Alloy Steels

A. 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 steel.

B. 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:

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.

Up : XXX :x.xx% average carbon content

Down : 51100

13	xx:1.75Mn	Manganese
23	xx:3.50Ni	Nickle
31	xx:1.25Ni, 0.65-0.80Cr	Nickle - 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	Nickle - 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	Nickle - 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.