Refractory Metals

Refractory metals are characterized by their extremely high melting points, which range well above those of iron, cobalt, and nickel. They are used in demanding applications requiring high-temperature strength and corrosion resistance. The most extensively used of these metals are tungsten, tantalum, molybdenum, and columbium (niobium). They are mutually soluble and form solid-solution alloys with each other in any proportion. These four refractory metals and their alloys are available in mill forms as well as products such as screws, bolts, studs, and tubing. Although the melting points of these metals are all well above 4,000°F, they oxidize at much lower temperatures. Accelerated oxidation in air occurs at 190°C for tungsten, 395°C for molybdenum, and 425°C for tantalum and columbium. Therefore, protective coatings must be applied to these metals if they are to be used at higher temperatures. Tensile and yield strengths of the refractory metals are substantially retained at high temperature. These metals are usually considered together because most of their working characteristics are similar. They can be fabricated by most conventional methods at room temperature. Heavy sections for forging can be heated, without protection, to approximately 425°C. Out of several commercial-grade tantalum alloys, those containing tungsten, columbium, and molybdenum generally retain the