There are two major numbering systems used by the steel industry, the first developed by the American Iron & Steel Institute (AISI), and the second by the Society of Automotive Engineers (SAE). Both of these systems are based on four digit code numbers when identifying the base carbon and alloy steels. There are selections of alloys that have five digit codes instead.
If the first digit is a one (1) in this designation it indicates a carbon steel. All carbon steels are in this group (1xxx) in both the SAE & AISI system. They are also subdivided into four categories due to particular underlying properties among them. See below:
- Plain Carbon Steel is encompassed within the 10xx series (containing 1.00% Mn maximum)
- Re-Sulfurized Carbon steel is encompassed within the 11xx series
- Re -Sulfurized and Re-Phosphorized Carbon Steel is encompassed within the 12xx series
- Non-Re-Sulfurized High-Manganese (up-to 1.65%) carbon steel is encompassed within the 15xx series.
The first digit on all other alloy steels (under the SAE-AISI system), are then classified as follows:
2 = Nickel steels.
3 = Nickel-chromium steels.
4 = Molybdenum steels.
5 = Chromium steels.
6 = Chromium-vanadium steels.
7 = Tungsten-chromium steels.
8 = Nickel-chromium-molybdenum steels
9 = Silicon-manganese steels and various other SAE grades
The second digit of the series (sometimes but not always) indicates the concentration of the major element in percentiles (1 equals 1%).
The last two digits of the series indicate the carbon concentration to 0.01%.
For example: SAE 5130 is a chromium alloy steel containing about 1% of chromium and approximately 0.30% of carbon.