Explanation of the A.I.S.I. System of Identification

A system of symbols is used to identify the grades of standard steel.  In those symbols a capital letter prefix “B” is used to indicate acid Bessemer steel, and “L” within the grade number is used to identify leaded steels.  Numbers are used to indicate grades of steel by chemical composition as described below.

Numerical Designations.  A four-numeral series designates carbon steel or alloy steel specified to chemical composition ranges.  Five numerals are used to designate certain types of alloy steels.

The last two digits of four-numeral series are intended, so far as feasible, to indicate approximate middle of the carbon range; for example, 35 represents a range of 0.32 to 0.38 percent.  It is necessary however to deviate from this rule and to interpolate numbers in the case of some carbon ranges; and for variations in manganese, phosphorous, sulphur, chromium, or other elements.

The first two digits of the four-numeral series for the various grades of steel and their meaning are as follows:

SERIES DESIGNATION TYPES AND CLASSES
10xx Nonsulphurized Carbon Steel Grades.
11xx Resulphurized Carbon Steel Grades.
12xx Rephosphorized and Resulphurized Carbon Steel Grades.
13xx Manganese 1.60 to 1.90 Per Cent.
*15xx Manganese 1.00 to 1.35 Per Cent, Sulphur .20 to .30 Per Cent.
23xx Nickel 3.50 Per Cent.
25xx Nickel 5.00 Per Cent.
31xx Nickel 1.25 Per Cent—Chromium 0.65 or 0.80 Per Cent.
33xx Nickel 3.50 Per Cent—Chromium 1.55 Per Cent.
40xx Molybdenum 0.25 Per Cent.
41xx Chromium 0.95 Per Cent—Molybdenum 0.20 Per Cent.
43xx Nickel 1.80 Per Cent—Chromium 0.50 or 0.80 Per Cent—Molybdenum 0.25 Per Cent.
46xx Nickel 1.80 Per Cent—Molybdenum 0.25 Per Cent.
48xx Nickel 3.50 Per Cent—Molybdenum 0.25 Per Cent.
50xx Chromium 0.30 or 0.60 Per Cent.
51xx Chromium 0.80, 0.95 or 1.05 Per Cent.
5xxxx Carbon 1.00 Per Cent—Chromium 0.50, 1.00 or 1.45 Per Cent
61xx Chromium 0.80 or 0.95 Per Cent—Vanadium 0.10 or 0.15 Per Cent.
86xx Nickel 0.55 Per Cent—Chromium 0.50 Per Cent—Molybdenum 0.20 Per Cent.
87xx Nickel 0.55 Per Cent—Chromium 0.50 Per Cent—Molybdenum 0.25 Per Cent.
92xx Manganese 0.85 Per Cent—Silicon 2.00 Per Cent.
93xx Nickel 3.25 Per Cent—Chromium 1.20 Per Cent—Molybdenum 0.12 Per Cent.
94xx Manganese 1.00 Per Cent—Nickel 0.45 Per Cent—Chromium 0.10 Per Cent—Molybdenum 0.12 Per Cent.
97xx Nickel 0.55 Per Cent—Molybdenum 0.20 Per Cent—Chromium 0.17 Per Cent.
98xx Nickel 1.00 Per Cent—Chromium 0.80 Per Cent—Molybdenum 0.25 Per Cent.
* Not a part of any A.I.S.I. or S.A.E. system of identification.