From Halopedia, the Halo wiki
Tungsten, also called wolfram (/ˈwʊlfrəm/), is a chemical element that has the symbol W (German: Wolfram) and atomic number 74. A very hard, heavy, steel-gray to white transition metal, tungsten is found in several ores including wolframite and scheelite and is remarkable for its robust physical properties, especially the fact that it has the highest melting point of all the non-alloyed metals and the second highest of all the elements after carbon. The pure form is used mainly in electrical applications but its many compounds and alloys are widely used in many applications, most notably in light bulb filaments, in X-ray tubes (as both the filament and target), and in superalloys. Tungsten is the only metal from the third transition series that is known to occur in biomolecules.
Pure tungsten is steel-gray to tin-white and is a hard metal. Tungsten can be cut with a hacksaw when it is very pure (it is brittle and hard to work when impure) and is otherwise worked by forging, drawing, extruding, or sintering. Of all metals this element has the highest melting point (3,422°C) (6,192°F), lowest vapor pressure and the highest tensile strength. Tungsten has the lowest coefficient of thermal expansion of any pure metal. Its corrosion resistance is excellent and it can be attacked only slightly by most mineral acids. Tungsten metal forms a protective oxide when exposed to air but can be oxidized at high temperature. Steel alloyed with small quantities of tungsten greatly increases its toughness.
Tungsten is used by the UNSC in the manufacture of Magnetic Accelerator Cannon rounds, combined with ferrous material, and used by UNSC Navy warships as high-penetration projectiles. The weapons technology was eventually miniaturized and the metal was also used in the composition of the M645 FTP-HE round used in the ARC-920 railgun. The 14.5×114mm Armor-Piercing Fin-Stabilizing Discarding Sabot (APFSDS) rounds of the S2 AM sniper rifle series are composed of tungsten. The metal can also be layered with Titanium-A armor to give the armor a radiation absorbtion rating of 5 and presumably grant greater protection against plasma weaponry.
The blade on the reverse end of the Gravity hammer is usually made with a tungsten-alloy.