Refractory Metals: Part One

Refractory metals are different. As a group they provide a number of unique characteristics – such as resistance to high heat, corrosion and wear – making them useful in a multitude of applications. The next time you climb into your automobile take note that you are surrounded by components which are made of refractory metals or have been cut or formed by them.

Your car’s electrical or electronic systems may also make use of the metals’ electric and heat-conducting qualities. In contrast, refractory metals were also used in the tools which helped to drill the well which produces the gasoline in your tank. Piping of refractory metal alloys helped process it. And your oil may contain a refractory metal compound to increase its lubricating ability.

The paradox of refractory metals is that, despite their wide and constantly growing list of applications, many people – sometimes even engineers, working with one of the metals – do not fully understand how and where each is mined, how it is processed, how it is formed, or even understand the full extent of the diversity of refractory metals’ applications.

What are refractory metals?

Refractory metals have one characteristic in common: an exceptionally high melting point. Tungsten, for example, melts at 3410°C (6170°F), which is more than double that of iron and ten times that of lead. As a group, they are found in one section of the periodic table of elements. Although there are twelve refractory metals, only five are widely used: Tungsten, Molybdenum, Niobium, Tantalum and Rhenium.

All but Rhenium have a body-centered cubic structure. Despite the fact that refractory metals have many similar qualities-such as high density, resistance to wear and corrosion-each metal is different in its own way, each providing its own individual combination of qualities.

Many of these individual qualities are rather unique, such as an ability to combine with gases and then release them under heat or possessing remarkable lubricating qualities. This selection of unusual characteristics provides engineers with a virtually uncountable number of potential applications ... each a key to solving a problem.

If refractory metals have such a high metal melting point, how is anything ever fabricated from them?

Refractory metals are extracted from ore concentrates, processed into chemicals and then into powders. The powders are consolidated into finished products or mill shapes and ingots for further processing. Because of their high melting points and ease of oxidation, refractory metals are usually worked in powder form.

The science of modern powder metallurgy (PM) actually started in the early 1900's when incandescent lamp filaments were made from tungsten powders. Another early PM product was cemented tungsten carbide used in the manufacture of cutting tools.

From this early era working with refractory metals, has grown one of the most depended upon methods of metal part fabrication and is incredibly important because of PM technology's manufacturing productivity advantages, material conservation, wide range of engineering properties, design flexibility and energy savings.

What other important properties do these metals have?

The mechanical and physical properties of refractory metals are compared to other common metals in Table 1. The strength values of the metals in Table 1 are given in ranges because the strengths of these metals may vary considerably with form and processing.

Strength is not the only thing which can vary because of processing methods. Alloying or combining the metals in composites can often provide properties which are, in some respects, superior to those inherent in the base metal itself. So, although there are only five principal refractory metals, they serve as major constituents in dozens of important metal and alloy compositions.

Table 1: Mechanical and physical properties comparison

Alloys containing varying amounts of refractory metals are vital to virtually every major industry, including automotive, mining, aerospace, chemical and petroleum processing, electrical and electronics, medical electronics and prosthetics, metal processing, nuclear technology and ordnance. The principal areas of uses for the five refractory metals are shown in Table 2.

Table 2: Major Uses of Refractory Metals

Molybdenum, symbol Mo, is a metallic element with chemical properties similar to those of chromium. Molybdenum is one of the transition elements of the periodic table. The atomic number of molybdenum is 42.

Molybdenum was discovered in 1778 by the Swedish chemist Carl Wilhelm Scheele. It is a silvery white, tough, malleable metal. Molybdenum is dissolved by dilute nitric acid and aqua regia, and is attacked by fused alkalies; it is not attacked by air at ordinary temperatures, but at elevated temperatures it oxidizes to form molybdenum oxide.

Molybdenum melts at about 2610°C (about 4730°F), boils at about 4640°C (about 8380°F), and has a specific gravity, or relative density, of 10.2.

Molybdenum does not occur free in nature, but in the form of its ores, the most important of which are molybdenite and wulfenite. It ranks 56th in order of abundance of the elements in the crust of the earth and is an important trace element in soils, where it contributes to the growth of plants.

Niobium or Columbium, symbol Nb, is a steel-gray, lustrous, ductile, and malleable metallic element. The atomic number of niobium is 41. Niobium is also one of the transition elements of the periodic table.

This metal was discovered in 1801 by the British chemist Charles Hatchett. Niobium burns when heated in air and combines with nitrogen, hydrogen, and the halogens. It resists the actions of most acids. Its principal use is as an alloying element in stainless steel, to which it lends additional corrosion resistance, particularly at high temperatures.

Niobium ranks about 32nd in natural abundance among the elements in crustal rock. It occurs, associated with the similar element tantalum, in various minerals, the most important of which is called columbite or tantalite, depending on which of the two elements predominates. Pure niobium has excellent characteristics as a construction material in nuclear power plants.

Niobium melts at about 2468°C (about 4474°F), boils at about 5127°C (about 9261°F), and has a specific gravity of 8.57. The atomic weight of niobium is 92.906.