Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0 nd 50 y weight to improve its mechanical properties. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels. The difference between the two is somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0 %. Most commonly, the phrase "alloy steel" refers to low-alloy steels.