Unraveling the SAE/AISI Index System for Carbon and Alloy Steels

July 27, 2016

SAE designations are four- and five-digit codes that identify the chemical composition of carbon and alloy steels.

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For years, a four-digit AISI/SAE (American Iron and Steel Institute/Society of Automotive Engineers) index system identified certain grades of carbon and alloy steels according to standard chemical compositions. The AISI stopped issuing specifications back in the 1990s, and AISI grade designations have been renamed as SAE designations.

The SAE designations are four-digit codes that identify the chemical composition of carbon and alloy steels. But there are some alloy steels that carry a five-digit designation (such as 30XXX and 51XXX). The first digit identifies the major alloying agent(s), with 1 standing for carbon, 2 for nickel, 3 for nickel and chromium, 4 for molybdenum, 5 for chromium, 6 for chromium and vanadium, 8 for nickel, chromium, and molybdenum, and 9 for silicon and manganese steel.

The second digit of the series indicates the approximate concentration of the major alloying element in percentiles. And the last two digits indicate the carbon concentration (in 0.01% units). For example, 5130 designates a chromium steel with 1% of chromium and 0.3% of carbon.

There can also be letters B or L between the second and third digits. “B” indicates that 0.0005% to 0.003% of boron was added for better hardenability. The “L” means 0.15% to 0.35% of lead was added for better machinability.
Here’s a look at some common carbon and alloy steel designations:

1XXX: Carbon steels

10XX: Plain carbon

11XX: Free machining, resulphurized (screw stock)

12XX: Free machining, resulphurized, resphosphorized

13XX: Manganese steels

15XX: High manganese carburizing steels

2XXX: Nickel steels

23XX: 3.5% nickel

25XX: 5.0% nickel

3XXX: Nickel-chromium steels

31XX: 1.25% nickel, 0.60% chromium

32XX: 1.75% nickel, 1% chromium