Refractory metals and alloys are well known for their extraordinary resistance to heat and wear. Key requirement to withstand high temperatures is a high melting point and stable mechanical properties (e.g. high hardness) even at high temperatures.
The most common refractory metals include five elements: niobium and molybdenum of the fifth period and tantalum, tungsten, and rhenium of the sixth period. They all share some properties, including a melting point above 2000 °C and high hardness at room temperature.
Poor low-temperature fabricability and extreme oxidability at high temperatures are main disadvantages of most refractory metals. Application of these metals requires a protective atmosphere or coating.
The strength of refractory metals at high temperatures, in combination with their hardness, makes them ideal for cutting and drilling tools. Refractory metal-based alloys are used in virtually all major industries, including electronics, aerospace, automotive, chemicals, mining, nuclear technology and metal processing. Today usage is not limited to lamp filaments, electron tube grids and heating elements.
- Tungsten is an extremely dense metal and it has the highest melting point of all metals, at 3,410 °C. Moreover, tungsten is an excellent alloying agent. The major use of tungsten is in cemented carbide metal cutting tools and wear-resistant materials.Produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications. The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains those properties at the high temperatures generated when cutting metals. Tungsten and molybdenum are interchangeable at an atomic level and both promote resistance to tempering which gives improved tool cutting performance at higher temperatures.
- Molybdenum increases hardenability and strength, particularly at high temperatures due to the high melting point of molybdenum. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel. It retards the transformation of austenite to pearlite far more than it does the transformation of austenite to bainite; thus, bainite may be produced by continuous cooling of molybdenum-containing steels. Most common molybdenum-based alloy is is the Titanium-Zirconium-Molybdenum alloy TZM, composed of 0.5% titanium and 0.08% of zirconium (with molybdenum being the rest). It is typically manufactured by powder metallurgy or arc-casting processes. The alloy exhibits a higher creep resistance and strength at high temperatures, making service temperatures of above 1060 °C possible for the material.