Alloy steel is a steel alloyed by adding a variety of elements with a total amount between 1.0% and 50% in order to improve its mechanical properties. Alloy steels are divided into two categories: low alloy steels and high alloy steels. Most commonly, the term "alloy steel" refers to low alloy steel.

Common Alloys in Alloy Steel
Strictly speaking, every steel is an alloy, but not all steels are called "alloy steels". The simplest steel is an alloy of iron (Fe) and carbon (C) (about 0.1% to 1%, depending on the type). However, the term "alloy steel" is the standard term and refers to steel to which other alloying elements besides carbon have been intentionally added. Common alloying elements include manganese (the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloying elements include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium.

Alloying elements are added to alloy steel castings to achieve certain properties of the material. As a guideline, add lower percentages (below 5%) of alloying elements to increase strength or hardenability, or higher percentages (over 5%) to gain special properties such as corrosion resistance or extreme temperature stability. Manganese, silicon or aluminum are added during steelmaking to remove dissolved oxygen, sulfur and phosphorus from the melt.

The following are a series of improved properties of alloy steel castings (compared to carbon steel): strength, hardness, toughness, wear resistance, corrosion resistance, hardenability and hot hardness. To achieve some of these improved properties, the metal may require heat treatment.

Some of these find use in exotic and demanding applications such as turbine blades for jet engines, spacecraft and nuclear reactors. Due to the ferromagnetic properties of iron, some steel alloys find important applications where their response to magnetism is important, including in electric motors and transformers. The uses and applications of alloy steel castings span a wide variety of industries and casting types.