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General Information
Alloy steels comprise a wide variety of steels which have compositions that exceed the limitations of C, Mn, Ni, Mo, Cr, Va, Si, and B which have been set for carbon steels. However, steels containing more than 3.99% chromium are classified differently as stainless and tool steels.

Alloy steels are always killed, but can use unique deoxidization or melting processes for specific applications. Alloy steels are generally more responsive to heat and mechanical treatments than carbon steels.

AISI Designation
Typically, alloy steels are designated by distinct AISI (American Iron and Steel Institute) four-digit numbers. The first two digits indicate the leading alloying elements, while the last two digits give the nominal carbon content of the alloy in hundredths of a percent. Occasionally we see five-digit designations where the last three digits tell that the carbon is actually over 1%. Here is an example:
 XXX :x.xx% average carbon content
13 xx:1.75Mn  Manganese
23 xx:3.50Ni  Nickel
31 xx:1.25Ni, 0.65-0.80Cr  Nickel-Chromium
40 xx:0.20-0.25Mo  Molybdenum
44 xx:0.40-0.52Mo
41 xx:0.50-0.95Cr, 0.12-0.30Mo  Chromium-Molybdenum
46 xx:0.85-1.82Ni, 0.20-0.25Mo  Nickel-Molybdenum
48 xx:3.5Ni, 0.25Mo
50 xx:0.27-0.65Cr  Chromium
51 xx:0.80-1.05Cr
50 xxx:0.50Cr, 1.00C
51 xxx:1.02Cr, 1.00C
52 xxx:1.45Cr, 1.00C
61 xx:0.60-0.95Cr, 0.10-0.15V  Chromium-Vanadium
92 xx:1.4-2Si, 0.65-0.85Mn, <0.65Cr  Silicon-Manganese
43 xx:1.82Ni, 0.50-0.80Cr, 0.25Mo  Nickel-Chromium-Molybdenum
47 xx:1.05Ni, 0.45Cr, 0.20-0.35Mo
81 xx:0.30Ni, 0.40Cr, 0.12Mo
86 xx:0.55Ni, 0.50Cr, 0.25Mo
87 xx:0.55Ni, 0.50Cr, 0.25Mo
88 xx:0.55Ni, 0.50Cr, 0.20-0.35Mo
93 xx:3.25Ni, 1.20Cr, 0.12Mo
94 xx:0.45Ni, 0.40Cr, 0.12Mo

If a B shows up between the second and third digits of an AISI number, it means that this grade is a Boron steel; Sometimes a suffix H is attached to a AISI number to indicate that the steel has been produced to prescribed hardenability limits.