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.
The
white metals are also any of several light-colored alloys used as a base for plated silverware, ornaments or novelties, as well as any of several lead-base or tin-base alloys used for things like bearings, jewellery, miniature figures, fusible plugs and metal type.
Some
of the metals that make up a white metal alloy are antimony, tin, lead, cadmium, bismuth, and zinc. Not all of these metals are found in all white metal
alloys but are mixed to achieve a desired goal or need. An example would be for
jewellery. You would need it to be castable, polishable, have good flow characteristics, have the ability to
cast fine detail without a excessive amount of porosity and cast at between 230°C and 300°C (450°F and 575°F).