Hardness is a relative term when talking about materials, both metal and non-metal. Diamonds are commonly known as the hardest substance on earth, used in drill bits for cutting applications. Of the natural elements titanium is considered to be extremely hard, if not the hardest. Alloys, a combination of elements, can make the hardness of metal even higher. Factors for considering hardness might include melting point, which usually points to a high hardness; scratch resistance; density; tensile strength; yield strength; and a high value on different hardness scales.
Steel Alloys
Steel is a very common alloy and is combined with other metals and elements to produce different properties. Carbon is a primary hardening ingredient for steel and is used to increase its hardness and tensile strength. Chromium is added to increase corrosion and oxidation resistance as well as hardening and high-temperature strength. Boron, nickel, molybdenum, niobium and titanium can all add strengthening and hardening properties. A combination of these different substances can produce some of the hardest-known metals.
Tungsten Carbide
Tungsten carbide 857 is made of 85.7 percent tungsten carbide, 9.5 percent nickel, 1.8 percent tantalum, 1.5 percent titanium, 1 percent niobium and 0.3 percent chromium. This tungsten carbide measures between 8 and 9 on the Mohs scale (see “Hardness Scale” section for definition of Mohs), and ring form is said to be the most scratch resistant in the world, does not deform and is the only metal that can be permanently polished. Tungsten carbide is four times harder than titanium.
Titanium
Titanium is considered very hard for an element and can have tensile properties between 35,000 psi and 100,000 psi. Titanium hardness is comparable to some heat-treated alloys. Iodide titanium has a Vickers value of 90 VHN, unalloyed titanium is about 160 VHN and heated-treated titanium can reach 250 to 500 VHN. Commercial alloys can have around 320 VHN (see “Hardness Scale” section for definition of Vickers).
Uranium
Uranium, although not as hard as titanium or metal alloys, is very useful for its hardness and density. Uranium can be made into alloys with titanium and molybdenum to be used as armor-piercing projectiles. They have the mass and physical properties that allow them to penetrate armor better and at longer distances. When tungsten rounds hit a target they can blunt, meaning some of the energy is wasted and deforms the round, unlike uranium’s self-sharpening properties.
Hardness Scales
Hardness is not really a property but rather an artificial method used to compare materials to each other. So if material A can scratch material B, then A must be harder than B. If material C can scratch A, then on this particular hardness scale B falls between A and C. The Mohs scale is a system that compares the scratch resistance of materials compared to each other. The Vickers scale uses a diamond indenter in the shape of a right pyramid, which is then pressed into the test material for 10 to 15 seconds and reported as VHN or Vickers Hardness Number. The value can also be reported as HV/x where HV is the harndess and x is the force used.
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