AISI 4140 Alloy Steel (UNS G41400) Alloy steels are designated by AISI four-digit numbers and comprise different kinds of steels, each with a composition which exceeds the limitations of B, C, Mn, Mo, Ni, Si, Cr, and Va set for carbon steels. AISI 4140 alloy steel is a chromium-, molybdenum-, and manganese-containing low alloy steel.Web