Steel is a metal alloy consisting mostly of iron, in addition to small amounts of carbon, depending on the grade and quality of the steel. Alloy steel is any type of steel to which one or more elements besides carbon have been intentionally added, to produce a desired physical property or characteristic. Common elements that are added to make alloy steel are molybdenum, manganese, nickel, silicon, boron, chromium, and vanadium.
Alloy steel is often subdivided into two groups: high alloy steels and low alloy steels. The difference between the two is defined somewhat arbitrarily. However, most agree that any steel that is alloyed with more than eight percent of its weight being other elements beside iron and carbon, is high alloy steel. Low alloy steels are slightly more common. The physical properties of these steels are modified by the other elements, to give them greater hardness, durability, corrosion resistance, or toughness as compared to carbon steel. To achieve such properties, these alloys often require heat treatment.
If the carbon level in a low alloy steel is in the medium to high range, it can be difficult to weld. If the carbon content is lowered to a range of 0.1% to 0.3%, and some of the alloying elements are reduced, the steel can achieve a greater weldability and formability while maintaining the strength that steel is known for. Such metals are classified as high strength, low alloy steels.
Perhaps the most well-known alloy steel is stainless steel. This is a steel alloy with a minimum of 10% chromium content. Stainless steel is more resistant to stains, corrosion, and rust than ordinary steel. It was discovered in 1913 by Harry Brearley of Sheffield, England, but the discovery was not announced to the world until 1915. Stainless steel is commonly used in table cutlery, jewelry, watch bands, surgical instruments, as well as in the aviation industry. Its familiar luster has also been appropriated for many famous architectural designs, such as the Gateway Arch in St. Louis, Missouri, and the pinnacle of the Chrysler Building in New York City.
In all types of alloy steel, the alloying elements tend to either form carbides or compounds, rather than simply being uniformly mixed in with the iron and carbon. Nickel, aluminum, and silicon are examples of the elements that form compounds in the steel. Tungsten and vanadium will form carbides, both of which increase the hardness and stability of the finished product.