Page 184 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 6.8 Refractory Metals and Alloys

6.8 Refractory Metals and Alloys

There are four refractory metals: molybdenum, niobium, tungsten, and tantalum.
These metals are called refractory because of their high melting points. Although they
were discovered about 200 years ago and have been used as important alloying ele-
ments in steels and superalloys, their use as engineering metals and alloys did not
begin until about the 1940s. More than most other metals and alloys, the refractory
metals maintain their strength at elevated temperatures. Therefore, they are of great
importance in rocket engines, gas turbines, and various other aerospace applications;
in the electronic, nuclear-power, and chemical industries; and as tool and die materi-
als. The temperature range for some of these applications is on the order of 1100 to
2200°C, where strength and oxidation are of major concern.

6.8.l Molybdenum

Molybdenum (Mo) is a silvery white metal discovered in the 18th century and has a
high melting point, high modulus of elasticity, good resistance to thermal shock, and
good electrical and thermal conductivity. Molybdenum is used in greater amounts
than any other refractory metal, in applications such as solid-propellant rockets, jet
engines, honeycomb structures, electronic components, heating elements, and dies
for' die casting. The principal alloying elements for molybdenum are titanium
and zirconium. Molybdenum is itself also an important alloying element in cast and
wrought alloy steels and in heat-resistant alloys, imparting strength, toughness, and
corrosion resistance. A major limitation of molybdenum alloys is their low resist-
ance to oxidation at temperatures above 500°C, which necessitates the use of pro-
tective coatings.

Production. The main source of molybdenum is the mineral molybdenite (molyb-
denum disulfide). The ore first is processed and the molybdenum is concentrated;
later, it is chemically reduced-first with oxygen and then with hydrogen. Powder-
metallurgy techniques also are used to produce ingots for further processing into
various shapes.

6.8.2 Niobium (Columbium)

Niobium (Nb, for niobium, after Niobe, the daughter of the mythical Greek king
Tantalus) was first identified in 1801; it is also referred to as columbium (after its
source mineral, columbite). Niobium possesses good ductility and formability and
has greater oxidation resistance than other refractory metals. With various alloying
elements, niobium alloys can be produced with moderate strength and good fabrica-
tion characteristics. These alloys are used in rockets and missiles and in nuclear,
chemical, and superconductor applications. Niobium is also an alloying element in
various alloys and superalloys. The metal is processed from ores by reduction and
refinement and from powder by melting and shaping into ingots.

6.8.3 Tungsten

Tungsten (W for u/olfrarn, its European name, and from its source mineral,
wolframite; in Swedish, tung means “heavy” and sten means “stone”) was first
identified in 1781; it is the most abundant of all the refractory metals. Tungsten has
the highest melting point of any metal (3410°C). As a result, it is notable for its high
strength at elevated temperatures. However, it has high density (hence it is used for
balancing weights and counterbalances in mechanical systems, including self-wind-