How to read SAE / AISI steel numbers (1018, 1045, 4140, 4340, 304)

The four digits

The SAE/AISI index is based on chemical composition. The first digit points to the main alloying system and the second to its level or variant within that system. Together the first two digits name the family.

The last two digits give the nominal carbon content in hundredths of a percent. A 1060 steel is a plain carbon steel with about 0.60% carbon. The actual specified carbon is a range around that figure, for example 0.43–0.50% for 1045.

AISI and SAE ran parallel lists for decades and merged them into the joint AISI/SAE system. In 1995 AISI handed maintenance of the system to SAE, so current composition lists are SAE documents, and bar standards such as ASTM A29 use the same grade numbers.

Letters inside and after the number

Letters add information without changing the base chemistry. They are not part of a separate system; they modify the four-digit grade.

• L between the second and third digits means lead is added for machinability: 12L14 is the leaded version of 1214-type free-cutting steel.
• B between the second and third digits means a small, deliberate boron addition for hardenability: 10B21, 15B37.
• H after the number means the steel is bought to a hardenability band measured by the Jominy end-quench test: 4140H, 8620H.
• E before the number means electric-arc-furnace steel, as in E52100 bearing steel.
• M marks merchant-quality carbon steel in older lists.

Stainless steels: the three-digit AISI types

Stainless steels use their own three-digit series. The 200 series are chromium-nickel-manganese austenitic steels, the 300 series chromium-nickel austenitic steels, and the 400 series ferritic and martensitic chromium steels.

In a stainless type the digits identify the grade only. 304 does not mean 0.04% carbon; 304 allows up to 0.08% carbon. Suffix letters mark variants: L for low carbon (304L, 316L), H for a controlled higher carbon range for high-temperature strength.

UNS numbers

The Unified Numbering System, run jointly by SAE and ASTM, gives every alloy a letter and five digits. G covers AISI/SAE carbon and alloy steels, H covers the H-steels, S covers stainless and heat-resisting steels, and T covers tool steels.

For carbon and alloy steels the UNS number is G followed by the four SAE digits and a fifth digit, which is 0 for the standard grade. Variants use other final digits: leaded 12L14 is G12144. Stainless types take S plus the AISI type and two digits: 304 is S30400 and 304L is S30403.

Worked examples

Each row reads the designation part by part. Carbon ranges are the published SAE limits; the two-digit figure is only the nominal midpoint.