American Iron and Steel Institute (AISI) together with Society of Automotive Engineers (SAE) have established four-digit (with additional letter prefixes) designation system:
First digit 1 indicates carbon steel (2-9 are used for alloy steels);
Second digit indicates modification of the steel.
0 - Plain carbon, non-modified
1 - Resulfurized
2 - Resulfurized and rephosphorized
5 - Non-resulfurized, Mn over 1.0%
Last two digits indicate carbon concentration in 0.01%.
Example: SAE 1030 means non modified carbon steel, containing 0.30% of carbon.
A letter prefix before the four-digit number indicates the steel making technology:
A - Alloy, basic open hearth
B - Carbon, acid Bessemer
C - Carbon, basic open hearth
D - Carbon, acid open hearth
E - Electric furnace
Example: AISI B1020 means non modified carbon steel, produced in acid Bessemer and containing 0.20% of carbon.
According to the four-digit classification SAE-AISI system:
First digit indicates the class of the alloy steel:
2- Nickel steels;
3- Nickel-chromium steels;
4- Molybdenum steels;
5- Chromium steels;
6- Chromium-vanadium steels;
7- Tungsten-chromium steels;
9- Silicon-manganese steels.
Second digit indicates concentration of the major element in percents (1 means 1%).
Last two digits indicate carbon concentration in 0,01%.
Example: SAE 5130 means alloy chromium steel, containing 1% of chromium and 0.30% of carbon.
AISI has established three-digit system for the stainless steels:
2XX series – chromium-nickel-manganese austenitic stainless steels;
3XX series – chromium-nickel austenitic stainless steels;
5XX series – low chromium martensitic stainless steels;
Designation system of one-letter in combination with a number is accepted for tool steels.
The letter means: