of the alloy, mainly weldability.

A relatively small number of elements—titanium, zirconium, hafnium, niobium, and vanadium—are used for alloying molybdenum (they are introduced in amounts of 0.1–1.5 percent). Such amounts provide high heat-resistance and sufficient plasticity (rhenium and tungsten may be introduced in amounts up to 50 percent, with retention of satisfactory deformability of the alloy). The heat resistance of molybdenum alloys increases with additional alloying with carbon (up to 0.4 percent), which leads to the formation of clearly heterophase alloys with carbide hardening. Small quantities of boron, chromium, nickel, tantalum, and a number of rare earths are also used to improve the technological properties of molybdenum alloys.

Industrial molybdenum alloys are used to produce rods, forgings, stampings, sheets, wire, and pipes. An important feature of molybdenum alloys is retention of considerable strength at high temperatures (see Table 1).

Table 1. Mechanical properties of molybdenum alloys in short-term tests (average values for various alloys)
 Temperature
Property20°C1200°C
*top values are in GN/m2; values in parentheses are in kgf/mm2
**top values are in MN/m2; values in parentheses are in kgf/mm2
Elastic modulus Ed* ...............330
(33,000)
270
(27,000)
Ultimate strength σb** ..............700‒800
(70‒80)
300‒450
(30‒45)
Relative elongation δ(%) ....................7‒3010‒15

The long-term strength of molybdenum alloys (100-hr tests at 1200°C) reaches 350 meganewtons per sq m (MN/m2), or 35 kilograms-force per sq mm (kgf/mm2). Cold shortness is characteristic of both molybdenum alloys and pure molybdenum. The cold-shortness threshold in impact testing of molybdenum alloys is 150°-300°C, although the alloys are fairly plastic in tensile testing at room temperature and retain plasticity even at –70°C. The physical properties of low molybdenum alloys are similar to the properties of the pure metal.

In the production of semifinished articles by deformation at temperatures below the recrystallization point (1300°-1600°C), low molybdenum alloys are hardened by cold working. The main type of heat treatment for low molybdenum alloys is annealing, such as annealing of finished products at 1000°-1200°C to remove strains, and also recrystallization annealing over a period of a few hours at a temperature slightly exceeding the recrystallization point, and homogenizing annealing of ingots at 1800°-2000°C. Age-hardenable heterophase molybdenum alloys are annealed at 1900°-2000°C for a few hours.

Because of their low melting point and the volatility of molybdenum oxides, molybdenum alloys are not heat-resistant. Protective coatings have been developed that make possible use of the alloys under the most varied conditions at temperatures up to 2000°C for certain periods that depend on the type of coating, the temperature, and the medium. Without protective coatings, molybdenum alloys may be used only in a neutral or reducing medium and in a vacuum.

Molybdenum alloys have satisfactory engineering properties. They are suitable for machining. Various items with a high degree of drawing may be made by stamping from sheets of the most ductile alloys at 200°-500°C. Sheets of such alloys are welded satisfactorily by contact welding, as well as by fusion welding, including argon-arc welding in chambers with a neutral atmosphere and electron-beam welding in a vacuum. The welding seams produced by these methods are resilient, and the best alloys have a bending angle of 50°-160° at room temperature.

Molybdenum alloys are used to make parts for rockets and other aircraft, as well as for special apparatus (inserts for the throat sections of nozzles, and also wing edges, jet vanes, radio antennas, casings, atomic reactor parts, and cathodes and anodes for thermoemission transformers). In addition, molybdenum alloys are used to make die inserts in pressure casting, equipment used in pipe production, parts of equipment for the petroleum and glass industries, and parts for the electrotechnical and radio-electronics industries.

REFERENCES

Molibden: Sbornik statei [translations]. Moscow, 1962.
Tugoplavkie materialy v mashinostroenii: Spravochnik. Edited by A. T. Tumanov and K. I. Portnoi. Moscow, 1967.
Savitskii, E. M., and G. S. Burkhanov. Metallovedenie splavov tugoplavkikh i redkikh metallov, 2nd ed. Moscow, 1971.

A. S. STROEV

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