The hardest known metal is steel alloy, which is often made even harder by adding carbon and other elements. With a tensile strength of 0.84 GPa (122,000 psi) and a yield strength of 0.64 GPa (67,000 psi), carbon steel is surpassed in hardness only by very hard nonmetals, such as rubies, diamonds, or aggregated diamond nanorods. Hardness is defined as the resistance of a matter to abrasion or indentation. The Mohs scale ranks elements according to comparative hardness, on a scale from 1 to 10, 10 being the hardest. The best high-carbon steels rate an 8 to 9 on the scale, relative to ruby at 9 and diamond at 10. The hardest metals are still somewhat soft in comparison to the strongest nonmetals, based on the strongest bonds in chemistry, the sp2 bond between carbon atoms.
High-strength carbon steel goes through a process of tempering and annealing, which uses heat, to make it harder. While heat makes the metal stronger, it also makes it less ductile, meaning it is less able to bend and move when under stress. Carbon is also added to the steel, at which point the steel alloy metal becomes carbon steel, or strengthened steel, the hardest metal available. Carbon contents as great as 2 percent may be present in the hardest metal.
Carbon steel is one of the most common types of steel, the other most common type being stainless steel. Due to it's high melting point of 2,800°F (1,537°C), most carbon steels are used in construction tools such as rock drills and construction dowels. Sometimes very hard metals, such as carbon steel, are undesirable because of their other properties; carbon steel, for instance, is notoriously prone to corrosion. While it is the hardest metal, it is not necessarily the most useful material for every job.
Other Hard Materials
While often mistaken to be the hardest metal, tungsten carbide, is actually a composite material made of tungsten and carbon, often with cobalt added. It is used in both the military and for gun reloading equipment, and is also a popular choice for men's wedding rings.
Iridium and titanium are also sometimes incorrectly called the hardest metals; though they are hard, they are not the hardest. Titanium has been used to build the world's deepest-diving submarines, however, and iridium is known to be the most corrosion-resistant metal. Chromium, which rates 8.5 on the Mohs scale, is also extremely hard, and is often added to alloy steels for this quality. Depleted uranium, the metal of choice for tank shells, is not harder than tungsten carbide or carbon steel but it is more dense, which is preferable for projectiles. The strongest single element is carbon, but only when it is in the form of a diamond.