The melting point of a substance is the temperature at which it transforms from a solid to a liquid state. Metals show a high melting point as they exist in a crystalline solid form. High melting point metals have strong intermolecular forces between atoms. Electrostatic attraction forces between metal ions and free electrons create strong metallic bonds with stronger bonds resulting in higher melting temperatures.
There are two accepted definitions for refractory metals. One states a metal must have a melting point above 2200 °C, whilst the other states all metals with a melting point above 1850 °C are considered refractory metals. Using the broader definition, the following 14 metals are classified as refractory metals.
The following have a melting point above 2200 °C
The following have a melting point above 1850 °C but below 2200 °C
- Chromium (Cr)
- Hafnium (Hf)
- Iridium (Ir)
- Osmium (Os)
- Rhodium (Rh)
- Ruthenium (Ru)
- Titanium (Ti)
- Vanadium (V)
- Zirconium (Zr)
Refractory metals have highly specialised applications such as in lighting, tools, lubricants and nuclear reaction rods. They cannot be cast into shapes and can only be processed using powder metallurgy.
Lutetium, Lawrencium and Protactinium also have high melting points. But they are highly radioactive or have very limited applications and are not commonly used.
Other common metals with a high melting point
The following four metals are the most commonly used metals with high melting points but below 1850 °C, and as such are not considered refractory metals:
Palladium is a shiny, silvery-white metal that melts at 1555 °C and has a density of 12.02g/cm3. The metal is highly corrosion resistant in air but may tarnish in moist air containing sulphur. It has no biological role and is non-toxic.
The metal is produced as a by-product during the refining of copper and nickel ores. It is extremely ductile and can easily be beaten into a thin leaf shape used for decorative purposes or as jewellery.
It is most commonly used in the manufacture of automobile catalytic converters. It is also widely used to decolourise gold when making white gold jewellery. Other popular uses include dentistry, ceramic capacitors, the making of electrical contacts and surgical instruments.
Scandium is a silvery-white metal that melts at 1541 °C and has a density of 3.0g/cm3. It is a soft metal that slowly changes colour to yellowish or pinkish when exposed to air due to the formation of scandium oxide (Sc2O3) on the surface. It has no known biological role but is a suspected carcinogen.
Scandium is the main element of thortveitite, a highly collectable mineral found in Scandinavia. Scandium is considered a rare earth element as it has similar chemical properties to other rare earth elements and is found in the same ores.
Scandium increases the temperature at which aluminium recrystallizes to more than 600 °C. This is well above the temperature range of heat treatable aluminium alloys. It is a potent alloying element that dramatically improves an aluminium alloy’s mechanical and physical characteristics. These alloys are gaining popularity in aeronautical and transportation industries.
Iron is a silvery-grey metal that melts at 1535 °C and has a density of 7.87g/cm3. It is a ductile, soft metal and is a relatively good conductor of heat and electricity. In its pure form, it is highly reactive and readily oxidises in air to form red-brown iron oxides, or rust. It has known biological roles and is vital in the functioning of living organisms. It is considered non-toxic.
Iron is produced through the smelting/reduction of iron ore (hematite and magnetite) into pig iron, containing a large amount of carbon and other impurities, in blast furnaces at about 2000 °C, followed by the removal of these impurities.
Iron (together with its alloys) is the world’s most common industrial metal. The majority of iron produced is used to manufacture various types of steel. Adding nickel, chrome, vanadium and tungsten improves corrosion resistance, whilst adding 3-5 wt.% carbon creates a low-cost alloy for pipes and other non-structural applications.
Yttrium is a silvery-white metal that melts at 1525 °C and has a density of 4.47g/cm3. It is moderately soft and ductile. It has no known biological role but can be highly toxic to humans and animals.
The metal is produced by reducing yttrium fluoride with a calcium-magnesium alloy in an arc furnace at 1600 °C, sufficient to melt the yttrium.
Yttrium is often used as an alloying element to increase the strength of aluminium and magnesium alloys. Its oxide is used as an additive in camera lens glass to make it heat and shock resistant.