Tungsten, also known as wolfram, is a hard, rare metal that is found naturally only in chemical compounds. The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements. Tungsten in pure form has the highest melting point (3422 °C, 6192 °F), lowest vapor pressure (at temperatures above 1650 °C, 3000 °F) and the highest tensile strength. Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons. Alloying small quantities of tungsten with steel greatly increases its toughness.