Alloy steel, with added carbon and other elements, is the hardest known metal with a tensile strength of 0.84 GPa and yield strength of 0.64 GPa. Carbon steel is surpassed only by non-very hard metals like rubies and diamonds. Carbon steel is commonly used in construction tools but is prone to corrosion. Tungsten carbide, iridium, and titanium are also hard materials, but not the hardest. Chromium is extremely hard and is added to alloy steels. The hardest element is carbon in diamond form.
The hardest known metal is alloy steel, which is often made even harder by the addition of 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 non-very hard metals, such as rubies, diamonds or diamond nanotube aggregates . Hardness is defined as the resistance of a material to abrasion or fingerprinting. Mohs’ scale ranks elements by comparative hardness, on a scale of 1 to 10, with 10 being the hardest. The best high-carbon steels score an 8 to 9 on the scale, compared to ruby at a 9 and diamond at a 10. Harder metals are still somewhat softer than stronger nonmetals, based on the bonds strongest in chemistry, the sp2 bond between carbon atoms.
Carbon steel
High-strength carbon steel goes through a tempering and annealing process, which uses heat, to make it harder. While heat makes metal stronger, it also makes it less ductile, meaning it’s less able to bend and move when under stress. Carbon is also added to steel, at which point the steel alloy metal becomes carbon steel or reinforced steel, the hardest metal available. Carbon contents as low as 2% may be present in the harder metal.
Carbon steel is one of the most common types of steel, the other most common type being stainless steel. Because of its high melting point of 2,800°F (1,537°C), most carbon steels are used in construction tools such as rock drills and building dowels. Sometimes very hard metals, such as carbon steel, are undesirable due to their other properties; carbon steel, for example, is notoriously prone to corrosion. While it’s the hardest metal, it’s not necessarily the most useful material for every job.
Other hard materials
While often mistaken to be the hardest metal, tungsten carbide, it is actually a composite material made of tungsten and carbon, often with cobalt added. It is used in both military and 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; even if they are difficult, they are not the most difficult. However, titanium has been used to build the deepest submarines in the world and iridium is known to be the most corrosion resistant metal. Chromium, which has a value of 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 no harder than tungsten carbide or carbon steel, but it is denser, which is preferable for shells. The single strongest element is carbon, but only when it’s in diamond form.
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