AISI/SAE Steel Numbers Explained: Reading 4140, 1045 and 304

The four-digit carbon and alloy steel system

For carbon and alloy steels, the SAE/AISI code has four digits:

• First digit — alloy family: 1 = carbon, 2 = nickel, 3 = nickel-chromium, 4 = molybdenum (including Cr-Mo and Ni-Mo), 5 = chromium, 6 = chromium-vanadium, 7 = tungsten, 8 = nickel-chromium-molybdenum, 9 = silicon-manganese.
• Second digit — approximate content of the principal alloying element in alloy steels: 23xx carries roughly 3.5% Ni, 31xx roughly 1.25% Ni with chromium. In plain carbon 10xx grades it is simply 0.
• Last two digits — nominal carbon in hundredths of a percent: 4140 ≈ 0.40% C, 1045 ≈ 0.45% C, 8620 ≈ 0.20% C.

So 4140 reads as "Cr-Mo family, about 0.40% carbon" and 4340 as "Ni-Cr-Mo family, 0.40% carbon" — the system tells you the recipe class at a glance, which is why engineers still think in it.

Carbon steel sub-families

• 10xx — plain carbon steels.
• 11xx — resulfurized free-cutting steels.
• 12xx — resulfurized and rephosphorized free-cutting steels.
• 15xx — carbon steels with elevated manganese (typically 1.00–1.65%).

Letters inside the code carry real information: B marks boron-treated steel (50B44), L marks leaded free-cutting steel (12L14), and a trailing H (4140H) means hardenability limits are guaranteed to a standard band rather than chemistry alone.

Stainless: a different three-digit logic

Stainless and heat-resistant steels use three digits, where the first digit is a family, not an element:

• 2xx — austenitic chromium-nickel-manganese.
• 3xx — austenitic chromium-nickel: 304, 316, 321. The last two digits are serial numbers within the family.
• 4xx — chromium steels: martensitic (410, 420) and ferritic (430, 409).
• 5xx — low-chromium heat-resistant grades.
• 6xx — precipitation-hardening grades (630, better known as 17-4 PH).

Suffix letters matter as much as the digits: L = low carbon (≤ 0.03%), H = controlled higher carbon for creep service, N = nitrogen-strengthened, Ti/Nb = stabilized grades (316Ti, 347).

Two more conventions worth knowing

• The letter E in front of a code (E52100, the bearing steel) historically denotes electric-furnace melting — a process guarantee attached to the same alloy, not a different one.
• Codes sharing the first three digits but differing in carbon are different steels in behaviour: 4130, 4140 and 4150 carry one Cr-Mo recipe family into tubing, shafting and spring duties respectively, purely through carbon content and the heat treatment it permits.

Caveats

The code describes nominal chemistry only — it says nothing about product form, heat treatment or guaranteed properties, which come from the ASTM/ASME specification ordered. AISI itself no longer maintains the system; stewardship passed to SAE and ASTM, and the numbers survive formally as UNS designations (G41400, S30400) that modern specifications cite. Two grades sharing a four-digit code across different specifications are the same alloy family, not automatically interchangeable products.