Metals refractory

The Zond VD - 96 is intended for the detection of cracks, precipitates, inclusions etc. and corrosive defects with the preliminary estimation of their depth in various objects of magnetic and non - magnetic steels, non-ferrous and refractory metals and welds including coatings. 

The basic device is very simple. A tip of refractory metal, such as tungsten, is electrically heat-polished to yield a nearly hemispherical end of about 10" cm radius. A potential of about 10 kV is applied between the tip and a hemispherical fluorescent screen. The field, F, falls off with distance as kr, and if the two radii of curvature are a and b, the total potential difference V is then... 

Refractory linings Refractory materials Refractory metals... 

A. P. Lama2e and D. Charquet, in K. C. Liddell and co-workers, eds.. Refractory Metals Extraction, Processing and Applications, The Minerals, Metals Materials Society, Warrendale, Pa., 1990, pp. 231—253. 

Short-term (100 h) internal pressure creep mpture testsof small bore cylinders having diameter ratios of 3 to 4, made of a number of different refractory metals, have been investigated at temperatures in the range 900—1300°C (73). 

In the case of refractory metals, coatings generaHy are sHicides, appHed by pack cementation or slurry processes. Typical sHicide compositions are... 

Fig. 11. Test temperature vs ultimate tensile strength for pure refractory metals (52) (---) rhenium (—...

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

Both molybdenum and tungsten can be worked in air without ductiHty loss. AH refractory metals can be made into tubing by extmsion, and most refractory metals, except chromium, are available as wine. Tungsten wines were attempted as fiber reinforcement for experimental nickel-base composites. 

A more extensive comparison of many potential turbine blade materials is available (67). The refractory metals and a ceramic, sHicon nitride, provide a much higher value of 100 h stress—mpture life, normalised by density, than any of the cobalt- or nickel-base aHoys. Several intermetaHics and intermetaUic matrix composites, eg, aHoyed Nb Al and MoSi —SiC composites, also show very high creep resistance at 1100°C (68). Nevertheless, the superaHoys are expected to continue to dominate high temperature aHoy technology for some time. 

At elevated temperatures, CaH2 reacts with halogens, sulfur, phosphoms, alcohols, and ammonia. At high temperatures, it reacts with refractory metal oxides and haUdes. Calcium hydride is substantially inert to organic compounds that do not contain acidic hydrogens. 

Siliconizing is yet another process used especially for coating of the refractory metals Ti, Nb, Ta, Cr, Mo, and W (see Refractories). These metals form siHcides which have a surface oxidation protection layer of Si02. Siliconizing is especially effective on molybdenum against air oxidation up to 1700°C. 

Chlorination. In some instances, the extraction of a pure metal is more easily achieved from the chloride than from the oxide. Oxide ores and concentrates react at high temperature with chlorine gas to produce volatile chlorides of the metal. This reaction can be used for common nonferrous metals, but it is particularly useful for refractory metals like titanium (see Titanium and titanium alloys) and 2irconium (see Zirconium and zirconium compounds), and for reactive metals like aluminum. 

Slip Casting. Slip casting of metal powders into useful articles is an interesting process but has only limited industrial application (30,31). It is sometimes used to produce large, very complicated parts from refractory metals (see Refractories). 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Refractory metals are associated with powder metallurgy because these metals are not easily melted. Therefore in smelting the ores, the metal is recovered in powder form rather than melted. Refractory metals are used mainly to produce filament wire for incandescent lamps. 

Space technology has always demanded materials that can operate at temperatures between those of superaHoys and refractory metals and that have high temperature strength during operation and room temperature ductility for fabrication. The development of dispersion-strengthened and oxide alloy systems has solved part of this problem. 

What are Refractory MetaM Refractory Metals Association, Metal Powder Industries Eederation, Princeton, N.J., 1980, pp. 4—5. 

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

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