Alloy steel refers to steel with various elements between 1.0% and 50% by weight added to improve its mechanical properties. Alloy steels are divided into two classes: low alloy steels and high alloy steels.
In general, "alloy steel" refers to low alloy steel. Strictly speaking, every kind of steel is an alloy, but not all steel is called "alloy steel". The simplest steel is iron (Fe) alloyed with carbon (C) (approx. 0.1% to 1%, depending on the type) and nothing else (except slight negligible traces of impurities); These are called carbon steels.
However, the term "alloy steel" is the standard term for steel with deliberately added alloying elements other than carbon. Common alloys include manganese (the most common), nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloys include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium.
The following is a list of improved properties of alloy steel (compared to carbon steel) : strength, hardness, toughness, wear resistance, corrosion resistance, hardenability and thermal hardness.
In order to achieve some of the improved properties, the metal may need heat treatment. Although alloy steels have been made for centuries, their metallurgy was not well understood until advanced chemical science revealed their composition in the 19th century. Early alloy steel was an expensive luxury, made on a "secret recipe" model and forged into tools such as knives and swords.
Modern alloy steels of the machine age have evolved into improved tool steels and newly available stainless steels. Today, alloy steel finds use in a wide range of applications, from everyday hand tools and cutlery to demanding applications such as turbine blades in jet engines and nuclear reactors. Because of iron's ferromagnetism, some steel alloys have important applications in their response to magnetism, including in electric motors and transformers.